Startseite Experimental study on the influence of clay content on the shear strength of silty soil and mechanism analysis
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Experimental study on the influence of clay content on the shear strength of silty soil and mechanism analysis

  • Haibiao Xu , Xinming Li ORCID logo EMAIL logo , Pengpeng Liu , Song Yin , Kebin Ren , Yuzhou Sun und Jinming Geng
Veröffentlicht/Copyright: 31. Mai 2024
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Abstract

Shear strengths of silty soil were determined for shallow destruction of the soil sites frequently occurring in the Central Plains area. Specimens were prepared with five different clay contents (5, 10, 15, 20, and 25%) prior to compaction at dry densities of 1.60, 1.70, and 1.80 g/cm3. Soil specimens were saturated and then the consolidated undrained shear test was conducted with eight confined pressures ranging from 1 to 400 kPa. Results indicate that the shear strength increases significantly as the clay content increases from 5 to 25%, and the cohesion c shows bilinear function with the inflection point at the clay content of 10%. The difference of cohesion in the high and low stress sections decreases gradually to almost the same value until 25% of the clay content, while the internal friction angle φ decreases with the increase in clay content. Within the range of dry density and clay content tested, the shear strength of silty soil in the low stress range obtained is higher than the measured value. Therefore, for the shallow damage of soil site, the shear strength parameters should consider the low stress test conditions. The bilinear growth of cohesion c with clay content can be attributed to the changes from “sand-like soil” to “clay-like soil” with the skeleton of soil specimen transitioning from sand particles to “clay grid” when the clay content exceeds approximately 10% combined with the results of scanning electron microscopy.

1 Introduction

With social progress and economic development, the concept of cultural relics protection is increasingly enhanced, and a large number of earth sites are restored. After the ecological protection and high-quality development of the Yellow River Basin have become a national strategy, it is necessary to carry out the Yellow River Cultural Heritage System protection project. Located in the middle and lower reaches of the Yellow River Henan Province is the birthplace of Chinese civilization. There are more than 320 earth sites, most of which are rammed by silt or silty sand. Due to long-term exposure to the natural environment, the site is very easy to deteriorate, the shear resistance is poor, and it is easy to produce cracks, erosion, collapse, and other types of failure [1]. The method of adding clay particles conforms to the principles of “using ancient materials” and “repairing the old as the old” in the protection of soil sites [2,3,4]. And it is widely used in the preventive protection of soil sites. However, unlike pure sand or clay, the existence of fine particles, especially clay particles, makes the shear behavior of fine-grained sand (such as silty sand, etc.) very complex [5], and the mechanism of clay action is still unclear. Therefore, it is necessary to study the influence of clay content on the mechanical properties of soil sites in the Central Plains.

Scientists at home and abroad have studied the mechanical properties and formation mechanisms of soil samples with different clay contents. For example, Papadopoulou and Tika [6] conducted monotonic and cyclic triaxial tests on sand with fine grains, and studied the influence of the plasticity of fine grains on the strength characteristics of sand. The results showed that the undrained shear strength of sand with fine grains increased with the increase in plasticity index of fine grains. The research by Thevanayagam et al. [7,8,9] shows that the fine-grained content has a great influence on the stress–strain relationship of sand. Kim et al. [10] conducted undrained triaxial tests on clay sand with different mixing ratios. The results showed that the shear strength of low-density samples increased with the increase in fine-grained content, while that of high-density samples decreased. Lai et al. [11] conducted the influence of dry density and shear rate on the shear strength of saturated remolded loess. The results showed that with the increase in dry density, the cohesive force and internal friction angle of reshaped saturated loess samples increase. Dafalla [12] conducted direct shear tests on clayey sand and loess and found that the cohesion of the samples increased gradually with the increase in the clay content. It can be found that the effect of clay content on the mechanical properties of soil is more complex, and the academic community has not reached a consensus at present. Therefore, it is of great theoretical and practical significance to systematically study the influence of clay content on the silty soil behavior in the Central Plains for the protection and restoration of soil sites.

The resistance of soil sites to destruction is closely related to their strength and deformation parameters. Cohesion value can indirectly reflect the tensile strength of soil, and it is the main index reflecting the ability of soil sites to resist destruction [13]. The cracks, undercutting, collapse, and other deterioration of the earth sites in the Central Plains mainly occur in the shallow part of the earth sites, with the depth range generally not exceeding 3 m [1], as shown in Figure 1. And the stress on the sliding surface is less than the normal stress in the shear strength test (50–400 kPa) [14]. Therefore, the shear strength of soil under low-stress conditions can be used to characterize its real strength [15,16].

Figure 1 
               Characteristics of shallow types of failure: (a) collapse and (b) undercutting (photo taken in Yuanling ancient city).
Figure 1

Characteristics of shallow types of failure: (a) collapse and (b) undercutting (photo taken in Yuanling ancient city).

On the other hand, for soil materials, the difference in physical and mechanical properties is mainly caused by the difference in its microstructure. For instance, Farulla et al. [17,18,19] investigated the impact of the change in particle arrangement characteristics of silty sand and clay on its macro mechanical properties through scanning electron microscopy (SEM) analyses. Consequently, an analysis of the mechanical properties of the site soil with various clay contents should be complemented by a description of its corresponding microscopic characteristics.

This study takes the silt of typical sites in the Central Plains as the research object, and uses a triaxial apparatus to carry out consolidated undrained shear tests of silt with different clay contents (5–25%), stress conditions (1–400 kPa), and dry density (1.6, 1.7, and 1.8 g/cm3). Through SEM test, the microstructure of some samples is analyzed to explore the influence of clay content on the shear strength parameters of site soil under both low-stress and high-stress conditions. This research provides valuable insights for the preventive protection of soil sites.

2 Materials and methods

2.1 Materials

Located in Zhengzhou city, Henan Province, Yuanling ancient city is the seventh batch of national key cultural relics protection units, which has a high research and reference value for the study of politics, economy, and military affairs in the Central Plains. According to the specifications of investigation for preservation engineering of earthen sites (WW/T 0040-2012) [20], the destruction of earth sites should be avoided as much as possible in the process of soil sampling based on the principle of “minimum intervention” in the protection of earth sites. Therefore, the soil samples used in the test are silty sand from the scattered soil of Yuanling’s old city wall. The soil index properties are shown in Table 1 according to the Chinese Standard for geotechnical testing method (GB/T 50123-1999, GB 1999) [21].

Table 1

Basic properties of soil sample

Liquid limit w L (%) Plastic limit w P (%) Plasticity index I P Optimum moisture content w pt (%) Maximum dry density ρ dmax (g/cm3)
19.20 8.22 10.98 9.30 1.96

The clay particle is 6,000 mesh kaolin with a particle size of less than 0.005 mm. The particle size was measured by laser diffraction technique. The results showed that the proportion of clay was 81.15%. To reduce experimental errors, the sandy soil and kaolin clay particles were dried to constant weight in a 105°C oven before the experiment. Then, different contents (5, 10, 15, 20, and 25%) of kaolin clay are set to prepare soil samples. Figure 2 shows the particle grading curves of soil samples with different clay contents.

Figure 2 
                  The grain size distribution curve of silty soil with different clay contents.
Figure 2

The grain size distribution curve of silty soil with different clay contents.

2.2 Liquid limit and plastic limit

Clay content is an important factor affecting the liquid limit of soil specimens. There is a positive linear relationship between the liquid limit and the clay content, and the correlation between the liquid limit and the clay content are significantly higher than the plastic limit (Figure 3). This is consistent with the conclusions of Jong et al. and Constantinescu and Constantinescu [22,23]. The main reason is that the particles size of clay particles is much smaller than that of powder and sand particles, and there are a lot of adsorbed water film on the surface, while the pores between clay particles are very small. Therefore, the water film adsorbed by clay particles may fill these tiny pores, and the adsorbed water film may also be protected under the action of capillary force. Holding a certain amount of water affects the liquid limit of the silty sand.

Figure 3 
                  Relationship between liquid/plastic limit and clay content.
Figure 3

Relationship between liquid/plastic limit and clay content.

The plasticity index indicates the plasticity degree of soil, which is the water content of the difference between the liquid limit and the plastic limit. It indicates that the range from semi-solid to liquid state of soil can absorb water is closely related to the particle size distribution and clay content of the soil. There is a significant linear relationship between plasticity index and clay content, as shown in Figure 4.

Figure 4 
                  Relationship between plasticity index and clay content.
Figure 4

Relationship between plasticity index and clay content.

2.3 Methods

The static compaction method is adopted to produce cylindrical soil specimens with a diameter of 50 mm and a height of 100 mm by jack and sampling setup. After the sample is vacuumed and saturated, the triaxial consolidated undrained shear test shall be carried out according to specification for geotechnical testing (sl237-1997) [24]. During the experiment, the samples are sequentially subjected to steps such as soil sample installation, back pressure saturation, saturation check, sample consolidation, and shearing. Among them, when the sample is saturated, the back pressure is 100 kPa and the confining pressure is 105 kPa. After saturation for a period of time, check the saturation state of the sample. Only when the saturation is greater than 0.95, the test will continue downwards. Otherwise, continue to saturate, which can effectively ensure the reliability of the test results. During the shearing process, the shear rate is 0.1 mm/min and the test is completed when the axial strain reaches 15%. The standard stress path triaxial test system (Figure 5) was used in this test. In this work, the low-stress refers to four levels of load, i.e., 1, 5, 10, and 25 kPa and the high-stress is 50, 100, 200, and 400 kPa. The specific experimental design parameters are shown in Table 2.

Figure 5 
                  Experimental system.
Figure 5

Experimental system.

Table 2

Triaxial test scheme

Dry density (g/cm3) Clay content (%) Confining pressure (kPa) Stress conditions
1
5 Low stress
5 10
1.60 10 25
1.70 15 50
1.80 20 100 High stress
25 200
400

3 Results and discussions

3.1 Deviator stress–strain curve

The soil samples were prepared with varying clay content and subjected to triaxial shear tests. Considering the similarity in deviatoric stress–strain curves between high and low stress sections with varying clay content, only the deviatoric stress–strain curves of dry densities 1.60, 1.70, and 1.80 g/cm3 were selected for further analysis at a low stress level of 25 kPa and a high stress level of 200 kPa (as shown in Figures 6 and 7). When there is peak stress, the peak stress is taken as shear strength; when there is no peak, the stress value corresponding to 15% of strain is taken as shear strength, and the test results are listed in Table 3.

Figure 6 
                  Deviator stress–strain curve under low-stress at confining stress of 25 kPa: (a) Dry density of 1.60 g/cm3, (b) dry density of 1.70 g/cm3, and (c) dry density of 1.80 g/cm3.
Figure 6

Deviator stress–strain curve under low-stress at confining stress of 25 kPa: (a) Dry density of 1.60 g/cm3, (b) dry density of 1.70 g/cm3, and (c) dry density of 1.80 g/cm3.

Figure 7 
                  Deviator stress–strain curve under high-stress at confining stress of 200 kPa: (a) Dry density of 1.60 g/cm3, (b) dry density of 1.70 g/cm3, and (c) dry density of 1.80 g/cm3.
Figure 7

Deviator stress–strain curve under high-stress at confining stress of 200 kPa: (a) Dry density of 1.60 g/cm3, (b) dry density of 1.70 g/cm3, and (c) dry density of 1.80 g/cm3.

Table 3

Results of shear strength tests

Clay content (%) Dry density (g/cm3) Shear strength (kPa)
1 5 10 25 50 100 200 400
5 1.60 16.32 27.03 28.86 38.31 52.44 80.06 115.23 232.48
1.70 16.82 33.63 45.06 48.20 81.56 91.68 157.30 292.96
1.80 29.53 50.15 60.49 67.23 83.79 131.61 265.43 430.27
10 1.60 18.39 34.53 43.58 47.33 71.25 86.05 117.81 236.75
1.70 32.28 43.58 58.23 61.90 99.91 105.10 171.79 300.21
1.80 35.53 49.42 63.44 75.84 101.71 134.77 298.48 431.28
15 1.60 31.53 45.73 56.73 57.78 84.73 105.79 137.13 260.14
1.70 44.75 54.43 81.65 71.29 113.97 125.55 230.45 373.25
1.80 52.03 65.23 89.03 106.57 129.48 208.48 340.57 483.27
20 1.60 40.65 50.56 64.44 68.63 97.18 115.14 163.09 297.07
1.70 55.82 65.73 80.69 86.19 115.68 152.13 261.59 418.72
1.80 65.20 76.98 104.48 126.28 178.47 244.32 364.57 517.39
25 1.60 47.87 56.05 67.88 69.53 103.08 133.54 175.27 316.99
1.70 73.38 83.64 100.80 104.55 142.09 167.12 284.53 441.99
1.80 103.64 111.79 127.98 148.03 220.79 291.90 419.08 546.29

As can be seen in the Figures 6 and 7, under different clay contents and dry densities, the deviator stress–strain curves of soil are different, which are strain softening type or strain hardening type. Under the same dry density, the deviator stress–strain curve was similar when the content of clay was 5 and 10%, but the deviator stress value increased significantly when the content of clay increased to 15%.

Under high and low stresses, when the dry density is 1.60 g/cm3, the deviator stress–strain curve of 5–20% clay content has an obvious peak value, which is the strain softening type. When the clay content continues to increase to 25%, the softening is weakened, and the curve begins to change to strain hardening type. When the dry density increases to 1.80 g/cm3, the pores between the soil particles are compressed, the particles are closely combined, and the peak value of the deviator stress is significantly increased. The stress–strain curves shift to hardening type.

At the same dry density, as the consolidation stress of the sample increases, some pores between soil particles have been adjusted or closed at this stage, and the stress–strain relationship curve gradually changes to hardening type. Moreover, with the increase in clay content, the stress–strain curve hardening becomes more significant.

3.2 Influence of consolidation stress and clay content on shear strength

Based on the experimental study of shear strength of expansive soil under low stress conditions by Yang et al. [25], the Mohr circles were first drawn for the measurement points of four high stresses according to the standard method to obtain the straight-line formula of shear strength, and the corresponding shear stresses of four low stress points were calculated accordingly. Meanwhile, the measured shear stresses of each low stress point were listed for comparison. The results are shown in Table 4 when the confining pressure is 1, 5, 10, and 25 kPa, the calculated value obtained by the standard method is 1.91, 1.50, 1.21, and 1.37 times of the measured value for the samples of 10% clay content and 1.70 g/cm3 dry density. And when the clay content is 20% and the dry density is 1.60 g/cm3, the calculated value is 1.84, 1.51, 1.22, and 1.24 times of the measured value. The shear strength under the low-stress obtained by the fitting formula of the standard method is far greater than the measured value. Therefore, it is not safe enough to use the calculated strength by conventional stress (or high-stress) to analyze the shallow stability of earth sites.

Table 4

Measured value and corresponding calculated value of shear stress under low-stress

Clay content (%) Dry density (g/cm3) Measured value (kPa) Calculated value (kPa)
1 5 10 25 1 5 10 25
5 1.60 16.32 27.03 28.86 38.31 36.69 38.26 40.22 46.09
1.70 16.82 33.63 45.06 48.20 40.49 44.49 49.49 64.49
1.80 29.53 50.15 60.49 67.23 53.87 55.80 58.22 65.47
10 1.60 18.39 34.53 43.58 47.33 50.46 51.93 53.76 59.26
1.70 32.28 43.58 58.23 61.90 61.69 65.53 70.32 84.69
1.80 35.53 49.42 63.44 75.84 72.73 74.53 76.78 83.53
15 1.60 31.53 45.73 56.73 57.78 66.38 67.88 69.76 75.38
1.70 44.75 54.43 81.65 71.29 85.44 87.81 90.77 99.64
1.80 52.03 65.23 89.03 106.57 109.81 113.61 118.36 132.61
20 1.60 40.65 50.56 64.44 68.63 74.77 76.50 78.67 85.17
1.70 55.82 65.73 80.69 86.19 94.51 97.18 100.51 110.51
1.80 65.20 76.98 104.48 126.28 136.94 140.78 145.57 159.94
25 1.60 47.87 56.05 67.88 69.53 84.62 86.42 88.67 95.42
1.70 73.38 83.64 100.80 104.55 116.76 119.43 122.76 132.76
1.80 103.64 111.79 127.98 148.03 205.86 209.26 213.51 217.93

According to the specification for geotechnical testing (sl237-1997), the effective stress Mohr circles of the triaxial test shear strengths under four high stresses and four low stresses were drawn, respectively, and the nonlinear characteristics of strength under high stress and low stress were compared and analyzed. Given the similarity in nonlinear characteristics observed in the test, only the broken line representing sectional strength for samples with 10 and 20% clay content and a dry density of 1.80 g/cm3 is presented (Figure 8). From Figure 8(a), it can be seen that the fitting formula of the low-stress section is y = 0.95x + 13.8, and that of the high-stress section is y = 0.58x + 25.4. From Figure 8(b), it can be seen that the fitting formula of the low-stress section is y = 0.85x + 36, and that of the high-stress section is y = 0.57x + 41.79. The slope and intercept of the fitting formula of the high and low stress sections are different significantly.

Figure 8 
                  Shear stress curve for samples with dry density of 1.80 g/cm3: (a) Clay content of 10% and (b) clay content of 20%.
Figure 8

Shear stress curve for samples with dry density of 1.80 g/cm3: (a) Clay content of 10% and (b) clay content of 20%.

3.3 Relationship between clay content and shear strength index

The cohesion and internal friction angle exhibited similar changes with the clay content under three dry density conditions. Therefore, only the relationship curves between the strength parameters of high and low stress ranges and the clay content were plotted for a dry density of 1.70 g/cm3 (refer to Figure 9). The cohesion value of soil samples increased significantly with the corresponding rise in clay content, while the internal friction angle value decreased gradually, which was consistent with the conclusions of Dafalla [12].

Figure 9 
                  Relationship between strength parameters cohesion and friction angle with clay content: (a) Relationship between cohesion and clay content and (b) relationship between internal friction angle and clay content.
Figure 9

Relationship between strength parameters cohesion and friction angle with clay content: (a) Relationship between cohesion and clay content and (b) relationship between internal friction angle and clay content.

The cohesion increases with the increase in clay content in bilinear, regardless of the state of high-stress or low-stress consolidation. The inflection point occurs at 10% clay content, and the curve slope of the high and low stress section on the right side of the inflection point is significantly larger than that on the left side. In addition, with the increase in clay content from 5 to 25%, the cohesion value of low-stress section increased more significantly, from 8.7 to 28 kPa.

Several scientists [26] have emphasized the crucial role played by the cohesion c value in determining whether shallow collapse occurs on soil slopes. The cohesion c value of silty sand with clay content below 10% exhibits significant variations under high and low stresses. As the clay content increases, the disparity between cohesion c values in high and low stress sections gradually diminishes, becoming essentially equal at a clay content of 25%. Given that soil sites in the Central Plains predominantly consist of silty soil or silty sand (with clay content <10%), and that soil site deterioration are concentrated in shallow areas, it is imperative to investigate comparative analysis between low stress and high stress conditions during soil site protection efforts. This will enable obtaining reasonable strength parameters based on actual protection conditions.

In the earth site restoration project, the sampling work to obtain the strength parameters contradicts the principle of “minimum intervention” and other cultural relics protection principles. However, some soil sites in Henan province have silty sand with varying clay content. Therefore, the strength parameters obtained from the test can serve as a reference for determining the mechanical parameters of the site.

3.4 Relationship between dry density and shear strength index

Considering the consistent variation patterns between shear strength indexes and dry density observed in multiple test groups, only the relationship curves depicting the strength parameters of high- and low-stress segments with respect to dry density at 25% clay content were plotted (refer to Figure 10).

Figure 10 
                  Relationship between strength parameters and dry density: (a) Relationship between cohesion and dry density and (b) relationship between internal friction angle and dry density.
Figure 10

Relationship between strength parameters and dry density: (a) Relationship between cohesion and dry density and (b) relationship between internal friction angle and dry density.

The cohesion and internal friction angle of the sample increase with the increase in dry density. The reason is that with the increase in dry density, the sample becomes denser and the friction between aggregates increases gradually. The force required to overcome the friction between aggregates by external force will increase rapidly, so the shear strength parameters will increase rapidly.

With the increase in dry density from 1.60 to 1.80 g/cm3, the cohesion and internal friction angle of low-stress section increased by 84.2 and 18.4%, respectively, while those of high-stress section showed an increase of 88% for c and 17.1% for φ. This indicates that the effect of dry density on cohesion was more significant than that of internal friction angle, while the effect of dry density on cohesion and internal friction angle was similar across the low-stress and high-stress sections.

3.5 Absolute growth rate of shear strength

To quantitatively analyze the change rule of strength (the increased range of shear strength τ i) under different dry densities and different clay contents, the definition of absolute increase rate of strength Δ i,j is given (equation (1)):

(1) Δ i , j = τ i τ 0 τ 0 × 100 % ,

where i is the clay content, j is the vertical stress of the sample, τ 0 and τ i are the shear strengths when the clay content of the sample is 0 and i, respectively.

Considering the consistent variation patterns in the absolute growth rates of shear strength under different dry densities, Figure 11 only presents the absolute growth rates for various clay contents and vertical stresses at a dry density of 1.70 g/cm3; while Figure 12 focuses on the absolute growth rates for different clay contents and dry densities within a low-stress interval of 5 kPa and a high-stress interval of 100 kPa. Under the same consolidation pressure, the absolute growth rate of shear strength increases with the increase in clay content. However, the absolute growth rate of shear strength varies with different levels of clay content, showing higher rates at lower stress sections and lower rates at higher stress sections. The strength of the soil site can be improved by adding clay content.

Figure 11 
                  Strength growth rate with different clay contents at dry density equal to 1.70 g/cm3.
Figure 11

Strength growth rate with different clay contents at dry density equal to 1.70 g/cm3.

Figure 12 
                  Strength growth rate with different dry densities: (a) Absolute growth rate of shear strength of soil sample at 5 kPa and (b) absolute growth rate of shear strength of soil sample at 100 kPa.
Figure 12

Strength growth rate with different dry densities: (a) Absolute growth rate of shear strength of soil sample at 5 kPa and (b) absolute growth rate of shear strength of soil sample at 100 kPa.

Under the same clay content, the absolute growth rate of shear strength increases with the increase in dry density in the low-stress section and decreases with the increase in dry density in the high-stress section. For the samples at 15% clay content, the absolute growth increases from 44 to 94%, while it decreases from 50 to 30%. Under the same increase in dry density, the shear strength increases more efficiently under low stress in comparison.

4 Mechanism analysis

The mechanical properties of silty sand with clay particles are different from those of pure sand and typical clay [6]. Gao GR [27] found in the study of soil spatial structure systems that silty soil or silt is a granular structure system, while cohesive soil is a sheet structure system. With the increase in clay content, silt with clay undergoes the transition process from “sand-like” to “clay-like,” and finally tends to clay [28,29], that is to say, its particle skeleton evolves from granular structure system to sheet structure system. With the process of clay increasing, the stress skeleton of silt with clay first depends on the sand particles in the soil sample, and then the effect of clay begins to appear and gradually increases.

Figure 13 shows the adjustment process of silt particles under the action of clay. With the increase in clay content, the adjustment process of particles in silty sand is mainly divided into two stages.

Figure 13 
               Adjustment of soil particles under the action of clay. (a) Sand like stage and (b) clay like stage.
Figure 13

Adjustment of soil particles under the action of clay. (a) Sand like stage and (b) clay like stage.

The first stage is the “sand-like” stage, which belongs to the granular structure system. Some scientists have pointed out that sand with a clay content of less than 10% does not have the properties of cohesive soil. Currently, the “lubrication” between the sand particles plays a leading role, so that the silty sand with clay particles has a lower cohesion, which is consistent with the test results in Figure 9. According to Donald Wood Taylor [25], the shear strength of sand is composed of the shear resistance (friction component) provided by the sliding and rolling friction of particles and the shear resistance (shear expansion component) caused by the occlusion between particles. Under the action of low-stress, the sand presents the phenomenon of shear expansion, and under the action of high-stress, it presents the phenomenon of shear shrinkage (as shown in Figure 14). Under the action of horizontal shear with low vertical stress, the occlusal structure between particles is destroyed, and the particles on the shear plane cross the adjacent particles; under the action of high vertical stress, the sand is dense under the lower stress, and under the action of horizontal shear stress, the particles are staggered with each other, thus consuming more energy, which is shown as relatively large cohesion, which is similar to 5–10% clay in Figure 9. The results of large difference of cohesive force in high-and low-stress section under low amount of “sand-like” soil are consistent. To verify the rationality of the above analysis, the soil samples with different clay contents were tested by SEM, as shown in Figure 15. Under the clay content of 0–10%, the distribution form of clay is mainly filled with sand pores, and a small amount of clay adheres to the particle surface.

Figure 14 
               Pore water pressure–strain curve (a) under low-stress and (b) under high-stress.
Figure 14

Pore water pressure–strain curve (a) under low-stress and (b) under high-stress.

Figure 15 
               Microstructure of soil under the action of clay: (a) Clay content 0%, (b) clay content 10%, and (c) clay content 25%.
Figure 15

Microstructure of soil under the action of clay: (a) Clay content 0%, (b) clay content 10%, and (c) clay content 25%.

The second stage is the “clay-like” stage, which belongs to the flaky structure system for higher clay content. Lamb [27] divided the shear strength of cohesive soil into three basic components from the submicroscopic scale, i.e., cohesion component, dilatancy component, and friction component, in which cohesion component is provided by the shear resistance caused by adhesion or cementation between clay particles. With the increase in clay content, the clay particle begins to wrap the sand and act as a part of the stress skeleton of the soil. The soil properties transition from “sand-like” to “clay-like,” so the cohesive force gradually increases, which is consistent with the rule that when the clay content in Figure 8 is greater than 10%, the cohesive force value increases significantly. When the clay content is more than 10%, although “clay-like” also has the phenomenon of shear expansion and shear shrinkage (as shown in Figure 14), its shear action mechanism is different from “sand-like.” At this time, the clay wrapped on the surface of the sand plays a leading role in the shear process. Under the action of shear stress, the magnitude of shear expansion and shear contraction is smaller than that of “sand-like.” In addition, the cohesion of the soil sample is mainly caused by the short-range connection fracture of the solid clay. After the clay content is greater than 10%, with the increase in the clay content, the clay on the surface of the sand in the soil sample is distributed from scattered to scattered uniform distribution and development until the formation of “clay grid,” resulting in a small difference of cohesion c in the high- and low- stress sections when the clay content is 10–25% (“clay-like”) (Figure 9). Therefore, it can be inferred that when the clay fully covers the sand, the increase in the clay content will not cause the change in cohesion. For example, Yang et al. [25] found that the difference value of cohesion c between high- and low-stress sections was only 1.9 kPa in the shear strength test of expansive soil with 43.56% clay content, which also indirectly proved the above speculation. It can also be seen from Figure 14 that when the clay content is 25%, the clay adheres to the sand surface and wraps the sand, which is mutually verified with the adjustment process of soil particles in Figure 13.

5 Conclusion

  1. Under the same consolidation stress condition, with the increase in clay content or dry density, the stress–strain curve shows a trend from strain softening type to hardening type.

  2. According to the standard method, the shear strength of the low-stress section is not consistent with the actual results, so the low-stress condition must be included in the soil at the site strength test to obtain the true strength parameter values especially when the site soil is in a shallow failure state.

  3. The increase in the strength of specimens is mainly due to the increase in cohesion and internal friction angle. With the increase in clay content, the cohesion increases in a bilinear (the inflection point is at 10% of the clay content). And the difference of cohesion in the high- and low-stress sections decreases gradually to almost the same value until 25% of the clay content.

  4. When the dry density is the same, with the increase in clay content, the absolute growth rate of shear strength in the low-stress range is higher than that in the high-stress section.

  5. When the content of clay particles is greater than 10%, the silty soil at the site experiences a transformation from “sand-like soil” to “clay-like soil,” and the skeleton under the force is transformed from sand particles, and surface clay particles to “clay-like grid,” with a significant increase in cohesion and shear strength.

Acknowledgements

This work was supported by the Open Research Fund of State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences (No. SKLGME022027), the Basic Research Project of Henan Provincial Key Scientific Research Project (20ZX009), the Postgraduate Education Reform and Quality Improvement Project of Henan Province (YJS2022SZ16), the Scientific and Technological Project of Henan Province (222102320060) and National Natural Science Foundation of China (51509274).

  1. Funding information: This work was supported by the Open Research Fund of State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences (No. SKLGME022027), the Basic Research Project of Henan Provincial Key Scientific Research Project (20ZX009), the Postgraduate Education Reform and Quality Improvement Project of Henan Province (YJS2022SZ16), the Scientific and Technological Project of Henan Province (222102320060) and National Natural Science Foundation of China (51509274), the Henan Outstanding Youth Science Foundation (242300421153).

  2. Author contributions: All authors contributed to the study conception and design. The first draft of the manuscript was written by Haibiao Xu, Xinming Li, Pengpeng Liu, Yin Song, Kebin Ren, and Yuzhou Sun. All authors read and approved the final manuscript.

  3. Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  4. Data availability statement: All data, models, or codes generated or used during the study are available from the corresponding author by request. All data shown in the figures and tables can be provided on request.

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Received: 2023-12-27
Revised: 2024-02-28
Accepted: 2024-04-16
Published Online: 2024-05-31

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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