Geotechnical investigation

Geotechnical investigation

Geotechnical investigation 9.1 9 Introduction As discussed in the previous chapters, the success rate of any ground-improvement technique depends u...

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Geotechnical investigation 9.1

9

Introduction

As discussed in the previous chapters, the success rate of any ground-improvement technique depends upon how much we have understood the subsurface conditions at the project site. Understanding the index and engineering properties of soils is essential before using the soil-stabilization techniques. The proportioning of materials and fixing the dose of stabilizers depends upon the properties of the soils to be stabilized. The effectiveness of many of the deep compaction methods (e.g. blasting, freezing, dynamic compaction, etc.) largely depends upon the presence of groundwater flow and/or site drainage conditions. The dynamic compaction method is not very effective if the fine content exceeds a certain limit. Selection of a dewatering method will largely depend upon the soil permeability. Similarly, the size and distribution of voids/fissures/cracks, presence of joints in rocks, filling materials present in joints, extent of weathering, and groundwater flow conditions comprise the details that must be assessed before doing grouting at a site. As already discussed, some of the techniques are applicable for cohesive soils, whereas some are for the noncohesive soils. In addition to understanding the effectiveness of various ground-improvement methods, the site exploration or site investigation will reveal a lot of information that is helpful in every stage of a project, starting from the project feasibility study to post failure investigation. If sufficient investment has been made into site investigation, the cost of a project will be lower (for ground improvement as well as for construction), especially in cases where uncertainty over ground conditions is expected. The depth and type of foundation to be used, skilled/unskilled manpower, and expertise required to implement the project, techniques, and equipment needed to execute the work on sites, are decided upon based on the information collected from the site investigation. Moreover, a good site investigation will minimize the chances of disputes that may arise between the contractors and owners due to insufficient subsurface information in the tender documents or even due to unpredictable ground conditions encountered during the execution of the project. Otherwise, this might cause delay in completion and also an increase in the cost of the project. In short, the time and cost required to complete the geotechnical work as well as the safety of the structures (including the effectiveness of various ground-improvement techniques) largely rely on the information collected from the site investigation. The main objectives of a site investigation are as follows: l

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To understand the feasibility of the project for the site To determine of depth, thickness, extent and composition of different soil/rock strata and assessment of groundwater conditions Selection of suitable construction techniques and ground improvement techniques as required

Geotechnical Investigations and Improvement of Ground Conditions. https://doi.org/10.1016/B978-0-12-817048-9.00009-3 © 2019 Elsevier Inc. All rights reserved.

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Geotechnical Investigations and Improvement of Ground Conditions

To determine of various engineering properties of the soil/rock strata and other geotechnical parameters regarding the allowable bearing capacity, settlement, earth pressure, etc. To identify any possibility of interference with the existing structures or the surrounding conditions To identify the presence of any subsurface anomalies and possibility of any kind of geotechnical hazards To avoid any kind of financial or contractual disputes between the owner and contractor that may arise from ground-related problems.

In this chapter the methodology for site investigation along with field-related studies are explained. However, considering the scope of this book, laboratory testing methods are not included here.

9.2

Planning of investigation programme

The different stages of a site investigation along with the geotechnical activities carried out during the preconstruction, construction, and postconstruction periods are presented in Fig. 9.1.

9.3

Methods of site investigation

The different steps in site investigation are outlined in Fig. 9.1. Broadly, the subsurface information is collected via (1) geophysical exploration, (2) field identification and laboratory testing of soil/rock samples collected by drilling or boring, and (3) field tests.

9.3.1 Geophysical exploration Geophysical investigation refers to the study of the subsurface physical properties using different sensing equipment placed above or below the earth’s surface. The geophysical tests are quick and cheaper to cover a very large area compared with the geotechnical field tests. However, soil/rock properties cannot be obtained conclusively using this method and subjective interpretation is required. A wide variety of geophysical methods (e.g. electrical resistivity, seismic refraction, electromagnetic, ground penetrating radar, microgravity survey, etc.) are available. However, each method has its own advantages and disadvantages. Hence, selection of a particular method or a combination of more than one of these methods for a particular project site, will depend upon several factors. These are l

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Objectives of the investigation Size of the study area Requirements of the design elements Geologic conditions Topography and site accessibility Limitations of budget and time, etc.

Desk study, walkover survey and site reconnaissance Study of available maps and aerial views; case studies of similar projects; collection of available data from different agencies/organizations Observation of the project location (built-up area/barren land); site accessibility (legal and physical aspects, natural and manmade constraints); topography; ecology; nearby amenities and availability of services (food, water, electricity, manpower, earthwork plants, etc.) Buried and overhead services at the construction site; extent of interferences with surrounding environment (volume of earthwork excavation, water discharge, trees cutting, etc.) or manmade establishment (damage to nearby building, monuments, etc. noise pollution, requirement of traffic control and/or traffic diversion) Visual identification of soils and rock outcrops on surface and from nearby cuttings Location of nearby water bodies, channels and water level in nearby wells Observation of surrounding structures (presence of any cracks, settlements, seepage, etc.)

Geotechnical investigation

Geotechnical investigation for construction projects

Preliminary site investigation Digging of a few trial pits and borehole drilling; geophysical investigations; field identification of soils and rocks in trial pits and samples collected from different depths in boreholes; determination of soil index properties in laboratory as applicable; conduction of a small nos. of field tests which are quick and easy to conduct

Preliminary investigation is sufficient (For minor projects)

Yes

Yes

Project feasibility?

Method of exploration, scope of work decided; initiation of tendering process

STOP

Factual/preliminary report preparation; trial pit and borehole loggings; determination of depth, thickness, extent and composition of different soil/rock strata and groundwater table; planning, arrangement and preparation of technical specification for the next stage

Detailed site investigation

Yes

Post construction failure?

Monitoring of subsurface information gathered and review/changes in investigation work

Further investigation as per requirement (forensic geotechnical investigation)

Detailed analysis of data and evaluation of design parameters for excavation work and construction of foundation; selection of ground improvement techniques, etc.; preparation of final geotechnical report

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Fig. 9.1 Geotechnical investigation planning and works.

Starting construction

Yes

No Sufficient nos. of trial pit digging and drilling of boreholes; soil/rock sampling, preservation and transportation to laboratory; conduction of laboratory tests on DS /UDS samples collected from site; adequate nos. of field tests as per site and project conditions

Any deviation in actual ground condition during construction excavation?

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The advantages and disadvantages of some of the geophysical tests are briefly discussed in Section 11.4 in Chapter 11. Seismic refraction and electrical resistivity are the two commonly used geophysical tests for different geotechnical projects. A brief explanation of these two methods is presented in the following section.

9.3.1.1 Seismic refraction method The seismic refraction method is a quick and economical technique of site investigation covering a large area. This method is also widely practiced in petroleum engineering to explore hydrocarbon trapping structures in sedimentary basins. This method is used to determine the thickness of different underlying strata and the material characteristics in terms of the wave velocity. This method is based on the principle that (1) seismic waves have different velocities in different soil/rock strata, and (2) the waves get refracted at the interface between two soil/rock strata. The test basically requires a energy source (hammer), seismometers (geophones) and a recording seismograph. The body waves, such as the primary (P) or secondary (S) wave, are propagated into the subsurface as shown in Fig. 9.2. A metal plate is first located on the ground surface at a distance of about 3–4 m from the recording instrument and geophones are inserted at suitable regular distances. The plate is then struck with a sledge hammer. The metal plate and geophones are connected to the recording instrument using a connecting wire. The wave travels through the underlying soil/rock and the arrival times are recorded on a digital recorder or seismograph. To better understand the concept a simple representation of the wave reflection phenomena is shown in Fig. 9.3. A source wave (P wave) is generated using a hammer and the arrival time is measured at the receiver end using a geophone placed at a distance ‘a’ from the source. The waves may travel in the following manner: Case I. A part of the wave that follows the direct path from source to receiver travels a distance ‘a’ along the ground surface with velocity Vp and travel time tt as:

Fig. 9.2 Seismic refraction test.

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Fig. 9.3 Wave reflection.

tt ¼

Distance travelled by the wave Wave velocity

tt ¼

a Vp

In this way, the Vp can be computed by knowing tt and a. Case II. The wave radiates away from the source with a hemispherical wave front and is reflected from the subsurface soil at depth ‘z’ as shown in Fig. 9.3. The angle of incidence ‘2i’ is imprinted at the bottom surface whilst striking the bottom of the strata horizontally. From Fig. 9.3, it can be written as: tan i ¼

a 2z

Hence, i ¼ tan 1

nao 2z

The time (tr) required for the wave to reach the receiver (geophone) at the surface after being reflected from the bottom can be represented as: tr ¼

¼

Distance travelled wave velocity 2

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi z2 + ða=2Þ2 Vp

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4z2 + a2 tr ¼ Vp

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Simplifying the above expression to calculate the thickness of soil strata below the ground surface: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi z ¼ 0:5 t2r Vp2  a2 The expression in terms of ‘z’ can be used once Vp is identified from case I, and wave travel time ‘tr’ and distance ‘a’ from transmitter to receiver are calculated. Studies have shown that the travel time obtained by using direct and indirect methods shows little difference as the distance between source and receiver increases. Further, by assuming a two-layered strata, as shown in Fig. 9.4, points A and D can be identified as the source and the receiver, respectively. Line AD describes the direct ray path with distance ‘a’. The time required can be written as: tdr ¼

a V1

where V1 is the wave velocity of soil layer 1 as shown in Fig. 9.4. The other refracted wave travels from both soil layer 1 and soil layer 2. Now, the refracted wave travels from point A to point B making an angle ic at the layer interface and moves back to point D following path CD making an angle ic. A portion of refracted wave travels along the interface from B to C with velocity V2 in soil layer 2. Hence, the total time required by the wave to reach from source to receiver following the loop ABCD is t ¼ tAB + tBC + tCD t¼

z z fa  2z tan ic g + + V2 V1 cos ic V1 cos ic

The above travel time equation can be expressed in a simplified way by substituting sin ic ¼ V1/V2 and cos ic ¼ (1  V21/V22)1/2 based on Snells law as:

Fig. 9.4 Wave propagation in a two layered soil.

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a 2z cos ic + V1 V2

 1  a 2z V22  V12 2 t¼ + V1 V2 V2 If the receiver was placed at critical distance ‘ac’, then by considering the direct wave from transmitter to receiver as shown in Fig. 9.3, the above equation can be simplified to obtain the thickness of soil layer as follows:  ð1=2Þ   2z V2 2  V1 2 ac ¼ + Substituting ‘a’ with ‘ac ’ and t with V 1 V2 V1 V 2 V1  0:5 a c V 2  V1 or , z ¼ 2 V2 + V 1 ac

ac

In a similar manner, the thickness and wave velocity in different layered strata can be calculated. However, it is worth mentioning that the seismic refraction method may not work under the following conditions: l

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When there is a lack of sufficient contrast between the seismic velocity of the different strata If a layer has a seismic velocity less than that of its overlying layer If the individual layers are not homogeneous and not isotropic If noise is present (related to subsurface or any ambient conditions) during measurement at the site If a layer does not have sufficient thickness to be detected by the geophysical test.

9.3.1.2 Electrical resistivity method In this method, subsurface resistivity (resistivity refers to the resistance to the flow of current through the subsurface or earth material) is determined by measurement from the ground surface, which then can be correlated to different geotechnical parameters, such as the porosity and degree of saturation, etc. The applications of electrical resistivity in field geotechnology include l

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Assessment of layer thickness of organic deposits in inaccessible areas To locate an underground salty water boundary Identifying the fluctuations in groundwater quality in homogenous granular soil deposits subjected to chemical contamination Assessment of bedrock depth Variation in the properties of soil/rock strata.

In the field, an electric current (DC) is applied to the ground and the resulting potential difference is measured as shown in Fig. 9.5. The electrical resistivity setup consists of a battery for current supply, potentiometer for voltage measurement, ammeter for current measurement, and two outer and

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Fig. 9.5 Electrical resistivity test.

two inner electrodes, as shown in Fig. 9.5. The electrodes used to flow currents are known as the current electrodes and the electrodes between which the potential difference is measured are called as the potential electrodes. With the current source connected to the two outer electrodes, the resulting potential drop in the inner electrodes is recorded using a potentiometer. Resistivity then can be calculated by using the equation: R¼K

ΔV I

where R is the soil resistivity, K is a geometric constant that depends upon the spacing and arrangement of the electrodes, △V is the potential difference between inner electrodes, and I is the current flowing through the outer electrodes. Wenner and Schlumberger arrays are the two commonly used arrangements of electrodes. The method employs four equally spaced electrodes in a straight line, for the measurement of electrical resistivity and is called the Wenner array. For the Wenner array the K value can be calculated as 2πl where l is the distance between two electrodes. This array is suitable for a noisey environment, such as in urban areas. However, all the four electrodes need to be moved for each of the successive test. In the Schlumberger array, the potential electrodes are kept fixed, whereas the current electrodes are expanded in a line about the centre of the arrangement. In this array the K value can be calculated as πL2/4l where L and l are the distances between the current and potential electrodes, respectively. This method is faster for use in the field. The apparent resistivity (as determined from field measurement of △V, I and the geometric factor),

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when measured with a fixed electrode spacing moving progressively in traverse to give a series of horizontal readings, is known as profiling. On the other hand, measurement with successively greater electrode spacing with one fixed point in the array, which gives a series of data on depth, is known as sounding. The limitations of the electrical resistivity method are as follows: l

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Interpretation of electrical resistivity along the vertical profile may yield misleading results in absence of bore log data. The electrical resistivity largely depends on the concentration of salts and water content. However, soils with different engineering properties may yield the same results. The method is limited to a certain depth depending upon the electrical power that can be introduced at the site. Topography and surface features may affect the results.

9.3.2 Boring and drilling techniques For any construction projects, soil or rock excavation is required (1) to understand the ground profile and soil/rock stratification, (2) to collect samples for laboratory tests, (3) to facilitate various field tests at different depths, (4) to understand the groundwater conditions, and (5) to identify if any subsurface anomalies or utilities are present below the ground surface. There are different methods of boring or drilling available, which are explained in the following section. However, the selection of drilling methods for a particular project and/or site is based upon the following factors: 1. 2. 3. 4. 5. 6.

Depth of drilling required Types and frequency of sampling Lithology and aquifer characteristics Disposal of drilling fluid Health and safety and noise issues Time and project cost.

The different types of soil/rock excavation methods commonly used are presented in Fig. 9.6.

Fig. 9.6 Soils/rocks excavation methods.

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Geotechnical Investigations and Improvement of Ground Conditions

Test pit, trenches, shaft tunnels, and drifts are the ground excavation methods used for the in situ examination of soils and rocks. Test pit and trenches can be excavated easily by hand or machine. This is lower in cost compared with many other boring/ drilling methods and is relatively quick. Detailed ground observations can be done using these methods and they facilitate the sampling process. However, the extent of soil disturbance, depth limitation (3–5 m), and difficulties where there are unstable ground conditions and hard rock are some of the limitations of these methods. In addition, the excavation of shaft tunnels/drifts is quite expensive. Confined working space and difficulty in identifying discontinuities are other limitations.

9.3.2.1 Auger boring Augers are helical screws that are driven into the ground with rotation. Hand augers are operated manually. It is an easy and portable method of sampling in soft-to-stiff soil near ground surface. However, it is labour intensive and there is depth limitation (2–3 m). Also, it is impractical in difficult ground conditions and in fully saturated cohesion-less soils and utmost care is required to ensure good-quality samples. However, a higher depth can be reached in less time using the power auger drilling, whereby the auger is connected to drill rods subjected to a torque. Although using this method it is not possible to collect representative samples due to the mixing of soils during boring. Moreover, heavy downward pressure during power drilling disturbs the soils in advance of the auger. Auger boring is quick, lower in cost, and there is no access restriction, unlike the drilling method.

9.3.2.2 Wash boring In this method the soil particles are loosened and moved up to the surface by jetting water through the chopping bit attached to the end of the drill rod. The drill rod in turn is continuously rotated and surged as the borehole is advanced in a downward direction. The advantages of this method are (1) it is lower cost than percussion and rotary core drilling, and (2) it is portable due to the use of a limited drilling accessories. A depth of 8–15 m can be easily reached by wash boring. However, in cemented soil and hard rock this method will not work. It is not possible to collect undisturbed samples using this method, although a change of soil strata can be identified by observing the changes in colour of the soil mud expelled during drilling. The availability of sufficient water for drilling is an important criterion for selection of this method. Moreover, poor drainage conditions may create a messy surface.

9.3.2.3 Percussion drilling Percussion drilling is employed when auger or wash boring is not possible in very stiff soil or rock. It also can be used in most soil types. Here the advancement of a hole is achieved by alternatively lifting and dropping a heavy cutting or hammering bit that is attached to a rope or cable that is lowered into an open hole or inside a temporary casing (casings are hollow cylindrical pipes used for borehole stability and to prevent

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the loss of drilling fluid through the boreholes). Usually a tripod is used to support the cable. The stroke of bit varies according to the ground condition. The major disadvantage of this method is that it is not possible to get good-quality undisturbed samples. In very hard rock (and especially fractured hard rock), down-the-hole (DTH) drilling can be employed. In this case the hammer, applying repeated percussive pressure, is located just behind the drill bit inside the hole, unlike the open percussion drilling, where the hammer is on top of the drilling string. The drilling string provides the necessary force and rotation to the hammer and bit, as well as compressed air or fluids to the hammer and for the flushing of cuttings. This arrangement also allows for much deeper percussion drilling. However, the DTH drills are typically more expensive.

Drill bit: Drill bits, as mentioned previously, are the tools for cutting soils/rocks. These drill bits come in many different sizes and shapes (e.g. spoon bit, spiral pointed, cone type, diamond coated, forstner bit, etc.) as shown in Fig. 9.7. Drilling fluid: Different types are drilling fluids (e.g. air, air/water, water-based mud, oil-based mud, and synthetic based fluids) are used during the drilling of boreholes into soil or rocks. The purpose of drilling fluid is as follows: (i) (ii) (iii) (iv)

Providing hydrostatic pressure to prevent the inflow of ground water inside the borehole. Cooling and lubricating the drilling bit. Cleaning of boreholes. Removing the rock cores and fragments excavated by the drilling bit and carrying them to the surface. (v) Transmitting the hydraulic energy to the measurement and logging tools and bit. (vi) Stabilizing the boreholes (casing is not required if an appropriate drilling fluid is used).

Fig. 9.7 Different types of drilling bits.

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Geotechnical Investigations and Improvement of Ground Conditions

9.3.2.4 Rotary core drilling In this method the combined action of downward force and rotary action is applied to the cutting bit (usually diamond or tungsten tipped) fitted down to a core barrel. The broken rock cores or fragments are removed by the circulating drilling fluid and pumped through the drill rods and bit up through the bore hole, from which it is collected in a settling tank for recirculation. This is the only method to collect good-quality samples from soft rock and it can also be used on hard rocks. Rock strata, along with the presence of cracks, joints, and fissures, can be assessed using this method. Important geotechnical parameters like core loss, total core recovery, solid core recovery, and rock quality designation can be determined in this method.

Core barrel: The core barrels (as shown in Fig. 9.8) are the tubes inside the drill pipes and they are supported by the drilling bits to receive the sample during core boring. The standard length of a core barrel varies from 1.5 to 3 m. It is available with a blank reaming shell and thread protector. Core barrels are available in single tube, double tubes, or triple tubes. In the case of a single tube the core barrel of the sampler rotates and thus there is a chance that the samples may be disturbed by shearing. However, the double tube samplers do not rotate with the core barrels and the samplers, and hence samples are not disturbed. The triple core barrel consists of a double-core barrel sampler along with a stationary liner, which protects the sample cores during extraction (useful in case of fractured rocks).

Fig. 9.8 Core barrels.

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Fig. 9.9 Drilling rig.

Generally, all the accessories or systems required for borehole drilling are nowadays assembled in one machine known as the drilling rig (as shown in Fig. 9.9). These machines are available with different capacities and heights, and can be operated using mechanical, electrical, hydraulic, or pneumatic power, depending upon the availability and site conditions. The machines are mounted onto a tracked vehicle and hence no manual work is required to move it from one location to other, Mobilization costs and time are therefore reduced. Advanced drilling rigs are equipped with GPS for exact positioning and to keep a permanent record of the borehole locations. Automated drilling rigs also help to record in situ the essential drilling parameters (i.e. variation in the rate of penetration of drilling rod, the rotational force, downward thrust, etc.), as shown in Fig. 9.10. It is worth mentioning that the magnitude of these drilling parameters provides a lot of information about the ground conditions (e.g. presence of subsurface voids or cavities, hardness of rock, etc.). In addition, cone penetration testing rigs, as well as direct push sampling rigs such as power probe and geo-probe, can be used for the characterization of unconsolidated soils and very soft rocks at shallow depths.

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Penetration rate (m/h) 0

250

500

Rotational pressure (bar) 0

25

50

75

100

Downward pressure (bar) 0

25

50

75

100

0

1

2

3

Depth (m)

4

5

6

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8

9

10

11

12

Fig. 9.10 A typical variation in drilling parameters obtained from the down-the-hole-hammer drilling (automatic logging).

9.3.3 Geotechnical field testing 9.3.3.1 Dynamic cone penetration test In situ penetration tests are popularly used in geotechnical engineering for subsoil investigations. The dynamic cone penetration test (DCPT) is used to evaluate the in situ resistance of soils to penetration. DCPT is performed by dropping a hammer from a certain height. The penetration is continuously recorded to measure the shearing resistance up to 5 ft below the ground surface. The results from the test are further correlated with the California Bearing Ratio (CBR) values, in situ density, resilient modulus, and soil-bearing capacity, etc. Fig. 9.11 shows the typical configuration

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Fig. 9.11 Dynamic cone penetration test.

of a dynamic cone penetration test. The dynamic cone penetrometer consists of an upper and lower shaft. The upper shaft has a 8 kg drop hammer with a 575 mm falling height and it is attached to the lower shaft through the anvil. The lower shaft portion has a replaceable cone at the bottom with a 60 degrees cone angle. A cone can be driven 3–4 ft at each location. Each test requires 5–30 min depending upon the type of surface. The test generally requires two to three skilled persons, one person for raising and dropping the hammer and another to record the cone penetration at each blow. The DCP elements are assembled by attaching the cone tip, connecting the upper and lower shaft, fixing the position of the anvil using a scale guide, setting the position of hammer, etc. The driving rod needs to be aligned prior to beginning the test. When testing highway pavement sites a hole must be cut through the upper bound layers of bitumen or concrete, etc., to avoid the stooping of the driving rod. The hole dimension should be at least 50 mm in diameter. The DCP is inserted gently by hand. The driving rod is held in one hand and the hammer weight is allowed to drop 575 mm onto the anvil until the widest part of the cone projects below the reference level. Then blows are administered and the penetration is recorded for each blow count. After the penetration value of each blow is recorded, the penetration value of a second blow also must be recorded by resetting the shaft reading to zero. Care should be taken to keep the rod straight for accurate measurements. Penetration recording continues in this way until the cone is driven to the full depth of lower shaft, then the DCP is extracted

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from the hole with a specially equipped jack tongue. The following points are important to remember when carrying out the DCP tests: There is a risk of pinching fingers between the hammer and anvil whilst working with DCP. It is important to apply the blows at the interval of 2–4 s to transfer the full energy of hammer to the lower shaft. Testing in cohesive and silty fine-grained soils containing clay requires the rotating of the shaft after every blow to prevent the sticking. The test points should be located at an interval of 30 m. The cone should be replaced when the diameter of the widest section is reduced by 2 mm or more due to testing in hard soils.

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The DCP test results are expressed in DPI (dynamic penetration index) related to the vertical movement of the cone produced by each drop in mm/blow. Soils with higher stiffness show a lower DPI, whereas soft soils show a higher DPI value. The field data are recorded onto a spreadsheet by making two separate columns for blow count and rod penetration. The penetration index can be calculated by subtracting the previous rod penetration from the present one and dividing it by the difference between present and previous blow counts, as shown in Table 9.1. Further, the other soil parameters can be established based on empirical relationships available in literature which correlate the DCP test results with California bearing ratio and standard penetration test results. A graph showing the DPI versus depth can be developed to assess the strength of different subgrade layers.

9.3.3.2 Pressure metre test The pressure metre test is another important in situ method for identifying the stiffness of soils and rocks. A pressure metre probe is a cylindrical device designed to apply uniform lateral pressure to the ground using a flexible membrane. The cylindrical device with a flexible membrane is installed vertically below the ground surface and is used to apply lateral pressure. The probe is connected by cabling or tubing to a test control unit, which is used to apply the pressure and to control the volume, and to a compressed gas chamber unit at the ground surface. The basic aim of conducting a pressure metre test is to acquire information on the stiffness of the soil by measuring the relationship between the radial applied stress and the resulting deformation. A pressure metre can be inserted into the ground by pushing, by preboring a hole, or by self-boring. A recently designed setup drives the pressure metre probe using compressed gas as shown in Fig. 9.12. This test is suitable mostly for Table 9.1 Spreadsheet format for calculating dynamic cone penetration index Blow counts (a)

Cone penetration (mm) (b)

Penetration index (mm/blow) (c)

0 1

5.6 6.1



6:15:6

2

6.3

10

¼ 0:5

21

¼ 0:2

6:36:1

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Fig. 9.12 Pressure metre test.

homogenous clay, silts, and sands. However, ground consisting of soft rocks can also be tested. In the case of the preboring method, a borehole is drilled to minimize lateral wall disturbance and to keep the diameter of the hole compatible with the probe size. The probe is lowered into the borehole to the test depth. After the probe reaches the desired level, pressure is applied in equal increments. The resulting changes in pressure– volume are recorded at 15, 30, 60, and 120 s, etc., after each pressure increment has been applied. Pressure and volume readings can be taken from the control unit. If a water table is encountered a probe can be inserted in a specially designed slotted tube that is driven into the ground using hammer blows or vibration. Some corrections that might be required whilst performing the test are as follows: l

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The probe consists of a rubber membrane and protective metal cover. Hence, a calibration test must be carried out with the probe on the surface to determine the relationship between applied pressure and volumetric expansion. The corrections can be ascertained by conducting a surface test in which the probe is confined using a steel cylinder and the volume change measured results from the expansion of the leads and the pressure and volume metre. A hydrostatic effect can be observed when the water table is encountered and the measuring cell and leads are filled with water. In such a case the pressure in the measuring cell is higher than that recorded by the pressure–volume metre.

Each test method consists of 10 approximately equal pressure increments. The number of increments depends upon the judgement of the operator based on limit pressure. Generally, 4 to 15 increments are considered. After the application of calibration corrections, the results can be plotted (pressure metre curve) as presented in Fig. 9.13.

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Geotechnical Investigations and Improvement of Ground Conditions Ep = k dp/dv = 1810 ´ 6/20 = 544 kg/cm2 700 60

500

50

Pressure – volume curve

Limit pressure pL

400

Creep

300 200

100

40 30

20

10

Creep curve

0 0

10

Creep volume (V60 –V30) (cc)

Plastic phase

pressure, pf

DV (cc)

Elastic phase 600

20

0

p (kg/cm2)

Fig. 9.13 Pressure metre test curve on mudstone (Meigh and Greenland, 1965).

A pressure metre curve can be divided into three parts based on the three phases of the tests as follow: l

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Bedding of the probe against the borehole wall and reestablishment of in situ stress Elastic linear stress–strain behaviour with low creep level (upto ρf in Fig. 9.13) Plastic deformation with increasing creep stress, which is the measure of soil failure (limit pressure).

In the case of clayey soils, the pressure metre curve starts from the in situ stress level and then proceeds through an elastic phase. There is later shift to an elasto-plastic phase. The advantages of pressure metre test are as follows: l

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A large number of fundamental and basic soil properties can be obtained from the single test. It is useful for testing a large volume of material at the appropriate confining stress. The results can be obtained quickly due to the automated data logging system. The test is useful for predicting the performance of laterally loaded piles. Pressure metre tests are regularly used to calibrate the FE models of complex advanced geotechnical problems.

The limitations of pressure metre test are as follows: l

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The pressure metre test is not suitable for gravels and rocks. The operation of pressure metre test instruments and its associated equipment requires skilled and trained personel. Only two stress paths can be followed in practice, i.e. undrained and fully drained. The use of inappropriate analysis to interpret a pressure metre test may result in misleading parameters.

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9.3.3.3 Lugeon test The Lugeon test (or Packer test) is one of the most widely used in situ testing methods employed for determining the average hydraulic conductivity of the rock formations. The test is named after Maurice Lugeon (1933). The test basically involves measuring the volume of water taken in a section of a test hole when the same is pressurized at a given level (10 bars–150 psi). The test is used for the evaluation of fracturing and discontinuities in rock formations. Conceptually the test is more or less similar to the constant head permeability test. The test is carried out in a borehole section isolated by the pneumatic packers. Water is injected using perforated pipes, which are bounded by the inflated packers, as shown in Fig. 9.14. The packers seal the portion of the borehole where the testing is conducted. Additionally, a pressure transducer is equipped with the perforated pipes to measure the water pressure exerted during the test. Before starting the test a maximum test pressure (Pmax) needs to be defined, which should be lower than the in situ stress to avoid rock fracturing or failure. The test is conducted in five stages, with the application of five different pressures at an interval of 10 min whilst pumping water. The pressure loading consists of cycles of loading and unloading, as shown in Table 9.2. However, the water pressure and flow rate must be measured at every minute interval.

Fig. 9.14 Lugeon test.

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Table 9.2 Pressure loading cycles for Lugeon test Stages

1st

2nd

3rd

4th

5th

Pressure

0.50Pmax

0.75Pmax

Pmax

0.75Pmax

0.50Pmax

A single Lugeon value is then calculated for each of the five different stages and pressure applied using the formula as follows: 1 litre 10 ðbarsÞ C B Lugeon value ¼ water taken in test @meterA  min test pressure ðbarsÞ 0

After calculating the five individual Lugeon values, they are compared and inspected so that an appropriate decision can be made. Further, depending upon the condition of the bores and rocks, various flow patterns can be observed. Interpretations are then made of the various patterns of Lugeon values obtained, as presented in Fig. 9.15. Further, the coefficient of permeability for the formation can be determined as explained later in this chapter (see Packer test).

9.3.3.4 Flat plate dilatometer The flat plate dilatometer test (DMT) is an in situ method used to determine the in situ lateral stress and stiffness of the soil. This is a relatively simple test. The main part of the flat plate dilatometer consists of a flat, stainless-steel, thin steel blade, with a circular expandable steel membrane of 60 mm in diameter on one side, as shown in Fig. 9.16. The test involves driving the steel blade to the testing depth and the circular steel membrane located on one side of the blade is expanded horizontally into the soil. The corresponding pressure and deformation are recorded during the test. The penetration of the steel blade requires conventional field penetration equipment, as used for cone penetration test. The membrane of the plate is connected to the gas pressure unit on the surface, which provides the pressure to expand the steel membrane. The cutting steel plate works as an electric switch. The circular membrane is insulated electrically from the underlying steel blade of dilatometer. The membrane rests against the sensing disc prior to the expansion. The centre of the membrane then expands 1.1 mm into the soil with the help of external gas pressure, as shown in Fig. 9.17. The operational procedure of DMT can be summarized as follows: l

l

l

The test is performed by pushing the blade to the desired depth at a typical rate of penetration of 20 mm/s. The test depth is taken as 200 mm and depending upon the ground conditions, it can be extending up to 300 mm. The blade is pushed using a rotary drilling rig, hammer, or an SPT hammer.

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Fig. 9.15 Lugeon test results and interpretations. (Modified from Houlsby, A.C., 1976. Routine Interpretation of the Lugeon Water Test. Q. J. Eng. Geol. 9, 303–313.)

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After achieving the desired depth, the control valve is operated from the surface to expand the membrane using high-pressure nitrogen gas. Two readings are recorded using audio and visual signals at the control box. The first reading represents the pressure at which the membrane lifts off, which ensures the initial contact with the soil. The second reading is recorded after 1 mm deflection. After this stage, the operator releases the pressure. The third reading is recorded as the closing pressure known as total pore water stress. The third reading is similar to the first reading. After this, the plate is pushed to the next depth level to repeat the test procedure.

The advantages of the DMT are as follows: l

l

The test is simple and rapid. It can be performed on wide variety of soil types. The blade-like shape of the flat plate helps to reduce the shear and volumetric strains compared with other geophysical field-testing methods.

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Fig. 9.16 Flat plate dilatometer test.

Fig. 9.17 Working principle of DMT: (A) initial, (B) push, (C) pressure reading-A, and (D) pressure reading-B.

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The test provides a reasonable estimate of horizontal stress and overconsolidation ratio, which are difficult to measure. The test equipment is relatively inexpensive.

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The disadvantages of the DMT are as follows: l

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The DMT test involves limited field exposure hence the validity of the soil property correlations are uncertain. DMT has limited use in very dense and cemented soils and in soils containing gravels or coarser fragments. In the case of gravelly deposits, the blade may deviate from vertical penetration, causing difficulty in interpreting the stresses. The results may be affected due to various instrumental defects like leaking seals, deformed membranes, deformed push rods, damaged blades, poor electrical ground, method of driving, rod friction, etc.

9.3.3.5 Plate load test The load carrying capacity and extent to which a soil undergoes settlement under a given load must be known beforehand in order to design a foundation for the structures. In the field the ultimate bearing capacity and corresponding settlements can be determined with the help of plate load tests. The plate load test, which is used to determine bearing capacity and settlement, may be carried out by either using a gravity loading or truss loading method. A test pit of the same depth as the foundation and width equal to five times the size of bearing plate is dug in the ground. A bearing plate (usually 30 cm square or diameter and 25 mm thick) is then placed in the test pit. The size of the bearing plate chosen for a test depends on the type of soil. A square plate of 45 cm size may be adopted for clayey or silty soil and also for loose to medium-dense sandy soils having a standard penetration resistance number <15. A bearing plate of 30–75 cm may be adopted for carrying out the test in dense sandy or gravelly soil the standard penetration resistance number of which is between 15 and 30. The size of test plate further depends on the maximum size of grains. The test plate is loaded with the help of the hydraulic jack, using either a gravity loading frame or truss loading frame to bear the reaction. The gravity loading method of the plate load test is shown in Fig. 9.18A and B. The loading frame rests on the columns built on the sides of the test pit. The loading frame is loaded with sand bags, rocks, or concrete blocks. The applied load is transmitted to the ground using an extension pipe and through the bearing plate. Four dial gauges are placed diagonally on each corner of the bearing plate to measure the settlement. The applied load is controlled using a hydraulic jack placed between the loading frame and extension pipe. The arrangement of plate load test when a truss is used for loading the test plate is shown in Fig. 9.19A and B. The truss is adequately anchored to the ground using mild steel anchors and a holder channel. The less complex nature of the truss loading method has made it a preferable choice for engineers. The test plate is placed in such a way that the centre of test plate, hydraulic jack, and the loading frame coincide with each other. A seating pressure of about 7 kN/m2 is applied before starting the test. The load on the soil is then increased in increments (20% of the estimated safe load or 1/10th of the ultimate load). The settlement of the bearing plate is measured at intervals of 1, 5, 10, 20, 40, 60 min, etc., until no significant change in settlement is observed. Once the rate of settlement is observed to be <0.2 mm/min, the next load increment is applied, and the settlement observation is started again. The load increment is continued till failure or until a settlement of

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Loaded platform

Hydraulic jack Steel girder Extension pipe

Test pit

Dail gauge

(A)

(B) Fig. 9.18 (A) Gravity loading plate load test vertical section. (B) Gravity loading plate load test top view.

25 mm is achieved under normal conditions; however, in special cases the test may be continued till a settlement of 50 mm is reached. The typical load settlement curve for the plate load test in different types of soil is shown in Fig. 9.20. A zero correction to the load settlement curve may be required sometimes prior to the calculation of the bearing capacity of the soil. It is done by drawing a straight line intersecting the zero load line from the early straight line of the curve, which is then subtracted from the settlement readings. The limitations of the plate load test are as follows: l

As the width of the bearing plate is very small compared with the actual foundation, it only provides an estimate of the bearing capacity up to a depth of twice the width of the bearing plate.

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Fig. 9.19 (A) Reaction truss loading vertical section. (B) Reaction truss loading top view.

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Since the plate load test is conducted over a very short duration, the settlement measured during plate load should not be considered as the ultimate settlement. This is particularly relevant to clayey soils. The plate load test underestimates the bearing capacity of dense sandy soils, because the bearing capacity of dense sandy soils increase with an increase in the size of the footing. The failure load is often not well defined in the load settlement curves obtained from the test. Hence, errors may arise based upon personal interpretation. The effect of the water table may not be taken into account properly in the test. It is advised to lower the water level by pumping, if it is encountered at the testing depth.

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Fig. 9.20 Typical load settlement curves for different types of soil.

In sandy soils, bearing capacity calculation can be done as follows: QF ¼ QP

WF WP

where QF is the bearing capacity of the footing WF is the width of the footing QP is the bearing capacity of the soil as determined by plate load test and WP is the width of the bearing plate. However, the bearing capacity of clayey soil is not dependent on the size of the footing or bearing plate. Thus for clayey soils the bearing capacity is equal to the bearing capacity of soil determined by plate load test. Hence, QF  QP The size of the bearing plate also affects the settlement of the footing. The settlement of the footing in granular soil can be determined by the following equation:

WF ðWP + 0:3Þ SF ¼ S P WP ðWF + 0:3Þ

2

where SP is the settlement of the bearing plate as determined during plate load test, SF is the settlement of the actual footing. The settlement of footing in clayey soil can be estimated using the equation as follows: SF ¼ S P

WF WP

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9.3.3.6 Standard penetration test The standard penetration test is the most common and widely used in geotechnical investigations for various projects. It is used to determine the in situ density and angle of shearing resistance of cohesion-less soils and also the strength of cohesive soils. It is found to be of great use in cases where it is difficult to obtain undisturbed samples for testing, for example, in gravelly, sandy, silty, sandy clay, or weak rock formations. The standard penetration test uses a split spoon sampler for obtaining soil samples from the subsurface. The test is conducted by placing the split spoon sampler on the surface at the point at which the test is going to be conducted, as shown in Fig. 9.21. A hammer of 63.5 kg is dropped from a height of 760 mm (at a rate of 30 blows per minute) to drive the sampler into the ground to a depth of 150 mm. After the sampler is driven to the initial 150 mm depth, the split spoon sampler is further driven into the ground till it has penetrated into the ground to a total depth of 450 mm. The number of blows required to penetrate the sampler to 150 mm is noted down. Whilst the number of blows required to penetrate the ground to a depth of 150 mm is termed as seating drive, the number of blows required to penetrate the remaining 300 mm depth is termed the penetration resistance. Penetration resistance is generally denoted by N. When a hard stratum is encountered, the N value might exceed 50, then it is termed as refusal and in such cases the test is discontinued. The N value obtained by standard penetration must be corrected for overburden pressure of cohesionless soil. The corrected value of N denoted by N0 can be determined as: N0 ¼ Cn  N (Cn is the normalizing correction factor that can be obtained from Fig. 9.22).

Drop hammer Anvil Drill rod

Split spoon sampler

150 mm 150 mm 150 mm

Fig. 9.21 Standard penetration test.

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Fig. 9.22 Correction factor (Peck et al., 1974).

The obtained N0 value is further corrected for dilatancy. This correction should be applied only if (a) the strata below the water table consist of fine sand and silt, and (b) the penetration resistance N is >15. If the above two conditions are applicable, the dilatancy correction (Ne) can be calculated as: Ne ¼ 15 + (N0  15). Using the empirical relationship given by Meyerhof (1956) the approximate values of angle of shearing resistance ∅ can be estimated using Table 9.3. Similarly, the values of unconfined compressive strength can also be estimated using Table 9.4. Since the standard penetration is conducted in a bore hole, there are a number of potential sources of error in measuring the standard penetration resistance value N. The following precautions should be taken into consideration whilst conducting a standard penetration test (taken from NAVFAC, 1982): l

Cleaning the bore hole properly: It should be ensured that the bottom of the bore hole is free from any mud or sludge, which may get pushed into the sampler after each blow. It may further be compressed into the sampler, occupying a significant amount of the space meant for sample recovery, which leads to the loss of sample for testing.

Table 9.3 Empirical relationship of cohesive soils and penetration resistance Penetration resistance N (blows)

Approximate ∅(degrees)

Description

4 10 30 50 >50

25–30 27–32 30–35 35–40 38–43

Very loose Loose Medium Dense Very dense

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Table 9.4 Empirical relationship of unconfined compressive strength and penetration resistance

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Penetration resistance N (blows)

Unconfined compressive strength (t/m2)

Description

0 2 4 8 16 32

0 2.5 5 10 20 40

Very soft Soft Medium Stiff Very stiff Hard

Seating the sampler at the bottom of the bore hole: A proper contact between the tip of the sampler and the bottom of the bore hole should be established, otherwise it may lead to the recording of false N values. Driving started even before reaching casing bottom: It should be ensured that the bottom of the casing is reached before the test is begun, as if the test is started too early, a lower values in cohesive soil and higher values in sands may result. Maintain sufficient hydrostatic head: Whilst conducting a standard penetration test in sandy soil, the water table must be maintained at the piezometric level so as to maintain the same level of stiffness in the sand at the bottom of the borehole. Operators’ attitude: The mood of the operator and time of operation may significantly affect the number of blows counted. Overdriven sampler: The operator on some occasions may overdrive the sampler into the ground; this gives a higher blow count. Gravel plugging in sampler: Gravel plugging the mouth of the spoon sampler offers resistance to the penetration of the sampler even in loose sand, giving higher N values. Plugged casing: This problem frequently occurs in cases where the standard penetration test is to be carried out below the groundwater level in sandy soil. The hydrostatic pressure of the water causes the sand to plug the casing, resulting in higher N values. Over washing below casing: Over washing below the casing may loosen the dense sand present below the casing, thus causing it to record a lower blow count. Method of drilling employed: The bore holes for standard penetration tests may be held in position either by inserting a casing or by stabilizing it with mud. Thus, the N values may differ, depending upon the method of drilling employed.

In addition, the following precautions should be taken into consideration regarding the drilling rig in order to obtain reliable N values: l

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Hammer not falling freely: It should be ensured that the rope used to lift the hammer should not be turned >1.5 times around the drum. This restricts the free fall of the hammer, thus giving a higher blow count. Drop hammer weight: The weight of the drop hammer should neither be more than nor <64 kg. Drop hammer not striking anvil centrally: If the drop hammer does not strike the anvil centrally then a smaller amount of energy is transferred into the sampler. Thus increasing the penetration resistance values.

116 l

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Guide rod not used: If a guide rod is not used, then incorrect N values may be obtained. Damaged sampler tip: The damaged part of the sampler may either increase or decrease the area at the tip of the sampler, resulting in inaccurate N values. Heavier drill rods: Using heavier rods leads to a higher blow count as a heavier rod absorbs more energy. Incorrect drilling procedure: Drilling methods that disturb the soil should be avoided, as this will affect the recorded N values. Too large drill holes: Boreholes >102 mm in diameter are not recommended for a standard penetration test, as it may lead to decreased blow count. Careful supervision: A sampler may be impeded by gravel or cobbles, resulting in an increased number of blows. In such cases an inexperienced observer may not recognize the problem and may report an incorrect blow count. Incorrect logging: The samples must be described correctly. Avoid using a large-capacity pump: The use of a high-capacity pump will loosen the soil base causing the number of blow counts to be lower.

9.3.3.7 Packer test The packer test has already been discussed previously as the Lugeon test. Generally, the test is termed the Lugeon test for rocks and the packer test in soils by most researchers. The packer test is most commonly used in rocks and occasional in soils. The permeability measurements using the packer test can be conducted either by using a single- or double-packer system. If the hole can stand without a casing, a doublepacker test can be conducted; otherwise a single-packer test is preferable. The test setup of a single-packer test is shown in Fig. 9.23A. The single-packer system uses a single inflatable packer to separate the test section from the bottom of the bore hole. Ground level

Packer Perforated pipe Test section

Packer Perforated pipe Test section

(A)

Packer

(B)

Fig. 9.23 Test setup for (A) single packer and (B) double packer method.

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Water is then injected into the test section under pressure and maintaining a constant head. The rate of flow of water is then measured under a steady-state condition. The double-packer system is used to separate a length of bore hole using two packers, as shown in Fig. 9.23B. The separated test section is about 3–6 m in length. The hydraulic conductivity or permeability of the bore hole under consideration is determined as follows, when piezometers are installed for observation: K¼

  Q L ln if L  10r 2πLh r

  Q 1 L K¼ sin h if 10r > L  r 2πLh 2r where K ¼ hydraulic conductivity Q ¼ inflow rate L ¼ length of the hole tested r ¼ inside radius of hole h ¼ difference in water level at the entry and the water table, if the test is conducted below the water table. If test is conducted above the water table, then h is the difference of water level at the entry and the middle of the test section. If water is applied under pressure ( p), then h becomes (h + p/γw).

9.3.3.8 Pocket penetrometer A pocket penetrometer is a small hand-held piece of equipment used to test the unconfined compressive strength of soil. An example of a pocket penetrometer is shown in Fig. 9.24. The unconfined compressive strength of soil is determined by pressing the needle of the pocket penetrometer against the in situ soil, until the indicator line meets the soil surface. The scale on the penetrometer enables the engineer to directly estimate the strength of the soil base on the penetration resistance of the soil immediately. The results may, however, be affected if the test is carried out on soil containing granular material.

9.3.3.9 Field vane shear test It is an in situ test to find out the undrained shear strength of clay soils. It consists of a rod with four radial vanes at the end (Fig. 9.25). This method is sometimes applied to determine the strength of mine tailings, organic muck, and other such materials. The test is often conducted on bores drilled into the ground or with pushed method.

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

Fig. 9.24 Pocket penetrometer.

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(B)

(A)

(C)

(D)

(E)

Ground level Vane rod

Four bladed Vane shear Device

Insertion of vane

Measure peak torque Tmax (Rotate at 6 degree/min)

Perform 8 to 10 Additional rotations Measure residual torque, T

Fig. 9.25 Schematic diagram of conducting field vane shear test. (A) Vane shear rod. (B) Pushing in the rod to the bottom of borehole. (C) Measuring peak torque with its rotation at 6 degree/min. (D) Additional 8–10 numbers of rotation. (E) Measurement of residual torque.

The vane consists of four blades, each of a diameter varying from 35 to 100 mm and the height is greater than the vane diameter and <2.5 times the vane diameter. The vane blades have a maximum thickness of about 3 mm. The apparatus also consists of a torque measuring device. The vane shear test is performed in a predrilled bore hole, the vane is penetrated into the ground in a single thrust up to the test depth without applying any torque. The undrained shear strength is determined by measuring the resistance of the rod to rotation. The rod is rotated in such a way that it rotates 6 degrees in 1 min. The maximum resistance offered by the soil to the rotation of the vane is noted at an interval of 15 s each. The undrained shear strength of soil is given as follows: Su ¼

T π ðD2 H=2 + ðD3 =12Þ

where Su is the undrained shear strength of the soil, T is the torque applied, H is the vane height, and D is the vane diameter.

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The vane shear test is simple and quick, and it gives good results in clay. However, the test cannot be conducted in fissured and vertically laminated clay, or if the clay contains coarse materials.

9.3.3.10 Sand replacement test The sand replacement method is used to determine in-place density. The sand replacement method of determination of in situ density uses a sand-pouring cylinder, cylindrical calibrating container, tray with a central circular hole, and a chisel. Determination of field density using the sand replacement method involves three steps. The first step involves calibration of a sand-pouring cylinder. The soil density is measured in the second step and then the water content and dry density is measured in the third step. The various stages of the sand replacement method are shown in Fig. 9.26. The sand-pouring cylinder is calibrated using the following steps. The sandpouring cylinder is filled with uniformly graded sand (passing 600 μ sieve and retained on 300 μ sieve) up to a height of 10 mm below the top. The mass of the sand-pouring cylinder filled with sand is recorded as M1. The sand-pouring cylinder is then placed over a plane surface and the shutter is opened to allow the sand to fill the cone. The weight of the sand remaining in the pouring cylinder is measured and noted as M2. The sand on the plane surface is collected and transferred back into the pouring cylinder to make the weight of the sand-pouring cylinder equal to M1. The sand-pouring cylinder is then placed over the cylindrical can concentrically and the sand is allowed to fall into the cylindrical can till it is full. After the sand flow has ceased the shutter is closed and the weight of the pouring cylinder is measured again and noted as M3. The unit weight of the sand is calculated using the following formula: γ sand ¼

Mc V

Fig. 9.26 Sand replacement method.

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where V is the volume of the calibrating cylinder and Mc is the weight of sand required to fill the calibrating cylinder, which is given by the following formula: Mc ¼ M1  M2  M3 The second step of the sand-replacement method is to measure the field density of the soil. The place at which field density is to be measured, is cleaned and the surface is levelled. A pan with the central hole is set in place. Taking the central hole as a guide, a pit of a diameter approximately equal to the central hole and about 12 cm in depth is excavated. The excavated soil is collected, weighed, and noted as MS. The soil is then kept in an oven to determine the moisture content. The sand-pouring cylinder having a weight of M1 is then placed over the excavated pit. The shutter of the sand-pouring cylinder is opened to allow the sand to flow freely into the pit till no further sand flow is observed. The weight of the sand-pouring cylinder is measured and noted as M4. The weight of the sand filling the excavated pit Mpit is determined by using the equation below: Mpit ¼ M1  M4  M2 The volume of the sand required to fill the pit is determined by dividing the weight of sand required to fill the excavated pit by the unit weight of sand determined, as follows: Vpit ¼

Mpit γ sand

The bulk unit weight of soil is determined by dividing the weight of sand required to fill the pit by volume of pit as follows: γ¼

MS Vpit

The dry unit weight of the soil is then determined as: γ dry ¼

γ 1 + Mc

where Mc is the moisture content of the excavated soil.

9.4

Field identification of soils and rocks properties

From the core pieces recovered during a drilling process, useful information regarding the rock quality can be interpreted. Some of the important rock parameters that can be directly assessed based on core recovery during drilling are the solid core recovery

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(SCR), total core recovery (TCR), core loss (CL), and rock quality designation (RQD). These parameters are determined as outline in the following section. During the drilling process the bit cuttings are removed in the flush system. The sample that passes up into the core barrel may be divided into five parts: Solid core >0.1 m in length Solid core <0.1 m in length Fragmental material not recovered as core (no full diameter pieces) Additional material that may have been lost from the previous core run. It includes the core pieces left when the barrel was pulled, material dropped from the core barrel during its withdrawal from the hole or cuttings that settled when circulation of drilling fluid was stopped. (v) A portion made up of sand silt from the erosion of soft or friable material and entirely removed along with flushed drilling fluid.

(i) (ii) (iii) (iv)

P TCR (%) ¼ (P Length of (i), (ii), (iii) and (iv)/total core run length)  100. SCR (%) ¼ ( Length of (i) and P (ii)/total core run length)  100. CL (%) ¼ 100-TCR (%) Or [ equivalent core Length of (v)  100/total core run length]. P RQD (%) ¼ ( Length of (i)/total core run length)  100. The RQD value is an important geotechnical parameter; it basically represents the percentage of good-quality rock recovered from a borehole during the drilling process and hence is an indicator of the rock quality at the site. It is difficult to obtain lengthy core pieces and hence the RQD value is less in soft, fractured, jointed, and/or weathered rocks. Rocks are rated as very poor to excellent based on the RQD value, as presented in Table 9.5. However, it is important to note that the length of the core pieces recovered will also depend upon the method of drilling, size of core barrel used, and the skill of the driller. Hence, assessment of rock samples based upon the RQD value is subjective up to a point and hence the strength of two rocks cannot be compared precisely. As such, the NX-size and the NQ-size remain the optimal core sizes for measuring RQD and are mostly used for geotechnical site investigation purposes. Moreover, some rocks, such as clay stones and shale, break up into small pieces with time due to slaking, desiccation, stress relief cracking, and/or swelling. Hence, it is very important also to keep a record of the exact time of core logging. Rocks with 100% RQD just after drilling may convert to a rock with 0% RQD after a few days or months because of the reasons mentioned above. Moreover, the core length (i.e. 10 cm) in the

Table 9.5 Rating of rocks as per rock quality designation (RQD) RQD value

Rock mass rating

RQD value

Rock mass rating

0%–25% 25%–50% 50%–75%

Very poor Poor Fair

75–90% 90–100%

Good Excellent

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definition of RQD mentioned above is not based upon any solid technical ground. The length of core pieces will depend upon the fracture pattern during drilling and the rock stratification and joint conditions in the site. Hence, proper engineering judgement is required in the selection of core pieces when determining the RQD of rock mass. Nevertheless, RQD value is extremely helpful for the comparison of one borehole with another, one depth with another, and one site with another. During the planning and investigation stages of any engineering project, the RQD value is going to be helpful in selecting an appropriate technique for excavation and in deciding the optimal depth of excavation to reach the suitable foundation strata and thus saving a good amount on project cost. Sometimes a stability index (SI) is also determined to express the rock quality in terms of the drilling parameters as follows: SI ¼ (0.1  core loss) + (Number of fractures per foot) + (0.1  broken core, i.e. core <7.5 cm in length) + Weathering (graded 1 to 4 from unweathered to severely weathered) + hardness (graded 1 to 4 from very hard to incompetent). The SI number is then related to a rock-grade classification from 10 (good rock, index <8) to 1 (incompetent rock, index >18).

9.4.1 Rock mass classification The extent of weathering in rocks and their hardness are the two important engineering parameters required for any construction project. The purpose of the grading of rocks based on these two parameters is to provide an engineering classification of a rock mass for a specific purpose or project. However, such a grading system is based on a subjective scale and, if incorporated in a log, it is the only part of the log that is a statement of opinion.

9.4.1.1 Weathering The weathering of rocks is a purely subjective term and is defined differently by different authors. However, weathering in general is a process that brings about several changes to the properties of a rock mass due to its exposure to changing environmental (physical, chemical, biological) conditions and hence from an engineering point of view, grading of weathering becomes very important. The alteration in the engineering properties of rocks due to weathering may occur on a material scale (rock strength, swelling potential, slaking index, compressibility, consolidation characteristics, etc.) and/or mass scale (discontinuities, fracture, joint, settlement, permeability, etc.), as reported in the literature (Dearman, 1986; Price, 1995; Lempe et al., 2010). Moreover, the behaviour of the weathered rocks depends upon the environmental conditions to which it is further exposed. For example, weathered rocks in dry conditions may swell or start to disintegrate when they come into contact with water. Similarly, different applications place emphasis on different attributes of weathered rocks. When constructing a building on weathered rocks, the engineer will be interested more in their bearing capacity; whereas on slopes and embankments, he or she is more concerned

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with changes in permeability as a result of weathering. However, the gradation of weathered rocks is a very difficult task and, as a result, the modification in various engineering properties of a weathered rock is quite unpredictable and cannot follow any calculation. In addition, engineering may expose the rock mass to a new environment and weathering may continue even after the construction has been completed. Efforts have been made by the researchers, engineers, and geologists to classify weathered rocks and the effects on the various engineering properties of rock mass. Moreover, codes and guidelines have been formulated by the individuals (as presented in Table 9.6) and also by the different international societies like International Society for Rock Mechanics (ISRM) and the Engineering Group of Geological Society Working Party. As mentioned previously, the descriptive terms used for classifying weathered rocks are purely subjective. For example, the colour seen by one person will depend on the type of light source, the background, the size of the object, and the colours that have been seen immediately before (Clayton et al., 1995). Moreover, it has been seen that often there is a mismatch in the descriptive terms (e.g. colours, decomposition, disintegration, discontinuities, fracture spacing, etc.) in the core logs and core photographs due to their subjective nature. The concept of the soil/rock mass ratio seems to be a better option to quantify the grade of weathering. However, the soil mass ratio again depends upon the friability or slaking nature of the rock mass and the environmental conditions to which it is further exposed. Interestingly, most of the classification systems are based upon the changes in the index properties of the weathered rock mass. A very few attempts have been made to relate weathered rocks to their engineering properties. It should be mentioned here that some of the field test results, like the standard penetration test (SPT) blow count and the shear and compression wave velocities, can precisely represent the weathering state in different rock mass. In contrast to the other classification systems, Price (1995) pointed out that in the description of mass weathering for engineering purposes, the uniformity or regularity and style of weathering are key issues. The Engineering Group of Geological Society Working Party (1995) has proposed a way of identifying the grade of weathering in rocks as presented in Table 9.7.

9.4.1.2 Hardness Rock hardness is another important parameter that needs to be identified for various engineering purposes. It is worth mentioning that rock hardness is not the same as mineral hardness (measured on Mohs scale). Rock is an aggregate of many minerals and rock hardness is expressed in terms of its compressive strength for engineering purposes. The hardness of rocks can be assessed for various engineering purposes as presented in Table 9.8. It should be noted that the overall behaviour of a rock mass for a particular engineering purpose will depend upon several factors, including the rock-strength properties, geological features of the project site, boundary conditions, location of the groundwater table, water pressure, etc. Hence, in order to assess the overall quality

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Table 9.6 Rating of rocks as per rock quality designation References

Moye (1955)

Roxton and Berry (1957) Little (1967) Dearman (1976)

Stapledon (1976)

Dames and Moore (1983) Komoo and Mogana (1988) Price (1995)

Engineering Group of Geological Society Working Party (1995) Hudec (1998)

Lempe et al. (2010)

Major descriptive terms

Origin/applicability

Joints strain, fock and soil material strain, feldspar decomposition, strength of NX core samples, disintegration in water, biotite decomposition and presence or absence of original texture Concept of rock:soil ratio

Weathering of granites in the Snowy Mountains, Australia

Concept of rock:soil ratio Discoloration, decompositions, disintegration and freshness Condition terms combined with descriptive terms for rock-material strength Modification to BS 5930

Material fabric (texture), discoloration, discontinuity surface Uniformity or regularity and style of weathering (uniform, complex, corestone and solution weathered) Changes in colour; changes in fracture state; reduction in strength; and presence, character and extent of weathering products Cluster analysis combining several tests (water absorption, magnesium sulphate, petrographic number, rock expansion, high–low T-cycles, freeze–thaw cycles, wettingdrying cycles, absorption SSD, impact strength, Los Angeles abrasion, bulk density) with similar results into groups Decalcification and associated browning, disintegration of dolomitic components, acidification and an increase of the clay content, loss of volume/mass

Weathered granite in Hong Kong Weathered granite Mass weathering grade

Wide range of rock types in varying structural conditions, Australia Granitic residual soil and sedimentary residual soil, Singapore Metamorphic rock (clastic metasediment) in Peninsular Malaysia Mass weathering for engineering purposes

Engineering purpose

Engineering purpose

Pleistocene glaciofluvial gravel rich in carbonate northern Alpine foreland (Bavaria/Germany)

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Table 9.7 Classification of rocks based on weathering Grade of weathering Fresh Slightly weathered Moderately weathered Highly weathered Completely weathered Residual soil

Field identification Unchanged from original states Slight discoloration on rock surface; slight weakening Discoloration penetrated inside the rock, but large pieces cannot be broken by hand Large pieces can be broken by hand; does not slake when dry samples immersed in water Considerably weakened; slakes; original texture still apparent Soil derived by in situ weathering but retaining none of the original texture/grain fabric

Table 9.8 Classification of rocks based on hardness

Hardness category

Typical range of UCS (MPa)

Very weak

0.6–1.25

Weak

1.25–5

Moderately weak

5–12.5

Field test on sample Scratched with fingernail; slight indentation by light blow of point of geologic pick; peels with pocket knife; requires power tools for excavation Permits denting by moderate pressure of the fingers; handheld specimen crumbles under firm blows with point of geologic pick Shallow indentation (1–3 mm) by firm blows with point of geologic hammer; peels with difficulty with pocket knife; resists denting by finger but can be abraded and pierced to a shallow depth by a pencil point; crumbles by rubbing with finger

Field test on outcrop

Easily deformable with finger pressure

Crumbles by rubbing with finger

Continued

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Table 9.8 Continued Moderately strong

12.5–50

Strong

50–100

Very strong

100–250

Extremely strong

>250

Can be scratched or peeled with pocket knife; intact handheld specimen breaks with single blow of a geologic hammer; can be distinctly scratched with a 20D (400 length and 13/3200 head size) common steel nail; resist a pencil point but can be scratched and cut with a knife blade Handheld specimen requires more than one hammer blow to break it; can be faintly scratched with 20D common steel nail; resistant to abrasion/cutting by knife blade but can be easily dented or broken by light blow of a hammer Specimen breaks only by repeated heavy blows of geologic hammer; can’t be scratched with a 20D common steel nail Specimen can only be chipped off and not broken by repeated heavy blows of geologic hammer

Unfractured outcrop crumbles with light hammer blows

Outcrops withstand a few firm blows before breaking

Outcrops withstand a few heavy ringing hammer blows, but will yield large fragments Outcrops resist heavy ringing hammer blows and yield only dust and small fragments that too with difficulty

Geology National Engineering Handbook, USDA Natural Resources Conservation Service.

and suitability of rock mass for different purposes (e.g. tunnelling, construction excavation, slope stabilization, structural support, erosion resistance capacity, scouring of foundation bed, etc.), different types of rock mass classification systems have been developed. Two of the common methods are presented in Tables 9.9–9.11. Other methods for classification of rock mass include Q-system or Norwegian Geotechnical Institute (NGI) system, unified rock classification system (URCS), rock material field classification procedure (RFMC), new Austrian tunnelling method (NATM), coal mine roof rating (CMRR), etc.. However, the selection and applicability of these methods are subject to the engineering purposes and the accuracy of subsurface information gathered from the site investigation.

Table 9.9 Rock mass classifications

Rock mass rating (RMR) system/geomechanics classification (Bieniawski, 1989)

1. Uniaxial compressive Strength of rock material 2. Rock quality designation (RQD) 3. Spacing of discontinuities 4. Joint character 5. Groundwater conditions 6. Orientation of discontinuities

Rock structure rating (RSR) system (Wickham, et al. 1972)

A:Geological parameters Rock origin Rock hardness Geologic structure B:Geometry parameters Joint spacing Joint orientation Direction of tunnel drive C: Effect of ground water inflow and joint condition

Methods ØRating of the first five parameters (in left column) is done (on a scale of 0 to 15 for strength; 3 to 20 for RQD; 5 to 20 for discontinuities; 0 to 30 for joints character; and 0 to 15 for groundwater condition) ØRating adjustment is done for the last parameter, i.e. orientation (on a scale of 0 to 12 for tunnels and mines; 0 to 25 for foundation and 0 to 60 for slopes) ØRMR ¼ rating sum of these six parameters (the RMR value varies from 0 to 100) ØBased on the RMR values, rock is graded as very poor (for an RMR value below 20) to very good (for an RMR value above 80) ØDepending upon the rock origin (i.e. igneous/ metamorphic/sedimentary) and hardness (i.e. hard/medium/soft/decomposed), a rating value (let’s say that A varies from 6 to 30) is assigned to the rock mass, which again depends upon its geologic structure (fault/ folding)

Tunnels, slopes, foundations, and mines

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127

ØDepending upon the geometry parameters another rating value is assigned (let’s say that B varies from 7 to 45) ØDepending upon the value (A + B), groundwater condition, and joint condition, a rating value (let’s say that C varies from 6 to 25) is assigned ØRSR ¼ A + B + C

Purpose for which the classification system can be used

Geotechnical investigation

Classification system

Parameters considered for classification of rock mass

Table 9.10 Rating of different geotechnical parameters in rock mass rating system Uniaxial compressive strength (MPa)

Rating

RQD (%)

Rating

Discontinuities spacing (m)

Rating

Joint character

Rating

>250

15

100–90

20

>2

20

30

250–100

12

90–75

17

2–0.6

15

100–50

7

75–50

13

0.6–0.2

10

50–25

4

50–25

8

0.2–0.06

8

25–5

2

<25

3

<0.06

5

Very rough surface; not continuous; no separation, unweathered rock walls Slightly rough surface; separation <1 mm, slightly weathered rock walls Slightly rough surface; separation <1 mm, highly weathered walls Slicken-sided surface or gouge filling <5 mm thick or separation 1–5 mm, joints extending more than several metres Gouge filling >5 mm thick or separation >5 mm; joints extending more than several metres

5–1 <1

1 0

Groundwater conditions

Rating adjustments for orientation of discontinuities (strike and dip)

Inflow per 10 m (l/m) Joint water pressure/major principal stress General condition Rating

Tunnels and mines Foundations Slopes

Nil 0 Completely dry 15

<10 <0.1 Moist 10

10–25 0.1–0.2 Wet 7

25–125 0.2–0.5 Dripping 4

25 20 10

0

>125 >0.5 Flowing 0

Very favourable

Favourable

Fair

Unfavourable

Very unfavourable

Rock mass classification

0

2

5

10

12

Class

Description

RMR

0 0

2 5

7 25

15 50

25 60

I II III IV V

Very good Good Fair Poor Very poor

100–81 80–61 60–41 40–21 20–0

Based on Bieniawski, Z.T., 1989. Engineering Rock Mass Classifications: A Complete Manual for Engineers and Geologists in Mining, Civil, and Petroleum Engineering, Wiley-Interscience. pp. 40–47.

Rating of parameter A Rock type based on hardness

Geologic structure

Rock origin

Hard

Medium

Soft

Decomposed

Igneous Metamorphic Sedimentary

I I II

II II III

III III IV

IV IV IV

Rock type I II III IV

Massive

Slightly faulted/ folded

Moderately faulted/ folded

Intensively faulted/ folded

30 27 24 19

22 20 18 15

15 13 12 10

9 8 7 6

Geotechnical investigation

Table 9.11 Rating of different geotechnical parameters in RSR system (Wickham et al., 1972)

Rating of parameter B Orientation and direct of drive Strike perpendicular to axis

Strike parallel to axis

Drive direction

Drive direction

With Dip

Both

Against dip

Either direction

Dip of joints

Dip of joints

Flat (0–20 degrees)

Dipping (20–50 degrees)

Vertical (50–90 degrees)

Dipping

Vertical

Flat

Dipping

Vertical

< 2 in 2–6 in 6–12 in 1–2 ft 2–4 ft >4 ft

9 13 23 30 36 40

11 16 24 32 38 43

13 19 28 36 40 45

10 15 19 25 33 37

12 17 22 28 35 40

9 14 23 30 36 40

9 14 23 28 24 38

7 11 19 24 28 34 Continued

129

Joint spacing

130

Table 9.11 Continued Rating of parameter C Sum of rating A + B Joint condition 13–44

45–75

Good

Fair

Poor

Good

Fair

Poor

Nil <200 200–1000 >1000

22 19 15 10

18 15 22 8

12 9 7 6

25 23 21 18

22 19 16 14

18 14 12 10

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Ground water inflow (gpm/1000 ft)

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9.4.2 Field identification of cohesive and noncohesive soils There are many ways to evaluate whether the soil on site is cohesive or noncohesive and to what extent. This can be done without conducting any field or laboratory testing; using only physical observation, mixing/dispersing the soils with water and applying pressure with the help of fingers and/or hammer or reinforcing rod, it can be done in the site. The best way to begin is to spread the soil sample onto a flat surface. If >50% is visible to the naked eye, the soil is coarse grained. Some other methods for field identification of fine-grained soils are as follows:

9.4.2.1 Dispersion test Pour a handful of the soil sample into a jar full of water and shake it well before allowing the soil particles to settle. The rate of settlement and thickness of the various soils can be used to judge the gradation. Fine-sand settles in a minute or so, whereas silts take 15 min or more. The interface between different soil layers can be easily seen with the naked eye. Clay particles will remain unsettled overnight and the cloudiness of the water indicates a relative clay content.

9.4.2.2 Dilatancy test or shaking test Mix a small amount of soil with water (about the size of a 25 mm cube) to a very soft consistency in the palm of the hand. The back of the palm is then lightly tapped. If the soil is silty, water rises quickly to its surface and gives it a shiny or glistening appearance. If the soil is deformed by squeezing and stretching, the water flows back into it and leaves the surface with a dull appearance.

9.4.2.3 Toughness test Roll a long thread (about 3 mm diameter) of moist soil with the palms of the hands and fingers. Fold the thread of soil and repeat the procedure a number of times. With this process moisture is lost and the soil approaches a nonplastic condition and begins to stiffen and crumble. This transition state is referred to as the plastic limit of the soil. Just before the crumbly state is reached, a highly plastic clay can be rolled into a long thread. However, it is not possible to make this type of long thread with silty soil. Moreover, for higher clay contents the threads are stiffer and lumps are tougher at the plastic limit than for lower plastic clays.

9.4.2.4 Dry strength test Mould a pat of soil to the consistency of putty and allow it to dry in the air. It is then broken and a small fragment about 1 cm in size is pressed between thumb and forefinger. The effort required to break the fragment provides a basis for describing the strength. A clay fragment can be broken only with great effort, thus exhibiting a high dry strength. Silty sands and silts have only slight dry strength and can be distinguished by feel when powdered. Fine sands feel gritty whereas silts feel smooth like flour. It is worth mentioning that expansive soil may develop shrinkage cracks after

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drying along which the samples may break. In such cases this should not be misunderstood as the real dry strength of the soil.

9.4.2.5 Slaking test Place the soil sample in the sun to dry completely and then soak in a jar of water. Certain shales and soft rock and some large soil aggregates of 2–5 mm size disintegrate by drying and/or soaking. Slaking indicates the stability of soil aggregates and resistance to erosion. If the soil contains enough organic particles, this influences the index and engineering properties of soils. The importance of the organic content of soil is demonstrated by the fact that some of the chemical stabilization properties that work perfectly in soils with no organic content are not effective in organic clays. Thus it is very important to assess the organic content of soils along with other soil properties. In the field, organic soils can be identified by their dark brown to black colour and they may have an organic odour. Moreover, organic soils often change colour, for example, black to brown, when exposed to air. Some organic soils lighten in colour significantly when air dried. The response of organic soils to the dilatancy/toughness/dry strength tests is included in the Table 9.12. The in situ dryness/wetness of soil samples can also be assessed using physical observation in the field, as presented in Table 9.13. The in situ density of coarse-grained soil can be assessed as per the observations presented in Table 9.14. Similarly, the in situ strength of fine-grained soil can be assessed according to Table 9.15.

9.5

Sampling techniques, preservation, and transportation of samples to laboratory

Under the following conditions, field tests are preferable to laboratory tests: 1. When good-quality sampling is not possible or will be costly (e.g. granular soil, fractured rock mass, very soft and sensitive clayey, stoney soil, etc.) 2. When good results cannot be obtained from laboratory tests for a particular soil parameter (e.g. in situ horizontal stress, percolation value, etc.). The effects of large particle size and discontinuities are represented in field test results that are otherwise not possible from testing a small sample in the laboratory. 3. When in situ tests are cheap and quick relative to the process involved in sampling, preservation of samples and laboratory testing (e.g. SPT test in soft clays). 4. For soil/rock profiling and to find out the soil/rock stratification. 5. During the initial planning stages of projects, when the soil/rock parameter needs to be obtained quickly and there is no time available for conducting laboratory tests.

On the other hand, laboratory tests are required under the following conditions: 1. When the site is not accessible for carrying out a field test. 2. Depending upon the availability of facility, skilled manpower and equipment, when it is cheaper to obtain a certain soil/rock parameter from a laboratory test compared with the field test.

Geotechnical investigation

Table 9.12 Response of different soil types to field identification tests Test

ML

CL

OL

MI

CI

OI

MH

CH

OH

Dilatancy/ shaking Toughness

Quick

None–very slow Medium

Slow

None

Slow

Medium

Low

Medium

Low

Low

Medium– high

Low– medium

Slow– none Low– medium Low– medium

None

Low

Quick– slow None

None–very slow Low– medium Medium– high

Dry strength

None None– low

High High–very high

ML, low plastic silt; CL, low plastic clay; OL, organic silt and clay of low plastic; MI, silt of medium plastic; CI, clay of medium plastic; OI, organic silt and clay of medium plastic; MH, high plastic silt; CH, high plastic clay; OH, organic silt and clay of high plastic.

133

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Table 9.13 Field identification of in situ dryness/wetness of soils In situ state Wet Very moist Moist Slightly moist Dry

Field identification Seepage is visible. Water films present over the soil aggregates. If clay, it will be sticky and can be easily moulded by hand Seepage and water films as mentioned for wet soils are absent. Clay is less sticky Colour variation will be there. Clay is not sticky but can be moulded by strong hand pressure Damp and slightly moist to touch. Colour is paler than the wet and moist state, but darker than the soil in dry state. Clay is not sticky and difficult to mould by hand. Not dusty Dry to touch. Sands loose, silts weak and brittle, clays cannot be moulded and are frequently shattered. Often dusty

Table 9.14 Field identification of density of coarse-grained soils Apparent density

Field identification

Very loose (SPT: <4 and RD: < 20%) Loose (SPT: 4–10 and RD: 21%–40%) Medium dense (SPT: 11–30 and RD: 41%–70%) Dense (SPT: 31–50 and RD: 71%–85%) Very dense (SPT: >50 and RD: 85%–100%)

Easily penetrated by hand Penetrated when pushed by hand/easily penetrated with a 13 mm diameter reinforcing rod pushed by hand Easily penetrated with a 13 mm diameter reinforcing rod driven with a 2–3 kg hammer Penetrated to about 30 cm with a 13 mm diameter reinforcing rod driven with a 2–3 kg hammer Penetrated only a few cm with a 13 mm diameter reinforcing rod driven with a 2–3 kg hammer

3. When it is not possible to obtain certain soil parameters from in situ tests (e.g. cohesion value, angle of shear resistance, etc.) which are otherwise essential for designing of foundation. It is important to mention that control of drainage conditions, ascertaining stress path and strain level, and the measurement of pore water pressure are not possible through field tests. 4. For research purposes, where it is intended to use the results for different related projects.

So, when it is required to carry out laboratory tests to obtain the index and engineering properties of soils, we need to collect quality disturbed and/or undisturbed samples from the site. This process is known as sampling. There are various sampling methods. The selection of the sampling method mainly depends upon the types of soils and the quality of samples (disturbed and undisturbed) required. A list of suggested sampler types and methods appropriate for the different types of soils is presented in Table 9.16.

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Table 9.15 Field Identification of strength of fine-grained soils Apparent strength

Field identification

Very soft (SPT: <2 and UCS: <25 kPa) Soft (SPT: 2–4 and UCS: 25–50 kPa) Firm (SPT: 4–8 and UCS: 50–100 kPa) Stiff (SPT: 8–15 and UCS: 100–200 kPa)

Specimen extrudes between fingers, specimens (aspect ratio 2) sags under its own weight Specimen can be moulded by light finger pressure

Very stiff (SPT: 15–30 and UCS: 200–400 kPa) Hard (SPT: >30 and UCS: >400 kPa)

Can be imprinted easily with finger. Specimen can be moulded with strong finger pressure only Can be imprinted with considerable pressure from finger. Cannot be moulded by fingers but can be indented by thumb only Can be barely imprinted with considerable pressure from finger. Can be hardly indented by thumb nail Cannot be imprinted with fingers or difficult to indent by thumbnail

Table 9.16 Suitability of different soil samplers Soils type

Sampler type/method

Very soft cohesive soils, organic soils, varved clays (undisturbed) Soft-to-medium cohesive soils (undisturbed) Fine-to-medium sands above the water table (undisturbed)

Stockinette sampler, foil sampler, or fixedpiston sampler Fixed-piston sampler

Fine-to-medium sands below the water table (undisturbed) Alternating layers of soil and rock Hard or dense cohesive soils Rock (undisturbed) Sand (undisturbed) Clays, silt, fine-grained soil, clayey sands (undisturbed) Stiff to hard cohesive soil (undisturbed) Sampling from formations which are too hard for thin-wall shelby samplers or too brittle, soft or water-sensitive (undisturbed) Sand, silt and clays (partially undisturbed) Sand silt and clays (disturbed) Sandy deposits containing pebbles, cohesion-less soils below water table (disturbed) Cohesive soil, loose gravels, sand silts (disturbed)

Hand trimming using the cylinder with advanced trimming technique. Fixed-piston sampler in a cased and/or mudded borehole In situ freezing and coring. Fixed-piston sampler in a mudded borehole Rotary core-barrel sampler

Open-tube samplers under compressed air, in situ freezing, and impregnation Thinned walled shelby tubes Denison sampler Pitcher sampler

Hydraulic push with plastic lining Split spoon sampler Scrapper bucket sampler

Auger boring (hand or power driven)

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Fig. 9.27 Different types of soil samplers.

Some of the commonly used soil samplers are presented in Fig. 9.27. It is worth mentioning that sample disturbance is significantly affected by the various dimensions of the samplers. The same are explained as follows: (i) Area ratio (Ar): Ar (%) ¼ [(D22  D21)/D21]  100, where the dimensions are as per Fig. 9.28

For obtaining good-quality samples, the permissible area ratio should be less than equal to 10%, as per Hvorslev (1949). The permissible area ratios depend upon the soil type, its strength and sensitivity, and the purpose of the sampling operations. However, small area ratios may result in bending or buckling during sampling operations. D4

D3

Sampling tube

Cutting edge

D1 D2

Fig. 9.28 Sampler section.

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(ii) Inside clearance ratio (Ci): Ci (%) ¼ [(D3  D1)/D1]  100

The friction drag when samples enter the sampling tube depends upon the inside clearance ratio. In order to get good-quality samples this ratio should be between 0% and 1% for very short samples and 0.5% and 3% for medium-length samples, as per Hvorslev (1949). (iii) Outside clearance ratio (Co): Co (%) ¼ [(D2  D4)/D2]  100

In order to minimize the driving force and hence sample disturbance, the outside clearance ratio should be about 0% to 3% for cohesive soil and zero for noncohesive soils. (iv) Length-to-diameter ratio: The ratio of length to diameter of the samplers is generally limited to 5 to 10 for cohesionless soils and 10 to 20 for cohesive soils, depending upon the soil types and laboratory requirements.

In addition, the samples are subjected to disturbance at different stages due to various reasons as follows: (i) Before sampling: Base heave, piping, caving, swelling, stress relief, displacement (ii) During sampling: Failure to recover, mixing or segregation, remoulding, stress relief, displacement, stones along cutting edge (iii) After sampling: Chemical changes, migration of moisture, changes of water content, stress relief, freezing, overheating, vibration, disturbance caused during extrusion, disturbance caused during transportation and handling, disturbance caused due to storage, disturbance caused during sample preparation.

The level of disturbance caused before, during, and after sampling must be determined based on site observation and subsurface conditions and should be taken into consideration when rating the quality (i.e. disturbance level) of the samples. A sample with a particular rating should be used for a particular purpose only, as mentioned in the Table 9.17. Otherwise, if the results are used for other purposes, a suitable factor of safety must be applied depending upon the quality or disturbance level of the soil samples. On the other hand, with the use of a proper sampler and sampling techniques, the disturbance level during sampling can be minimized. In order to avoid sample disturbance (due to improper sample preservation and the wrong method of sample transportation from site to the testing laboratory) after sampling, precautions must be taken as outlined in the following section. Table 9.17 Rating of soil samples (German Standard DIN 4021, Rowe, 1972) Quality rating 1 2 3 4 5

State of soil samples No geometric distortion, shear strength and compressibility are unaffected Geometric distortion, density and water content unaffected Density altered, water content and particle size distribution unaffected Water content and density altered, particle size distribution unaffected Particle size distribution altered by loss of fines or grain crushing

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The procedures required for the preservation and transportation of soil samples would primarily depend upon the types of laboratory tests to be conducted on the samples, the types of samples and the climatic conditions. Accordingly, grouping of samples are done and separate procedures are followed for different groups. However, the samples from each of these group should be clearly labelled and a record of the following information should be maintained (in a field notebook dated and signed by the site engineer/in-charge) prior to transporting. The sample containers should be properly tagged and labelled: l

l

l

l

Borehole/tripit number and location details from where sample was been collected Depth and orientation of samples Timing and method of sampling Any special site observation and weather conditions, etc.

Group A: consists of samples used for the visual identification of the soils. During transportation it is important to ensure that no sample is lost or adulterated. Group B: consists of samples used for laboratory testing for the determination of moisture content, grain size analysis, soil classification, obtaining Proctor compaction results and relative density, as well as other tests like swelling potential, consolidation, permeability, CBR, shear strength, etc., on remolded samples. Samples from this group should be preserved and transported in sealed moistureproof containers like plastic bags, glass jars, thin-walled tubes, etc. The cylindrical and cube samples should be wrapped with aluminium foil and coated with several layers of wax and cheesecloth. Whilst shipping, the samples should be kept in a wooden cardboard or metallic box. Group C: consists of intact undisturbed samples used for conducting laboratory tests to get parameters like swelling potential, consolidation characteristics, in situ density, permeability, shear strength, etc. This group of samples need all the care given to Group B samples. In addition, these samples need to be protected from any kind of disturbance due to vibration and shock, as well as extreme heat and cold. The sealed samples are fitted into cardboard boxes with proper cushioning and/or insulation arrangements inside. Group D: consists of intact undisturbed samples of the same type as Group C, but these sample are also fragile or highly sensitive in nature. This group of samples needs all the care given to Group C samples. In addition, these samples should be transported by maintaining an orientation that is the same as when they were sampled. All the stages of transportation of these samples should be supervised by a geotechnical professional (i.e. an engineer, geologist of geoscientist).

9.6

Borehole and trial pit logging

A boring log is a description of the exploration procedures and subsurface conditions encountered during drilling, sampling, and coring. Similarly, the logging also can be done for a trial pit. The following is a brief list of the items that should be incorporated into a log for an adequate engineering geological description:

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1. Drilling crew, project name, name of client, consultants, and any other parties involved 2. The date and time that the borings were started and completed along with the weather conditions 3. Topographic survey data including boring location and surface elevation, and bench mark location and datum 4. The identification of the subsoil and bedrock including density, consistency, colour, moisture content, strength of rock mass, extent of weathering, etc. (expressed in a standard sequence of systematic description). The standard sequence generally followed for soils is as presented below:

Consistency or relative density, fabric or fissuring, colour, subsidiary constituents, angularity or grading of principal SOIL TYPE (in capitals/brackets), more detailed comments on constituents or fabric, geologic origin if known (in brackets), soil classification symbol. The standard sequence generally followed for rocks is as presented below: Weathered state, structure, colour, grain size, subordinate particle size, texture, alteration state, cementation state as relevant, rock hardness, mineral type as relevant, ROCK NAME (in capitals), more detailed comments on the spacing and angles of rock fractures, filling materials in the joints and joint surface roughness, any significant anomalies if observed (optional). 5. The depths of the various generalized soil and rock strata encountered 6. Sampler type, depth, penetration, and recovery 7. Sampling resistance in terms of hydraulic pressure or blows per depth of sampler penetration, size and type of hammer, and height of drop 8. Soil sampling interval and recovery 9. Rock core run numbers, depths and lengths, core recovery (core loss, solid core recovery as well as total core recovery) and rock quality designation (RQD) 10. Type of drilling operation used to advance and the process to stabilize the hole 11. Comparative resistance to drilling (rate of excavation in case of trial pit along with the method of excavation, i.e. manual or using an excavator); drilling parameters (rate of penetration, downward thrust, rotary pressure, drilling fluid used, etc.) can be obtained in situ in case of automated drilling rig used 12. Loss of drilling fluid 13. Water level observations with remarks on possible variations due to nearby water bodies 14. Field and laboratory test results if available (in factual reports that are submitted every week, or month, or so, depending upon the project requirement, laboratory test results might be missing in the logs; however, these can be included later on in the final report when the tests results are ready) 15. Drilled or excavated soil/rock in pictorial form can be incorporated (optional) in the bore log itself for better visualization of the geologic strata; otherwise, core/trial pit photographs are presented along with the bore logs/trial pit logs 16. Closure of borings.

For a clearer understanding, typical core samples and trial pit photographs, as obtained from drilling/excavation, are presented in Figs 9.29 and 9.30, respectively. Further, logging of this borehole (from which the cores in Fig. 9.29 are obtained) is presented in Fig. 9.31. Similarly, logging of the trial pit (corresponding to that in Fig. 9.30) is presented in Fig. 9.32.

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End of bore hole at 6.00 m

Fig. 9.29 Core photographs.

For proper visualization and hence interpretation of the subsurface condition, a summary of the bore logs can be prepared, as shown in Fig. 9.33 (for the borehole locations as shown in Fig. 9.34).

9.7

Assessment of ground water table

In simple terms, the groundwater level can be defined as the uppermost extent of groundwater. It is expressed as a height above a datum, generally the mean sea level (or the depth below the natural ground surface level). In construction projects the importance of groundwater level is due to the following reasons:

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Fig. 9.30 Trial pit photographs. 1. The position of the groundwater level below the ground surface affects the bearing capacity of soils 2. Water pressure will depend upon the position of the water level. If the water pressure is high, it will adversely affect the stability of sloping ground and earth pressure will increase for retaining walls 3. A high water table generally means that construction excavation work is going to be difficult and dewatering of the site is required 4. Lowering of the groundwater table can cause the soil to consolidate and thus induce ground settlement, which may be more severe in soft compressible soil

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and

and

Fig. 9.31 Borelogging.

and

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Fig. 9.32 Trial pit logging.

5. Land surface subsidence may occur due to a sudden drawdown of the water table 6. If the building site is surrounded by areas with higher ground surface, water may flow to the building site and thus damage the floor of the building, if proper drainage provisions are not put in place.

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Fig. 9.33 Summary of bore logs.

Moreover, if the relative groundwater levels in a number of boreholes are known, the flow direction can also be established. However, the numbers, relative elevations, and spatial locations of the monitoring bore wells are required to be determined very judiciously in this case. Groundwater levels are measured after the water level in the

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BOREHOLE NO-7 5m

U/S

(RL 274.250 m) 93 m WEIR 43.5 m

46.5 m

D/S

21 m BOREHOLE NO-1 45 m

(RL 269.110 m)

45 m

90 m BOREHOLE NO-6

BOREHOLE NO-5

(RL 271.235 m)

(RL 272.445 m)

BOREHOLE NO-2 (RL 271.025 m)

90 m

90 m

BOREHOLE NO-4 (RL 269.140 m)

BOREHOLE NO-3 (RL 267.670 m)

Fig. 9.34 Borehole locations.

monitoring wells has been stabilized. This generally takes from 24 h to several days. In the case of coarse-grained soils, it may even get occur in less than an hour; however, in very fine-grained soils it may take several weeks. Construction of a typical monitoring well is shown in Fig. 9.35. The groundwater level can be measured in monitoring wells or in a piezometer in the following manner:

9.7.1 Chalked steel tape In this method a weighted tape is rubbed with coloured chalk (bottom 0.9–1.2 m length) and is lowered down the monitoring well until lower part (about 0.6 m) is submerged in water. By lowering down the tape, contact of the weight (usually lead, brass or stainless steel) with the water surface can be heard. For wells with a deep water level, it must be ensured that the tape is not submerged below its chalked length. Now, the depth of the water table is obtained by subtracting the length of wet tape (indicated by wet chalk) from the total length of the tape below a reference point on top of the well.

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Fig. 9.35 Ground water monitoring bore well.

This method is very accurate and measurement can be done with an accuracy of about 3 mm. However, the tape needs to be checked for thermal expansion or stretching. This method is not suitable for rapidly changing water table and also the tape needs to be chalked for each measurement.

9.7.2 Float and pully sensor system This consists of a small (5–10 cm) float with a pulley on top of the borehole, as shown in Fig. 9.36. The moving float is looped over the geared pulley with a counterweight that activates a pen marking the level on a chart driven round on a horizontal clockwork drum. This method is less convenient, especially in narrow boreholes and higher maintenance is required. However, results can be obtained with moderate accuracy. The time scale of the chart is usually designed to last a week, but the trace continues around the drum until the chart is changed or the clock stops.

9.7.3 Electric tape The electric tape (as shown in Fig. 9.37) consists of a pair of insulated wires whose ends are separated by an air gap in an electrode. A circuit is completed when the electrode comes into contact with the groundwater surface, indicated by an ammeterneedle deflection, light and/or buzzer.

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Clockwork drum

Geared pulley

Counterweight

Fig. 9.36 Float recorder (Musy, 2001). Calibrated cable with insulated wires

Cable reel

Crank handle

Indicator buzzer or light Weighted end Inner electrode

Fig. 9.37 Electric cable water-level indicator (Sanders, 1998).

The accuracy of electric tape varies from about 3 mm to 6 mm. For better results, it is suggested that these electric tapes are calibrated with a chalked steel tape. One major limitation of this method is that low or high conductivity of water due to the presence of hydrocarbons in water may give a false reading.

9.7.4 Pressure transducers Pressure transducers use a silicon-based strain gauge that generates an electric current. The current is calibrated to pressure that can be related to the water level: 9.8 kPa ¼ 1 m of water height. The pressure transducers generally use vented cables to eliminate the effect of atmospheric pressure changes. Pressure transducers are equipped with an automatic digital data logger and hence are useful for the long-term

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monitoring of water levels. Moreover, data-recording intervals can be set from a fraction of seconds to a few days as per requirements. The limitation of pressure transducers includes (1) clogging of transducers and hence errors in readings due to the precipitation of iron hydroxides and other materials, (2) damage to the pressure transducer casings due to acidic drainage environments, (3) corrosion and damage to the transducer parts due to methane and hydrogen sulphide gas, etc., in the boreholes in underground mining environment, and (4) the effect of temperature variations.

9.7.5 Interface probe Unlike the pressure transducers, the interface probes can detect the smallest amount of anhydrous liquids on water and hence are useful for monitoring the groundwater level when it is contaminated with oil or hydrocarbons. Interface probes use infra-red refraction to detect hydrocarbons (liquid/air interface) and conductivity for distinguishing between water and hydrocarbon. For protection, these probes are designed to be chemically resistant.

9.8

Geotechnical specification writing

The technical specification is a written document given by the owner to the contractor who is charged with carrying out the site investigation for a particular project. Whilst carrying out the site investigation, the contractor is supposed to follow the guidelines of the specification, as per the agreement. Technical specifications are prepared by geotechnical experts or professional engineers with adequate knowledge and experience in similar fields. The reasons for the technical specifications of a geotechnical investigation are as follows: 1. To ensure that the investigation is carried out smoothly and completed on time 2. To ensure the site investigation is conducted in a proper manner so as to collect the correct information regarding the soil/rock properties in the project site 3. To minimize project cost that might increase otherwise due to ignorance of subsurface conditions 4. To avoid any dispute between the owner and contractor (on financial matters related to the scope of work, extra payments due to unexpected subsurface conditions encountered at the site or due to low-quality work, or completion by the contractor not being on time).

In short, a good specification will help to collect adequate and accurate information about the project site saving time and money. If there is any fault in the specification, it may be reflected in the design and construction or even during postconstruction periods. Hence, it is highly advisable that the technical specification be prepared precisely and in a very careful manner. Two sample of technical specifications for (1) borings, and (2) undisturbed soil sampling, as part of a geotechnical investigation, are presented in the following section.

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Example 1. Boring Boring shall be carried out in accordance with the provisions of BS5930 or equivalent standard with shell and auger, rotary drilling, or adopting a method that suits the prevailing site conditions. The boreholes shall have a minimum diameter of 76 mm and shall be suitably lined throughout. The toe of the lining shall at no time be >1 m above the level to which the material has been removed from the borehole. Before taking any undisturbed samples or making any in situ test, the lining shall be carried down to the bottom of the borehole. Auger of a proper size shall be used in soft to firm clays and silts to avoid suction. The use of a shell-tube shall be restricted only to very stiff to hard clays and sandy strata below the water table. The use of a chisel bit shall be permitted only in boulder or rock formations or through local obstructions. Uncased boreholes may be permitted only up to a depth where the sides of the holes can stand unsupported. In the case of side falls or squeezing being noticed, steps shall be taken immediately to stabilize the sides of the boreholes by casing pipes as directed by the geotechnical engineer. Bentonite slurry may be permitted to stabilize the boreholes as directed by the geotechnical engineer. Wash boring or any similar methods of boring, employing a water jet and/or a percussion bit may be permitted by the geotechnical engineer if sufficient progress in boring becomes impossible, considering the subsoil condition. No water shall be added whilst boring through cohesive soils and cohesionless soils above the water table. The cutting brought up by the auger, shell, the cutting shoe of the split-spoon, or the undisturbed sampler shall be carefully examined and the soil description duly recorded after performing field identification tests. After completion of the boring at any borehole, a bore log shall be prepared in a performa approved by the geotechnical engineer. After observing the position of the water table, backfilling of the borehole shall be carried out in an approved manner as directed by the geotechnical engineer. Example 2. Undisturbed Sampling Undisturbed samples shall generally be taken from the trial pits or boreholes at every identifiable change of strata unless otherwise instructed by the geotechnical engineer. In the case of sandy strata, the intervals of sampling shall be suitably increased. Selection of samplers and sampling procedure for undisturbed samples should be in accordance with provisions in BS1377 or equivalent unless otherwise instructed by the geotechnical engineer. In case of cohesive deposits, undisturbed samples shall be taken by an open tube sampler or piston sampler. The size of the sampler should be such that a sample having a minimum size of 50 mm diameter and 300 mm long can be recovered. The contractor is required to ascertain the diameter and length of the sample in accordance with the laboratory testing before taking the samples. The sampler shall be pushed strictly by hand or by jacking in soft deposits and no hammering is allowed. The area ratio of all samplers shall be limited to 10% in soft-to-firm cohesive deposits and use of thick-walled samplers may be permitted in case of deposits having very

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high consistency, subject to the approval of geotechnical engineer. Recovery ratios should be observed and recorded in the boreholes for every sample. The samples shall be sealed where necessary by wax, packed and properly labelled, and transported to the laboratory in accordance with BS1377 or equivalent. The top and bottom of the samples must be indicated clearly on the sample tube to facilitate the laboratory testing in proper orientation as specified by the geotechnical engineer. The samples that are 200 mm long in sampling tubes will not be paid for. The writing is very clear with little chance of the reader misunderstanding or there being a communication gap. In many cases a third party (consultant) is employed to check whether the work has been carried out as per the specification. It should be noted here that the technical specification for a particular item of work may vary from site to site, project to project, or even from client to client.

9.9

Geotechnical report writing

A geotechnical report is a way of communicating the subsurface information, along with the design and construction recommendations, to the planners, design engineers, architects, and other technical personnel involved with a project. The information included in the geotechnical reports is required during various stages of a project (for the selection of ground-improvement techniques if required, during structural design and construction, and even during post construction periods, whenever there is a failure of structures or loss of serviceability, for resolving claims and disputes). Incomplete and/or erroneous data in the geotechnical report may cause construction delays, overruns, claims, and disputes. The practice of geotechnical investigation in the case of a ground-related failure of structures within the rules and practice of the legal system is known as forensic geotechnical engineering, which is explained in detail in Chapter 10.

A geotechnical report may be a factual (preliminary) report or a detailed interpretive report. A factual geotechnical report is based only on the trial pit/bore log data, along with field observations and any laboratory test result, if available. It does include any interpretation of data or design or construction recommendation. These types of reports are prepared at regular intervals during the investigation and are used for constant monitoring of the ground situation and review of the investigation strategy. Factual geotechnical reports are generally required for large projects or in the case of complex ground conditions. At the end of a site investigation, a detailed report is prepared including the interpretation of soils/rocks data and geotechnical engineering recommendations. All these reports must be stamped, signed, and dated by the geotechnical engineer who can be contacted later whenever any clarification is required related to any information in the report.

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The geotechnical report should be very clear, concise, and accurate. The following information is generally expected to be included in a geotechnical report in sequence: l

l

l

l

l

l

l

l

l

l

l

l

Introduction of the report, i.e. about the project stage and its contents Project overview, i.e. name and background of the project Regional geology, local stratigraphy, groundwater occurrence, and description of the location Scope of work, methods and equipment used, and work schedule Summary of all subsurface exploration data including information about the groundwater table and soil stratification All field and laboratory test results Interpretation and analysis of the subsurface data (with methods and formulae used for calculations and the codes and standards used for analysis) Major geotechnical issues and possible alternatives based on site investigation data Engineering recommendations for design Recommendations for geotechnical special provisions (as applicable) supported with proper reasoning and justifications Conclusions Annexure ✓ Location map and site plan ✓ Layout map of the trial pits and boreholes ✓ Trial pit and borehole logs ✓ Core photographs in colour ✓ Worksheets of site engineer ✓ Certified laboratory test reports along with graphs if any produced from field or laboratory tests ✓ Any other visual aids or drawings as applicable to the project.

However, it must be kept in mind that a geotechnical report is meant for the particular project for which it was carried out, for a particular site where it was carried out, and for a particular client for whom it was carried out. The report can be referred to for a similar project or site condition, but it should be used very carefully whilst understanding the limitations of the report for other projects and site conditions. Even for the same site and project for which it was prepared, it can be subject to misinterpretation. Hence, a reader should follow the points below when using the geotechnical report of his or her project: l

l

l

l

l

A geotechnical report is subject to misinterpretation if not read carefully and completely. The information regarding subsurface conditions in the report is subject to change with time (e.g. groundwater table, boundary conditions, state of stress in the case of new constructions in nearby areas, etc.). The information in the report may not be reliable if used for a different purpose or project at the site. The reports are based on a few representative numbers of trial pit/borehole excavations. Hence, the ‘out of sight, out of mind’ often applies and a clear picture of the ground conditions is not known unless a complete excavation is done at the construction stage. The report should be always produced as the original. It should not be rewritten or the graphs and pictures should not be redrawn or reproduced. In such cases, there is a possibility of inviting errors into the report which will be communicated further.

152 l

l

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The geoenvironmental issues are not covered in the report. Such issues are to be addressed (as applicable to the project) in conjunction with the recommendation in the EIA (environmental impact analysis) report. Good coordination between the geotechnical engineer, geophysicists, geologists, design team members, and the contractors executing the work at site is essential for the successful use of the findings from the site investigation.

9.10

Geotechnical risk and hazards

Geotechnical risk is the key term associated with any construction projects and is related to site investigated. Moreover, the ground-improvement techniques adopted for different construction projects are preventive measures for minimizing the geotechnical risks. Hence, a brief explanation has been included in this chapter. The geotechnical risk is a problem associated with a construction as a result of ground conditions. It may be because either the ground conditions/response is poor or mistakes have been made by the project personnel in understanding the ground conditions. In either case, this adversely affects the project costs, completion time, and safety of the structure. Moreover, it may result in health issues or even loss of life, and may damage the environment. There are many causes of geotechnical risk, known as hazards (Fig. 9.38). Geotechnical risk is basically the multiplication of the extent of damage caused to a project by the hazard and the probability or frequency of the hazards. Keeping in view the consequences, first, it is important to realize the importance of geotechnical risk in any construction project. Second, a strategy must be made for the assessment of risk and its mitigation. What is the likelihood of a hazard causing damage and what would be the extent of that damage are quantified, which is an important factor in geotechnical risk management. This can be done mathematically using optimization techniques, reliability theories, and various probabilistic and statistical analyses. However, the accuracy of these analyses will again depend upon the source of data and information used. Many of the parameters are purely subjective and hence can be based on engineering judgments and experience gathered from other projects. A simple risk assessment plan is presented in Fig. 9.39.

Fig. 9.38 Source of geotechnical risk.

Geotechnical investigation 153

Fig. 9.39 Typical risk assessment plan.

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References Bieniawski, Z.T., 1989. Engineering Rock Mass Classifications: A Complete Manual for Engineers and Geologists in Mining, Civil, and Petroleum Engineering. Wiley-Interscience, New York, pp. 40–47. Clayton, C.R.I., Matthews, M.C., Simons, N.E., 1995. Site Investigation, second ed. Blackwell Science, Oxford, p. 584. Dames, Moore, 1983. Singapore mass rapid transit system – detailed geotechnical study interpretative report. Prepared for Provisional Mass Rapid Transit Authority, Singapore. Dearman, W.R., 1976. Weathering classification in the characterization of rock: a revision. Bull. Int. Assoc. Eng. Geol. 13, 123–127. Dearman WR (1986) State of weathering: the search for a rational approach, ln: Hawkins, A. B. (ed.) Site Investigation Practice: Assessing BS5930, Geological Society, London, Engg Geol. Special Publication, vol. 2, 132–142. Engineering Group of Geological Society Working Party, 1995. Description and classification of weathered rock for engineering purposes. Q. J. Eng. Geol. 28 (2), 207–242. Hudec, P.P., 1998. Rock properties and physical processes of rapid weathering and deterioration. In: 8th International IAEG Congress, Balkema, Rotterdam. ISBN 90 5410 990 4. Hvorslev, M.J., 1949. Subsurface exploration and sampling of soils for civil engineering purposes: Committee on Sampling and Testing. Soil Mechanics and Foundations Division, American Society of Civil Engineers: U.S. Corps of Engineers Waterways Experiment Station, Vicksburg, MS. Komoo, I., Mogana, S.N., 1988. Physical Characterization of Weathering Profiles of Clastic Metasediments in Peninsular Malaysia. In: Proceedings of the 2nd Conference on Geomechanics in Tropical Soils, Singapore, vol. 1. pp. 37–42. Lempe, B., Scholz, H., Thuro, K., 2010. The influence of weathering on the geotechnical properties of pleistocene coarse-grained glacio-fluvial deposits in the Northern Alpine Foreland (Bavaria/Germany). In: Williams, et al., (Ed.), Geologically Active. Taylor & Francis Group, London. ISBN 978-0-415-60,034-72,043. Little, A.L., 1967. Laterites. In: Proceedings of the 3rd Asian Conference on Soil Mechanics and Foundation Engineering, Haifa. vol. 2. pp. 61–71. Lugeon, M., 1933. Barrage et Geologie. Dunod, Paris. Meigh, A.C., Greenland, S.W., 1965. In situ testing of soft rocks. In: Proceedings of 6th International Conference on Soil Mechanics and Foundation Engineering, Montreal.1, pp. 73–76. Meyerhof, G.G., 1956. Penetration tests and bearing capacity of cohesionless soils. J. Soil Mech. Found. Div. 82 (1), 1–19. Moye, D.G., 1955. Engineering geology of the snowy mountains scheme. J. Inst. Eng. Aust. 27, 281–299. Musy, A., 2001. e-drologie. Ecole Polytechnique Federale, Lausanne. NAVFAC DM-7.1, 1982. Soil mechanics. In: Design Manual 7.1. Department of the Navy, Alexandria, VA. May. Peck, R.B., Hanson, W.E., Thornburn, T.H., 1974. Foundation Engineering, second ed. Wiley, New York. Price, D.G., 1995. Weathering and weathering processes. Q. J. Eng. Geol. 28, 243–252. Rowe, P.W., 1972. The relevance of soil fabric to site investigation practice: 12th Rankine Lecture. Geotechnique 22 (2), 195–300. Roxton, R.B., Berry, L., 1957. Weathering of granite and associated erosional features in Hong Kong. Bull. Geol. Soc. Am. 68, 1263–1292.

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Sanders, L.L., 1998. A Manual of Field Hydrogeology. Prentice Hall, Upper Saddle River, NJ. ISBN 0-13-227,927-4. Stapledon, D.H., 1976. Geological hazards and water storage. Bull. Int. Assoc. Eng. Geol. 14, 249–262. Wickham, G.E., Tiedemann, H.R., Skinner, E.H., 1972. Support determination based on geologic predictions. In: Lane, K.S., Garfield, L.A. (Eds.), Proc. North American Rapid Excav. Tunnelling Conf., Chicago. Soc. Min. Engrs, Am. Inst. Min. Metall. Petrolm Engrs, New York, pp. 43–64.

Further reading Anon, 1990. Geological Society Engineering Group Working Party Report: Tropical Residual Soils. Q. J. Eng. Geol. 23, 1–101. ASTM D 4220-95, 2007. Standard Practices for Preserving and Transporting Soil Samples. ASTM International, West Conshohocken. ASTM D 4719-07, 2007. Standard Test Method for Prebored Pressuremeter Testing in Soils. ASTM International, West Conshohocken. ASTM D 5777-00, 2000. Standard Guide for Using the Seismic Refraction Method for Subsurface Investigation. ASTM International, West Conshohocken. ASTM D 5878-00, 2000. Standard Guide for Using Rock-Mass Classification Systems for Engineering Purposes. ASTM International, West Conshohocken. BS5930, n.d. Code of Practice for Ground Investigations. British Standards Institution, London. Deere, D.U., Deere, D.W., 1998. The rock quality designation (RQD) index in practice. In: Kirkaldie, L. (Ed.), Rock Classification Systems for Engineering Purposes. 1998, ASTM, Philadelphia, pp. 91–101. ASTM STP 984. United States Department of Agriculture, 2012. Engineering Classification of Rock Materials, Part 631 Geology, National Engineering Handbook. Natural Resources Conservation, Washington DC. Geological Society Engineering Group Working Party, 1970. Report on the logging of rock cores for engineering purposes. Q. J. Eng. Geol. Hydrogeol. 3 (1), 1–24. Geological Society Working Group, 1995. Description and classification of weathered rock for engineering purposes, summary of recommendations of Engineering Group of Geological Society working party. Q. J. Eng. Geol. Hydrogeol.. 28(2). Houlsby, A.C., 1976. Routine Interpretation of the Lugeon Water Test. Q. J. Eng. Geol. 9, 303–313. IS 15736, 2007. Geological Exploration by Geophysical Method (Electrical Resistivity). Bureau of Indian Standard, Manak Bhavan, 9 Bahadur Shah Zafar Marg, New Delhi. Mayne, P.W., Christopher, B.R., DeJong, J., 2001. Geotechnical site characterization. In: Manual on Subsurface Investigations, National Highway Institute Publication No. FHWA NHI-01-031. Federal Highway Administration, Washington, DC. US Army Corps of Engineers, 2001. Geotechnical Investigations. Engineering Manual No. 1110-1-1804, US Army Corps of Engineers, 449 p.