Analysis of ground surface settlement induced by the construction of a large-diameter shallow-buried twin-tunnel in soft ground

Analysis of ground surface settlement induced by the construction of a large-diameter shallow-buried twin-tunnel in soft ground

Tunnelling and Underground Space Technology 83 (2019) 520–532 Contents lists available at ScienceDirect Tunnelling and Underground Space Technology ...

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Tunnelling and Underground Space Technology 83 (2019) 520–532

Contents lists available at ScienceDirect

Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust

Analysis of ground surface settlement induced by the construction of a largediameter shallow-buried twin-tunnel in soft ground Zhe Wanga, Wangjing Yaoa, Yuanqiang Caia, Bin Xua, Yi Fuc, Gang Weib,

T



a

Institute of Geotechnical Engineering, Zhejiang University of Technology, Zhejiang, Hangzhou 310014, China The Department of Civil Engineering, Zhejiang University City College, Zhejiang, Hangzhou 310015, China c Hangzhou Qianjiang New City Construction and Development Co., Ltd., Zhejiang, Hangzhou 310015, China b

ARTICLE INFO

ABSTRACT

Keywords: Shallow-buried tunnel Twin tunneling Closely-spaced Large-diameter Muddy silty clay Surface settlement

Soft soil poses more challenges to the construction of large-diameter shallow-buried tunnels than other soils: the surrounding rock mass is more easily disturbed, and the tunnel stability is more influenced by various factors. This paper focuses on the excavation of a large-diameter shallow-buried twin-tunnel in soft ground using the CRD (Cross Diagram) method and the four-step method. Based on field monitoring results, it analyzes the effects of both construction factors, such as tunneling methods and construction speed, and environmental factors, such as soil conditions and continuous rainfall, on the ground surface and tunnel vault settlements. For construction factors, it is indicated that the ground surface settlement caused by the four-step method is 332% of that caused by the CRD method in argillaceous siltstone. When the twin-tunnel is excavated from opposite directions, the shape of the ground surface settlement curve of the second tunnel excavation is affected by whether or not the surface settlement caused by the first tunnel excavation is stable. For environmental factors, it is indicated that the ground surface settlement is generally large (most of the monitoring values are larger than 100 mm) with a long duration under poor soil conditions. In muddy silty clay, the range of the longitudinal surface settlement affected by the tunnel excavation face is between −0.5D and 4D, and the ratio of the vault settlement to the surface settlement is less than 1. That ratio obtained when the CRD method was used in muddy silty clay is less than that obtained when the four-step method was used in the weathered argillaceous siltstone. In other words, the lower the strength of the soil is, the smaller the ratio of the vault settlement to the surface settlement becomes. The CRD method-induced ground surface settlement in muddy silty clay is 324% of that in weathered argillaceous siltstone. Continuous rainfall increases the load on the upper soil while reducing the strength of the upper soil and surrounding rocks, which causes significant deformation of the surface settlement within a short period of time.

1. Introduction Shallow-buried tunnels have been widely adopted in regions with good soil conditions, which cause relatively small ground surface settlement as the warning value is set as 30 mm according to the Beijing experience (Yao and Wang, 2006). It is rarer to construct shallowburied tunnels in soft ground, which tends to cause relatively large ground surface settlement. Especially when the tunnel diameter is large, the ground surface settlement becomes more sensitive, thus causing nearby surface to collapse more easily. Since changes of the ground surface settlement indicate the stability levels of both the ground surface and surrounding rock mass, it is very important to analyze these changes when a large diameter shallow-buried tunnel is excavated in soft ground. ⁎

Relying on numerical modeling, scholars have conducted meaningful studies on ground surface settlement induced by tunneling (Han et al., 2011; Huang and Zhang, 2004; Karakus and Fowell, 2003; Lee et al., 2006). Finite element modeling, however, involves simplifying some parameters and assuming the operating conditions. As a result, the simulated results are not as accurate as the field monitoring results. As far as field studies are concerned, small-diameter tunnels are more extensively investigated than large-diameter tunnels (Addenbrooke and Potts, 2001; Chen et al., 2011; Fang et al., 2016). With regard to the few field studies involving large-diameter tunnels, they are mainly concerned with the shield tunneling method in soft clay and the New Austrian tunneling method in good soil conditions (Fang et al., 2017; Min et al., 2015; Xie et al., 2016). As the literature shows, there is a lack of field studies investigating the use of the shallow-buried method to

Corresponding author. E-mail address: [email protected] (G. Wei).

https://doi.org/10.1016/j.tust.2018.09.021 Received 21 August 2017; Received in revised form 4 May 2018; Accepted 30 September 2018 0886-7798/ © 2018 Elsevier Ltd. All rights reserved.

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Fig. 1. The location of the Zizhi Tunnel.

Fig. 2. Cross-section views of the tunnel and high pressure jet grouting pile reinforcement area.

excavate large-diameter tunnels in muddy silty clay. The larger the tunnel diameters are, the more sensitive the ground surface settlements become to the changes of the influence factors. Some of those factors, such as tunnel depths, drainage conditions, and groundwater, have been investigated by researchers (Hajjar et al., 2015; Migliazza et al., 2009; Yoo et al., 2012). Few field studies concerning large-diameter tunnels, however, have analyzed such influence factors as construction methods, geological conditions and rainfall. This paper focuses on a large-diameter shallow-buried twin-tunnel. Based on the field monitoring results, it summarizes some common developments of ground surface settlement caused by tunneling. Specifically, it analyzes show the tunneling method, geological conditions, and continuous rainfall affected the ground surface settlement. Finally, it discusses the reasons for the sudden change of the ground surface settlement during the rainfall and proposes certain reinforcement measures.

shallow-buried tunneling method was used for excavation (Fig. 1). At the tunnel entrance, the axes of the parallel tunnels were 7.19 m apart, the overburden thickness was about 8 m, and the single tunnel diameter was12.8 m (Fig. 2). The construction of the Zizhi Tunnel began on April 9, 2014 and completed on June 27, 2016. 2.2. Geological and hydrogeological conditions The geological reconnaissance team obtained the geotechnical test samples by drilling boreholes in the field. They used thin wall sampler to take soft soil samples, sand sampler to take silt and sand samples, and took core samples as rock samples. Then they waxed all the samples and brought them from the field to the laboratory, where we carried out geotechnical tests on those samples. With regard to the soil samples, we obtained data on moisture, natural unit weight, horizontal permeation coefficient, vertical permeation coefficient, liquid limit, plastic limit, compression modulus, cohesion, and friction angle. With regard to the core samples, we obtained data on compressive strength, shear strength, cohesion, internal friction, and RQD values. The physical parameters of the soil and rock are shown, respectively, in Tables 1 and 2. The longitudinal section view is shown in Fig. 3.

2. Field measurements 2.1. Project overview The measured field is located in the first section of the Zizhi Tunnel of Hangzhou, China. Hangzhou’s latitude is 30.18 N and longitude 120.10E. It has subtropical monsoon climate. The 14.4 km long Zizhi Tunnel lies to the west of the West Lake in Hangzhou. The measured length was about 737.4 m (from K0 + 792 to K1 + 530) and the

2.2.1. Geological conditions Geomorphologically speaking, the K0 + 792-K0 + 860 section belongs to Qiantang River alluvial plain with backfill on the top. The K0 + 860-K1 + 440 section is lake plain with artificial fill on the top 521

522

Project

Samples number Max Min Mean

Samples number Max Min Mean

Samples number Max Min Mean

Samples number Max Min Mean

Number

Silty clay ②-1

Muddy silty clay ③

Muddy Silty clay ⑤

Clayey soil ⑥

Table 2 Soil properties.

⒂-2 ⒂-3

⒂-1

⑿-3

2 20.9 17.5 19.2

41.1 17.1 29.1

18.8 16.9 17.2

58.5 37.3 42.8

2

104

18.6 16.8 17.3

54.6 36.6 45.1

104

18

20.9 17.3 19.3

32.2 20.5 26.9

18

28

Natural unit weight kN/m3

-6

/ / /

/

6.84 × 10 2.51 × 10-6 5.81 × 10-6

7

2.5 × 10 1.5 × 10-7 2.22 × 10-7

-7

-6

/ / /

/

8.24 × 10 3.15 × 10-6 6.35 × 10-6

7

/ / /

/

7.23 × 10-6 3.52 × 10-6 5.27 × 10-6

6.56 × 10-6 3.16 × 10-6 4.71 × 10-6 7

6

6

Vertical permeation coefficient cm/s

/ / /

/

51.0 25.0 36.1

104

47.4 29.8 36.8

18

35.3 28.3 32.0

28

Liquid limit/%

/ / /

/

28.3 14.2 20.8

104

26.3 18.8 20.9

18

20.1 16.4 18.4

28

Plastic limit/%

22.7 10.8 15.3

104

21.1 11.0 15.9

18

15.5 10.8 13.5

28

Liquidity index

2.40 1.08 1.43

104

2.08 1.09 1.45

18

0.76 0.25 0.63

28

Plasticity index

3.6 1.64 2.94

95

2.95 1.85 2.64

10

10.26 5.12 7.68

5

Compression modulus/MPa

13 9 10

16

13 10 11

12

31 23 27

6

Cohesion/kPa

6 3 5

16

4 2 3

12

22 15 17

6

Friction angle/°

It has no bedding planes and the material is unevenly distributed. It is composed mainly of silty clay and construction waste It has no bedding planes and the material is unevenly distributed. It is composed mainly of silty clay, gravel and rubble It is soft, plastic, dry and of medium intensity. It shows no reaction to shaking test It is saturated, shiny, and tough, showing good flow plasticity, viscoelasticity, uniformity, and high dry strength. It shows no reaction to shaking test It is saturated and slightly lustrous, showing good flow plasticity, medium toughness, and high dry strength. It shows no reaction to shaking test It shows low to medium density and high humidity. Gravel accounts for 50–60% of this layer and their diameters are between 2 and 5 cm It shows visible signs of mineral weathering and alteration. The original rock is weathered into plastic clay and can be cut with a knife. The original rock structure is clear It is composed mainly of silt and contains some shale cementation, showing visible signs of mineral weathering and alteration. The rock is soft and easy to break with a hammer It is composed mainly of silt. The rock is soft and easy to break with a hammer. It softens when encountering water. The integrity of the rock mass is normal It has a porphyritic structure and shows an intense mineral differentiation. The original rock is weathered into plastic silty clay and can be cut with a knife. The current structure is not clear It has a porphyritic or block structure, showing visible signs of mineral weathering and alteration. It has joint fissures and breaks when hammering It has a porphyritic, matrix interlacing or block structure. The core is hard, and not easy to break with a hammer

Characteristic

Horizontal permeation coefficient cm/s

Dark gray, gray-yellow and brown PURPLISH grey and brown Brown and black grey

28

Moisture/%

Strong weathered basaltic porphyrite Intermediary weathered basaltic porphyrite

purplish red and crimson

Strong weathered argillaceous siltstone Intermediary weathered muddy siltstone. Fully weathered basaltic porphyrite Purplish red and crimson

Mainly brown and gray Mainly brown and gray Yellow Yellow Gray Gray and yellow Celadon and purple

Miscellaneous fill Plain fill Silty clay Muddy silty clay Muddy silty clay Clayey gravel Fully weathered argillaceous siltstone

①-1 ①-2 ②-1 ③ ⑤ ⑥ ⑿-1

⑿-2

Color

Soil type

Number

Table 1 Soil characteristics.

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Fig. 3. Geologic profile of the Zizhi Tunnel (K0 + 843 ∼ K1 + 353).

Fig. 4. Profile of the tunnel.

Fig. 5. Schematic view of vertical high pressure jet grouting pile.

and muddy silty clay at the bottom. The K1 + 440-K1 + 530 section is diluvia-bevel geomorphology with a mix of clay, gravel and breccia. The main characteristics of the soil are shown in Table 1. The main properties of the rock are shown in Table 2. The geologic profile is shown in Fig. 3.

(1) Porephreatic aquifer of silt Located between K0 + 792 and K0 + 860, the pore phreatic aquifer is composed of silt. Its permeability is weak, runoff is slow, and water storage is abundant. The well water output is generally 20–50 m3/d. Groundwater of this aquifer is supplied by the infiltration of both atmospheric and surface water. It has a mutual supply relationship with the nearby ponds. It is discharged by lateral runoff into the nearby ponds as well as evaporation, which causes significant changes of its water level.

2.2.2. Hydrogeological conditions We divided the groundwater in the concerned area into three aquifer groups according to water-bearing media, occurrence conditions, water’s physical properties and water conservation features. 523

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Fig. 6. Construction steps and support parameters of the CRD method.

(2) Confined aquifer of gravel

2.3. Construction methods

Located between K0 + 792 and K0 + 860, the confined aquifer is composed of gravel of medium to high density. Its permeability is strong and water storage is abundant, which may be related to the old Qiantang water course. It is baffled by an impermeable roof composed of muddy soil.

The tunnel overview is shown in Fig. 4, which focuses on the west (first) tunnel and introduces the construction methods, pre-reinforcement methods, monitoring points and sections. As shown in Fig. 4, both the CRD (cross diaphragm) and four-step methods were used for the excavation: the CRD method was used in the section from K0 + 792 to K1 + 043 and the four-step method was used in the section from K1 + 043 to K1 + 530. In the section from K1 + 073 to K1 + 530, temporary middle wall and invert were constructed at the beginning of the excavation. The schematic view of the high pressure jet grouting pile, constructed between K0 + 822 and K1 + 043, is shown in Fig. 5. This pilewas16m long and 38.8 m wide (3 m wider than the tunnel on each side). Fig. 5 shows the different arrangements of the pile: Φ600 high

(3) Porephreatic aquifer of silty clay and gravel Located between K0 + 860 and K1 + 530, the pore phreatic aquifer is composed of silty clay and gravel. Its density is medium, permeability is weak, and water storage is poor. The changes of water levels are significant: high in rainy season and low in dry season, with an annual amplitude of 1–3 m.

524

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Fig. 7. Construction steps and support parameters of the four steps method.

pressure jet grouting [email protected] mm (Style 1) was adopted along the dividing lines; Φ600 high pressure jet grouting [email protected] × 900 mm plum-blossom stake (Style 2) was adopted between the dividing lines; and Φ600 high pressure jet grouting [email protected] × 900 mm rectangle (Style 3) was adopted on the outskirts. The construction steps and support parameters of the CRD method is shown in Fig. 6.The pre-reinforcement measures include large-pipe shed and grouting. The primary lining consisted of the shotcrete and steel arches. The secondary lining consisted of the reinforced concrete. Between the two linings was a waterproofing membrane. The large pipe shed reinforcement was adopted above the tunnel crown between K0 + 792 and K1 + 043. It was 30 m long, with a diameter of 108 mm, a wall thickness of 6 mm, and a center-to-center spacing of 30 cm. The small grouting pipes installed in the longitudinal direction were 4.5 m long and with a center-to-center spacing of 30 cm. Those installed along the cross-section direction were 3.5 m long, with a center-to-center spacing of 1 m. With regard to the primary lining, the designed thickness was 30 cm, the type of the steel beam was I22a with a cross-section area of 4212 mm2, and the 28-days compressive strength of the

shotcrete was 25 MPa. With regard to the secondary lining, the designed thickness was 60 cm and the 28-days compressive strength of the cast-in-place concrete was35 MPa. Both the temporary middle wall and the invert consisted of I18 steel beams with a cross-section area of 3076 mm2. The excavation sequence of the CRD construction method is shown in Fig. 6. Firstly, section ①-1 was excavated. As ①-1 was excavated from top down, jet grouting was applied, which was followed by the welding of the steel arch and vertical support structure, completed by the reinforcement of a steel mesh. Then jet grouting was applied again to achieve the designed thickness. Secondly, section ①-2 was excavated for lateral support. Thirdly, section ②-1 was excavated. As section ②-1 was excavated, jet grouting was applied, which was followed by the welding of the steel arch and vertical support structure, completed by the reinforcement of a steel mesh. Then jet grouting was applied again to achieve the designed thickness. Fourthly, section ②-2 was excavated for lateral support. Fifthly, section ③-1 was excavated. As section ③-1 was excavated, jet grouting was applied, which was followed by the welding of the steel arch and vertical support structure, completed by the 525

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Fig. 8. The layout of the surface settlement monitoring points.

order to analyze the relationship of the vault settlement with the ground surface settlement. Fig. 8 shows the ground surface settlement points between cross sections S14 and S24. The layout of other monitoring points in other sections was the same as shown in Fig. 8. For the ground surface, the allowable settlement was set as 30 mm and the allowable settlement rate was 3 mm/d. For the tunnel vault, the allowable settlement was set as 20 mm and the allowable settlement rate was 2 mm/d.

reinforcement of a steel mesh. Then jet grouting was applied again to achieve the designed thickness. Sixthly, section ③-2 was excavated for lateral support. Seventhly, section ④-1 was excavated. As section ④-1 was excavated, jet grouting was applied, which was followed by the welding of the steel arch and vertical support structure, completed by the reinforcement of a steel mesh. Then jet grouting was applied again to achieve the designed thickness. At last, section ④-2 was excavated for lateral support. The tunnel drilling cycle was 0.5 m and the distance between each steel frame was 0.5 m. The speed of CRD method was 1 m/d. The construction steps and support parameters of the four-step method is shown in Fig. 7. Except for the adoption of the double-tube double packer grouting method for the four-step tunneling, other prereinforcement measures were the same as the CRD method. As it is shown in Fig. 7, Section ① was excavated first and the primary lining was constructed in the same way as the CRD method (K1 + 073〜K1 + 530, core soil was removed from the fourth segment but the vertical support system was constructed). Section ② and ➂ were then excavated. Section ⑤, ⑥, ④, ⑦, ⑧, and ⑨ were then excavated subsequently in that order. The advancing speed of the four-step method was 2 m/d.

3. Results 3.1. Surface settlement of single point on the center line The ground surface settlement measuring system consisted of multiple monitoring points that formed the settlement trough. Therefore, we could use the data obtained from those single points to analyze the maximum settlement, settlement duration, settlement rate, and the range of the influence of the tunnel face (Luo et al., 2006; Zhang and Lv, 2006). Fig. 9(a) shows the development of the surface settlement with time at monitoring point S14-0. The X-Axis represents the excavation date, and the Y-Axis represents the ground surface settlement. The positive direction of the Y-axis represents uplift and the negative direction of it represents settlement. S14-0 was defined as the origin. The numbers in Fig. 9(a) are the distances between the tunnel faces in part 1 and part 2 (as shown in Fig. 6) and the origin. A negative number means that the S14 cross section was not passed, while a positive number means that it was passed. Part 3 and part 4 was about 12 m behind part 1 and part 2. As shown in Fig. 9(a), on January 11, when the distances from the tunnel faces of the sections ① and ② to the origin were, respectively, -6m and -5m, the ground surface began to subside. On January 17, when those distances were, respectively, 8 m and 0 m, the settlement rate increased and the settlement value reached 33.49 mm (16% of the maximum settlement). On January 28, when those distances were, respectively, 25 m and 20m, the settlement rate decreased and the settlement value reached 156.85 mm (75% of the maximum settlement). On February 20, when those distances reached, respectively, 43 m and 53 m, the surface settlement became stabilized.

2.4. Arrangement of the monitoring points The DINI03 electronic level made by the Trimble Company of the US was used to obtain the surface and vault settlement values in the field. The accuracy of the electronic level is 0.3 mm/km. In addition, steel tapes and leveling staff were also used as secondary measuring tools. To investigate how the ground surface settlement changed, we arranged a total of eighty-seven monitoring cross-sections from k0 + 792 to k1 + 300. From S1 to S22, each section with an odd number had only one center point, while each section with an even number had fifteen center points with a 2.5 m distance between any two adjacent points. From S23 to S87, each section with an even number had only one center point, while each section with an odd number had fifteen center points with a 2.5 m distance between any two adjacent points. We numbered the corresponding vault settlements from VS1 to VS87 in 526

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by low-intensity, high thixotropy and high compression. With an overburden depth of only 9 m and a large diameter, tunneling caused obvious disturbance to the soil. Despite the adoption of the CRD method and pre-reinforcement measures such as the big-pipe shed, advanced small pipe grouting and full-face grouting, the maximum surface settlement still far exceeded the allowable value. Although the settlement values in those far exceeded the allowable value, the tunnel faces remained stable. Therefore, for excavation of large-diameter shallowburied tunnels in soft soil, it is to be discussed whether or not 30 mm should be the allowable value for ground surface settlement. Generally speaking, this value of 30 mm is based on statistical analysis of field data obtained in Northern China where the soils are composed mainly of silt and silty sand. Thanks to the good soil condition, in Northern China, it is relatively easy to control the ground surface settlement during excavation (Yao and Wang, 2006). In our case of tunneling in silty clay, the value of 30 mm is too small to be a meaningful reference. We argue that soil conditions and tunnel radiuses need to be taken into account when determining alert deformation values. Only then, we believe, those alert values such as the allowable ground settlement would be more relatable and play a better role as warning reference for tunneling projects. At S53-0, S54-0, and S56-0 where the soil consists of strong weathered argillaceous siltstone, both the stabilization time and maximum settlement were less than those at S16-0 to S19-0. Given the fact that the CRD method was used in the section from S16-0 to S19-0, we believe that in this study the influence of soil conditions on ground surface settlement may be greater than that of excavation methods.

(a) Center monitoring point of section K0+867

(b) Center monitoring points of section K0+872 to K0+892

3.2. Variations of ground surface settlement troughs Fig. 10 shows dates on which the excavation face passed the selected cross-sections. Fig. 11(a) shows the development of the transverse settlement trough curve with time at cross-section S85.The tunnel was excavated through strong weathered argillaceous siltstone, with an overburden thickness of 23.9 m. The west tunnel was excavated from the south side while the east tunnel was excavated from the north side. The west tunnel face passed S85 on February 13, 2016. Ninety-six days later, the east tunnel face passed S85 on May 20, 2016. As shown in Fig. 11, the tunnel face had passed S85 by February 15, 2016, while the monitoring of S85 did not start until February 18, 2016 due to construction reasons. Between February 18 and April 25, 2016, as a result, the ground surface settlement at S85 caused by the excavation of the west (first) tunnel was relatively small. The curve was shallow v-shaped and the maximum surface settlement of −36.81 mm is located at −2.5 month e X-axis. The settlement at S85 induced by the passing of the east (second) tunnel face, however, was bigger. The curve was deep vshaped and the maximum settlement of −177.35 mm is located at 9 m on the X-axis. Fig. 11(b) shows the development of the transverse settlement trough curve with time at cross-section S87 (20 m behind S85). The west tunnel face passed S87 on February 20, 2016. Sixty five days later, the east tunnel face passed S87 on April 25, 2016. Between February 18 and April 11, 2016, the maximum surface settlement induced by the west tunnel was −127.97 mm, which is located at 2.5 m on the X-axis. The settlement continued to increase with the passing of the east tunnel, reaching −235.99 mm that was located at 4 m on the X-axis. The surface settlement induced by the first tunnel accounted for 54% of the maximum settlement. Fig. 12 shows the development of the ground surface settlement with time at monitoring points ES85-0 and ES87-0. Comparing the settlements at those two points, we found that when the east tunnel face passed the concerned cross-sections, the settlement rate at ES85-0 was slower than that atES87-0, which indicated that the settlement rate at the cross section of ES85 was steadier than that of ES87. As shown in Fig. 11(a) and (b), the i (the distance from the tunnel centerline to the inflection point of the trough) of ES85 was smaller than that of ES87.

(c) Center monitoring points of section K1+065 to K1+080 Fig. 9. Development of surface settlement with time at selected center monitoring points of the west tunnel.

Luo concluded that during the excavation of large-diameter shallow-buried tunnels in soils composed of silt, sand and gravel, it was within the distance of −10 m to 10 m from the tunnel face that ground surface settlement occurred (Luo et al., 2006). In our case, however, it was within the distance of −6 m (about 0.5D) to 48 m (about 4D). Different soil conditions may explain those different findings. Fig. 9(b) shows the development of surface settlements with time at monitoring points from S16-0 to S19-0. The curves here are similar to that of S14-0. The settlements lasted for about 45 days, with the maximum settlement value of around 200–400 mm. Fig. 9(c) shows the development of surface settlements with time at monitoring points S53-0, S54-0, S56-0. S55-0 was destroyed during excavation. The curves here are similar to those in Fig. 9(b). But the settlements lasted about 26 days and the maximum value reached around 100–200 mm. The settlement values of S16-0 to S19-0 were large mostly due to the muddy silty clay soil condition. This muddy silty clay is characterized 527

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Fig. 10. Dates on which the excavation face passed the selected cross-sections.

The maximum surface settlement curves of ES85 and ES87 are, respectively, small v-shaped and big v-shaped. For ES85 and ES87, the maximum settlement points were respectively at the center of the second tunnel and between the two tunnels. Therefore, we believe when twin tunnels are excavated from opposite directions, whether or not the first tunnel-induced ground surface settlement rate is stable affects the settlement rate and the shape of the settlement trough induced by the second tunnel excavation.

settlement value when the soil consisted of clay and gravel sand (Huang and Zhang, 2005) Table 4. The relationship between the ground surface settlement and vault settlement in weathered argillaceous siltstone at monitoring points S530, S54-0 and S56-0 is shown in Fig. 13(b). The maximum vault and ground surface settlement values are shown in Table 3. The maximum vault settlements accounted for 57–63% of the maximum surface settlements. Comparing Fig. 13(a) and (b), it is evident that the proportions of the vault settlements in Fig. 13(a) are smaller than those in Fig. 13(b). In other words, the worse conditions the surrounding rock masses are in, the smaller percentages the vault settlement accounts for the maximum ground surface settlement.

3.3. Relationship between the surface settlement and the vault settlement The vault settlement monitoring points were set up one day after the passing of the tunnel face. Primary lining, temporary middle wall and temporary invert were installed three days after the passing of the excavation face. The date of the first point of every curve is zero at the time of reading. Fig. 13(a) shows the relationship between the surface settlement at S16-0 ∼ S19-0 and the vault settlement at VS16-0 ∼ VS19-0 where the soil consists of muddy silty clay. Table 3 tells us each point’s maximum vault and surface settlements. As it shows, the maximum vault settlement accounted for 31–41%of the corresponding maximum surface settlement. Theoretically, vault settlements should be larger than surface settlements. Our field data, however, showed the opposite. This might be due to the fact that we were unable to obtain the vault settlement data during two time periods: the first was before the tunnel face passed the concerned cross sections, and the second was from the beginning of the excavation at a concerned cross section to the completion of the installation of the monitoring points there. For VS16-0 to VS19-0, the pre-excavation surface settlement values accounted, respectively, for 8%, 10%, 9%, and 10% of the maximum surface settlement values. Assuming that the vault settlement values were similar to that of the surface settlement during the first time period, the settlement values during the second time period accounted, respectively, 54%, 49%, 60%, and 58% of the maximum settlement values. Those rather large percentages indicate whether or not the pre-reinforcement support system is completed in a timely manner has a great impact on tunnel deformation and ground surface settlement in such muddy silty clay. Jun Huang’s study also showed that the maximum vault settlement value accounted for about 40% of the maximum ground surface

3.4. Influence of the continuous rainfall Table 5 shows how many days each different type of rain lasted during the excavation. As it shows, during the eighteen days between February 19 and March 8, 2015, it rained every day except for March 6 and March 1. For four days on February 20, 25, 26, and March 7, the rain was moderate. For one day on March 27, the rain was heavy. And for two days on March 4 and March 5, the rain was mixed with snow. Fig. 14 is the schematic plan view of the rain-affected area that we divided into three colored parts: blue, yellow, and red. As it is shown in Fig. 14, the blue represents the unaffected part, the yellow represents the affected part, and the red represents the part of rapid subsidence. Fig. 15(a) shows the development of surface settlement with time at monitoring points from S16-0 to S19-0 as well as the duration of the rain. The maximum surface settlement values from S16-0 to S19-0 were, respectively, 322.69 mm, 359.76 mm, 377.05 mm, and 340.05 mm. At those points the rapid subsidence lasted 20 days and the settlement rate slowed down on February 10. When it began to rain on February 19, the primary support structure had gained enough strength so that the rainfall had little effect on this part. Fig. 15(b) shows the development of the surface settlement with time at monitoring points from S-22-0 to S-23-0 as well as the duration of the rain. The maximum surface settlement values from S-22-0 to S23-0 were, respectively, −480.53 mm and −388.8 mm. It started to rain on February 19 when the ground surface settlement had not stabilized and the primary support structure had not reached the desired 528

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(a) Center monitoring points and vault settlement point of section K0+872 to K0+887.

(a) Section K1+270 S85 .

(b) Center monitoring points and vault settlement points of section K1+065 to K1+080 Fig. 13. Relationship between the surface settlement and vault settlement in different surrounding soils.

Fig. 15(c) shows the development of the surface settlement with time at monitoring points from S24-0 to S26-0 as well as the duration of the rain. The settlement started to mutate on March 5 and by March 7, the settlement values had respectively increased to 166.13 mm and 369.77 mm 269.20 mm. Some cracks began to show on the ground surface and we observed the ups and downs around the field. By this point the tunnel face had become unstable and deformation was obvious in the primary support structure as wells as the ground surface of the tunnel. The tunnel face was located between S24 and S25: 4 m in front of S24 and 1 m behind S25. Although S25 was not passed by the tunnel face, its surface settlement value reached 369.77 mm. This indicates that the soils in front of the tunnel face flocked to the face and thus caused the large deformation of the excavation.

(b) Section K1+290 S87 Fig. 11. Comparison of surface settlement troughs measured at S85and S87.

3.5. Influence of geological conditions Fig. 16 shows the development of the surface settlement with time at monitoring points from S41-0 to S49-0. The section between S41-0 and S49-0 was excavated by the CRD method: S41-0 and S42-0 in muddy silty clay, S43-0 at the junction of muddy silty clay and weathered argillaceous siltstone, and S44-0 to S49-0 in weathered argillaceous siltstone. Table 6 shows the maximum surface settlement values at the monitoring points from S-41-0 to S49-0. The maximum surface settlement values at S41-0 and S42-0 were larger than those at S43-0 to S490. We calculated the average value of S41-0 and S42-0 and that of S440 ∼ S49-0, finding that the maximum surface settlement value in the muddy silty clay layer was 324% of that in the argillaceous siltstone

Fig. 12. Developments of surface settlement with time at selected center monitoring points of the east tunnel.

intensity. As a result, the settlement rate did not decrease with time. At S23-0, an increasing settlement rate led to displacement mutation that occurred on March 6. 529

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Table 3 Rock properties. Rock type

Weathering degree

Argillaceous Siltstone ⑿-2 Argillaceous Siltstone ⑿-3 Basaltic porphyrite ⒂-2 Basaltic porphyrite ⒂-3

Strong Intermediary Strong Middle

Compressive strength (MPa)

Shear strength (MPa)

Cohesion (kPa)

Internal friction (Angle°)

3

1.5 16.4

0.4 6

47.5 36.9

10

45.8

20.5

RQD

0–21 63–75

Table 4 Maximum vault settlements and surface settlements at the monitoring points. Points

16-0

17-0

18-0

19-0

53-0

54-0

56-0

Surface settlement/mm Vaults settlement/mm Proportion/%

322.69 122.16 38

359.76 146.63 41

377.05 115.83 31

340.05 107.27 32

171.68 97.08 57

179.06 110.29 62

193.89 122.13 63

The cross section of S24 is shown in Fig. 18. A river is within about 10–15 m distance from the tunnel. Due to the continuous rainfall, the water level of the river was only 1 m below the ground surface, and thus water penetrated into the backfill soil from both horizontal and vertical directions. Since the permeability coefficient of the backfill soil is larger than that of the muddy silty clay, the backfill soil became heavier. This led to excess pore-water pressure in the muddy silty clay soil and made dissipation difficult, thus causing significant deformation of the muddy silty clay. In response to large surface settlement and tunnel deformation, temporary support system was added. Bolts were inserted at the tunnel waist and foot, grouting was used to reinforce the ground surface, and the steel arches inside the tunnel were replaced. These measures evidently helped stabilize the ground surface settlement. Huang mentioned that when a large-diameter tunnel was excavated using the CRD method in Xiamen, China, those reinforcement measures were used when the ground surface settlement became too large. As a result, he pointed out, the tunnel deformation became less (Huang, 2014).

Table 5 Days of different degrees of rain. Light rain

Moderate rain

Heavy rain

Moderate snow

9d

4d

1d

2d

layer when the CRD method was adopted. In other words, soil conditions have great impacts on ground surface settlement. 3.6. Influence of construction methods Fig. 17 shows the development of surface settlement with time at the monitoring points from S44-0 to S56-0 in weathered argillaceous siltstone. The section from S44-0 to S49-0 was excavated by using the CRD method, the section from S50-0 to S53-0 was by the four-step method, and the section from S54-0 and S56-0, reinforced by vertical support system and temporary invert, was also by the four-step method. Table 7 shows the maximum surface settlement values of S44-0 to S560. The ratio of the four-step method-induced settlement to the CRD method-induced settlement was about 332%. Therefore, we believe that tunneling methods and primary support systems have great influence on ground surface settlement.

5. Conclusions In this paper we investigate the ground surface settlement as well as its relationship with the vault settlement during the excavation of a large-diameter shallow-buried twin-tunnel in soft soil. Our conclusions are as follow:

4. Discussion

(1) In muddy silty clay, the surface settlement is generally big and the settlement duration is long. The effect of the tunnel excavation face ranges from −0.5D to 4D. Both construction methods and soil conditions have significant effects on ground surface settlement. We believe that using the CRD method in soft clay still carries high risk. (2) When the first tunnel-caused surface settlement had not stabilized, excavation of the second tunnel causes large settlement. Therefore, the excavation of the second tunnel should wait until the first

Instability that occurred during the continuous rainfall were mainly due to the following factors: (1) When rain seeped into the backfill soil, it increased the soil weight and thus caused the overload to the muddy silty clay. (2) The seepage force had an effect of on the surrounding soils. (3) Continuous rainfall led to the decrease of the shear strength and modulus of the soils as well as the high-pressure jet grouting pipes. (4) The surrounding soils were disturbed by the construction-caused reduction in groundwater levels.

Fig. 14. Rainfall-affected area. 530

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(a) Center monitoring point of section from K0+872 to K0+887

(b) Center monitoring point of section between K0+907 andK0+917

(c) Center monitoring points of section from K0+922 to K0+932. Fig. 15. Three different kinds of surface settlement areas affected by rainfall.

Fig. 16. Development of surface settlement with time at selected monitoring points of different surrounding soils.

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(4) Continuous rainfall increases the load on the upper soil and thus reduce the strength of the upper soil and surrounding rocks. As a result, the primary support structure experiences significant deformation and the ground surface subsides dramatically within a short period of time. In this case, temporary support system should be added, such as inserting bolts at the tunnel waist and foot, to prevent further tunnel deformation, and grouting should be used to reinforce the ground surface to prevent cracks.

Table 6 Maximum surface settlement of S41-0 to S49-0. Points

S41-0

S42-0

S43-0

S44-0

Settlement/mm Points Settlement/mm

−201.83 S45-0 −68.95

−211.47 S46-0 −79.33

−106.73 S48-0 −27.09

−54.42 S49-0 −46.54

Acknowledgement The work presented in this paper is financially supported by The National Nature Science Foundation of China, China, (Grant No. 51778585), The National Key Research and Development Program of China, China, (Grant No. 2016YFC0800203). Thanks also go to Peiwu Jiang and Pengfei Weng for the data of the Zizhi Tunnel. We also thank Kaiwen Weng and Lei Jin for analysis of data and modification of pictures. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.tust.2018.09.021.

Fig. 17. Development of surface settlement with time at selected monitoring points excavated by different construction methods.

References

Table 7 Maximum surface settlement values of S44-0 to S56-0. Points

S44-0

Settlement/mm −54.42 Points S51-0 Settlement/mm −135.37

S45-0

S46-0

S48-0

−68.95 −79.33 −27.09 S52-0 S53-0 S54-0 −189.16 −171.68 −179.06

S49-0

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S50-0

−46.54 −123.62 S56-0 −193.89

Fig. 18. Diagram of cross-section in stability.

tunnel-caused surface settlement stabilizes. (3) The field data shows that the vault settlement is less than the corresponding surface settlement. In terms of the ratio of the vault settlement value to the surface settlement value, it is smaller in muddy silty clay than in weathered argillaceous siltstone. In other words, that ratio becomes smaller when the soil condition is worse. Therefore, more attention should be paid to both quality and construction speed when it comes to building primary support structures in muddy silty clay.

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