Research on the stabilization treatment of clay slope topsoil by organic polymer soil stabilizer

Research on the stabilization treatment of clay slope topsoil by organic polymer soil stabilizer

Engineering Geology 117 (2011) 114–120 Contents lists available at ScienceDirect Engineering Geology j o u r n a l h o m e p a g e : w w w. e l s ev...

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Engineering Geology 117 (2011) 114–120

Contents lists available at ScienceDirect

Engineering Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e n g g e o

Research on the stabilization treatment of clay slope topsoil by organic polymer soil stabilizer Jin Liu a,d, Bin Shi a,⁎, Hongtao Jiang b, He Huang c, Gonghui Wang d, Toshitaka Kamai d a

School of Earth Sciences and Engineering, Nanjing University, Nanjing, 210093, China School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing, 210093, China School of Earth & Environmental Sciences, Anhui University of Science and Technology, Huainan 232001, China d Research Centre on Landslides, Disaster Prevention Research Institute, Kyoto University, Kyoto, 611, Japan b c

a r t i c l e

i n f o

Article history: Received 18 July 2010 Received in revised form 3 October 2010 Accepted 9 October 2010 Available online 15 October 2010 Keywords: Soil stabilizer Organic polymer Stabilization Clayey soil Slope

a b s t r a c t The topsoil of clayey slope is easy to erosion because it is weak in its strength, water stability and erosion resistance. A new organic polymer soil stabilizer, which was developed for the stabilization treatment of clay slope topsoil and was named as STW, was introduced in this study. In order to understand the effect of STW on the stabilization of clayey soil, laboratory tests on the unconfined compressive strength, shear strength, water stability and erosion resistance of untreated and treated soil specimens are performed, The results indicated that STW soil stabilizer can significantly increased the unconfined compression strength, shear strength, water stability and erosion resistance of clayey soil. The unconfined compression strength increased with the increasing of curing time and the variation mainly occurs in the first 24-hour. With the addition amounts of STW increasing, the strength, water stability and erosion resistance increased at the curing time being 48 h, but in the case of friction angle, no major change was observed. Based on the scanning electron microscopy (SEM) analysis of the stabilized soil, the stabilization mechanisms of STW soil stabilizer in the clayey soil were discussed. Finally, a field test of the stabilization treatment of clay slope topsoil with STW was carried out, and the results indicated that the STW soil stabilizer on the stabilization treatment of clay slope topsoil is effective for improving the erosion resistance of slope topsoil, reducing the soil loss and protecting the vegetation growth. Therefore, this technique is worth popularizing for the topsoil protection of clay slope. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Clay slope is extensively distributed in the east part of China. During rainfall, slope surface is to suffer from soil erosion, because the topsoil is weak in its water stability, strength and erosion resistance. This often damages the plant growths of slope surface and influence the stability of slope on the side of railway, highway and other engineering structures. Some physical methods were used to protect slope surface, such as geotextiles, wire mesh, cable net and other membrane structures (Smoltczyk and Malcharek, 1985; Barker, 1988; Agassi and Ben, 1992; Liu and Li, 2003; Lekha, 2004; Shu et al., 2005). As well known, these methods are based on utilizing enforcing structures, on the other hand, it does not modify the soil property. Recently, a new method of chemical stabilization of the soil with soil stabilizer (stabilizing agent) has been developed to control the soil erosion by modifying the properties of the soil itself (Ajayi et al., 1991; William and Robert, 2000).

⁎ Corresponding author. Tel.: +86 25 83596220; fax: +86 25 83596220. E-mail address: [email protected] (B. Shi). 0013-7952/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2010.10.011

The inorganic types of stabilizing agents have been popularly applied to soil stabilization, such as cement (Bell, 1995; Chen and Lin, 2009), lime (Mckinley et al., 2001; Cai et al., 2006), fly ash (Dermatas and Meng, 2003), and their mixtures (Peter and Little, 2002; Ouhadi and Goodarzi, 2006; Zhu and Liu, 2008). These inorganic stabilizing agents are mainly used in non-ecological soil stabilization, such as foundations, roadbeds, embankments and piles. They improve the engineering properties of soils greatly, such as the strength and stiffness. But, their higher stiffness and inorganic material will inhibit the plant growth, such that these kinds of inorganic stabilizing agents cannot meet the requirements for the slope ecological stabilization (Zhou and Watts, 1999). Therefore, the organic polymer soil stabilizers as a new stabilizing agent applied in soil ecological stabilization have received recent attention (Barry et al., 1991; Nwankow, 2001; Liu et al., 2009). In this study, a new type of organic polymer soil stabilizers was introduced. This stabilizer was developed for the ecological stabilization treatment of clayey slope topsoil in Nanjing University and was named as STW. In order to understand the effect of STW on topsoil stabilization of clay slope, laboratory tests on unconfined compressive strength, shear strength, water stability and erosion resistance of untreated and treated soil specimens was performed. The potential

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mechanisms of STW modification of the soil were discussed. Finally, a field test of the topsoil stabilization treatment of clay slope with STW was carried out.

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Table 1 Unconfined compressive strength of specimens. Serial number

STW concentration (%)

Dry density (g/cm3)

2. Materials and test methods 2.1. Materials Clayey soils used for this study were acquired from the slope of Ninghuai Highway in Jiangsu province, China. This slope was selected as the site of field test. The soil sample has a liquid limit of 52.6%, a plasticity index of 19.7, a specific gravity of 2.73, an optimum moisture content of 15.4% and a maximum dry density of 1.71 g/cm3. STW is a type of organic polymer soil stabilizer (Fig. 1). Its main component is acetic-ethylene-ester polymer. It contains a large number of functional group –OOCCH3 and has a milky appearance. The solution has a pH of 6–7, solid content of 41%, 1.05 g/cm3 of specific gravity and 3000 Mpa.s of viscosity. As a new kind of soil stabilization material used in slope protection, STW has the following unique advantages: (i) it is a water-soluble material which can be diluted to different concentrations; (ii) its organic component is beneficial to plant growth; (iii) the presence of STW soil stabilizer can form an elastic and viscous membrane structure on soil surface at natural conditions. 2.2. Test methods Clayey slope surface suffers from soil erosion due to the low strength, water stability and erosion resistance of topsoil. In order to understand the usefulness of STW as a new soil stabilizer for stabilizing the topsoil of clayey slope by modifying the properties of soil itself, laboratory tests including unconfined compressive test, direct shear test, water stability test, and surface erosion test were performed to evaluate the performance of STW on clayey soil through examining the changes in the unconfined compressive strength (USC), shear strength, water stability and erosion resistance. Owing to different test requirements, there are differences in preparation of specimens and test procedures for different tests. Unconfined compressive test and direct shear test were carried out according to test methods of soil (GB/T 50123-, 1999, i.e. a national criterion for geotechnical tests in China which was set based on ASTM standards). The water stability test was performed by the static water measure method. The surface erosion test was performed by using

S1 S2 S3 S4 S5

0 5 10 20 30

1.7 1.7 1.7 1.7 1.7

Unconfined compressive strength(Kpa) 0

24 h

48 h

72 h

76.3 79.6 78.2 81.5 80.6

99.5 180.1 189.3 198.2 205.4

113.0 192.1 204.3 221.2 233.3

120.2 198.5 211.2 232.4 241.5

rainfall simulating method. In the following sections each of these four tests is introduced in turn. 2.2.1. Unconfined compressive test Soil samples were first oven-dried and screened of aggregates with a diameter smaller than 2 mm. Four concentrations 5%, 10%, 20% and 30% of STW dilutions were proposed for stabilization of clayey soil and the water (0%) as a control. The additive amount of each dilution (by weight of dry soil) and dry density of the prepared samples were 17.8%, 1.70 g/cm3, respectively. The preparation of specimens of unconfined compressive strength tests was shown in Table 1. The screened soil was mixed with the proposed STW dilution and then was prepared with static compaction method (GB/T 50123-, 1999). Four-layered compaction was adopted to keep the uniformity of test specimens with the diameter being 39.1 mm and height being 80 mm. After the specimen preparation, one group of specimens was tested immediately and others were air-dried at a temperature around 25 °C, until tested at 24, 48 and 72 h. The unconfined compression tests were carried out at the loading rate of 2.4 mm/min until samples failed. Additionally, unconfined compression test was performed on specimen triplicates and average values were used. 2.2.2. Direct shear test Soil samples were oven-dried and screened of aggregates with a diameter smaller than 2 mm. Four concentrations 5%, 10%, 20% and 30% of STW dilutions were proposed for stabilization of clayey soil and the water (0%) as a control. The additive amount of each dilution (by weight of dry soil) and dry density of the prepared samples were 17.8%, 1.70 g/cm3, respectively. The preparation of specimens of direct shear tests was shown in Table 2. The screened soil was mixed with the proposed STW dilution and then was prepared with static compaction method (GB/T 50123-, 1999). Two-layered compaction was adopted to test specimens with the 61.8 mm in diameter and 20 mm in height. Thereafter, one group of specimens were sheared immediately, another one was air-dried at around 25 °C for 48 h. The tests were carried out at a strain rate of 0.8 mm/min under the normal pressures of 50, 100, 200 and 300 KPa in order to define the shear strength parameters (c and ψ). And the shear strength parameters were obtained on specimen triplicate and average values were used. 2.2.3. Water stability test The screened aggregates with a diameter between 5 and 10 mm were dispersed into a thin layer in pots. Five groups of specimen were Table 2 Cohesion and angle of internal friction of specimens tested at different curing time. Serial number

Fig. 1. STW soil stabilizer.

S6 S7 S8 S9 S10

STW concentration (%)

0 5 10 20 30

Dry density (g/cm3)

1.7 1.7 1.7 1.7 1.7

Cohesion (kpa)

Angle of internal friction (degree)

0

48 h

0

48 h

56 75 70 65 66

260.0 347.7 367.3 373.8 384.4

29.5 23.5 24 26.5 30

58.5 58.5 56.5 61.5 58.5

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Table 3 Stability of aggregates modified with STW with different immersion time. Number of collapse Serial STW number concentration a1 a2 a3 a4 a5 (%)

a6 a7 a8 a9 a10 a∞

S11 S12 S13 S14 S15

6 2 0 0 0

0 5 10 20 30

34 0 0 0 0

6 0 0 0 0

16 2 2 0 0

8 2 0 0 0

30 4 4 0 0

0 0 4 0 0

0 4 2 2 0

0 2 0 0 0

0 4 2 2 0

Stability K (%)

0 28 80 90.8 86 94.3 96 99.4 100 100

shown in Table 3. The STW dilutions with concentrations being 0%, 5%, 10%, 20%, 30% were sprayed on the surface of the aggregates. The spray content of each dilution was 30% of the dry soil by weight. After spaying, the soil specimens were air-dried at a temperature around 25 °C with the time being 48 h, and then were used for examining the water stability by the static water measure method proposed by Andrea Arnold and improved by Kaчинcкий (1965). This method requires the aggregates to be immersed in water. Then the time was recorded to count the number of the collapses. In this study, one hundred aggregates were randomly selected from each group and were placed uniformly on a screen with filter paper. Then, water was added along the screen wall for submerging the aggregates. Thereafter, the number of collapsed aggregates was record at 1 min intervals from beginning to 10 min. When all of the edges or corners around the aggregate were collapsed or the interior soil effuses through the surface crackles of aggregate, it was considered to be collapsed. In order to evaluate the water stability of modified soil clearly, K calculated by the formula as follow is defined as a water stability index. K = ða1 × 5 + a2 × 15 + a3 × 25 + … a10 × 95 + a∞ × 100Þ = 100 ð1Þ In this formula, a1, a2, a3 … a10 express the number of collapses at the immersion time of 1 min, 2 min, 3 min…10 min, respectively. a∞ is the number of aggregates which remain integrated after an immersion time of 10 min. The coefficient of 5, 15, 25…100 represent the water stability of soil aggregate. If the aggregate collapses within 1 min, its coefficient is defined as 5(= (0 + 10) / 2), If the aggregate collapses during the immersion time of 1–2 min, its coefficient is defined as 15(= (10 + 20) / 2). …If the soil aggregate never collapses during the total immersion time of 10 min, its coefficient is defined as 100. It should be noted that a coefficient of water stability each aggregate is obtained firstly according to the recorded time and K is the average value of all the coefficients of one hundred aggregates. So the K-value has a range of 5 to 100. It can be seen that K-value increase with the immersing time of each aggregate before collapse, on the other hand, the more aggregates with higher water stability coefficient can raise the K-value and a higher K-value represents a stronger water stability of soil. 2.2.4. Surface erosion test Erosion test was performed with simulating rainfall method. The screened soil with a diameter smaller 10 mm was filled into a container with the dimension of 20 cm in length, 30 cm in width and 3 cm in height and compacted. The dry weight of each specimen was 2000 g. Then the proposed STW dilutions were sprayed on the surface uniformly. The spraying content was 3 L/m2. After spraying, the specimens were air-dried at the temperature around 25 °C for 48 h, and then laid on frame of erosion simulator to measure the erosion resistance. Fig. 2 illustrates the simulator comprised of rainfall simulator, specimen frame and soil collecting box. The angle of the frame was 30°, the simulating rainfall intensity was 50 mm/h and the duration of rain was 60 min. After simulating rainfall, the soil in

Fig. 2. Simulation equipment of erosion test.

collecting box was oven-dried and weighed (Δm). The erosion rate F defined as Δm/m0, was obtained to evaluate the erosion resistance of stabilized specimens. 3. Laboratory test results and analysis 3.1. Unconfined compressive strength The values of unconfined compressive strength of specimens with different STW concentration and curing time are presented in Table 1. The variation of strength of specimens is illustrated in Fig. 3. As seen, an increase in STW concentration from 5% to 30% induces a gradual improvement in strength for specimens tested at 24, 48, 72 h, but the strength is almost unchanged for specimens without curing. As shown in Fig. 3, the strength of each specimen increases with increasing the curing time, and this tendency becomes stronger with increase of STW concentration. Note that the water content of specimens decrease during the curing period in air-dried condition, so the untreated specimens also increase with curing time. Compared with the untreated specimens, the treated specimens have greater strength after curing. The maximum value of the strength as 241 KPa is observed at 30% STW concentration with 72-hour of curing time, which is 2 times of untreated specimen with 72-hour curing, 3 times that of the one with 30% STW concentration but without curing. Additionally, the relative increment of unconfined compressive strength was shown in Fig. 4. As seen, the increase of strength of treated specimens mainly occurs in the first 24-hour curing. With the

Fig. 3. Variation of unconfined compressive strength of samples with curing time.

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Fig. 4. Relative increment of unconfined compressive strength. Fig. 6. Effect of soil stabilizer concentration on cohesion of samples of stabilized soil.

concentration of 5% as an example, the relative increment of unconfined compressive strength tested at 24, 48, 72-hour is 1.26, 0.07, 0.03 times, respectively. Fig. 5 illustrates the typical axial stress– strain curves of untreated specimens and treated specimens with concentration of 20%. It is clearly shown that both the untreated and treated specimens take on strain-softening ductile failure and the residual strength increases with the curing time. 3.2. Shear strength parameters Direct shear test were performed to obtain the shear strength of specimens that were treated and curing time being 48 h and without curing. The obtained shear strength by mean of cohesion and internal friction angles are given in Table 2. It is observed that both cohesion and internal friction angle of specimens increase greatly after curing. The untreated specimens similarly have this tendency due to the specimens were curd in air-dried condition and the water content decreased from 17.8% to around 6.5%. Additionally, for the cuing specimens, the addition amounts of STW have the significant influence on the development of cohesion, but the internal friction angle is rarely affected. The variations of cohesion with STW concentration are shown in Fig. 6. As shown in Fig. 6, cohesion increases with increase of STW concentration with the curing time being 48 h, this tendency does not present in the group without curing. Contrarily, the cohesion of specimens without curing slightly decreases in the higher STW concentration condition. The internal friction angle of specimens treated with curing time being 48 h and without curing is illustrated in Fig. 7. It is observed that internal friction angle of these two groups of specimens both has little change with increasing of STW concentration. Therefore the clayey soil stabilized by STW has higher

Fig. 5. The typical axial stress–strain curves in unconfined compressive tests.

Fig. 7. Variation of internal friction angle of samples.

cohesion, but has no change in its internal friction angle. It should be noted that the larger value of specimens with curing might be due to low water content and the rubber friction occurring during the shearing process. In addition, it was found that the untreated and treated specimens show strain-softening ductile failure in the shear process. The presence of STW might not change the failure characteristics of clayey soil. 3.3. Water stability The values of collapses and water stability index K are presented in Table 3 and shown in Fig. 8. It is shown that number of collapse of

Fig. 8. Effect of soil stabilizer concentration on water stability of stabilized soil.

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Table 4 Soil flow rate of specimens. Serial number

STW concentration (%)

Weight of each specimen (g)

Weight of soil in collecting box Δm (g)

Soil erosion rate F (%)

S16 S17 S18 S19 S20

0 5 10 20 30

2000 2000 2000 2000 2000

1404.2 424.5 260.3 110.8 22.5

70.2 21.2 13.0 5.5 1.1

stabilized specimen decrease greatly and the number of aggregates that remain integrated after an immersion time of 10 min increases with increasing of STW concentration. STW concentration changes in the from 5% to 10%, 20% and 30% elevated the water stability index K from 90.8 to 94.3, 99.4 and 100% respectively, while the K-value of the untreated aggregates is only 28%. K-value of specimen treated with concentration 5% is 3.2 times of the untreated one. The K-value of the stabilized specimens increases with the increase of the STW concentration. These results indicate that treated specimens have better water stability. 3.4. Erosion resistance The test results are tabulated in Table 4 using the rainfall stimulator shown in Fig. 2. Four tests were performed to examine the erosion resistance for samples treated by STW with different concentration. The variation of soil erosion rate with STW concentration is illustrated in Fig. 9. It shows that the treated specimens with the concentration from 5% to 10%, 20% and 30% reduce the soil erosion rate F (defined as Δm/m0) from 21.2 to 13.0, 5.5 and 1.1%, respectively, and the untreated one reached a value of 70.2%. The soil erosion rate decreases while increasing of STW concentration. These results indicate that the stabilized specimens have a higher erosion resistance. 4. Mechanisms of stabilization Normally, the clayey soil stabilization mechanism of organic polymer soil stabilizer includes filling, chemical reaction and enwrapping (Barry et al., 1991; Nwankow, 2001; Zhu and Liu, 2008). STW is composed of polyvinyl-acetate that contains much of long-chain macromolecule and polarity carboxyl (–OOCCH3). When STW is applied to clayey soil, a part of them fill up the voids of soil (Fig. 10), and other part stay on the soil aggregates surface, The hydrophilic groups (–OOCCH3) in its molecular structure have chemical reaction with positive ions of clay grain and create physicochemical bonds between molecules and soil aggregates with

Fig. 9. Effect of soil stabilizer concentration on erosion resistance of stabilized soil.

Fig. 10. SEM image of soil void filled by STW with 3000 time magnification.

ionic, hydrogen, or Van der Waals bonds (Fig. 11). Through these bonds, long-chain macromolecules of polymers enwrap the aggregate's surface and interlink to form an elastic and viscous membrane structure. Therefore, the strength, water stability and erosion resistance are improved. The physicochemical reactions between soil stabilizer and soil usually require a few days. Thus, the unconfined compressive strength of the stabilized soil increases with increase of curing duration. The strength increase quickly at the first 24-hour, and then become slower until up to constant value. In addition, the formation of the voids filling and physicochemical bond in reactions of stabilized soil leads to the increases in bonding and interlocking forces between soil particles and as a result, the unconfined compressive strength and cohesion of the clayey soil is improved after STW treatment. In view of the influence of STW concentration on strength, more STW addition amount can fill up more voids and produce more bonds and hence result in the greater strength of the soil. Nevertheless, the addition amount of STW normally does not exceed 30% concentration, because it is related to the void ratio and clay content. At this concentration the molecules of STW might have already been enough to fill up most of soil voids and the hydrophilic groups (–OOCCH3) in molecule might have fully reacted with the clay particle. When STW is applied to the soil aggregates, the formation of physicochemical bonds and membrane structure leads to the increase in water stability and erosion resistance. With increase of STW concentration, the ability of interaction between molecules and aggregates is improved and the aggregate's surfaces are enwrapped by more long-chain macromolecules. This will help the formation

Fig. 11. SEM image of interaction of soil and STW with 1000 time magnification.

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of a more complete membrane structure on soil aggregate surface, and then raise the cohesion of the soil. Therefore, the water stability and erosion resistance increase with the increasing of the STW concentration. 5. Application

Fig. 12. Weather condition after spraying.

In order to evaluate the effects of STW on the stabilization treatment of clay slope topsoil, a field test was carried out. The field clay slope is located in a highway, Jiangsu, China. This slope has a slope angle of 35°. The erosion of the slope surface was very serious. Although seeds of grasses had been sprayed on the surface of the slope after the construction of the slope, they were washed away before germinating. In this test, considering the effect and cost, the optimum spraying concentration and content of STW soil stabilizer were 20%, 3 L/m2, respectively. Three types of seeds, i.e. bahia, bermuda and white clover, were selected.

Fig. 13. Field test results (a) stabilized with STW (b) Stabilized without STW.

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The implementing procedures were as follows. (i) the slope surface was leveled off and the seeds were sowed on the surface; (ii) the dilution concentration 20% of STW was made and sprayed on the slope surface; (iii) spray water on slope once every two days and take the comprehensive evaluation. The results of comprehensive evaluation show that STW soil stabilizers are effective for improving the erosion resistance of the slope topsoil, reducing the soil loss and protecting the vegetation growth. The weather condition in the coming 10 months after spraying is presented in Fig. 12. As shown, the rainfall larger than 100 mm/day occurred 16 times during this period. The comparison of field test results after 10 months were presented in Fig. 13. It is showed from Fig. 13a that the slope surface treated by STW had been lightly eroded, and the vegetation is well-growth, whereas, Fig. 13b shows that the untreated part had severer soil erosion with little vegetation. Therefore, it is concluded that the stabilization treatment of clay slope topsoil with STW soil stabilizers is an effective technology to reduce the soil erosion, improve the slope surface water ability and protect the vegetation growth. 6. Conclusions Based on the laboratory and field tests, the effects of an organic polymer soil stabilizer (STW) on stabilization treatment of clay slope topsoil have been examined. It is shown from the laboratory test results that the addition amount of STW causes the beneficial changes in the unconfined compressive strength, shear strength, water stability and erosion resistance of clayey soil used in this study. Field test results indicate that the stabilized topsoil has higher erosion resistance. It can satisfy the requirements of clay slope surface stabilization treatment. It is observed from lab testing that the properties of stabilized soil vary and depend on STW concentration, and also on the curing time. The unconfined compressive strength increases with increase of the curing time and the increase mainly accrues in the first 24-h. The unconfined compressive strength and cohesion also increase with the increase of STW concentration in the curing condition, but the variation of friction angles is little. The water stability and erosion resistance are improved greatly and increase with STW concentration. SEM images indicate that STW has an interaction between soil stabilizer and soil. These interactions greatly change the soil fabric and thereby result in the variation of strength, water stability and erosion resistance of clayey soil. The field test shows that STW on the stabilization treatment of clay slope topsoil is effective for improving the erosion resistance of the slope topsoil, reducing the soil loss and protecting the vegetation growth. Trough this investigation, it is clearly indicated that STW as a new type of organic polymer Soil Stabilizer is useful for ecological stabilizing the topsoil of clay slope, and the technique is worth popularizing for soil slope surface protection.

Acknowledgements This research is financially supported by the Natural Science Foundation of China (Grant No. 40672181), the State Key Program of National Natural Science of China (Grant No. 40730739) and the Scientific Research Foundation of Graduate School of Nanjing University (Grant No. 2009CL10). The authors gratefully acknowledge Dr. Chaosheng Tang and Prof. Baojun Wang in School of Earth Sciences and Engineering, Nanjing University, China, for their contribution to the field test. The authors would also like to acknowledge the editors and reviewers of this paper for their very helpful comments and valuable remarks. References Agassi, M., Ben, H.M., 1992. Stabilizing steep slopes with soil conditioners and plants. Soil Technology 5 (3), 249–256. Ajayi, M.A., Grissom, W.A., Smith, L.S., et al., 1991. Epoxy-resin-based chemical stabilization of a fine, poorly graded soil system[R]. Transportation Research Record. National Research Council, Washington D C. Barker, D H., 1988. Geotextiles in Slope Protection and Erosion Control: Civil Engineering, Lond, March, 52–55. Barry, P.P., Stott, D.E., Turco, R.F., 1991. Organic polymers' effect on soil shear strength and detachment by single raindrops ]. Soil Science Society of America Journal 55 (3), 799–804. Bell, F.G., 1995. Cement stabilization and clay soils, with examples. Environmental and Engineering Geoscience 1 (2), 139–151. Cai, Y., Shi, B., Ng, C.W.W., Tang, C., 2006. Effect of polypropylene fibre and lime admixture on engineering properties of clayey soil. Engineering Geology 87, 230–240. Chen, L., Lin, D., 2009. Stabilization treatment of soft subgrade soil by sewage sludge ash and cement. Journal of Hazardous Materials 162, 321–327. Dermatas, D., Meng, X.G., 2003. Utilization of fly ash for stabilization solidification of heavy metal contaminated soils. Engineering Geology 70, 377–394. GB/T 50123-1999. Standard for soil test method. Ministry of Construction, P.R. China (in Chinese). Kaчинcкий H. A., 1965. Физикa пoчвы, Изл. Bыcшaя Шкoлa, M. Lekha, K.R., 2004. Field instrumentation and monitoring of soil erosion in coir geotextile stabilized slopes—a case study. Engineering Geology 22, 399–413. Liu, D., Li, Y., 2003. Mechanism of plant roots improving resistance of soil to concentrated flow erosion. Journal of Soil Water Conservation 17 (3), 34–37. Liu, J., Shi, B., Huang, H., Jiang, H., 2009. Improvement of water-stability of clay aggregates admixed with aqueous polymer soil stabilizers. Catena 77, 175–179. Mckinley, J.D., Thomas, H.R., Williams, K.P., Reid, J.M., 2001. Chemical analysis of contaminated soil strengthened by the addition of lime. Engineering Geology 60, 181–192. Nwankow, K.N., 2001. Polyacrylamide as a Soil Stabilizer for Erosion Control. Wisconsin Department of Transportation, Madison. Report. Ouhadi, V.R., Goodarzi, A.R., 2006. Assessment of the stability of a dispersive soil treated by alum. Engineering Geology 85, 91–101. Peter, T.M., Little, D.N., 2002. Review of stabilization of clays and expansive soils in pavements and lightly loaded structures—history, practice, and future. Journal of Materials in Civil Engineering 14 (6), 447–460. Shu, S., Muhunthan, B., Badger, T.C., Grandorff, R., 2005. Load testing of anchors for wire mesh and cable net rockfall slope protection systems. Engineering Geology 79 (3–4), 162–176. Smoltczyk, U., Malcharek, K., 1985. Slope protection by membrane structures. Geotextiles and Geomembranes 2 (4), 323–336. William, J.O., Robert, E.S., Gregory, M.G., 2000. Biopolymer additives to reduce erosioninduced soil losses during irrigation [J]. Industrial Crops and Products 11, 19–29. Zhou, Y., Watts, D., 1999. Current development of Slope Eco engineering principle and application in Europe and America. Journal of Soil Water Conservation 5 (1), 79–83. Zhu, Z., Liu, S., 2008. Utilization of a new soil stabilizer for silt subgrade. Engineering Geology 97, 192–198.