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Front. Archit. Civ. Eng. China 2007, 1(2): 198–204 DOI 10.1007/s11709-007-0023-1 RESEARCH ARTICLE ZHAN Liangtong Soil-water interaction in unsaturated expansive soil slopes © Higher Education Press and Springer-Verlag 2007 occur in wet seasons; (2) most of the slips are shallow, i.e. in the order of 2–3 m in depth; (3) numerous slips follow the mode of retrogressive failures; and (4) slips do occur to slopes with slight inclinations. The characteristics of slope failure are closely related to the strong soil-water interaction in unsaturated expansive clay. During the seasonal wetting and drying cycles, continu- ous water exchange occurs between the atmosphere and the unsaturated expansive clay in the slope, particularly the shallow soil layer. The unsaturated expansive clay exhibits its inherent nature, i.e. swelling/shrinkage, upon the wetting/ drying cycles. As a result, the stress state, deformation char- acteristic, water permeability and shear strength of the soil mass in the slope continue to evolve in a direction adverse to the slope stability, finally leading to a slope failure. The soil-water interaction, involving the coupled effects of many aspects, is very complex. To improve our understanding on the soil-water interaction, an 11 m high cut slope of medium expansive clay in Hubei of China was selected for a well- instrumented field study [7,8]. The behavior of the unsatu- rated expansive clay of the natural samples was analyzed on the spot and in the laboratory [9]. On the basis of the field and laboratory test results, detailed discussion on the soil-water interaction of the unsaturated expansive clay is presented in this paper. 2 Relationship between matric suction and water content Expansive clay in a slope is generally unsaturated, at least for the shallow soil layer. Matric suction exists in unsaturated clay and plays an important role in the soil-water interaction. Matric suction, which equals to the difference between the pore-air pressure and the pore-water pressure (i.e. u a -u w ) across a water meniscus, is one of the two independent stress state variables for unsaturated soil. For a given soil, the value of matric suction primarily depends on the water content. As water content decreases, the water menisci withdraws into smaller and smaller pore spaces, the radius of curvature of the menisci reduces, and then the matric suction increases. The increase of matric suction generally results in the decrease of the driving potential for moisture flow and the increase Abstract The intensive soil-water interaction in unsatura- ted expansive soil is one of the major reasons for slope fail- ures. In this paper, the soil-water interaction is investigated with the full-scale field inspection of rainwater infiltration and comprehensive experiments, including wetting-induced softening tests, swelling, and shrinkage tests. It is demonstrat- ed that the soil-water interaction induced by seasonal wetting- drying cycles is very complex, and it involves coupled effects among the changes in water content, suction, stress, deforma- tion and shear strength. In addition, the abundant cracks in the expansive soil play an important role in the soil-water interaction. The cracks disintegrate the soil mass, and more importantly, provide easy pathways for rainfall infiltration. Infiltration of rainwater not only results in wetting-induced softening of the shallow unsaturated soil layers, but also leads to the increase of horizontal stress. The increase of horizontal stress may lead to a local passive failure. The seasonal wetting-drying cycles tend to result in a down-slope creeping of the shallow soil layer, which leads to progressive slope failure. Keywords expansive soil, slope, suction, deformation, stress, shear strength, cracks 1 Introduction Expansive clay exhibits significant swelling/shrinkage upon wetting/drying. The soil is widely distributed in the world, and often causes damages to slopes, pavements, and light buildings. Landslides greatly affect the serviceability of canals, and the remediation costs a lot of money. Numerous engineers and researchers have put efforts on the investiga- tion of slope failures in unsaturated expansive soil [1–5]. Site investigations indicate that the slope failures generally exhibit four characteristics [6]: (1) the slope failures usually Translated from Journal of Zhejiang University (Engineering Science), 2006, 40(3): 494–500 [译自: 浙江大学学报 (工学版)] ZHAN Liangtong ( ) Institute of Geotechnical Engineering, Zhejiang University, Hangzhou 310027, China E-mail: [email protected]

Soil-Water Interaction in Unsaturated Expansive Soil Slopes

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Page 1: Soil-Water Interaction in Unsaturated Expansive Soil Slopes

Front. Archit. Civ. Eng. China 2007, 1(2): 198–204DOI 10.1007/s11709-007-0023-1

RESEARCH ARTICLE

ZHAN Liangtong

Soil-water interaction in unsaturated expansive soil slopes

© Higher Education Press and Springer-Verlag 2007

occur in wet seasons; (2) most of the slips are shallow, i.e. in the order of 2–3 m in depth; (3) numerous slips follow the mode of retrogressive failures; and (4) slips do occur to slopes with slight inclinations.

The characteristics of slope failure are closely related to the strong soil-water interaction in unsaturated expansive clay. During the seasonal wetting and drying cycles, continu-ous water exchange occurs between the atmosphere and the unsaturated expansive clay in the slope, particularly the shallow soil layer. The unsaturated expansive clay exhibits its inherent nature, i.e. swelling/shrinkage, upon the wetting/drying cycles. As a result, the stress state, deformation char-acteristic, water permeability and shear strength of the soil mass in the slope continue to evolve in a direction adverse to the slope stability, finally leading to a slope failure. The soil-water interaction, involving the coupled effects of many aspects, is very complex. To improve our understanding on the soil-water interaction, an 11 m high cut slope of medium expansive clay in Hubei of China was selected for a well-instrumented field study [7,8]. The behavior of the unsatu-rated expansive clay of the natural samples was analyzed on the spot and in the laboratory [9]. On the basis of the field and laboratory test results, detailed discussion on the soil-water interaction of the unsaturated expansive clay is presented in this paper.

2 Relationship between matric suction and water content

Expansive clay in a slope is generally unsaturated, at least for the shallow soil layer. Matric suction exists in unsaturated clay and plays an important role in the soil-water interaction. Matric suction, which equals to the difference between the pore-air pressure and the pore-water pressure (i.e. ua-uw) across a water meniscus, is one of the two independent stress state variables for unsaturated soil. For a given soil, the value of matric suction primarily depends on the water content. As water content decreases, the water menisci withdraws into smaller and smaller pore spaces, the radius of curvature of the menisci reduces, and then the matric suction increases. The increase of matric suction generally results in the decrease of the driving potential for moisture flow and the increase

Abstract The intensive soil-water interaction in unsatura-ted expansive soil is one of the major reasons for slope fail-ures. In this paper, the soil-water interaction is investigated with the full-scale field inspection of rainwater infiltration and comprehensive experiments, including wetting-induced softening tests, swelling, and shrinkage tests. It is demonstrat-ed that the soil-water interaction induced by seasonal wetting-drying cycles is very complex, and it involves coupled effects among the changes in water content, suction, stress, deforma-tion and shear strength. In addition, the abundant cracks in the expansive soil play an important role in the soil-water interaction. The cracks disintegrate the soil mass, and more importantly, provide easy pathways for rainfall infiltration. Infiltration of rainwater not only results in wetting-induced softening of the shallow unsaturated soil layers, but also leads to the increase of horizontal stress. The increase of horizontal stress may lead to a local passive failure. The seasonal wetting-drying cycles tend to result in a down-slope creeping of the shallow soil layer, which leads to progressive slope failure.

Keywords expansive soil, slope, suction, deformation, stress, shear strength, cracks

1 Introduction

Expansive clay exhibits significant swelling/shrinkage upon wetting/drying. The soil is widely distributed in the world, and often causes damages to slopes, pavements, and light buildings. Landslides greatly affect the serviceability of canals, and the remediation costs a lot of money. Numerous engineers and researchers have put efforts on the investiga-tion of slope failures in unsaturated expansive soil [1–5]. Site investigations indicate that the slope failures generally exhibit four characteristics [6]: (1) the slope failures usually

Translated from Journal of Zhejiang University (Engineering Science), 2006, 40(3): 494–500 [译自: 浙江大学学报 (工学版)]

ZHAN Liangtong ( )Institute of Geotechnical Engineering, Zhejiang University, Hangzhou 310027, ChinaE-mail: [email protected]

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of inter-particle contact stress, shear strength and shear modulus.

The relationship between water content and matric suction is defined as the soil-water characteristic curve. Figure 1 shows the curves in term of gravimetric water content mea-sured from the natural expansive clay taken from the research slope. The curves were measured by using a five bar pressure plate and a two bar volumetric pressure plate extractor. As illustrated by the desorption curves, as matric suction increases in the clay, the water content starts to decrease at a quite low suction (less than 1 kPa). It indicates that the air-entry value of the natural clay is quite low due to cracks and fissures present. The hysteresis between the adsorption and desorption curves is relatively insignificant. As illustrated by the adsorption curve, while water content increases, matric suction in the unsaturated clay decreases significantly. For example, an increase of water content from 21.7% to 25.7% results in a decrease of matric suction from 200 to 0 kPa.

middle of the slope. It is indicated that prior to the first rainfall event, high negative pore-water pressures (equivalent with matric suction for the case of zero pore-air pressure) exist in the shallow soil layers. One and a half days after the first rain, the suctions in the unsaturated clay started to decrease. At the end of the first period of rain (i.e. seven days later), the suctions measured within the depth of 2 m all decreased to zero, and even a build-up of positive pore-water pressure was observed. The decrease in matric suction and build-up of pore-water pressure inevitably led to the decrease in the shear strength of the expansive clay.

3 Soil-water interactions associated with wetting and drying

3.1 Wetting-induced swelling

It is well known that expansive clay generally exhibits significant swelling upon wetting. For a given expansive clay, the magnitude of wetting-induced swelling strain primarily depends on the values of initial suction (or initial water con-tent), initial dry density and the confining pressure. Hence, the swelling characteristic should be described by a certain relationship curve (e.g. relationship of swelling strain to initial suction), rather than a constant value. Figure 3 shows a relationship between one-dimensional free swelling and initial suction obtained from the natural expansive clay. As expected, the measured swelling strain increases with the value of initial suction. The maximum value of one-dimensional free swelling for the natural clay was approxi-mately 8%, which was obtained from air-drying specimens with an initial suction of 105 kPa. Within the suction range of 0 to 500 kPa, the relationship between the swelling strain and initial suction appears to be bilinear on a linear scale. At suctions less than 100 kPa, the increase in swelling strain with initial suction is more significant than that of suctions greater than 100 kPa.

Fig. 1 Soil-water characteristic curves for the natural expansive soil

On the basis of the soil-water characteristic curve, it is inferred that the infiltration of rainwater into the unsaturated clay will result in a significant decrease in the initial matric suction. Figure 2 shows field measurements of matric suction in the research slope subjected to two artificial rainfall events. The field measurements are carried out by using tensiometers and thermal conductivity suction sensors installed at the

Fig. 2 Responses of pore-water pressure to rainfall recorded by four tensiometers at mid-slope

Fig. 3 Relationship between one-dimensional free swelling and initial suction for expansive soil

During the infiltration of rainwater, wetting-induced swell-ing occurs in shallow unsaturated expansive clay. Figure 4 shows field measurements of wetting-induced heave for the

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shallow soil layer subjected to the two artificial rainfall events. The field measurements are carried out by surveying vertical movement points installed at the middle of the research slope. It is indicated that a heave up to 20 mm was measured at the end of the second rainfall event. The heave in the shallower soil layer is greater than one in the deeper soil layer. This was attributed to the high initial suctions existing in the shallower soil layer (as shown in Fig. 2). The wetting-induced swelling of the shallow soil layer results in an increase of the void ratio, and hence the softening of the expansive clay.

suction ranging from 0 to 500 kPa, a bilinear relationship is also observed between the swelling pressure and the initial suction.

When unsaturated expansive clay is wetted on an anisotro-pic confinement condition, a passive failure may occur to the clay. Figure 6 shows the passive failure of an expansive soil specimen which had been wetted in an oedometer ring. The failure resulted from an anisotropic development of swelling pressure. The specimen was able to swell freely in the vertical direction, whereas in the horizontal direction, the swelling potential had been constrained by the oedometer ring. During the wetting process, the horizontal stress increased greatly, but the vertical stress had been kept at a small value. When the stress state reached the Mohr-Coulomb strength envelope, the passive failure occured in the specimen.

Fig. 4 Heave of shallow soil layers due to rainfall infiltration

Fig. 6 Passive failure of expansive soil specimen when wetted in an oedometer ring [10]

As mentioned above, the passive failure may occur in an unsaturated expansive soil slope subjected to rainfall infiltra-tion. During the rainwater infiltration, the shallow soil layer was able to heave in the vertical direction, but the swelling potential in the horizontal direction was constrained by the adjacent soil mass. The anisotropic constraint leads to the sig-nificant increase of horizontal stress over the constant vertical stress. Figure 7 shows field measurements of total stress ratio (σh /σv) in the research slope subjected to the two artificial rainfall events. The horizontal stresses were registered by two earth pressure cells installed at the depth of 1.2 m. One measures the horizontal stresses acting in the direction perpendicular to the inclination of the slope, and the other

Fig. 7 Changes of horizontal stress with time recorded by the two earth pressure cell at mid-slope

3.2 Swelling pressure

When confined unsaturated expansive clay is wetted (i.e., the swelling tendency of the soil is constrained), a swelling pressure will develop. The magnitude of swelling pressure for a given clay also depends on the values of initial suction (or initial water content), initial dry density and the confining pressure. Figure 5 shows a change of swelling pressure with initial suction measured in the natural expansive clay. The measurement was carried out by free swelling and then loading method. As shown in the figure, the measured swell-ing pressure increases with the value of initial suction. The maximum value of swelling pressure for the natural clay is app roxi mately 400 kPa, which was obtained from air-drying specimens with an initial suction of 105 kPa. Within the

Fig. 5 Relationship between swelling pressure and initial suction for the expansive soil

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records horizontal stresses acting in the direction parallel to the inclination of the slope. It is indicated that a significant increase in σh /σv is observed one and a half days after the first rain. The measured stress ratio in the direction perpendicular to the inclination of the slope is larger than one in the direction parallel to the inclination of the slope. This is probably related to a higher constraint imposed as a result of the slope ground in the direction perpendicular to the inclination of the slope. At the end of the first rainfall, the magnitude of horizontal stress is about two to three times as large as the vertical stress. On the basis of theoretical calculation, the measured stress ratios are close to the stress ratio required for a passive failure.

The significant increase of in situ stress ratio (σh /σv) in the slope subjected to rainfall infiltration may be one of the main reasons for the retrogressive shallow failures commonly observed in regions of expansive soil. It is postulated that passive failure is likely to first occur in local soil with stress ratios (σh /σv) approaching the limiting condition. Once the local soil has failed with strain softening, the part of stress in excess of the residual strength would be transferred to the adjacent soil. The excessive shear stress may lead to a failure of the adjacent soil. Thus, the rupture surface may extend progressively under a condition, such as continuous rainfall, resulting in a landslide. On the other hand, seasonal cycles of wetting and drying may also result in continuous develop-ment of rupture surfaces. This is discussed in the next section.

3.3 Drying-induced shrinkage

As water content decreases, the expansive clay desiccates and shrinks. Figure 8 shows the shrinkage curve for the natural expansive clay. It is indicated that the void ratio of the clay significantly decreases with the decrease of water content until water content approaches the shrinkage limit, which is equal to 13.0%. In the end, the shrinkage strain reached a magnitude of 10%. During the drying process, numerous cracks and fissures developed in the unsaturated expansive clay. Figure 9 shows a picture of a cracked specimen at an air-drying state. The development of cracks and fissures directly disintegrated the clay, and hence decreased its shear

strength. In addition, the cracks and fissures in the shallow soil layers provided numerous bypass flow channels for ease of rainwater infiltration.

3.4 Deformation induced by wetting and drying cycles

Having been subjected to seasonal cycles of wetting and drying, expansive clay experiences repeated swelling and shrinking. The role of seasonal moisture cycles in the progressive failure of an over-consolidated clay slope was investigated in Cambridge University by using a geotechnical centrifuge modeling [11]. To simulate the seasonal moisture cycles, the research group designed an environmental cham-ber in which the relative humidity boundary condition was controlled. Wet seasons were modeled through the provision of a finely automated mist over the model surface at a proto-type intensity of approximately 5 mm/h. Dry seasons were simulated by exchanging the moisture laden air within the package with warm dry air to promote evaporation from the soil surface. The high acceleration produced by the centrifuge was able to accelerate the moisture flow in the model, so that several years of seasonal moisture cycles can be simulated by several hours of centrifuge modeling tests.

Figure 10 shows the track of deformation for the shallow soil during the drying and wetting cycles. The drying-induced shrinkage vectors are largely normal to the slope, but are slightly biased down-slope by gravity. The swelling deforma-tions observed during a wetting season are also mostly verti-cally upwards. With each moisture cycle, the clay in the slope has a net down-slope displacement vector. The irrecoverable down-slope deformation will accumulate over numerous cycles of wetting and drying. This builds up tensile strains at the crest of the slope, and results in a shear zone at the toe of the slope. The seasonal cycles can, therefore, drive progres-sive failure by initiating tensile failure at the crest and shear failure at the toe. Therefore, the seasonal moisture cycles play an important role in the progressive shallow failure of a slope, which is the most common failure mode in the regions of expansive soils.Fig. 8 Shrinkage curve for the natural expansive clay

Fig. 9 Photo showing cracks developed in air-drying soil sample

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4 Effect of suction change on shear strength

4.1 Contribution of suction to shear strength

As mentioned previously, the increase of matric suction generally results in the increase in inter-granular contact stress and shear strength. The contribution of suction to shear strength is represented by an apparent cohesion, i.e. (ua-uw) ⋅ tan wb. Figure 11 shows the contribution of suction to shear strength for the natural expansive clay taken from the research slope. The data were obtained by performing suction-controlled direct shear tests. A nonlinear increase of apparent cohesion with suction was observed for the expan-sive clay. The magnitude of apparent cohesion first increased with suction at a slope equal to tan wp (wp = 28.7°) up to a suc-tion of 50 kPa, and then increased at a significantly reduced rate. The increase in suction from 0 to 100 kPa resulted in the increase of shear strength with a magnitude of 40 kPa. Hence, the significant contribution of suction to shear strength (particularly at suctions less than 100 kPa) enhances the slope stability during the dry seasons.

4.2 Wetting-induced softening

As stated previously, rainwater infiltration into a slope results in a decrease or total loss of matric suction. Wetting-induced softening occurs in the unsaturated expansive clay. It should be noted that the wetting-induced softening for expansive clay includes two mechanisms. One is the reduction of apparent cohesion due to loss of matric suction, and the other is the reduction of shear strength as a result of wetting-induced swelling (i.e. increase in void ratio).

The wetting-induced softening is simulated by performing constant-q wetting tests in a suction-controlled triaxial apparatus. The natural expansive clay taken from the research slope was used for the triaxial tests. Suction of 200 kPa was first applied to the soil specimen. After a suction equilibrium was achieved, the specimen was consolidated to the required confining stress with the suction being kept constant. Then, the specimen was sheared to the required stress ratio (e.g. q/p = 2.0), at which the specimen still un-failed due to the significant contribution of suction to shear strength. Lastly, the suction in the specimen was reduced gradually by lowering the pore-air pressure until a failure occurred to the specimen. The constant-q wetting test results are presented in Fig. 12. The three curves were obtained from three specimens applied with three different values of stress ratio (q/p = 2.52, 2.0 and 1.82). Figure 12(a) shows a development of axial strain with the decreasing suction during the wetting process. A threshold value of suction can be identified on the axial strain versus suction curves, namely the point with greatest curvature (marked by the arrows). When the suction value is greater than the threshold value, the development of axial strain is insignificant. However, once the suction value becomes lower than the threshold value, the axial strain starts to increase substantially with the additional decrease of suction, and the specimen fails quickly. The threshold value of suction increases with the increase in the stress ratio (q/p). The threshold values corresponding to q/p = 2.52, 2.0 and 1.82 are approximately 58 kPa, 9 kPa and 6 kPa, respectively. Figure 12(b) shows a development of volumetric strain with the decreasing suction. Prior to the threshold value of suction, the decreasing suction resulted in the swelling of the soil specimen (i.e. increase in void ratio). When the suction value becomes lower than the threshold value, a significant dilation occured in the soil specimen. The wetting-induced swelling softened the specimens, and the shear-induced dilation tended to accelerate the failure process. The results from the con-stant-q wetting tests reveal the wetting-induced softening associated with rainfall infiltration. The threshold values of suction identified from the tests are significant to develop an early-warning system against the slope instability in expansive soils.

5 Role of cracks in soil-water interaction

An expansive soil is generally a clayey soil with high plasticity, so the expansive soil mass without cracks or

Fig. 10 Track of soil deformation in a slope subject to seasonal wetting-drying cycles [11]

Fig. 11 Contribution of suction to shear strength for the natural expansive clay

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fissures has a low coefficient of water permeability (e.g. ks<10−7 m/s). However, the abundant cracks and fissures existing in the natural expansive clay lead to a significant increase in water permeability as well as infiltration rate.

During the site investigation, double-ring infiltration tests were carried out at the site to investigate the infiltration characteristics (i.e. permeability) of the soil layer near the ground surface. Figure 13 shows the change of infiltration rate with time measured in a ground with abundant cracks. The initial infiltration rate measured by the double-ring infiltration test was in the rate of 10−5 m/s initially due to the rapid ingress of water into the opened cracks. However, the infiltration rates decreased dramatically with test duration. After the first half day, the rate reduced towards a steady value (i.e. in the order of 10−7 m/s). The main reason for the sharp decrease in infiltration rate appears to be related to the filling and closing up of open crack channels due to soil swelling upon wetting. Another reason is that the hydraulic gradient decreased gradually with the test duration due to a loss of initial suction.

a system of regularly spaced surface cracks tends to appear due to soil desiccation. The opened cracks expose more surfaces for evaporation and greatly reduce the length of the evaporation path, which lead to a significant increase in evap-oration rate. As evaporation continues, these cracks gradually deepen, and secondary cracks develop [12]. The depth of cracks can vary from several centimeters to perhaps more than one meter, depending on the nature of the clay, the climate (i.e. the duration of the drought period) and the veg-etation on the ground surface [13]. When it is rainy, the opened cracks provide an easy pathway for rainwater to infiltrate the soil. The rainwater first bypasses the regions between the cracks as it ingresses directly into the cracks. However, the number of opened cracks and their opening tend to decrease significantly with depth as discussed before, so that the ingress of rainwater tends to be impeded on the deeper un-cracked soil layer or any other less permeable layers. Then the rainwater rises up from the bottom of open cracks or the impeding layer and flow in all directions. Thus, a perched water table may develop on the less permeable layer without open cracks. On the other hand, the opened cracks tend to close up due to the swelling of the expansive clay upon wetting. This results in a great decrease in the infiltration rate (or an increase in surface runoff).

There are many aspects of mechanism associated with the formation of cracks in an unsaturated expansive soil slope. In addition to the tension cracks due to desiccation, cracks or fissures may take place due to the vertical stress relief associ-ated with the excavation in an over-consolidated soil. These cracks are due to local shear failure and are generally parallel to or nearly parallel to the slope surface. As stated previously, shear-induced ruptures may also be caused by the local passive failure associated with the high stress ratio (σh /σv) during rainwater infiltration. In addition, seasonal wetting and drying cycles tend to result in a tension cracking zone in the upper part of the slope and a shear-induced rapture zone near the toe of the slope. As the seasonal moisture cycles continue, the abundant cracks and fissures resulting from dif-ferent mechanisms tend to extend progressively, and finally merge into a slip surface.

Fig. 12 Results from the constant-q wetting tests on for the expansive clay

Fig. 13 Results from double-ring infiltration test on the ground with abundant cracks

The previous discussion indicates that the abundant cracks and fissures in the expansive soils play important roles on the soil-water interaction in the unsaturated expansive soil slope. During dry seasons, if expansive clay is allowed to dry on top,

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6 Conclusions

On the basis of the previous discussion, the following conclusions can be drawn.

(1) In dry seasons, cracking occurs in the expansive clay due to desiccation. The cracks disintegrate the soil mass, and more importantly, provide easy pathways for rainfall infiltration.

(2) In wet seasons, rainwater infiltration results in loss of soil suction and wetting-induced swelling in the shallow unsaturated soil layer. The loss of soil suction leads to the decrease in the inter-particle contact stress, and the decrease in shear strength of the clay. The swelling leads to the increase in void ratio, and the decrease of shear strength. The dual softening effect associated with rainfall infiltration is one of the major reasons for the rain-induced slips in expansive clays.

(3) Rainwater infiltration into the unsaturated expansive clay also results in significant increase in horizontal stress due to the horizontal constraint effect on the swelling poten-tial. The significant increase of stress ratio (σh /σv) may lead to local passive failure in the slope, which may continue under a climate condition with a prolonged rain.

(4) During each seasonal wetting-drying cycle, an irre-coverable down-slope deformation occurs in shallow expan-sive clay. The down-slope deformation accumulated over a long term (i.e. creep) tend to drive a progressive failure by initiating tensile failure at the crest and shear failure near the toe of the slope. The down-slope creep may be the important cause for the progressive failure of the expansive soil slope.

(5) The soil suction in the unsaturated expansive clay has a significant contribution to shear strength, particularly at suctions less than 100 kPa. The wetting-induced softening can be simulated by a constant-q wetting tests in a triaxial apparatus with suction control. The constant-q wetting test results indicate that a critical value of suction exists during the wetting-induced softening process. The critical value of suction decreases with the increase in the stress ratio (q/p) applied on the soil mass.

(6) The abundant cracks in the expansive soil, resulting from different rapture mechanisms, play important roles in the soil-water interaction. As the seasonal wetting and drying cycles continue, the abundant cracks and fissures tend to extend progressively, and finally merge into a slip surface.

(7) The intense soil-water interaction induced by the seasonal moisture cycles is the major cause for the slope failure in unsaturated expansive clay. The mitigation

measures, which can weaken the soil-water interaction, are extremely effective for the hazards of expansive soil slopes.

Acknowledgements The authors would like to acknowledge financial support from research grants (Grant No. 50408023) provided by National Natural Science Foundation of China and research grant (Y104394) provided by Natural Science Foundation of Zhejiang Province.

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13. Richard B G, Peter P, Emerson W W. The effect of vegetation on the swelling and shrinkage of soils in Australia. Géotechnique, 1983, 33(2): 127–139

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