The phyllite-weathered soil is a regional speciality. It is essential to study the changes in shear strength of phyllite-weathered soil under dry-wet cycles to understand the changes in mechanical properties of phyllite-weathered soil in the process of dry-wet climate and to manage the slope of phyllite-weathered soil. This paper simulated 12 dry-wet cycles on the specimens of remodelled phyllite-weathered soil. Direct shear and SEM tests were conducted on the specimens in the 0th, 3rd, 6th, 9th, and 12th drying paths. The effects of moisture content and the number of dry-wet cycles on the shear strength of phyllite-weathered soil were analysed macroscopically and microscopically. The following conclusions were obtained: (1) The cohesion of the weathered soil of phyllite will be reduced by increasing the number of cycles, and the more the number of dry-wet cycles, the more pronounced the reduction; the internal friction angle of the weathered soil of phyllite will be reduced by increasing the number of cycles, but the pattern of the decrease in the internal friction angle is not obvious. (2) The increase in the number of dry-wet cycles will increase the stiffness and brittleness of the phyllite-weathered soil specimen, and it will change from the weak hardening type of plastic damage to the solid softening type of brittle damage after a certain number of cycles. (3) The SEM test found that phyllite-weathered soil particles in Longsheng, Guilin are large, and most of the particles are in face-to-face and angle-to-face contact, which is easy to form a hollow structure, and the dry density value of the soil in the natural state is small. At the same time, the soil is reddish-brown in colour because of the leaching of Fe2 O3. The shear strength index of the cemented phyllite-weathered soil with Fe2 O3 is more significant than that of phyllite-weathered soil in other areas. The soil has a good shear strength index and a small dry density.
1 Introduction
Longsheng County is located in the mountainous area northwest of Guilin City, which is a typical phyllite rock area in China. The “Longji Terraces” is a famous tourist scenic spot in the area. Longsheng County has less land and more mountains, an inch of land, and sizeable topographic relief, resulting in natural landslides and slope instability caused by cutting slopes to build houses, which is one of the most developed areas of landslide disasters in northern Guilin. The main soil layer in Longsheng County is the residual clayey soil of the Quaternary System weathered by the phyllite rock. The lower rock layer is the phyllite rock of the Arch cave group (Pt3ng) of the proterozoic danzhou group. As a particular regional soil, phyllite-weathered soil is formed by metamorphic phyllite rock through long-time physicochemical and weathering transportation. Longsheng County of Guilin belongs to a typical subtropical climate zone with a hot and rainy environment. Under the specific climate conditions of the region, the soil of the phyllite-weathered soil slope of phyllite is often in a saturated and unsaturated state and has experienced a long period of dry-wet cyclic action. The shear strength of the soil will be affected by the damp and dry cyclic action, thus affecting the stability of the landslide or slope.
Allam [1] found that the soil’s stiffness and brittleness would increase with the number of dry-wet cycles, and the earth’s compression modulus and shear strength would decrease with the number of dry-wet cycles. Zeng [2] obtained from the test of swelling soil in the Nanning area that the shear strength of swelling soil will be weakened continuously with the increase of the number of dry-wet cycles until it tends to be stable. Liu [3] summarised the empirical formula of shear strength decay of expansive soil by increasing the number of dry-wet cycles through the direct shear test and also proposed that the shear strength decay is due to the development of fissures during the dry-wet cycles. Chen [4] Through the test, it is concluded that the cohesive force of red clay tends to decrease with increasing dry density, and the angle of internal friction continues to grow under the same water content. After the remodelled red clay cement bond is broken, the cementing force is weakened and cannot be recovered in a short time. The increase in dry density causes the effective cementing area of soil particles to decrease to a greater extent than it increases, leading to a decrease in cohesion. The analysis of mechanical properties of swelling soil during dry-wet cycles by Xu [5] found that its strength is closely related to the number of dry-wet cycles and water content status. Zhang [6] argues that the mechanical properties of unsaturated soils undergo irreversible changes after dry-wet cycles by finding that the hygroscopic-dehygroscopic cycle process not only decreases the effective internal friction angle ϕ′ of unsaturated soils but also has a specific effect on the value of the suction internal friction angle ϕb. Liang [7] measured soil microstructure parameters rapidly and accurately using scanning electron microscopy and IPP image processing techniques, and fractal theory contributed Ideas to studying the pore microstructure of red clay soils. Wan [8] conducted a systematic experimental study on the mechanical properties and microstructural characteristics of compacted clay under the action of dry-wet cycles (indoor simulated landfill climate) to reveal the intrinsic nature of deformation characteristics and strength decay of compacted clay under the act of dry-wet cycles from the microscopic level, in response to the problems such as damage of compacted clay impermeable structure of landfill closure cover system under the action of dry-wet cycles. The influence of factors such as threshold size, analysis area size, scan point location, and magnification on the quantitative study of soil microstructure was investigated by calculating its apparent porosity and soil particle morphology fractional dimensional number from a series of SEM photographs by Tang [9], and the influence mechanism of each factor was explored. In addition, about there are also changes in the shear strength of various types of soils under the action of dry-wet cycles. Liu [10‐12] investigated the effects of dry density and dry-wet cyclic activity on the water-holding capacity of soils. Zhang [13‐15] studied the law of fracture evolution of soils under the action of dry-wet cycles. Zhao [16‐19] researched the shear strength test and the fracture evolution law of phyllite-weathered soil and red clay mixture.
Anzeige
There are few studies on the mechanical properties of phyllite-weathered soil, and the mechanisms affecting the mechanical properties of phyllite-weathered soil are not very clear, especially no studies on the soil–water interaction of phyllite-weathered soil (including dry-wet cycles). According to previous research [16‐19], phyllite-weathered soils are powdered soils or powdered clay soils with a small plasticity index, low clay content, quiet strength, poor water stability and water retention, and poor soil adhesion. In contrast, the phyllite-weathered soil in the Longsheng region has a high Fe2O3 content after leaching, and following cementation, the soil’s characteristics alter in numerous ways. This paper used the shear test to investigate the strength change of unsaturated phyllite-weathered soil during dry-wet cycles. The test results were analysed from unsaturated soil mechanics and soil structure perspectives. The research results are significant for understanding the change law of the mechanical properties of phyllite-weathered soil during dry-wet climatic processes. The significance of the research findings lies in their contribution to a better understanding of the variations in mechanical characteristics of phyllite-weathered soil in dry and wet climates and the management of phyllite-weathered soil slopes.
2 Test Material
The test soil was taken from the weathered soil of phyllite rock on the south side of the unstable slope of Heng Yi Road, Long Ji Xue Fu, Longsheng, Guilin City. The soil is reddish brown, with a depth of 1–2 m, a natural moisture content of 30–55%, and a natural dry density of 1.15–1.35 g/cm3.
The main clay minerals were kaolinite and illite. Other primary physical property indexes are shown in Table 1.
Table 1
Basic physical property indexes of phyllite-weathered soil
Specific gravity (g/cm3)
Free expansion rate (%)
Sand mass fraction (%)
Powder particle mass fraction (%)
Clay mass fraction (%)
Liquid limit (%)
Plastic limit (%)
Plasticity index (%)
2.68
44
18.2
48.2
43.6
57
34.5
22.5
3 Test Method
3.1 Specimen Preparation
The air-dried moisture content of the soil samples was measured to be 3.7% after being air-dried, crushed, and sieved through a 2-mm mesh. The original moisture level of the crushed soil sample was adjusted to 28, 30, 32, 34, 36, 38 and 40% by adding the appropriate amount of water and mixing carefully. Soil samples were sealed in plastic bags, placed in a humidifying cylinder, and allowed to stand for 48 h to ensure that the moisture in the models was evenly distributed. Then, weigh the required amount of soil sample, pour it into the ring knife, and crush it with a hydraulic jack such that the specimen’s diameter is 61.8 mm and its height is 20 mm. Referring to the density index of the in-situ soil, the initial dry density of the test design specimen is 1.25 g/cm3.
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3.2 Dry-Wet Cycle Test Scheme
The experiment was structured to include 12 dry-wet cycles. Seven moisture content control points were established during the drying process: 28, 30, 32, 34, 36, 38 and 40%. The process of the dry-wet cycle is shown in Fig. 1. Firstly, some pressed heavy plastic soil specimens were sealed and stored with an initial moisture content of 40%. The remaining heavy plastic soil specimens were placed on a porous plate and dried at room temperature (temperature: (20 ± 3) °C). The water content was calculated by weighing the specimens and obtaining the change in mass. When the desired moisture content control point of the specimens was reached, the specimens were taken out and sealed with cling film for 48 h to allow for a more uniform distribution of moisture inside the specimens, thus reducing the experimental error. When the desired moisture content control point is reached, the specimens are taken out and stored in cling film for 48 h so that the internal moisture of the specimens can be distributed more evenly to reduce the experimental error. For the straight shear test, four parallel samples were prepared for each moisture content control point. The remaining specimens were dried until the residual moisture content; then immersed in water to saturate the specimens under side-limited conditions for 48 h. After that, the specimens were removed, and the drying process was repeated. The straight shear test was performed at the corresponding moisture content control point at 0, 3, 6, 9, and 12 cycles.
Fig. 1
Schematic diagram of the dry-wet cycle process
×
3.3 Direct Shear Test
The test instrument is the ZJ-type strain-controlled straight shear instrument produced by Nanjing Ningxi Soil Instrument Co. To determine the effect of dry-wet cycles on the shear strength indexes (cohesion and internal friction angle), two sets of parallel direct shear tests were conducted for each group of specimens at a shear rate of 0.8 mm/min under normal stress conditions of 100, 200, 300 and 400 kPa when the specimens’ moisture content reached the control point. A total of 280 samples were pressed to carry out the above dry-wet cycle tests, and the test protocol is shown in Table 2. Figure 1 depicts the process of the dry-wet cycle in schematic form.
Table 2
Test scheme
Water content
Direct shear test options under different positive pressures
Dry-wet cycle 0 times
Dry-wet cycle 3 times
Dry-wet cycle 6 times
Dry-wet cycle 9 times
Dry-wet cycle 12 times
28
√
√
√
√
√
30
√
√
√
√
√
32
√
√
√
√
√
34
√
√
√
√
√
36
√
√
√
√
√
38
√
√
√
√
√
40
√
√
√
√
√
3.4 Scanning Electron Microscope Experiments
The electron microscope scanning instrument used in this experiment is Field Emission Scanning Electron Microscope (FESEM), model S-4800. The technical parameters are the magnification range from 25× to 800,000×, and the chemical element analysis range from 4Be to 99Es. In this experiment, the effect of dry-wet cycles on the weathered soil of Longsheng phyllite was mainly studied, so the test group with 34% moisture content was taken as an example, and tiny cubes of about 1 cm3 were selected from the middle part after the dry-wet cycles for freeze-drying with liquid nitrogen. Since the phyllite-weathered soil of Longsheng is not a conductive material, the sample was sprayed with gold to complete the SEM preparation. At 1000×, 5000×, 10,000× and 20,000× magnification, soil samples of phyllite rock subjected to 0, 3, 6, 9, and 12 cycles of dry-wet conditions were photographed.
4 Test Results and Discussion
4.1 Direct Shear Test Results and Analysis
The specimens meet the test requirements after completing the fabrication and dry-wet cycle process according to the scheme in Sect. 3.2. By geotechnical criteria [20], specimens of phyllite-weathered soil were submitted to direct shear testing, and the test results are presented in Tables 3 and 4.
Table 3
Direct shear test of phyllite-weathered soil with dry-wet cycles under different water content conditions (cohesion)
Water content (%)
The cohesion of phyllite-weathered soils (KPa)
Dry-wet cycle 0 times
Dry-wet cycle 3 times
Dry-wet cycle 6 times
Dry-wet cycle 9 times
Dry-wet cycle 12 times
28
32
41.3
39.6
35.9
31.1
30
39
50.7
47.2
45.3
39.6
32
46
54.2
52.7
51.3
43.2
34
53.2
58
56.8
54.9
48.2
36
48
53.3
51.7
50.2
46.1
38
42
47.1
46.3
44.7
38.2
40
38
38.2
36.7
35.1
29.2
Table 4
Direct shear test of phyllite-weathered soil with dry-wet cycles under different water content conditions (internal friction angle)
Water content
Internal friction angle of phyllite-weathered soil (°)
Dry-wet cycle 0 times
Dry-wet cycle 3 times
Dry-wet cycle 6 times
Dry-wet cycle 9 times
Dry-wet cycle 12 times
28
26.7
23.2
24
26.2
27.5
30
24.8
21.7
22.3
23.1
24.5
32
22
17.9
18.6
19.7
20.3
34
18.7
16.2
17.1
18.5
19.2
36
20.1
18.2
17.8
18.9
19.2
38
20.5
16.2
16.7
18.9
20.1
40
18.7
14.3
15.2
16.4
17.1
The graphs of the variation of cohesion and internal friction angle with moisture content under the controlled number of dry-wet cycles (Fig. 2a and b) and the graphs of the variation of cohesion and internal friction angle with moisture content under the controlled number of dry-wet cycles (Fig. 3a and b) are plotted according to the contents of Tables 3 and 4.
Fig. 2
Variation of cohesiveness and the angle of internal friction with the number of dry-wet cycles under-regulated moisture content
Fig. 3
Variation of cohesion and internal friction angle with moisture content
×
×
From Fig. 2a, it can be observed that the cohesive forces of the remodelled phyllite-weathered soil specimens increased significantly from 0 times dry-wet cycles to 3 times dry-wet cycles due to the action of dry-wet cycles in the case of controlled water content, except in the case of 40% water content, and that the cohesive forces of the remaining six controlled water content specimens underwent varying degrees of cohesive reduction from 3 times dry-wet cycles. Among these, the extent of the drop in force of cohesion is shown to be at its greatest between 9 and 12 cycles. The cohesive strength of the specimen group with 34% water content was the largest group in this test, and the maximum cohesive force of 54.2 kPa was obtained after the third cycle of the test. The magnitude and direction of the change in cohesion with the number of dry-wet cycles for these two test groups with a control moisture content of 32 versus 36% were comparable. The cohesive forces of the two groups of tests with 28 and 40% control moisture content had lower values in this test. Under the condition of constant water content, the cohesive force of the weathered soil specimens of phyllite showed an overall trend of increasing and gradually decreasing after the dry-wet cycles.
From Fig. 2b, it can be seen that the angle of internal friction shows a general trend of gradually decreasing after the action of dry-wet cycles with constant water content. The value of the angle of internal conflict for the group with low water content is more significant than that for the group with high water content.
From Fig. 3a, it can be observed that the cohesive force of the phyllite-weathered soil tends to increase and then decrease as the water content increases over the same number of cycles. The phyllite-weathered soil with three dry-wet cycles had the highest cohesion among the group. In this test, the cohesive force of the phyllite-weathered soil sample after 12 wet–dry cycles has the lowest value. And it can be observed that: the cohesive force after 3 dry-wet cycles > the coherent force after 6 dry-wet cycles > the cohesive force after 9 dry-wet cycles > the coherent force after 0 dry-wet cycles > the cohesive force after 12 dry-wet cycles.
From Fig. 3b, it can be observed that the internal friction angle of the phyllite-weathered soil decreases, then increases by a certain amount with the increase of water content over the same number of cycles, and then decreases again. The values of internal friction angle are more significant for the two groups of phyllite-weathered soil after 0 times cycles and after 3 times cycles and smaller for the group after 12 times cycles.
4.2 Comparative Microscopic Analysis of Phyllite-Weathered Soil
Figure 4 shows the SEM images of the weathered soil specimens with water content controlled at 34% phyllite-weathered soil after 0 cycles. The electron microscope scan can be used to obtain the microstructure of the phyllite-weathered soil, observe the particle morphology of the soil particles, and estimate the division size of the soil particles from the scale in the picture. It can be clearly marked from Fig. 4d that the grains of the phyllite-weathered soil without circulating remodelled maintain a lamellar structure with flaky, long flat lamellar grains with a grain length of 10–40 μm and a thickness of 0.5–3 μm. Most soil particles are in face-to-face and edge-to-face contact with each other. Therefore, the interior of the phyllite-weathered soil will form a fly-over structure. The natural dry density is small. At the same time, it can be seen that the arrangement of the particles of the phyllite-remoulded soil is basically disordered, and the directional arrangement trend of the particles cannot be obviously found. The overall structure of this combination of soil aggregates is unstable in geotechnical mechanics. A laminar structure dominates the microstructure of phyllite-weathered soil. The soil comprises varying-sized lamellar units, primarily in face-to-face contact but partially in edge-to-face and edge-to-edge connections to pile up and gather, forming erected lamellar agglomerates. The particles of cemented oxidised cemented clay are dispersed on the surface of the soil or fill the pores above. Under the contact bonding of these cement, the flake aggregates are connected into the laminated structure. The contact between the particle units is not close, the distribution is irregular, the pores between the particles are large, and the structural compactness is poor. The soil samples were damaged under pressure, which is mainly due to the damage to the body of the contact unit, that is, the damage to the bonding force between the contact soil particles, the change of the soil structure, the transformation of the contact mode, the arrangement direction between the soil particles and other tissues.
Fig. 4
SEM images of specimens of phyllite-weathered soil at different magnifications after 0 times cycles
×
Figure 5 is the SEM image of the phyllite-weathered soil sample with a water content of 34% after different cycles of 20,000 times. Figure 5a demonstrates that the particles of remodelled soil without a dry-wet cycle are relatively complete, with more large particles, a greater pore depth, and a large number of small particles on the surface of large particles. After three dry-wet cycles, as depicted in Fig. 5b, the pore depth becomes shallow, the large particles are shattered, and the small particles on the surface of the large particles are drastically diminished. After 6 and 9 cycles, the particles were further broken, and the fine particles increased significantly. The arrangement and combination of particles cannot find clear rules and show anisotropy.
Fig. 5
Phyllite weathered soil sample magnified 20,000 times SEM image after different cycles
×
The SEM image scanning is binarised, and MATLAB calculates the pores and particles to obtain the proportion of pores and particles, and then the line chart is drawn.
It can be seen from Fig. 6 that the proportion of pores in phyllite-weathered soil samples under four different magnifications decreases first when the dry-wet cycle is 0 to 3 times. In the case of dry-wet cycles of 3–9 times, the pore ratio of samples amplification 1000 and 5000 times continue to increase. The proportion of pores in the samples magnified by 10,000 times and 20,000 times increased in the case of 3–6 times dry-wet cycles and decreased in the case of 6–9 cycles. This differs from the law of 1000 and 5000 times enlarged samples. Since the image of the sample magnified 10,000 and 20,000 times have larger soil particles and pores, the entire image is occupied by fewer soil particles and pores, and the data obtained by the proportion of pores in the image is more one-sided. So magnified 1000 and 5000 times, the response of the image results is more convincing. In the case of 9–12 wetting–drying cycles, the proportion of pores in phyllite-weathered soil continues to decrease. The proportion of pores in phyllite-weathered soil decreases when experiencing 0–3 wetting–drying cycles, increases when experiencing 3–9 cycles, and decreases again when experiencing 9–12 cycles. After 0, 3, 6, 9, 12 dry-wet cycles, the proportion of pores in SEM images is about 55%, 40%, 44%, 59% and 51%, respectively.
Fig. 6
Variation of pore proportion with cycle times under different magnifications
×
5 Discussion and Mechanism Analysis
In the direct shear test results, the corresponding strength indexes (c and φ) of the remoulded soil samples after 0 cycles are lower than those of the remoulded soil samples after 3 cycles. This is because when the cycle is not experienced, the particles of the phyllite-weathered soil are large, the degree of compaction is not very high (the dry density of the remoulded soil sample is designed to be 1.25 g/cm3), the overall structure of the remoulded soil sample is loose, and there are many aggregates. This leads to large pores and cracks in the remoulded soil sample. There will be bubbles and a large amount of free water in the pores, so the obtained shear strength index is low. After three dry-wet cycles, these pores and cracks will gradually be filled by the solid phase components of clay, non-clay minerals, organic matter, and precipitated salts. Therefore, the shear strength index of phyllite-weathered soil samples after 3 times of dry-wet cycle is higher than that of phyllite-weathered soil samples after 0 times the dry-wet cycle. This falls in line with the findings of other cohesive soils.
In most instances, the soil is an integrated body of soil particles, and the displacement between soil particles determines the deformation process. In the process of dry-wet cycles, the content of clay minerals and soil organic matter will be lost more and more with the increase of cycle times, which is one of the reasons why the shear strength index of phyllite weathered soil decreases with the increase of cycle times.
The 0 dry-wet cycle test with a relatively large test error was temporarily taken out. The shear strength indexes of phyllite-weathered soil samples after 3, 6, 9, and 12 wetting–drying cycles were analysed. In the same drying process, in the optimal moisture content (33.7%) dry side, with the decrease of moisture content, the cohesion of the sample decreases the internal friction angle increases. On the side higher than the optimal moisture content, as the moisture content decreases, the cohesion of the sample increases, and the internal friction angle increases [21‐23]. This is because the sample suction gradually increases during drying, contributing to cohesion and shear strength. In addition, the volume of the specimen shrinks during drying, the pore ratio decreases, and the structure become denser, which also contributes to the increase in the shear strength index. In addition, during the drying process, the sample volume shrinks, the void ratio decreases, and the structure becomes denser, which is also an important reason for the increase in the shear strength index.
As shown in Fig. 7, after 3, 6, 9, and 12 wetting–drying cycles, the shear stress-displacement curve (normal stress 300 kPa) of phyllite soil with 34% moisture content are taken as an example. It can be seen from the figure that after 9 and 12 dry-wet cycles, the phyllite samples with 34% water content showed apparent brittle failure. The peak strength loss of the sample with 12 dry-wet cycles is more significant than that of the sample with 9 dry-wet cycles. Combined with Fig. 8, phyllite weathered soil samples after 3 times, 6 times, 9 times, and 12 times dry-wet cycles after a direct shear test. It can be seen that after 3 and 6 dry-wet cycles, the sample has apparent displacement but no shear fracture, and the shear stress does not decrease. Combined with Fig. 7, it is inferred that the failure mode of the sample is weak hardening plastic deformation. After the 9th and 12th wetting–drying cycles, the sample was sheared off. But it is worth mentioning that after the 9th wetting–drying cycle, the specimen still has a weak connection after being cut. The sample after the 12th dry-wet cycle was utterly cut off. Combined with the shear stress-displacement curve of Fig. 7, it can also be seen that the residual shear strength value after the 9th dry-wet cycle is more significant than that after the 12th dry-wet cycle.
Fig. 7
Effect of wetting–drying cycles on shear behaviour of phyllite weathered soil with 34% water content (normal stress 300 kPa)
Fig. 8
Pictures of 34% water content phyllite weathered soil after direct shear test fter the dry-wet cycle
×
×
The stiffness and brittleness of phyllite-weathered soil samples showed an increasing trend during the dry-wet cycle. The failure mode of the direct shear test changed from a weak hardening type of plastic failure to a strong softening brittle failure. This is because, during the drying process of the remoulded soil sample, the suction of the soil leads to the shrinkage deformation and structural adjustment of the soil and the crystal replacement of the clay minerals in the dry-wet cycle, which is irreversible. The principal clay mineral of phyllite-weathered soil is kaolinite. Isomorphous replacement and exchange capacity occur in the process of the dry-wet cycles. These factors are deep and need to be analysed with geotechnical engineering soil properties.
In addition, the bound water film between soil agglomerates is an essential factor affecting the shear strength. During the wetting process of the sample, the thickness of the surface-bound water film between the soil particles is thickened, and the water film force is weakened. In Table 3, cohesion decreases in the optimal moisture content wet test, and internal friction angle decreases in the same dry-wet cycle with increased moisture content. During dry-wet cycles, the water film of soil particles becomes thinner with the increase of cycles. The soil develops cracks after experiencing multiple dry-wet cycles, and the number of cracks increases with the rise of dry-wet cycles, which is one of the reasons why the shear strength index decreases with the increase of dry-wet cycles.
The microscopic particles of the phyllite-weathered soil are large-sized particles with substantial occlusion, which may increase the dilatancy of the soil. Thus the shear strength is more significant. After the dry-wet cycles, the large-sized particles are broken, thus reducing the dilatancy and decreasing the strength of the soil. Figure 5 shows that the particles of the phyllite-weathered soil specimen after 12 cycles are significantly smaller than those of the phyllite-weathered soil specimen after 0 and 3 cycles. The clay minerals of the phyllite-weathered soil are mainly illite and kaolinite. Kaolinite is a hexagonal flake and correlates with its degree of crystallisation, while illite is lamellar. The water-holding capacity of the mineral particles of the clay is weakened after multiple dry-wet cycles, the particles are reduced, the needle-like particles become smooth after movement, and the cohesion and internal friction angle are reduced. Due to the dry-wet cycles, the clay minerals are leached. The conversion of secondary minerals and the weakening of Fe2O3 cementation are also reasons for weakening the shear strength of phyllite-weathered soil. The strength of phyllite-weathered soil is also related to the properties of adsorbed cations, the exchange of ions affects the stability of the soil, and the exchange of cations affects the shear strength.
6 Conclusion
(1)
In the case of the same water content, after the dry-wet cycling action, with the increase in the number of cycles, the cohesion of the phyllite-weathered soil will decrease. The more the number of dry-wet cycles experienced, the more pronounced the decrease is. Under the same water content, the internal friction angle of phyllite-weathered soil tends to increase and then decrease with the number of cycles in general, but the regularity is not obvious.
(2)
Under the same water content, the increase in dry-wet cycles increases the stiffness and brittleness of the phyllite-weathered soil specimens. When the 9th cycle is experienced, the failure mode of the specimen will change from weak hardening failure of plastic failure to muscular softening failure of brittle failure.
(3)
After 0, 3, 6, 9, and 12 dry-wet cycles, the proportion of pores occupied by pores in SEM images is about 55%, 40%, 44%, 59%, and 51%, respectively. During the cycling process, the small clay particles first fill part of the larger pores through the dry-wet cycling motion, and the proportion of the area occupied by the pores decreases. Then, after the subsequent cycles, the large particles are broken, increasing the area occupied by the pores.
(4)
As the number of cycles increases, the larger particles and agglomerates in the phyllite-weathered soil decrease, the fine particles increase, and the cementation strength decreases, thus reducing the shear strength of the phyllite-weathered soil.
(5)
The SEM test revealed that the phyllite-weathered soil in the Longsheng area has larger particles, and most of the particles are in surface-surface and corner-surface contact, which can easily form a hollow structure. The dry density value of the soil is small in its natural state. At the same time, the reddish-brown colour of the soil is due to the leaching of Fe2O3. The shear strength index (cohesion and internal friction angle) of the phyllite-weathered soil cemented by Fe2O3 is larger than that of the previously studied phyllite-weathered soil. At the same time, the soil has a better shear strength index while having a smaller dry density.
Acknowledgements
This research was funded by the National Natural Science Foundation of China (41867039), the Foundation of Technical Innovation Center of Mine Geological Environmental Restoration Engineering in Southern Area (CXZX 2020002), project funded by Guangxi Key Laboratory of Geotechnical Engineering (20-Y-XT-03).
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