Under the action of rainwater seepage, geological origin, and human activities, the soil shear strength parameters will have spatial variability along the slope direction. After collecting samples of silty clay at a slope in the Three Gorges Reservoir area as the research object, not only the large-scale direct shear test was carried out on the site but also the direct shear test, water content test, density test, and particle grading analysis test were carried out in the laboratory with the undisturbed soil. The variation law and mechanism of soil shear strength parameters along slope were studied. The results indicate the following: (1) The coefficient of variation of shear strength parameters along the slope is relatively large. With the decrease of the elevation of the test location, the cohesion value tends to be strengthened, while the friction angle tends to degrade. (2) The mechanism of the variation law of soil shear strength parameters along the slope, which is mainly due to the decrease of the elevation, the decrease of the edges and angles between the particles, and the increase of the clay content is determined. (3) The variation model of shear strength parameters along the slope is proposed, which can provide a reference for relevant projects.
The shear strength parameters of soil are very important in the calculation of soil slope stability. The accuracy of shear strength parameters directly affects the reliability of slope stability calculation results [
In recent years, scholars carried out series of studies on the spatial variability and probability distribution of soil shear strength parameters of the slope. Luo et al. [
The above studies mainly focus on the spatial variability and probability distribution models of the shear strength parameters of regional slopes, and most of them focus on the difference of the shear strength parameters of different soil types in the buried depth direction, while few studies were conducted on the variation of the shear strength parameters along the slope of the same geological layer in a single slope. In this paper, the silty clay of a sediment slope in Shazhenxi Town of the Three Gorges Reservoir area was taken as the research object, and the large-scale direct shear test, laboratory direct shear test, water content test, density test, and particle sieving analysis test were adopted, respectively. The variation law and mechanism of shear strength parameters of soil along the slope direction (from high to low) were studied.
The test locations were selected based on the following principles: ① The test locations should be relatively flat, and it is convenient to arrange the test equipment and carry out the in situ direct shear test. ② In order to study the variation law, there should be enough height difference between adjacent test locations and be in a line along the slope direction.
Based on the above principles, 4 test locations were selected for the in situ large-scale direct shear test. The numbers were ranged from high to low termed XCZJ1, XCZJ2, XCZJ3, and XCZJ4, and the corresponding elevations were 285 m, 271 m, 251 m, and 224 m. The elevation of slope foot was 183 m, as shown in Figure
Map of in situ direct shear test locations.
When preparing the in situ direct shear test samples, first remove the cover layer about 30 cm thick on the surface, then draw a square of 510 mm × 510 mm on the ground, excavate the grooves along the circumference of the square, and process it into a sample of 500 mm × 500 mm × 400 mm, as shown in Figure
In situ direct shear samples preparation.
The in situ direct shear tests were carried out using a self-made test equipment system, as shown in Figure
In situ direct shear test equipment.
Four plastic storage boxes were used to seal the undisturbed soil in four test locations on site and then transported back to the laboratory. According to the standards [
The ZJ strain-controlled quadruple direct shear instrument was adopted in the laboratory direct shear test, which can simultaneously perform shear tests of four samples under different normal stresses. The normal stress was, respectively, applied at 100 kPa, 200 kPa, 300 kPa, and 400 kPa. The direct shear instrument is shown in Figure
ZJ strain-controlled quadrupole direct shear instrument.
The samples in the in situ direct tests after shear failure are shown in Figure
Damaged samples of in situ direct shear tests.
Shear stress and shear displacement curves of in situ direct shear tests: (a) 20.00 kPa; (b) 33.33 kPa; (c) 46.67 kPa.
It can be seen from Figure
The peak strength curves of the four different elevation test locations are shown in Figure
Variation of in situ test peak strength with different test locations.
Based on the Mohr–Coulomb criterion, the in situ direct shear test results at four different elevation test locations under different normal stresses were analyzed, and the cohesion and friction angles of the four test locations were obtained, as shown in Table
Cohesion and friction angle of the in situ test.
Test location number | Elevation of test location (m) |
|
Total strengthening degree (kPa/m) | Stage strengthening degree (kPa/m) |
|
Total deterioration degree (°/m) | Stage deterioration degree (°/m) |
---|---|---|---|---|---|---|---|
XCZJ1 | 285 | 9.66 | 39.35 | ||||
XCZJ2 | 271 | 11.12 | 0.10 | 0.10 | 30.54 | 0.63 | 0.63 |
XCZJ3 | 251 | 13.01 | 0.10 | 0.09 | 16.70 | 0.67 | 0.69 |
XCZJ4 | 224 | 21.32 | 0.19 | 0.31 | 14.57 | 0.41 | 0.08 |
In order to analyze the variation of cohesion and friction angle along the slope direction, the degree of deterioration was used to indicate the degree of decrease of the parameters along the slope, and the degree of strengthening was used to indicate the degree of enhancement of the parameters along the slope, as shown in the following equations:
Taking the test data obtained from the test location with an elevation of 285 m as a reference, the total strengthening degree and the stage strengthening degree of the cohesion and the total deterioration degree and the stage deterioration degree of the friction angle are calculated. The results are summarized in Table
The evolution curve of cohesion and friction angle in in situ direct shear tests.
It can be seen from Table
In order to further study the variation law of soil shear strength parameters with the slope direction and analyze the mechanism of this variation law, the laboratory direct shear tests, water content tests, natural density tests, and particle sieving tests were carried out by using the undisturbed samples taken from the site.
The laboratory direct shear tests were carried out on the undisturbed soil samples taken from the site. The numbers were ranged from high to low termed SNZJ1, SNZJ2, SNZJ3, and SNZJ4, and the corresponding elevations were 285 m, 271 m, 251 m, and 224 m, respectively. The shear stress-displacement curves of four different elevation samples under four different normal stresses are shown in Figure
Shear stress-shear displacement curves of laboratory direct shear tests: (a) 100 kPa; (b) 200 kPa; (c) 300 kPa; (d) 400 kPa.
As can be seen from Figure
The peak strength curves of four different elevation test locations under four normal stresses were calculated separately, as shown in Figure
Variation of peak strength versus laboratory test location.
Based on the Mohr–Coulomb criterion, the results of laboratory direct shear tests under different normal stresses at four different test locations were analyzed, and the cohesion and friction angles of the four test locations were obtained. The total strengthening degree, the stage strengthening degree, the total deterioration degree, and the stage deterioration degree were calculated, as shown in Table
Cohesion and friction angle of the laboratory test.
Test location number | Elevation of test location (m) |
|
Total strengthening degree (kPa/m) | Stage strengthening degree (kPa/m) |
|
Total deterioration degree (°/m) | Stage deterioration degree (°/m) |
---|---|---|---|---|---|---|---|
SNZJ1 | 285 | 10.48 | 35.40 | ||||
SNZJ2 | 271 | 12.27 | 0.13 | 0.13 | 33.22 | 0.16 | 0.16 |
SNZJ3 | 251 | 16.94 | 0.19 | 0.23 | 21.80 | 0.40 | 0.57 |
SNZJ4 | 224 | 29.05 | 0.30 | 0.45 | 20.17 | 0.25 | 0.06 |
Evolution of shear strength parameters in laboratory direct shear tests.
It can be seen from Table
The main factors affecting the shear parameters of soil include mineral composition of the soil, particle gradation, water content, and compactness [
The same numbering principle as the laboratory direct shear tests is adopted. The water content test numbers are SNHS1, SNHS2, SNHS3, and SNHS4; the natural density test numbers are SNMD1, SNMD2, SNMD3, and SNMD4; and the particle sieving analysis test numbers are SNKF1, SNKF2, SNKF3, and SNKF4. The water content test results are shown in Table
Water content test results.
Number | SNHS1 | SNHS2 | SNHS3 | SNHS4 |
---|---|---|---|---|
Water content (%) | 18.42 | 18.14 | 18.40 | 18.30 |
Standard deviation | 0.111 |
Natural density test results.
Number | SNHS1 | SNHS2 | SNHS3 | SNHS4 |
---|---|---|---|---|
Natural density (g·cm−3) | 2.023 | 2.020 | 2.005 | 2.023 |
Standard deviation | 0.007 |
The water content test results in Table
Grading curves of different test location soils are shown in Figure
Grading curves of undisturbed soil samples.
Particles with a particle diameter larger than 2 mm are called the gravel group, and the particle with 2 mm diameter is the threshold with or without capillary force. Therefore, it can be considered that the percentage of particles larger than 2 mm is the main factor affecting the friction angle of soil [
The percentages of particles larger than 2 mm and smaller than 0.5 mm at different test locations are shown in Table
Percentage of particles larger than 2 mm and smaller than 0.5 mm.
Number | Larger than 2 mm (%) | Smaller than 0.5 mm (%) |
---|---|---|
SNKF1 | 30.44 | 36.96 |
SNKF2 | 29.47 | 38.16 |
SNKF3 | 26.85 | 39.59 |
SNKF4 | 25.10 | 42.56 |
Percentage of particles larger than 2 mm and the friction angle versus the elevation of test location.
Percentage of particles smaller than 0.5 mm and the cohesion versus the elevation of test location.
The percentage of particles larger than 2 mm decreases with the decrease of elevation, while the percentage of particles smaller than 0.5 mm increases with the decrease of elevation. This may be caused by the seepage of rainfall, which brings the fine particles in the soil from high to lower. The friction angle of the soil decreases with the decrease of the percentage of particles larger than 2 mm because the angular angle between the contact of soil particles decreases and the bite force decreases. The cohesion increases with the increase of the percentage of particles smaller than 0.5 mm mainly because the voids between the soil particles are reduced, the clay content is increased, and the adhesion is increased. The test results are consistent with the opinions of Li et al. [
The range of cohesion and friction angle variation coefficients involved in the existing literature is statistically summarized in Table
Cohesion and friction angle variation coefficients of soil.
Category | Cohesion (kPa) | Friction angle (°) |
---|---|---|
Literature | 0.19–0.55 | 0.05–0.40 |
In situ test | 0.33 | 0.40 |
Laboratory test | 0.42 | 0.24 |
Comparing and analyzing the data in Table
Assuming that the change of shear strength parameters along the slope is a continuous process, the evolution models of shear strength parameters along the slope can be established. Due to the integrity of the fracture network and high fracture rate of large-size samples, the shear strength parameters obtained by the in situ direct shear test are generally smaller than those obtained by the laboratory direct shear test. The weight of in situ test results and laboratory test results was taken as 50%, respectively, and the friction angle and cohesion were calculated to analyze the variation law of shear strength parameters. Taking 285 m test location as the reference location, the height difference, the total strengthening degree of cohesive, and the total deterioration degree of friction angle are calculated, and the results are summarized in Table
Synthetic shear strength parameters.
Elevation of test location |
Elevation difference |
|
Total strengthening degree of soil cohesion |
|
Total deterioration degree of soil friction angle |
---|---|---|---|---|---|
285 | 0 | 10.07 | — | 37.38 | — |
271 | 14 | 11.70 | 0.115 | 31.88 | 0.393 |
251 | 34 | 14.98 | 0.145 | 19.25 | 0.533 |
224 | 61 | 25.19 | 0.245 | 17.37 | 0.328 |
Evolution curve of shear strength parameters with elevation difference.
The evolution model of total strengthening degree of cohesion along the slope:
Soil cohesion along the slope to a certain elevation:
The evolution model of total deterioration degree of friction angle along the slope:
Soil friction angle along the slope to a certain elevation:
This paper is based on the in situ direct shear test, laboratory direct shear test, water content test, density test, and particle sieving analysis test, and the main conclusions are as follows: The curve of shear stress change with shear displacement shows the plastic deformation characteristics. Under the same normal stress condition, the lower the test location elevation, the earlier the peak strength of the soil appears. The peak strength of the soil generally decreases with the decrease of the elevation of the test location. The cohesion of the soil along slope shows a trend of continuous strengthening with the decrease of the test location elevation. The lower the elevation, the greater the total and the stage strengthening degree. The friction angle shows a continuous deterioration trend with the decrease of the test location elevation. When the elevation is lower, the total and the stage deterioration degree first increase and then decrease, and the overall trend is decreasing. The mechanism of the abovementioned variation law of the shear strength parameters of the soil along slope is mainly due to the dragging force of rainfall seepage. The fine particles in the soil are brought by the dragging force from the high level location to the low level location, resulting in the reduction in the edge angles of the soil particles at the lower level location and the increase in the clay content and adhesion. But the superposition effect of other occurrence conditions of the soil is not excluded. The mechanism is independent of the water content and natural density of the soil. In view of the large variation coefficient of the shear strength parameters of the soil along the slope, it is suggested to consider the variability of the shear strength parameters along the slope and the buried depth in the stability analysis of the slope. The evolution model of the shear strength parameters of soil along the slope is presented in this paper, which can serve as a reference for similar projects.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This research was sponsored by the National Natural Science Foundation of China (no. 51439003) and the Research Fund for Excellent Dissertation of China Three Gorges University (no. 2018BSPY008).