Stability Analysis of Embankment Slope Considering Water Absorption and Softening of Subgrade Expansive Soil
2. Calculation Program of Moisture Absorption and Softening
- Firstly, the strength of undisturbed expansive soil with different water content is measured by a laboratory test, and the relationship between water content (saturation) and strength is obtained.
- The boundary conditions are imposed on the slope model, and the shear strength parameters of expansive soil are automatically modified and calculated through subroutines, according to the saturation change of soil, during the change of rainfall or groundwater level.
- Finally, the new stability coefficient and potential sliding surface are obtained by using the strength reduction method in the finite element analysis.
2.1. Analysis of Moisture Absorption and Softening Characteristics
2.2. Development of ABAQUS Subroutine
- Apply rainfall load boundary on the top and outside of the slope, and calculate the slope saturation field and groundwater level after rainwater infiltration.
- According to the calculation results of the new slope saturation after rainfall, the strength of the soil element is updated.
- Based on the new soil element strength, the stress field and displacement field of the slope are updated, and the stability is calculated by the finite element strength reduction method.
3. Engineering Examples
3.1. Landslide Survey
- The strength of the weathered layer of the shallow expansive soil is low, and there is a weak interlayer, at the depth of 2.5 m, from the original surface. The inclination of the interlayer is approximately the same as that of the stratum, and the dip angle is slightly larger than that of the stratum.
- The long-term rainfall in the rainy season infiltrates through the weathered layer of expansive soil with high permeability, raising the groundwater level, and the weak interlayer further softens to form a potential sliding surface. The lower slope toe has higher humidity and low strength, which reduces the binding force on the upper soil mass.
3.2. Slope Model Considering Rainfall Moisture Absorption Softening of Foundation Soil
- Basic model
- Analysis of groundwater rising process caused by rainfall infiltration
- Initial boundary conditions: before embankment construction, the groundwater level was observed after site leveling and before the rainy season in June 2017. It is located at 3.5–5 m underground. This water level line is set as the initial water level before rainfall. The long-term rainfall in the rainy season corresponds to a process of continuous rise in the groundwater level of the slope. When the groundwater level exceeds the ground during the rise process, it becomes runoff and will not generate excess pore pressure. Therefore, the surface of the slope should be defined as the boundary of pore water pressure-free dissipation. The bottom and both sides of the slope are defined as impervious boundaries to simulate the process of groundwater rise. The long-term rainfall in the rainy season corresponds to a process of continuous rise in the groundwater level of the slope. When the groundwater level exceeds the ground during the rise process, it becomes runoff and will not generate excess pore pressure. Therefore, the surface of the slope should be defined as the boundary of pore water pressure-free dissipation. The bottom and both sides of the slope are defined as impervious boundaries to simulate the process of groundwater rise.
- Rainfall infiltration process: The embankment filling of this section began on 2 July 2017, and the slope began to lose stability when the elevation of the road construction unit was filled to 17 m on November 6. In order to get the impact of real rainfall on the overall humidity field, it is necessary to determine the amount of rainwater infiltration according to the local rainfall data. The permeability of the fill is large, so it is assumed that the rainfall will directly infiltrate into the foundation soil layer through the fill. The leveled ground surface is selected as the rainfall infiltration boundary. The infiltration intensity and time of the boundary is the rainfall minus evaporation. According to the theory of unsaturated soil infiltration, the permeability of unsaturated soil is modified according to saturation. In the process of rainfall, when the rainfall amplitude is less than the unsaturated permeability coefficient of expansive soil, all rainwater infiltrates. When it is greater than the unsaturated permeability coefficient, the actual infiltration intensity is equal to the unsaturated permeability coefficient, and the rest of the rainfall is discharged by generating surface runoff.
- Consideration of moisture absorption and softening
3.3. Stability Analysis of Embankment Slope
- Verification of groundwater rise calculation results
- Analysis of slope stress and displacement and comparison of monitoring data
- Site landslide analysis
- Landslide site analysis
4. Emergency Disposal and Reinforcement Measures
4.1. Analysis of Emergency Response Measures
4.2. Design of Slope Reinforcement
- restricted by the site conditions, the design height and grading gradient of the pavement cannot be changed;
- it is necessary to ensure the long-term stability of highway traffic;
- the uneven settlement shall be strictly controlled to protect the highway pavement and the pipe gallery facilities 3 m below the pavement.
- Layout of reinforced soil in the filling area: excavate the steps at the top of the subgrade, lay the geogrid at a spacing of 0.7 m, and the tensile strength is ft = 70 kn/m. A total of 10 layers of geogrid are laid.
- Anti-slide pile: two rows of manually dug piles (with a section size of 2 × 2.5 m, a spacing of 5 m, and a pile length of 12–16 m) are arranged 10 m away from the step. A row of bored cast-in-place piles (pile diameter 1.2 m, pile length 25 m, longitudinal pile spacing 4 m, horizontal pile spacing 3 m) are arranged at the lower part of the slope.
- Drainage: due to the moisture absorption and softening characteristics of expansive soil, drainage channels are excavated on the east and west sides of the road before the construction of anti-slide piles. During the construction process, attention should be paid to the sealing and waterproof of the surface to prevent the infiltration of surface water.
- The shallow foundation soil is fractured by weathering, has low strength, and is absorbed and softened in rainy season, which is the main cause of the embankment landslide. Considering the strength reduction caused by moisture absorption, the true stability of slope can be better evaluated.
- For high fill embankment on weathered expansive soil, its stability and failure mode are mainly affected by the shallow weathered layer and weak layer. With the increase in the filling height, the most dangerous sliding of the embankment slope moves towards the inner side of the embankment, and the instability and failure range increase, but the failure surface still develops along the bottom of the weathered layer.
- In the case of strict site conditions, the scheme of anti-slide pile-reinforced soil has better practicability for this kind of embankment slope. At the same time, attention should be paid to prevent the infiltration of surface water during the construction process to avoid the further decline in the strength of the expansive soil foundation.
Data Availability Statement
Conflicts of Interest
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|(Sr %)||c (kPa) (Matrix Suction Has Been Considered)|
|93~97||−1.925 Sr + 217.925|
|Soil Layer||Strength Parameters|
|c (kPa)||φ (°)||E (MPa)||υ|
|③1 weathered layer of expansive soil||20||12||30||0.3|
|③2 unweathered layer of expansive soil||38||12||40||0.28|
|④ hard soil||38||20||50||0.25|
|Soil Layer||Natural Strength||Saturation Strength|
|c (kPa)||φ (°)||c (kPa)||φ (°)|
|Monitoring Point||Slope Crest||Middle||Slope Toe|
|Monitoring value||0.47 m||0.19 m||0.09 m|
(Consider moisture absorption and softening)
|0.45 m||0.18 m||0.10 m|
(Moisture absorption softening is not considered)
|0.051 m||0.022 m||0.13 m|
|Computing Method||Fs (Before the Rainy Season)||Fs (After the Rainy Season)|
(Consider moisture absorption and softening)
(Moisture absorption softening is not considered)
|unload 1 m of fill||1.04|
|unload 3 m of fill||1.15|
|unload 5 m of fill||1.32|
|remove spoil only||0.95|
|remove spoil and unload 5 m||1.24|
|8 degrees of earthquake||1.20||0.86|
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Zhao, S.; Zheng, J.; Yang, J. Stability Analysis of Embankment Slope Considering Water Absorption and Softening of Subgrade Expansive Soil. Water 2022, 14, 3528. https://doi.org/10.3390/w14213528
Zhao S, Zheng J, Yang J. Stability Analysis of Embankment Slope Considering Water Absorption and Softening of Subgrade Expansive Soil. Water. 2022; 14(21):3528. https://doi.org/10.3390/w14213528Chicago/Turabian Style
Zhao, Siyi, Jiantao Zheng, and Jian Yang. 2022. "Stability Analysis of Embankment Slope Considering Water Absorption and Softening of Subgrade Expansive Soil" Water 14, no. 21: 3528. https://doi.org/10.3390/w14213528