The surficial failure of most expansive soil cutting slopes, subjected to the repeated wetdry cycles, often occurs during or after rainfall following a long drought. The reason for this, however, is still unclear. Therefore, the laboratory tests were conducted to gain the saturated drained shear strength of the natural Nanning expansive soil considering the combined effects of swelling with loading and wetdry cycles. The findings indicate that the envelope of shear strength, which significantly drops close or equal to zero, can be well fitted by the generalized power function. At the same time, the effect of shear strength parameters on the stability of the expansive soil cutting slope was investigated. The reasons for the shear strength attenuation of the natural expansive soil and the surficial failure of the expansive soil cutting slopes were analyzed. It is evident that the effective cohesion being small is a vital factor influencing the occurrence of surficial failure of an expansion soil slope. Moreover, an effective flexible support treatment measure was provided.
Expansive soils, which are regarded as a problem soil, are typically encountered around the world. The total distribution areas of expansive soil are more than one hundred thousand square kilometers in China [
The failure of expansive soil slopes, especially surficial failure, is one of the most serious geological disasters that frequently occurs during the construction of highways, railways, and hydraulic engineering projects in expansive soil areas in China [
Photo of surficial failure of an expansive soil cut slope.
For an engineering project, the shear strength of expansive soils is required to address the stability of expansive soil slopes. A review of the technical literatures revealed that the investigation of shear strength behavior of expansive soils was limited, especially, with respect to the effect of swelling. Dinesh and Chandra [
The effective normal stress on the failure plane is very small and commonly varies from approximately 10 to 30 kPa due to the surficial nature of slope failures. This stress is significantly lower than the actual range of stresses at which specimens are usually tested for investigating the slope stability of expansive soils. Therefore, the results of slope stability by applying the shear strength parameters obtained by universal tests could not be explained nationally since the saturated peak drained shear strength parameters of the expansive soil under wetdry cycles, or as the residual drained shear strength parameters [
In summary, little attention has been focused on the shear strength of natural expansive soils influenced by swelling (low stresses) and wetdry cycles. Therefore, the present paper highlighted the combined effect of swelling with loading and wetdry cycles on the saturated drained shear strength of natural expansive soils. The effect of shear strength parameters on the stability of expansive soil cutting slope was discussed. The reasons for the shear strength attenuation of natural expansive soils and the surficial failure of expansive soil cutting slopes were analyzed. Finally, an effective and flexible support treatment measure was provided.
The expansive soil used in this study was taken from a cutting slope at depths ranging from 2 to 3 m below the ground surface of the Nanning outer ring expressway in the middle of Guangxi Zhuang Autonomous Region, China. The block samples for natural samples were wrapped with a plastic membrane and waxed cloth to prevent moisture loss (ASTM D4220) [
Laboratory tests revealed that the soil has an average natural moisture content of 20.3% and an average natural density of 2.075 mg/m^{3}. The specific gravity, Gs, of the soil is 2.70. The particlesize distribution indicated that the soil consists of 0.3% sand, 56.1% silt, and 43.6% clay. Therefore, this soil could be classified as a silty clay. The liquid limit (LL) and the plasticity index (PI) are 46.0% and 22.3%, respectively. The results of the Xray diffraction test indicated that the predominant clay mineral in the soil is an illitesmectite (58%) mixed layer. The free swelling rate (FSR) is 62% obtained by free swell tests (T 01241993) performed according to
Characteristic index of expansive soil.
Proportion of different particle size (mm)/%  Montmorillonite content/%  Relative content of clay minerals/%  

>0.075  0.075–0.005  <0.005  <0.002 





0.30  56.14  43.56  42.29  11.78  58  11  20  11 
For the tested natural specimens, suitable sizes of soil blocks were first cut from the block samples and were then trimmed into appropriate dimensions for the saturated drained direct shear tests. The specimens were circular in shape with an inner diameter of 61.8 mm and a thickness of 20 mm. A sharpedged cutting tool with the same dimensions for both inner diameter and thickness was used for the trimming. All the cutting and trimming work was conducted in a humiditycontrolled room. The differences in dry density and moisture content did not exceed 0.04 g/cm^{3} and 1% for various soil specimens, respectively.
In a steel water tank, a properly sized filter was placed on a porous stone under a soil specimen, on which another filter under a porous stone was placed. Since the influence depth effected by atmospheric wetdry cycles is generally shallow, only fivelevel low stress was applied directly by a given loading, as shown in Figure
Wetdry cycle saturation with loading.
The direct shear apparatus used in this study was manufactured by Nanjing Soil Instrument Factory Technology Co., Ltd. The shear testing procedure follows T 01401993 in JTG E402007
To reflect the effect of the low normal stress on the drained shear strength, eight normal stresses of 5, 15, 30, 50, 75, 100, 200, and 300 kPa were applied. For the universal direct shear tests, the minimum normal stress was 50 kPa applied through a lever loading yoke. Therefore, the three low normal stresses of 5, 15, and 30 kPa were directly applied through equivalent dead weights.
Figure
Results of saturated drained direct shear tests on natural specimens subjected to different wetdry cycles. (a) Zero wetdry cycle. (b) Two wetdry cycles. (c) Four wetdry cycle. (d) Six wetdry cycles. (e) Eight wetdry cycles.
Except that two stressdisplacement curves at vertical stress of 300 kPa at 0 and 2 wetdry cycles have no peak, the others show a peak. For the two stressdisplacement curves having no peak, the maximum shear strength determined is the stress corresponding to the horizontal displacement of 6 mm. In general, the horizontal displacement corresponding to the peak value increases with the increase in vertical stress for different wetdry cycles. As expected, the maximum shear stress decreases with an increase in wetdry cycles. The saturation degrees of specimens after shear stageis slightly greater than that before shear, which is similar to the results achieved by Zhan and Ng [
Currently, different strength functions have been proposed to describe the nonlinear strength characteristics for soils, such as the bilinear function [
From (
According to (
Figure
Shear strength variations under wetdry cycles with loading.
Shear strength parameters of natural expansive soils under wetdry cycles with loading.
Number of wetdry cycles  Nonlinear fitting parameters  The intercept of nonlinear fitting curve (kPa)  Mohr strength parameters (75–300 kPa)  




Adj. 




0  0.5665  0.4617  0.9889  0.9956  26.7  30.4  28.5  0.997 
2  0.6111  0.2535  0.9495  0.9906  16.8  23.7  28.6  0.986 
4  0.6900  0.0724  0.8731  0.9898  7.1  21.6  28.4  0.988 
6  0.7074  0.0094  0.8417  0.9953  1.4  19.3  28.1  0.994 
8  0.7176  0.0000  0.8444  0.9904  0.0  20.8  28.2  0.985 
The increase in water content and water film thickness of clay particles in soils will decrease the strength of soils. Overburden pressures significantly influence the amount of water absorbed by the soils, and hence smaller overburden pressures lead to higher soil water absorption capacities. Therefore, it is evident that the water film between the clay particles subjected to lower overburden pressures will be thicker and the dry densities will be lower. When the soil absorbs water in dry conditions, most of the gases in soils could be discharged in the form of bubbles. This process may affect the action on the soil skeleton, resulting in microcracks in some weak bonding positions of clay particles. In addition, cementation substances in the soil may also be dissolved or softened, and the softened cementation substances and water film play lubricating roles in the soil, leading to a reduction in the friction resistance between clayey clay particles. These will destroy the bonding between soil structures and allow the sliding between clay particles. On the osmotic swelling stage, the smectite lattice spacing of the soils may increase due to the hydration. It results in the soils further dispersing into fine particles. Each fine particle could further absorb water molecules and hydration cations to form thicker hydrate films. Furthermore, the diffusion of the double layer repulsion should cause a lattice layer separation to seriously disperse into very thin sheets, resulting in a significant decrease in strength.
During the process of repeated wet and dry cycles, the occurrence of a large number of macro and microcracks, which gradually decreases from the surface of the slope to the interior of the active zone insitu, destroys the integrity, identity, and continuity of the soil mass. Alternatively, the continuous accumulation of the plastic strain will cause losses in cohesion of shear strength resulting from the bonding of clay particles. The cementation effect caused by the iron and manganese oxides existing in the natural samples may be destroyed, resulting in a loose structure. In addition, the natural original structure or the bonding of the soil might be irreversibly broken down by wetdry cycles [
Therefore, it should be noted that the expansive soil has a remarkable swelling characteristic being different from the common soils, the shear strength of which should account for the effect of wet and dry cycles as well as the effect of overburden pressures. Surface soil in shallower expansive soil slopes has more severe attenuation of shear strength.
To investigate the effect of shear strength parameters on the expansive soil slope stability, a series of limit equilibrium analyses were carried out using Slope/W [
A homogeneous slope was assumed for this limit equilibrium analysis with a height of 6.0 m and a slope ratio of 1 : 1.5 (V : H). The side and bottom boundaries were no flow boundaries. The others were flux boundaries to model the infiltration phases of rainfall, and constant water pressure was prescribed to simulate the field situations (i.e., runoff). It should be noted that the slope was not prescribed by layers with different saturated permeability coefficients according to the effect of wetdry cycles, because the fully saturation condition was focused on in this study. The initial ground water table was horizontal at the elevation of 1.0 m. A saturated steady seepage flow state calculated is shown in Figure
Slope calculation model.
Physical and mechanical properties of expansive soil.
Unit weight (kN)  Saturated permeability coefficient (m/s)  Effective friction angle (°)  Effective cohesion (kPa)  

Soil properties  20  2.3 
9, 12, 15, 18, 21, 24  0, 1, 2, 3, 5, 7.5, 10, 15, 20, 25, 30 
Figures
Relationship between factor of safety and effective cohesion
Relationship between factor of safety and effective friction angle
In the limit equilibrium state, the effective cohesion and the effective friction angle obtained, respectively, are approximately (7.0 kPa, 24°), (8.2 kPa, 21°), (9.3 kPa, 18°), (10.6 kPa, 15°), (12.2 kPa, 12°), and (13.8 kPa, 9°). The factor of safety increases from 0.34 to 1.48 with the effective cohesion changing from 0 to 15 kPa for the effective friction angle of 24°. However, the factor of safety only increases from 0.79 to 1.18, as the effective friction angle increases from 9° to 24°, respectively, for the effective cohesion of 10 kPa. These findings indicate that the variations in the effective cohesion have a greater impact on the factor of safety than changes in the effective friction angle. When the effective cohesion exceeds 15 kPa, the factor of safety is greater than 1.0.
Figures
Relationship between the maximum sliding vertical depth and effective cohesion
Relationship between the maximum sliding vertical depth and effective friction angle
There is a builtin universal datapoint strength function in Slope/W to describe the nonlinear shear strength envelope of the materials (soils or rocks) in a stability analysis, using the limit equilibrium method. For each slice, Slope/W first computes the effective normal stress at the slice base and then calculates the slope (tangent) of the nonlinear curve at the slice base effective normal stress as to be
General datapoint shearnormal function.
Based on the same slope calculation model and boundary conditions as seen in Figure
Factor of safety and the maximum vertical sliding depth (no wetdry cycles with loading).
Factor of safety and the maximum vertical sliding depth (after eight wetdry cycles with loading).
The factor of safety calculated using the nonlinear shear strength of a natural expansive soil cutting slope without wetdry cycles is 2.32, and the maximum vertical sliding depth is 4.38 m. The results clearly indicate that the newly excavated expansive soil slope without original structural fissures is stable even in a full saturation condition. However, the factor of safety reduces considerably to 0.94 when the expansive soils are subjected to eight wetdry cycles with loading, and the maximum vertical sliding depth is only 1.11 m. This result is fundamentally in agreement with the results observed insitu [
The previous research has shown that the strength reduces due to swelling of expansive soils after absorbing water, and this potential is controlled by the overburden pressure. Therefore, to a large extent, the shear strength relates to the overburden pressure. Studies of dams filled with expansion soils have indicated that the shear strength parameters should be different due to gravity stress variation in different depths [
Generally, a newly excavated expansive soil slope that is free of wetdry cycles is stable because there are no shrinkage cracks except for only a small number of unloading cracks or fissures; hence, the strength of more intact slope soil does not attenuate even if the saturated shear strength is also high. Moreover, the permeability coefficient of the surface soil is very small, and it can be considered impermeable. Therefore, the influence of rainfall on the surface soil is very limited, and it is still not saturated, leading to a stable slope. When treatment measures are not timely or invalid, numerous macro and microshrinkage cracks will occur on the slope surface soil after the wetdry cycles, undermining the integrity of the slope (as seen in Figure
Slaking of expansive clay soil due to swelling [
Therefore, there might be three important factors resulting in a surficial failure of an expansive soil slope: remarkable degreasing shear strength of surface soil subjected to wetdry cycles, increasing permeability coefficient of surface soil, and continuous or heavy rainfall after a long drought.
For a project, the technical measures used to treat expansive soil cutting slopes can be categorized as rigid and flexible treatment techniques. Common rigid treatment structures include a selfweight retaining wall, antislide piles, or a schistous slope wall. For these structures, the shear fracture or extrusion failure often occurs due to the swelling deformation of expansive soil slopes after wetting. It will result in a large swelling force exceeding the resistance of the rigid structure. Therefore, according to the characteristics of the surficial failure of expansive soil cutting slopes, flexible techniques using geotextiles, are more suitable for cutting slopes in expansive soil areas. This is because that a flexible structure allows a limited deformation of the slope surface and can adsorb the energy produced by the swelling and horizontal slope deformation. And then, a new balance for an expansive soil cutting slope will be reached.
A typical design diagram of the expansive soil cutting slope treated by the geogrid reinforced flexible support structures is shown in Figure
A typical design diagram of geogrid reinforced flexible support structure (unit: centimeter).
The construction of the geogrid reinforced flexible structure can be described as follows [
The designed cutting slope is overexcavated to a designed width, which depends on the depth obviously affected by the atmosphere and exceeds 3.5 m in general.
The geogrid is placed, and the fill layer is then backfilled and compacted, layer by layer with a thickness of 50 cm for each layer.
The layer with the geogrid is backenveloped with a 1 m lap length, and the next geogrid is placed on the layer.
The upper and lower geogrid layers are connected with connecting rods, and then the fill is backfilled and compacted with a new layer.
Steps 3 and 4 are repeated to the designed height, and the slope of the geogrid reinforced structure is adjusted to 1 : 1.5 (V : H).
During the process of construction, drainage facilities are constructed in the back and the subbase of the geogrid reinforced structure.
When the geogrid reinforced structure is completed, a moisture barrier is constructed at the top of the slope.
The working mechanism of the geogrid reinforced flexible structure can be summarized as follows:
The friction and interlocking between the geogrid and the filler, along with the enveloping and connecting of the layers of the geogrid, can reinforce the layers as a whole to stabilize the reinforced structure.
The swelling stress in the slope will be released through the deformation of the slope surface, which is permitted to a certain degree. Moreover, the large dead weight of the structure is beneficial to restrain the deformation [
The structure covering the face of a freshly cutting slope has a slope of 1 : 1.5 and a thickness exceeding 3.5 m, which prevents or considerably reduces the attenuation of shear strength due to the noticeable decrease in the effect of weathering (i.e., the wetdry cycles). At the same time, the selfweight of the reinforced structure can increase the strength of the slope soil as discussed above.
The comparison of factor of safety of expansive soil cutting slope treated by geogrid reinforced flexible structure or not are only focused in this paper. The slope calculation model and boundary conditions are assumed as the same as seen in Figure
Factor of safety of geogrid reinforced slope.
From Figure
According to the working mechanism for the flexible structure to treat cutting slopes mentioned above, fourteen expansive soil failure cutting slopes located in Nanning to Youyi Guan expressway were treated using the geogrid reinforced flexible support in 2003, which are stable till now after about fourteen years of seasonal wetdry cycles and several typhoon rainstorms (Figure
Photos of expansive soil cut slope treated by geogrid reinforced flexible support measures. (a) Nanning to Youyi Guan expressway (a ramp slope 350 m long and 20 m high). (b) Baise to Longlin expressway (a slope 160 m long and 8.7 m high). (c) Beijing west sixth ring expressway (a slope 1.2 km long and 22.3 m high). (d) Nanning outer ring expressway (a slope 246 m long and 9.2 m high).
Based on the laboratory tests on natural Nanning expansive soil, the stability of expansive soil cutting slope was analyzed and the application of flexible support treatment measure was described, and the following conclusions can be drawn:
The shear strength envelope of natural Nanning expansive soil subjected to repeated wetdry cycles with loading is nonlinear and can be well fitted by a generalized power function. The occurrence of the surficial failure of expansive soil cutting slopes is very different from general clays. This is because the shear strength of the expansive soil decreases significantly due to the effect of wetdry cycles. This decrease in the effective cohesion especially is considerable, and it can even be close to or equals zero.
When analyzing the stability of expansive soil cutting slopes, the selection of shear strength parameters must consider the combined effects of wetdry cycles and low stress. The results of slope stability, achieved using nonlinear shear strength parameters, are basically consistent with the actual conditions, indicating that the method is reasonable and reliable. The application of practical engineering methods for treatment expansive soil cutting slopes proves that the geogrid flexible support treatment measure is effective and environment friendly.
The authors declare that they have no conflicts of interest.
This work was supported by the National Key Research and Development Program of China (2017YFC0805300), the National Natural Science Foundation of China (51478054 and 51608053), Natural Science Foundation of Hunan Province (2017JJ3335), Key Project of Education Department of Hunan Province (17A008), Open Fund of State Engineering Laboratory of Highway Maintenance Technology (Changsha University of Science & Technology) (kfj150103), Open Fund of Engineering Research Center of Catastrophic Prophylaxis and Treatment of Road & Traffic Safety of Ministry of Education (Changsha University of Science & Technology) (kfj20160401), Jiangxi Communications Department Program (2013C0011), and the Scientific Research Fund of Hunan Provincial Education Department (15C0043).