# Deformation and Strength Characteristics of Structured Clay under Different Stress Paths

Authors: Lu Li; Meng Zang; Rong-Tang Zhang; Hai-Jun Lu
Journal: Mathematical Problems in Engineering
 (2022)

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## Abstract

In geotechnical applications such as deep foundation excavation, slope cutting, and tunnel construction, the stress state of the soil subjected to loading or unloading will change and undergo different stress paths with corresponding changes in its mechanical properties. Therefore, a series of comprehensive laboratory tests were conducted on the natural strong structured clay to evaluate the effects of stress paths on the stress-strain, pore water pressure-strain, deformation characteristics, shear strength behaviors, and the microscopic mechanism of the natural strong structured clay. The results show that the mechanical properties of structured clay under different stress paths are quite different and are closely related to its structure. The peak deviatoric stresses of the soil under both the compression path and the tensile path are affected by the stress path, and the order of its strength is INP > KCP > DEP. The variations of the pore water pressure reflect the shear dilation and the shear contraction characteristics of the soil under different stress paths; the pore pressure of the specimen is particularly complicated in the case of both axial stress and lateral stress unloading; therefore, the pore water pressure in the soil under unloading conditions should be monitored in real time in engineering practice. The stress path affects the total stress strength indices of the structured clay to a significant extent and the effect on soil cohesion is greater than that on the internal friction angle, and the microstructure of the specimens has been changed after shearing tests with different stress paths. Therefore, design calculations should be conducted according to the experimental mechanical parameters under the stress path experienced by the soil under the actual engineering conditions to ensure safety and stability during construction.


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## Body

## 1. Introduction

In engineering practice, when the soil elements are subjected to load, the stress state will change accordingly and different stress paths will govern its behavior, for instance, the excavation of deep foundation pits, the construction of tunnels, etc. [1]. Ng et al. [2] described the stress paths in the soil units around the pit walls in deep foundation-pit excavation, compared and examined the field stress paths with the laboratory stress path tests, and proposed that the mechanical behavior of many soils such as stiff clay depends on their effective-stress and stress history. Much research [3–15] has shown that the mechanical properties of the rock-soil body are very complex, with characteristics of nonlinearity, elastoplasticity, shear dilation, and anisotropy; the mechanical properties and failure modes not only depend on the current and final stress state of the rock-soil body, but also are closely related to the stress history, subsequent loading direction, and the rock-soil body types; the rock-soil body may show different mechanical properties under different stress paths.Since the concept of stress path was proposed by Lambe [16], many researchers have used the notion to characterize the mechanical properties of soils. To study the strength and deformation properties of soils under different stress paths, Fu et al. [17] investigated the strength and deformation characteristics of remolded strength-loss under nonlinear continuous unloading paths. The results found that the shear-displacement curve of the soil under nonlinear continuous unloading paths is different from that under an unloaded state, and the shear strength is related to the initial consolidation pressure. Huang et al. [18] explored the correlation between the tangent modulus of soils and stress path; the results showed that soil samples exhibit distinct stress-strain characteristics under different stress paths, each of which exhibits nonlinearity. Based on the test results, a hyperbola function was proposed to express stress-strain relation and the formulae of the tangent modulus under K0 consolidation unloading were established. Yang et al. [19] investigated the effects of different unloading stress paths and unloading rates on the stress-strain relationship, variations of pore pressure, and failure strength characteristics of soft dredger fill. The results showed that soil behavior is related to the unloading rates and stress paths. Cai et al. [20], Yin et al. [21], and Wang et al. [22] conducted a series of experiments to study the influences of different consolidation stress paths on the strength and pore pressure properties of soft clay. Their results showed that the mechanical properties of soils are related to the stress path. The aforementioned research implied the influences of stress paths on the mechanical characteristics of soils from different perspectives and the results showed that the mechanical properties of soils around a foundation pit are affected by complex stress paths, making it necessary to conduct research on the influences of stress paths on the mechanical properties of soil during the excavation of such foundation pits; moreover, the previous research was mostly focused on weakly structured clay.The macroscopic mechanical properties of soils are strongly affected by the microstructure feature. X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) are commonly used to observe and analyze the mineral composition, microparticle morphology, pore distribution, and interparticle connections of the soil. Hamidi et al. [23] investigated the effects of epoxy resin on the mechanical parameters and microstructural properties of clays by uniaxial tests and SEM image-processing technology and proposed a quantitative evaluation index of structural elements. Zhu et al. [24] studied the mechanism underpinning the stress-strain characteristics of structured clay under triaxial load by SEM. They found that the diameters of the pore and orientations of particles and pores are affected by different stress paths, while shape characteristics of particles and pores are little changed thereby. Zhao et al. [25] studied the local mechanisms related to creep behavior of the remolded kaolin clay under triaxial load by SEM. The results indicated that the structural evolution during creep depends on the state of the structure at the end of the monotonic loading stage and the stress history had a significant influence on the dilation during creep, and the microstructure evolution results of particles and pores identified by SEM photos were consistent with the tendencies at the sample scale. However, the trend in the evolution of the microstructure of the natural strong structured clay along different stress paths has only been investigated by a few researchers.In summary, previous studies on mechanical properties of soils under different stress paths are mostly focused on normal soft clay, remolded soil, artificially structured clay, etc. Nevertheless, little research on the stress path dependence has been reported relating to the natural strong structured clay under actual working conditions of foundation-pit excavation. Therefore, a series of consolidation undrained triaxial tests were conducted on the natural strong structured clay under different stress paths for the simulation of the actual working conditions of foundation-pit excavation, to study the stress-strain, pore water pressure, and strength properties of the natural strong structured clay. Moreover, the microstructural mechanism of the structured clay under different stress paths was investigated using SEM. The results were discussed and the effects of stress path on the mechanical properties of soils around the excavation of foundation pits were explored. The results may provide the theoretical basis for the design calculation and deformation calculation of foundation-pit excavation in regions with the same type of soil.


## 2. Test Material and Experimental Programs

### 2.1. Sampling and Sample Preparation of Test Materials

A representative high-quality undisturbed soil sample was obtained at depths of 8.0 to 10.0 m from an excavation site in the Xiashan district of Zhanjiang city, Guangdong Province, China. From site sampling photographs (Figure1(a)) and the cross-sections of fresh soil samples (Figure 1(c)), it is noted that the sample is grey-brown and uniform (Figure 1(c)); the sampling method was conducted in accordance with GB/T 50123-2019 [26] in an attempt to minimize the disturbance of the sample during sampling, transporting, and trimming. The sample was shaped into a cylinder with a diameter of 39.1 mm and a height of 80 mm using a hand-saw to prepare for the unconfined compression test and the triaxial consolidated undrained shear test.Figure 1
Site sampling photographs from Zhanjiang, China.


### 2.2. Experimental Program

#### 2.2.1. Physical Tests and Unconfined Compression Tests

Tests to measure the physical properties (including water content, density, specific gravity, liquid and plastic limits, permeability test, and particle size distribution analysis tests) and the unconfined compression test (UC) on a natural strong structured clay were conducted according to the GB/T 50123-2019 [26]. The specimens used for UC tests were saturated by applying vacuum to them and then injecting water to submerge the specimen while maintaining the vacuum; the specimen was then tested at a shear rate of 2.4 mm/min.The grain size distribution curve of the soil is shown in Figure2. This indicates that the Zhanjiang strong structured clay is mainly composed of clay particles and powder particles. Table 1 summarizes the physical and mechanical indices of the soil. The Zhanjiang natural clay was found to have a high water content, high plasticity index, large void ratio, and low permeability coefficient in terms of physical properties. However, it has an unconfined compressive strength of 114 KPa, a sensitivity of 6.7, and structural yield strength of 300–400 KPa. Therefore, the Zhanjiang natural clay has inferior physical properties but superior mechanical properties and is an ideal material for studying the mechanical properties of structured soil.Figure 2
Particle size distribution curve of test specimens.Table 1
Physical and mechanical indices of Zhanjiang strong structured clay.
Specific gravity Gs/(g·cm−3)Water contentw/%Void ratioeLiquid limitwL/%Plastic limitwp/%Plasticity indexIpDensityρ/(g·cm−3)Permeability coefficientK/(10−7 cm/s)SensitivityStUnconfined compressive strengthqu/kPaStructural yield stressσk/KPa2.6943.851.254.220.833.41.842.66.2114300–400


#### 2.2.2. Mineralogical Composition of Soil by X-Ray Diffraction Test

The X-ray diffraction test (XRD) was adopted in accordance with GB/T 50123-2019 [26] standard to study the mineralogy of Zhanjiang clay. The specimen was a dry powder that had been naturally air-dried and then finely ground in a pestle to pass through a 200-mesh sieve (giving particles finer than 0.075 mm). A slide containing the processed specimen was inserted on the test bench of the X-ray diffractometer and scanned. The X-ray diffraction results of Zhanjiang clay are listed in Table 2: the specimen mainly consisted of illite, kaolinite, and smectite, and the relative content of clay minerals was 32%; smectite was present in the form of a mixed layer, and other nonclay minerals were mainly quartz and feldspar minerals.Table 2
Mineral composition of Zhanjiang strong structured clay.
Relative mineral content/%Clay mineral content/%QtzKfsAbIllKlnIll-SmeIIll-SmeII62338.9610.245.127.6832


#### 2.2.3. Determination of Stress Paths

The deformation behaviors of the soil around the foundation-pit excavation are influenced by complex stress paths in engineering applications. Therefore, the stress paths of the soil within the influence of deep foundation-pit excavation are classified into passive or active Earth pressure zones (Figure3) [19, 27]. In the passive Earth pressure zone, the vertical stresses decrease with the excavation of the overlying soils of the foundation pit, i.e., axial unloading. However, in some cases, such as the uplift of the pit bottom and the displacement of the foundation-pit support structure, the horizontal may be unloaded or loaded or remain unchanged. Therefore, Figure 3 illustrates soil unit A near the excavation surface of the foundation pit since the vertical unloading is the most obviously affected, indicating that the axial stress unloading or the horizontal stress remains unchanged (σ1 decreases, σ3 remains unchanged), and the horizontal unloading (σ1 decreases, σ3 decreases) results from the uplift of the pit bottom. However, the stress state of soil unit B near the foundation-pit support structure is more complicated due to the offset of the foundation support structure into the pit. Therefore, it is thought that B1 is under axial unloading and horizontal loading (σ1 decreases, σ3 increases), while B2 near the bottom of the support pile is unloaded both axially and horizontally (σ1 decreases, σ3 decreases). In the active Earth pressure zone, with the excavation of the foundation pit, as for the soil on the outside of the foundation support, such as soil unit D, the horizontal stress decreases and the axial stress remains unchanged (σ3 decreases, σ1 remains unchanged), and due to the actual construction process with construction machinery and construction operators walking around the edge of the pit, it is considered that its horizontal stress decreases or the axial stress increases (σ3 decreases, σ1 increases). For soil unit C, which is far from the foundation pit, the axial loading and the horizontal stress remain unchanged as soil unit C is subjected to the action of uniformly distributed load. Figure 4 depicts the different stress paths in p-q space.Figure 3
Stress state in the process of foundation-pit excavation.Figure 4
Stress paths inp-q space.


#### 2.2.4. Triaxial Consolidated Undrained Shear Tests

Triaxial consolidated undrained shear tests (CU) were conducted in accordance with GB/T 50123-2019 [26] on undisturbed clay samples by using the GDS stress path triaxial apparatus. The test proceeded as follows: firstly, the specimens used for CU tests were saturated by a combination of vacuum saturation and backpressure saturation, with full saturation confirmed when Skempton’s B-value exceeded 0.98. Then, the specimens were isotropically consolidated under different pressures, and when the excess pore water pressure had completely dissipated, the consolidation of the soil sample was considered complete. Finally, undrained shear tests were carried out according to the set stress path until the specimen was damaged or reached the required axial strain. In the p-q plane, the variations in deviator stress q and confining pressure σ3 are Δq and Δσ3, respectively, and the variation of the average principal stress p is Δp, so(1)Δp=Δσ3+Δq3.Table3 lists the tests conducted, in which ɳ denotes the stress path slope expressed as(2)n=ΔqΔp=ΔqΔσ3+Δq/3=Δσ1−Δσ3Δσ1+2σ3/3.Table 3
Stress path test program.
Consolidation confining pressure/kPaStress path slope/ɳStress path typeShear pathsShear rateDrainage condition100/200/300/400/500 (compression path)3.0INPΔσ3 = 0, Δσ1 > 00.08 mm/min (strain control)∞KCPΔσ3 < 0, Δσ1 > 00.25 KPa/min (stress control)−5.0DEPΔσ3 < 0, Δσ1 > 0−1.5DEPΔσ3 < 0, Δσ1 = 0−1.0DEPΔσ3 < 0, Δσ1 < 0Undrained100/200/300 (tensile path)1.0DEPΔσ3 < 0, Δσ1 < 00.25 kPa/min (stress control)3.0DEPΔσ3 = 0, Δσ1 < 0∞KCPΔσ3 > 0, Δσ1 < 0−5.0INPΔσ3 > 0, Δσ1 < 0Note. INP refers to increasing pressure, KCP denotes keeping constant pressure, and DEP represents decreasing pressure.


#### 2.2.5. Microstructural Observations

SEM was used to study the microstructure of specimens before and after the triaxial tests. When the CU test was completed, the sheared specimen was carefully removed from the pressure chamber with minimal disturbance and the original state of the soil was ensured. The samples for SEM tests were prepared with a size of 10 mm × 10 mm × 10 mm and were obtained by freeze-drying using liquid nitrogen (at −196°C) to remove water with minimal changes to the soil structure. Then, the samples were sprayed with gold under vacuum to increase electrical conductivity and the SEM samples were obtained. The FEI QUANTA 200 environmental scanning electron microscope was adopted to scan and image the microstructure of the samples.


## 2.1. Sampling and Sample Preparation of Test Materials

A representative high-quality undisturbed soil sample was obtained at depths of 8.0 to 10.0 m from an excavation site in the Xiashan district of Zhanjiang city, Guangdong Province, China. From site sampling photographs (Figure1(a)) and the cross-sections of fresh soil samples (Figure 1(c)), it is noted that the sample is grey-brown and uniform (Figure 1(c)); the sampling method was conducted in accordance with GB/T 50123-2019 [26] in an attempt to minimize the disturbance of the sample during sampling, transporting, and trimming. The sample was shaped into a cylinder with a diameter of 39.1 mm and a height of 80 mm using a hand-saw to prepare for the unconfined compression test and the triaxial consolidated undrained shear test.Figure 1
Site sampling photographs from Zhanjiang, China.


## 2.2. Experimental Program

### 2.2.1. Physical Tests and Unconfined Compression Tests

Tests to measure the physical properties (including water content, density, specific gravity, liquid and plastic limits, permeability test, and particle size distribution analysis tests) and the unconfined compression test (UC) on a natural strong structured clay were conducted according to the GB/T 50123-2019 [26]. The specimens used for UC tests were saturated by applying vacuum to them and then injecting water to submerge the specimen while maintaining the vacuum; the specimen was then tested at a shear rate of 2.4 mm/min.The grain size distribution curve of the soil is shown in Figure2. This indicates that the Zhanjiang strong structured clay is mainly composed of clay particles and powder particles. Table 1 summarizes the physical and mechanical indices of the soil. The Zhanjiang natural clay was found to have a high water content, high plasticity index, large void ratio, and low permeability coefficient in terms of physical properties. However, it has an unconfined compressive strength of 114 KPa, a sensitivity of 6.7, and structural yield strength of 300–400 KPa. Therefore, the Zhanjiang natural clay has inferior physical properties but superior mechanical properties and is an ideal material for studying the mechanical properties of structured soil.Figure 2
Particle size distribution curve of test specimens.Table 1
Physical and mechanical indices of Zhanjiang strong structured clay.
Specific gravity Gs/(g·cm−3)Water contentw/%Void ratioeLiquid limitwL/%Plastic limitwp/%Plasticity indexIpDensityρ/(g·cm−3)Permeability coefficientK/(10−7 cm/s)SensitivityStUnconfined compressive strengthqu/kPaStructural yield stressσk/KPa2.6943.851.254.220.833.41.842.66.2114300–400


### 2.2.2. Mineralogical Composition of Soil by X-Ray Diffraction Test

The X-ray diffraction test (XRD) was adopted in accordance with GB/T 50123-2019 [26] standard to study the mineralogy of Zhanjiang clay. The specimen was a dry powder that had been naturally air-dried and then finely ground in a pestle to pass through a 200-mesh sieve (giving particles finer than 0.075 mm). A slide containing the processed specimen was inserted on the test bench of the X-ray diffractometer and scanned. The X-ray diffraction results of Zhanjiang clay are listed in Table 2: the specimen mainly consisted of illite, kaolinite, and smectite, and the relative content of clay minerals was 32%; smectite was present in the form of a mixed layer, and other nonclay minerals were mainly quartz and feldspar minerals.Table 2
Mineral composition of Zhanjiang strong structured clay.
Relative mineral content/%Clay mineral content/%QtzKfsAbIllKlnIll-SmeIIll-SmeII62338.9610.245.127.6832


### 2.2.3. Determination of Stress Paths

The deformation behaviors of the soil around the foundation-pit excavation are influenced by complex stress paths in engineering applications. Therefore, the stress paths of the soil within the influence of deep foundation-pit excavation are classified into passive or active Earth pressure zones (Figure3) [19, 27]. In the passive Earth pressure zone, the vertical stresses decrease with the excavation of the overlying soils of the foundation pit, i.e., axial unloading. However, in some cases, such as the uplift of the pit bottom and the displacement of the foundation-pit support structure, the horizontal may be unloaded or loaded or remain unchanged. Therefore, Figure 3 illustrates soil unit A near the excavation surface of the foundation pit since the vertical unloading is the most obviously affected, indicating that the axial stress unloading or the horizontal stress remains unchanged (σ1 decreases, σ3 remains unchanged), and the horizontal unloading (σ1 decreases, σ3 decreases) results from the uplift of the pit bottom. However, the stress state of soil unit B near the foundation-pit support structure is more complicated due to the offset of the foundation support structure into the pit. Therefore, it is thought that B1 is under axial unloading and horizontal loading (σ1 decreases, σ3 increases), while B2 near the bottom of the support pile is unloaded both axially and horizontally (σ1 decreases, σ3 decreases). In the active Earth pressure zone, with the excavation of the foundation pit, as for the soil on the outside of the foundation support, such as soil unit D, the horizontal stress decreases and the axial stress remains unchanged (σ3 decreases, σ1 remains unchanged), and due to the actual construction process with construction machinery and construction operators walking around the edge of the pit, it is considered that its horizontal stress decreases or the axial stress increases (σ3 decreases, σ1 increases). For soil unit C, which is far from the foundation pit, the axial loading and the horizontal stress remain unchanged as soil unit C is subjected to the action of uniformly distributed load. Figure 4 depicts the different stress paths in p-q space.Figure 3
Stress state in the process of foundation-pit excavation.Figure 4
Stress paths inp-q space.


### 2.2.4. Triaxial Consolidated Undrained Shear Tests

Triaxial consolidated undrained shear tests (CU) were conducted in accordance with GB/T 50123-2019 [26] on undisturbed clay samples by using the GDS stress path triaxial apparatus. The test proceeded as follows: firstly, the specimens used for CU tests were saturated by a combination of vacuum saturation and backpressure saturation, with full saturation confirmed when Skempton’s B-value exceeded 0.98. Then, the specimens were isotropically consolidated under different pressures, and when the excess pore water pressure had completely dissipated, the consolidation of the soil sample was considered complete. Finally, undrained shear tests were carried out according to the set stress path until the specimen was damaged or reached the required axial strain. In the p-q plane, the variations in deviator stress q and confining pressure σ3 are Δq and Δσ3, respectively, and the variation of the average principal stress p is Δp, so(1)Δp=Δσ3+Δq3.Table3 lists the tests conducted, in which ɳ denotes the stress path slope expressed as(2)n=ΔqΔp=ΔqΔσ3+Δq/3=Δσ1−Δσ3Δσ1+2σ3/3.Table 3
Stress path test program.
Consolidation confining pressure/kPaStress path slope/ɳStress path typeShear pathsShear rateDrainage condition100/200/300/400/500 (compression path)3.0INPΔσ3 = 0, Δσ1 > 00.08 mm/min (strain control)∞KCPΔσ3 < 0, Δσ1 > 00.25 KPa/min (stress control)−5.0DEPΔσ3 < 0, Δσ1 > 0−1.5DEPΔσ3 < 0, Δσ1 = 0−1.0DEPΔσ3 < 0, Δσ1 < 0Undrained100/200/300 (tensile path)1.0DEPΔσ3 < 0, Δσ1 < 00.25 kPa/min (stress control)3.0DEPΔσ3 = 0, Δσ1 < 0∞KCPΔσ3 > 0, Δσ1 < 0−5.0INPΔσ3 > 0, Δσ1 < 0Note. INP refers to increasing pressure, KCP denotes keeping constant pressure, and DEP represents decreasing pressure.


### 2.2.5. Microstructural Observations

SEM was used to study the microstructure of specimens before and after the triaxial tests. When the CU test was completed, the sheared specimen was carefully removed from the pressure chamber with minimal disturbance and the original state of the soil was ensured. The samples for SEM tests were prepared with a size of 10 mm × 10 mm × 10 mm and were obtained by freeze-drying using liquid nitrogen (at −196°C) to remove water with minimal changes to the soil structure. Then, the samples were sprayed with gold under vacuum to increase electrical conductivity and the SEM samples were obtained. The FEI QUANTA 200 environmental scanning electron microscope was adopted to scan and image the microstructure of the samples.


## 2.2.1. Physical Tests and Unconfined Compression Tests

Tests to measure the physical properties (including water content, density, specific gravity, liquid and plastic limits, permeability test, and particle size distribution analysis tests) and the unconfined compression test (UC) on a natural strong structured clay were conducted according to the GB/T 50123-2019 [26]. The specimens used for UC tests were saturated by applying vacuum to them and then injecting water to submerge the specimen while maintaining the vacuum; the specimen was then tested at a shear rate of 2.4 mm/min.The grain size distribution curve of the soil is shown in Figure2. This indicates that the Zhanjiang strong structured clay is mainly composed of clay particles and powder particles. Table 1 summarizes the physical and mechanical indices of the soil. The Zhanjiang natural clay was found to have a high water content, high plasticity index, large void ratio, and low permeability coefficient in terms of physical properties. However, it has an unconfined compressive strength of 114 KPa, a sensitivity of 6.7, and structural yield strength of 300–400 KPa. Therefore, the Zhanjiang natural clay has inferior physical properties but superior mechanical properties and is an ideal material for studying the mechanical properties of structured soil.Figure 2
Particle size distribution curve of test specimens.Table 1
Physical and mechanical indices of Zhanjiang strong structured clay.
Specific gravity Gs/(g·cm−3)Water contentw/%Void ratioeLiquid limitwL/%Plastic limitwp/%Plasticity indexIpDensityρ/(g·cm−3)Permeability coefficientK/(10−7 cm/s)SensitivityStUnconfined compressive strengthqu/kPaStructural yield stressσk/KPa2.6943.851.254.220.833.41.842.66.2114300–400


## 2.2.2. Mineralogical Composition of Soil by X-Ray Diffraction Test

The X-ray diffraction test (XRD) was adopted in accordance with GB/T 50123-2019 [26] standard to study the mineralogy of Zhanjiang clay. The specimen was a dry powder that had been naturally air-dried and then finely ground in a pestle to pass through a 200-mesh sieve (giving particles finer than 0.075 mm). A slide containing the processed specimen was inserted on the test bench of the X-ray diffractometer and scanned. The X-ray diffraction results of Zhanjiang clay are listed in Table 2: the specimen mainly consisted of illite, kaolinite, and smectite, and the relative content of clay minerals was 32%; smectite was present in the form of a mixed layer, and other nonclay minerals were mainly quartz and feldspar minerals.Table 2
Mineral composition of Zhanjiang strong structured clay.
Relative mineral content/%Clay mineral content/%QtzKfsAbIllKlnIll-SmeIIll-SmeII62338.9610.245.127.6832


## 2.2.3. Determination of Stress Paths

The deformation behaviors of the soil around the foundation-pit excavation are influenced by complex stress paths in engineering applications. Therefore, the stress paths of the soil within the influence of deep foundation-pit excavation are classified into passive or active Earth pressure zones (Figure3) [19, 27]. In the passive Earth pressure zone, the vertical stresses decrease with the excavation of the overlying soils of the foundation pit, i.e., axial unloading. However, in some cases, such as the uplift of the pit bottom and the displacement of the foundation-pit support structure, the horizontal may be unloaded or loaded or remain unchanged. Therefore, Figure 3 illustrates soil unit A near the excavation surface of the foundation pit since the vertical unloading is the most obviously affected, indicating that the axial stress unloading or the horizontal stress remains unchanged (σ1 decreases, σ3 remains unchanged), and the horizontal unloading (σ1 decreases, σ3 decreases) results from the uplift of the pit bottom. However, the stress state of soil unit B near the foundation-pit support structure is more complicated due to the offset of the foundation support structure into the pit. Therefore, it is thought that B1 is under axial unloading and horizontal loading (σ1 decreases, σ3 increases), while B2 near the bottom of the support pile is unloaded both axially and horizontally (σ1 decreases, σ3 decreases). In the active Earth pressure zone, with the excavation of the foundation pit, as for the soil on the outside of the foundation support, such as soil unit D, the horizontal stress decreases and the axial stress remains unchanged (σ3 decreases, σ1 remains unchanged), and due to the actual construction process with construction machinery and construction operators walking around the edge of the pit, it is considered that its horizontal stress decreases or the axial stress increases (σ3 decreases, σ1 increases). For soil unit C, which is far from the foundation pit, the axial loading and the horizontal stress remain unchanged as soil unit C is subjected to the action of uniformly distributed load. Figure 4 depicts the different stress paths in p-q space.Figure 3
Stress state in the process of foundation-pit excavation.Figure 4
Stress paths inp-q space.


## 2.2.4. Triaxial Consolidated Undrained Shear Tests

Triaxial consolidated undrained shear tests (CU) were conducted in accordance with GB/T 50123-2019 [26] on undisturbed clay samples by using the GDS stress path triaxial apparatus. The test proceeded as follows: firstly, the specimens used for CU tests were saturated by a combination of vacuum saturation and backpressure saturation, with full saturation confirmed when Skempton’s B-value exceeded 0.98. Then, the specimens were isotropically consolidated under different pressures, and when the excess pore water pressure had completely dissipated, the consolidation of the soil sample was considered complete. Finally, undrained shear tests were carried out according to the set stress path until the specimen was damaged or reached the required axial strain. In the p-q plane, the variations in deviator stress q and confining pressure σ3 are Δq and Δσ3, respectively, and the variation of the average principal stress p is Δp, so(1)Δp=Δσ3+Δq3.Table3 lists the tests conducted, in which ɳ denotes the stress path slope expressed as(2)n=ΔqΔp=ΔqΔσ3+Δq/3=Δσ1−Δσ3Δσ1+2σ3/3.Table 3
Stress path test program.
Consolidation confining pressure/kPaStress path slope/ɳStress path typeShear pathsShear rateDrainage condition100/200/300/400/500 (compression path)3.0INPΔσ3 = 0, Δσ1 > 00.08 mm/min (strain control)∞KCPΔσ3 < 0, Δσ1 > 00.25 KPa/min (stress control)−5.0DEPΔσ3 < 0, Δσ1 > 0−1.5DEPΔσ3 < 0, Δσ1 = 0−1.0DEPΔσ3 < 0, Δσ1 < 0Undrained100/200/300 (tensile path)1.0DEPΔσ3 < 0, Δσ1 < 00.25 kPa/min (stress control)3.0DEPΔσ3 = 0, Δσ1 < 0∞KCPΔσ3 > 0, Δσ1 < 0−5.0INPΔσ3 > 0, Δσ1 < 0Note. INP refers to increasing pressure, KCP denotes keeping constant pressure, and DEP represents decreasing pressure.


## 2.2.5. Microstructural Observations

SEM was used to study the microstructure of specimens before and after the triaxial tests. When the CU test was completed, the sheared specimen was carefully removed from the pressure chamber with minimal disturbance and the original state of the soil was ensured. The samples for SEM tests were prepared with a size of 10 mm × 10 mm × 10 mm and were obtained by freeze-drying using liquid nitrogen (at −196°C) to remove water with minimal changes to the soil structure. Then, the samples were sprayed with gold under vacuum to increase electrical conductivity and the SEM samples were obtained. The FEI QUANTA 200 environmental scanning electron microscope was adopted to scan and image the microstructure of the samples.


## 3. Test Results and Discussion

### 3.1. Failure Modes

All samples showed plastic failure characteristics and formed obvious shear bands before the axial deformation reached 15% (Figure5). The failure modes of the soil depend on many random factors such as the homogeneity of the soil, the initial boundary conditions, the disturbance of the soil, and the loading rates. In the test, it was observed that three types of shear band failures occurred in the structured clay under different stress paths as follows:(1)
Under the compression path (includingɳ = 3.0, and ɳ = −1.5), the failure modes of the specimen include a single shear band, inclined to one side, as shown in Figure 5(a).(2)
Part of the failure modes of the specimens under the compression path (includingɳ = ∞, ɳ = −5.0, and ɳ = −1.0) appear primary and secondary shear bands; the failure mode of the specimen is a double-cross asymmetric X-shaped shear band, as shown in Figure 5(b).(3)
Under the tensile loading path (ɳ = 1.0, ɳ = 3.0, ɳ = ∞, and ɳ = −5.0), the deformation of the specimen exhibits axial elongation and radial local contraction, which is a figure-of-eight-shaped shear band, as shown in Figure 5(c).Figure 5
Typical failure modes of samples under different stress paths. (a) Single shear band failure. (b) X-shaped shear band failure. (c) Figure-of-eight-shaped tensile failure.
(a)(b)(c)Under the compression path, the particle displacement increases with the increase in deviatoric stress during shearing, which makes the specimen produce an obvious shear band in the compression process; with the further increase of deformation, the specimen produces multiple shear bands and demonstrates a double-cross asymmetric X-shaped shear band. Under the tensile path, the failure modes of the specimens are figure-of-eight-shaped shear bands, due to the stress, the soil undergoes elastic deformation and internal structural dislocation, which causes the concentration of the stress in the soil, and the shear bands are generated rapidly, undergoing tensile deformation. The stress on the soil under different stress paths is different, which leads to the formation of different shear bands; however, for the Zhanjiang structured clay, the structure of the soil is a necessary condition for the generation of shear bands [9].


### 3.2. Stress-Strain Relationships

The deviatoric stress-axial strain curves of the strong structured clay under different stress paths are shown in Figure6. The results of the tests are listed in Table 4.Figure 6
Deviator stressq and vertical strain εa on structured clay under different stress paths. (a) ɳ = 3.0. (b) ɳ = ∞. (c) ɳ = −1.5. (d) ɳ = 1.0 (tensile path). (e) ɳ = 3.0 (tensile path).
(a)(b)(c)(d)(e)Table 4
Test results of structured clay under different stress paths.
Consolidation confining pressure/KPaStress path slope/ɳStress path typeFailure deviatoric stress/qfFailure pore pressure/ufFailure strainεa/%1003.0INP122.130.01.92∞KCP129.725.72.5−1.5DEP120.9−38.52.21.0DEP95.1−64.03.63.0DEP101.9−39.35.32003.0INP149.251.32.3∞KCP167.458.72.9−1.5DEP158.0−8.02.81.0DEP127.8−60.05.23.0DEP141.7−29.46.23003.0INP179.992.03.3∞KCP195.6123.04.14−1.5DEP199.13.13.21.0DEP158.1−57.06.63.0DEP171.8−13.46.964003.0INP225.4112.74.4∞KCP237.2170.84.3−1.5DEP230.934.04.25003.0INP266.1138.54.8∞KCP287.4219.54.93−1.5DEP258.965.04.4Figure6 shows that the stress-strain curves of the soil under different stress paths exhibit a similar trend in terms of deformation; the stress increases with strain in the initial stage of the test, when the stress increases to its peak value; then it decreases with strain and finally stabilizes (i.e., strain-softening occurs [28]). In general, all the stress-strain curves in Figure 6 present strain-softening. As the internal structure of the soil plays an important role in resisting the external load, with the increase of axial strain, the external stress exceeds the structural yield stress and the specimen exhibits distinct shear failure locally. This finding shows that the specimen enters an obvious failure stage after the elastic deformation and the yield stage, and the strength of the specimen decreases gradually with the increase in strain after the deformation reaches the structural yield strength.As can be seen from Figure6, the strain at the failure point under the compression path is mostly around 2% to 5%; however, the strain at the failure point under the tensile path is between 5% and 10%. At the initial stage of loading, i.e., when axial strain εa < 2%, the deviatoric stress increases rapidly with the increase in axial strain, the stress path has little effect on the strength of the specimen, and the deviatoric stress-axial strain curves of the specimens under different stress paths almost overlap and increase linearly. However, when εa > 2%, the deviatoric stress-axial strain curves are gradually separated, specimens under different stress paths show different peak strengths, and the influences of stress paths on soil are more obvious.The failure strength of the specimen under the same stress path is related to the consolidation pressure: the larger the consolidation pressure, the greater the failure strength of the specimen, and the strain at failure increases with the consolidation pressure. Table4 shows that the failure strength of soil is affected by the stress path, compared to the results of increasing P and constant P, the failure deviatoric stress under decreasing P is the smallest, except for triaxial experiment of strain control (ɳ = 3.0), and the order of the strengths is INP > KCP > DEP. Moreover, the failure strength of the stress paths with slopes ɳ = −1.0 (compression path) and ɳ = 1.0 (tensile path) under decreasing P is the smallest; the axial and lateral unloading during shearing cause a decline in the shear strength of the soil. The peak deviatoric stress of the soil under the compression path is higher than that under the tensile path. For instance, when the consolidation pressure is 100 KPa, the values of qf of the stress path slopes ɳ = 3.0, ∞, −1.5 under the compression path are 122.1 KPa, 129.7 KPa, and 120.9 KPa, respectively, and the values of qf of the stress path slopes ɳ = 3.0, 1.0 under the tensile path are 101.9 KPa and 95.1 KPa, respectively.In addition, these results demonstrate that the difference of stress-strain curves on strong structured clay under different stress paths is obvious, and the order of strength of the specimens under different stress paths is INP > KCP > DEP. This is mainly due to the uneven cementation between structured soil particles, and during shearing of the soil under different conditions, because the size and arrangement of soil particles constantly changed that the intergranular stress state has changed, which leads to the difference in stress-strain properties of the specimens.


### 3.3. Pore Pressure-Strain Relationships

The pore water pressure,u, and axial strain curves of the strong structured clay under different stress paths are illustrated in Figure 7.Figure 7
Relationships between pore pressure and strain of structured clay under different stress paths. (a)ɳ = 3.0. (b) ɳ = ∞. (c) ɳ = −1.5. (d) ɳ = 1.0 (tensile path). (e) ɳ = 3.0 (tensile path).
(a)(b)(c)(d)(e)Figure7 demonstrates that the pore water pressure-axial strain relationship of the strong structured clay is significantly affected by the stress path. Under different stress paths, for the strong structured clay at low consolidation pressure, such as 100 KPa, the pore water pressure shows a peak “softening-type” phenomenon. At higher consolidation pressures, the pore water pressure increases with axial strain and it does not indicate distinct peak values or sudden changes. Moreover, the pore water pressure-axial strain relationship of the strong structured clay is related to the consolidation stress. The failure strain of the soil is smaller under low confining pressure, with the strong cementation ruptures of the structured clay, and the strength of the soil decreases rapidly, the soil deforms and slips along the weak surface of the basic unit body causing the soil to generate a dilation potential, and the pore pressure also reduces rapidly before and after the failure strain. This indicates that the soil exhibits the characteristics of a sudden change from shear contraction to shear dilation in the failure stage.The pore water pressure increases with the axial strain and is always positive when the specimen is under increasingP. While in the case of constant P, u develops quickly with the increase in εa at the start of testing, with the destruction of the structure of the specimen, u reaches the peak values first and then decreases to a stable value. As a result, the pore water pressure is positive throughout the shear process under increasing p or constant p stress conditions (compression path and tensile path).The stress path slope is different under decreasingp, and the trend in the pore pressure is different, as shown in Figure 7: the development of u under decreasing P at ɳ = −1.5, ɳ = 1.0 (tensile path), and ɳ = 3.0 (tensile path) is very different from the cases of increasing or constant P, especially when ɳ = −1.5. When ɳ = −1.5 at lower confining pressure such as σ3 = 100 KPa or 200 KPa, u is negative in the shearing process, but when σ3 > 200 KPa, a positive pore water pressure is generated throughout the test. The curve at ɳ = 1.0 (tensile path) is very different from that at ɳ = −1.5; the pore water pressure u of the stress path with ɳ = 1.0 (tensile path) is always negative in the case of both axial and lateral unloading. The test curves at ɳ = 3.0 (tensile path) are very similar to those at ɳ = 1.0 except that the values of u at ɳ = 3.0 (tensile path) are lower and the curve has an inflection point after which u will decline. The variation in pore water pressure of the structured clay is closely related to the consolidation pressure and the stress path.The variation of complex pore pressure is a comprehensive reflection of the elastic deformation caused by the unloading of the confining pressure and plastic deformation caused by shear. The pore water pressure of the specimen is affected by both axial and lateral stresses under different stress paths. For example, the decrease in confining pressures will cause the elastic pore pressure to decrease; the axial pressure increase will cause the shear pore pressure to increase, and vice versa. Therefore, when the axial stress is increasing and the confining pressure remains unchanged under increasingP, the specimen will generate shear-induced pore pressure which increases continuously with increasing axial pressure. The variation of pore pressure under constant P is a comprehensive reflection of the decrease in elastic pore pressure caused by lateral unloading and the increase of shear pore pressure caused by the increase of deviatoric stress. Under decreasing P at ɳ = 1.0 (tensile path), i.e., the specimen is unloaded in both axial and lateral stresses during shearing, the total pore pressure decreases under this path, and the pore pressure is negative throughout the shearing process.The increase or decrease of the confining pressure and the deviatoric stress will cause the increase or decrease of the pore pressure [29]. For example, under stress paths with increasing p (e.g., along the compression path at ɳ = 3.0), the confining pressure remains unchanged during loading, while the axial stress increases, the pore pressure in the specimen is mainly driven by shearing caused by the increase of deviatoric stress. Nevertheless, the trend in the pore pressure with increasing strain is different from that in deviatoric stress with strain, and the stress-strain curves are all strain-softening types, while the pore water pressure-strain curves show peak softening only at low consolidation pressures. Therefore, for the structured clay, in addition to the pore pressure change caused by the confining pressure and the deviatoric stress, the structure of the specimen can affect the development of pore pressure.In summary, these results show that the pore water pressure of Zhanjiang strong structured clay is affected by the stress path and is closely related to the consolidation pressure of the soil. However, for the structured clay, the effect of the structure on the development regularity of total pore pressure must be considered.


### 3.4. Strength Index under Different Stress Paths

Figure8 depicts Mohr’s circles of stress and strength envelopes of the specimen under different stress paths. Table 5 lists the strength indices of the sample under different stress paths, in which c and c′ refer to the total cohesion and effective cohesion, respectively; φ and φ′ denote the total and effective internal friction angle, respectively. It can be found from Figure 8 and Table 5 that the shear strength indices of Zhanjiang clay under different stress paths are different before and after structural yielding. The cohesion c decreases and the internal friction angle φ increases as the structure of the sample is damaged, which indicates that the strength of the specimens after structural failure is mainly borne by friction between particles. This is due to the structure influence of Zhanjiang clay, the internal structure of the soil gradually densifies with the increase in the consolidation stress and the structural strength gradually weakens, and the frictional component of the strength of the soil is compensated by the apparent cohesion.Figure 8
Mohr’s circles of stress and strength envelopes of structured clay under different stress paths. (a)ɳ = 3.0. (b) ɳ = ∞. (c) ɳ = −5.0. (d) ɳ = −5.0 (tensile path). (e) ɳ = ∞ (tensile path). (f) ɳ = 1.0 (tensile path)
(a)(b)(c)(d)(e)(f)Table 5
Strength indices of structured clay under different test conditions.
Stress path typeStress path slope/ɳTotal shear strength indicesEffective shear strength indicesBefore structural yieldingAfter structural yieldingBefore structural yieldingAfter structural yieldingc (kPa)φ (°)c (kPa)φ (°)c′ (kPa)ϕ′ (°)c′ (kPa)ϕ′ (°)Compression pathINPɳ = 3.040.647.2821.6010.1938.219.9425.4712.64KCPɳ = ∞42.618.1523.2510.7831.9113.9113.3317.93DEPɳ = −5.032.919.9032.239.5929.0612.797.9516.18DEPɳ = −1.534.499.3922.1111.37DEPɳ = −1.025.9410.3911.4711.28Tensile pathINPɳ = −5.032.938.0522.8212.53KCPɳ = ∞33.527.1728.819.04DEPɳ = 3.029.528.5719.879.65DEPɳ = 1.027.907.8218.218.08The strength parameters of the specimens under different stress paths differ, especially in terms of the difference in the total stress strength index. The unloading and loading processes of different stress paths have a greater effect on the cohesionc in the strength index, while the effects on the internal friction angle φ are smaller, and the cohesion c of the specimen under the compression path is higher than the tensile path.The cohesionc obtained from the increasing P path and the constant P path is higher than the decreasing P path, which suggests that the stress path has an effect on the shear strength of the structured clay, especially when the axial and lateral stresses are unloaded at the same time; i.e., the cohesion c at ɳ = 1.0 (compression path), ɳ = −1.0 (tensile path) is the smallest: because the excavation of the foundation pit is a typical unloading process, the stress state of the soil under unloading conditions is extremely complex; hence, the mechanical properties of the soil under the unloading state are completely different from those under the loading state.Therefore, in engineering practice, the mechanical parameters obtained from the conventional triaxial compression test are unsafe when performing design calculations for lateral excavation works of foundation pits or slopes in the area of the structured clay, especially in regions of severe unloading in the excavation of a foundation pit: because the shear strength of the unloading soil decreases, the reasonable parameters should be used for design calculation and construction according to the actual working conditions, to ensure the safety and stability of the engineering works.


## 3.1. Failure Modes

All samples showed plastic failure characteristics and formed obvious shear bands before the axial deformation reached 15% (Figure5). The failure modes of the soil depend on many random factors such as the homogeneity of the soil, the initial boundary conditions, the disturbance of the soil, and the loading rates. In the test, it was observed that three types of shear band failures occurred in the structured clay under different stress paths as follows:(1)
Under the compression path (includingɳ = 3.0, and ɳ = −1.5), the failure modes of the specimen include a single shear band, inclined to one side, as shown in Figure 5(a).(2)
Part of the failure modes of the specimens under the compression path (includingɳ = ∞, ɳ = −5.0, and ɳ = −1.0) appear primary and secondary shear bands; the failure mode of the specimen is a double-cross asymmetric X-shaped shear band, as shown in Figure 5(b).(3)
Under the tensile loading path (ɳ = 1.0, ɳ = 3.0, ɳ = ∞, and ɳ = −5.0), the deformation of the specimen exhibits axial elongation and radial local contraction, which is a figure-of-eight-shaped shear band, as shown in Figure 5(c).Figure 5
Typical failure modes of samples under different stress paths. (a) Single shear band failure. (b) X-shaped shear band failure. (c) Figure-of-eight-shaped tensile failure.
(a)(b)(c)Under the compression path, the particle displacement increases with the increase in deviatoric stress during shearing, which makes the specimen produce an obvious shear band in the compression process; with the further increase of deformation, the specimen produces multiple shear bands and demonstrates a double-cross asymmetric X-shaped shear band. Under the tensile path, the failure modes of the specimens are figure-of-eight-shaped shear bands, due to the stress, the soil undergoes elastic deformation and internal structural dislocation, which causes the concentration of the stress in the soil, and the shear bands are generated rapidly, undergoing tensile deformation. The stress on the soil under different stress paths is different, which leads to the formation of different shear bands; however, for the Zhanjiang structured clay, the structure of the soil is a necessary condition for the generation of shear bands [9].


## 3.2. Stress-Strain Relationships

The deviatoric stress-axial strain curves of the strong structured clay under different stress paths are shown in Figure6. The results of the tests are listed in Table 4.Figure 6
Deviator stressq and vertical strain εa on structured clay under different stress paths. (a) ɳ = 3.0. (b) ɳ = ∞. (c) ɳ = −1.5. (d) ɳ = 1.0 (tensile path). (e) ɳ = 3.0 (tensile path).
(a)(b)(c)(d)(e)Table 4
Test results of structured clay under different stress paths.
Consolidation confining pressure/KPaStress path slope/ɳStress path typeFailure deviatoric stress/qfFailure pore pressure/ufFailure strainεa/%1003.0INP122.130.01.92∞KCP129.725.72.5−1.5DEP120.9−38.52.21.0DEP95.1−64.03.63.0DEP101.9−39.35.32003.0INP149.251.32.3∞KCP167.458.72.9−1.5DEP158.0−8.02.81.0DEP127.8−60.05.23.0DEP141.7−29.46.23003.0INP179.992.03.3∞KCP195.6123.04.14−1.5DEP199.13.13.21.0DEP158.1−57.06.63.0DEP171.8−13.46.964003.0INP225.4112.74.4∞KCP237.2170.84.3−1.5DEP230.934.04.25003.0INP266.1138.54.8∞KCP287.4219.54.93−1.5DEP258.965.04.4Figure6 shows that the stress-strain curves of the soil under different stress paths exhibit a similar trend in terms of deformation; the stress increases with strain in the initial stage of the test, when the stress increases to its peak value; then it decreases with strain and finally stabilizes (i.e., strain-softening occurs [28]). In general, all the stress-strain curves in Figure 6 present strain-softening. As the internal structure of the soil plays an important role in resisting the external load, with the increase of axial strain, the external stress exceeds the structural yield stress and the specimen exhibits distinct shear failure locally. This finding shows that the specimen enters an obvious failure stage after the elastic deformation and the yield stage, and the strength of the specimen decreases gradually with the increase in strain after the deformation reaches the structural yield strength.As can be seen from Figure6, the strain at the failure point under the compression path is mostly around 2% to 5%; however, the strain at the failure point under the tensile path is between 5% and 10%. At the initial stage of loading, i.e., when axial strain εa < 2%, the deviatoric stress increases rapidly with the increase in axial strain, the stress path has little effect on the strength of the specimen, and the deviatoric stress-axial strain curves of the specimens under different stress paths almost overlap and increase linearly. However, when εa > 2%, the deviatoric stress-axial strain curves are gradually separated, specimens under different stress paths show different peak strengths, and the influences of stress paths on soil are more obvious.The failure strength of the specimen under the same stress path is related to the consolidation pressure: the larger the consolidation pressure, the greater the failure strength of the specimen, and the strain at failure increases with the consolidation pressure. Table4 shows that the failure strength of soil is affected by the stress path, compared to the results of increasing P and constant P, the failure deviatoric stress under decreasing P is the smallest, except for triaxial experiment of strain control (ɳ = 3.0), and the order of the strengths is INP > KCP > DEP. Moreover, the failure strength of the stress paths with slopes ɳ = −1.0 (compression path) and ɳ = 1.0 (tensile path) under decreasing P is the smallest; the axial and lateral unloading during shearing cause a decline in the shear strength of the soil. The peak deviatoric stress of the soil under the compression path is higher than that under the tensile path. For instance, when the consolidation pressure is 100 KPa, the values of qf of the stress path slopes ɳ = 3.0, ∞, −1.5 under the compression path are 122.1 KPa, 129.7 KPa, and 120.9 KPa, respectively, and the values of qf of the stress path slopes ɳ = 3.0, 1.0 under the tensile path are 101.9 KPa and 95.1 KPa, respectively.In addition, these results demonstrate that the difference of stress-strain curves on strong structured clay under different stress paths is obvious, and the order of strength of the specimens under different stress paths is INP > KCP > DEP. This is mainly due to the uneven cementation between structured soil particles, and during shearing of the soil under different conditions, because the size and arrangement of soil particles constantly changed that the intergranular stress state has changed, which leads to the difference in stress-strain properties of the specimens.


## 3.3. Pore Pressure-Strain Relationships

The pore water pressure,u, and axial strain curves of the strong structured clay under different stress paths are illustrated in Figure 7.Figure 7
Relationships between pore pressure and strain of structured clay under different stress paths. (a)ɳ = 3.0. (b) ɳ = ∞. (c) ɳ = −1.5. (d) ɳ = 1.0 (tensile path). (e) ɳ = 3.0 (tensile path).
(a)(b)(c)(d)(e)Figure7 demonstrates that the pore water pressure-axial strain relationship of the strong structured clay is significantly affected by the stress path. Under different stress paths, for the strong structured clay at low consolidation pressure, such as 100 KPa, the pore water pressure shows a peak “softening-type” phenomenon. At higher consolidation pressures, the pore water pressure increases with axial strain and it does not indicate distinct peak values or sudden changes. Moreover, the pore water pressure-axial strain relationship of the strong structured clay is related to the consolidation stress. The failure strain of the soil is smaller under low confining pressure, with the strong cementation ruptures of the structured clay, and the strength of the soil decreases rapidly, the soil deforms and slips along the weak surface of the basic unit body causing the soil to generate a dilation potential, and the pore pressure also reduces rapidly before and after the failure strain. This indicates that the soil exhibits the characteristics of a sudden change from shear contraction to shear dilation in the failure stage.The pore water pressure increases with the axial strain and is always positive when the specimen is under increasingP. While in the case of constant P, u develops quickly with the increase in εa at the start of testing, with the destruction of the structure of the specimen, u reaches the peak values first and then decreases to a stable value. As a result, the pore water pressure is positive throughout the shear process under increasing p or constant p stress conditions (compression path and tensile path).The stress path slope is different under decreasingp, and the trend in the pore pressure is different, as shown in Figure 7: the development of u under decreasing P at ɳ = −1.5, ɳ = 1.0 (tensile path), and ɳ = 3.0 (tensile path) is very different from the cases of increasing or constant P, especially when ɳ = −1.5. When ɳ = −1.5 at lower confining pressure such as σ3 = 100 KPa or 200 KPa, u is negative in the shearing process, but when σ3 > 200 KPa, a positive pore water pressure is generated throughout the test. The curve at ɳ = 1.0 (tensile path) is very different from that at ɳ = −1.5; the pore water pressure u of the stress path with ɳ = 1.0 (tensile path) is always negative in the case of both axial and lateral unloading. The test curves at ɳ = 3.0 (tensile path) are very similar to those at ɳ = 1.0 except that the values of u at ɳ = 3.0 (tensile path) are lower and the curve has an inflection point after which u will decline. The variation in pore water pressure of the structured clay is closely related to the consolidation pressure and the stress path.The variation of complex pore pressure is a comprehensive reflection of the elastic deformation caused by the unloading of the confining pressure and plastic deformation caused by shear. The pore water pressure of the specimen is affected by both axial and lateral stresses under different stress paths. For example, the decrease in confining pressures will cause the elastic pore pressure to decrease; the axial pressure increase will cause the shear pore pressure to increase, and vice versa. Therefore, when the axial stress is increasing and the confining pressure remains unchanged under increasingP, the specimen will generate shear-induced pore pressure which increases continuously with increasing axial pressure. The variation of pore pressure under constant P is a comprehensive reflection of the decrease in elastic pore pressure caused by lateral unloading and the increase of shear pore pressure caused by the increase of deviatoric stress. Under decreasing P at ɳ = 1.0 (tensile path), i.e., the specimen is unloaded in both axial and lateral stresses during shearing, the total pore pressure decreases under this path, and the pore pressure is negative throughout the shearing process.The increase or decrease of the confining pressure and the deviatoric stress will cause the increase or decrease of the pore pressure [29]. For example, under stress paths with increasing p (e.g., along the compression path at ɳ = 3.0), the confining pressure remains unchanged during loading, while the axial stress increases, the pore pressure in the specimen is mainly driven by shearing caused by the increase of deviatoric stress. Nevertheless, the trend in the pore pressure with increasing strain is different from that in deviatoric stress with strain, and the stress-strain curves are all strain-softening types, while the pore water pressure-strain curves show peak softening only at low consolidation pressures. Therefore, for the structured clay, in addition to the pore pressure change caused by the confining pressure and the deviatoric stress, the structure of the specimen can affect the development of pore pressure.In summary, these results show that the pore water pressure of Zhanjiang strong structured clay is affected by the stress path and is closely related to the consolidation pressure of the soil. However, for the structured clay, the effect of the structure on the development regularity of total pore pressure must be considered.


## 3.4. Strength Index under Different Stress Paths

Figure8 depicts Mohr’s circles of stress and strength envelopes of the specimen under different stress paths. Table 5 lists the strength indices of the sample under different stress paths, in which c and c′ refer to the total cohesion and effective cohesion, respectively; φ and φ′ denote the total and effective internal friction angle, respectively. It can be found from Figure 8 and Table 5 that the shear strength indices of Zhanjiang clay under different stress paths are different before and after structural yielding. The cohesion c decreases and the internal friction angle φ increases as the structure of the sample is damaged, which indicates that the strength of the specimens after structural failure is mainly borne by friction between particles. This is due to the structure influence of Zhanjiang clay, the internal structure of the soil gradually densifies with the increase in the consolidation stress and the structural strength gradually weakens, and the frictional component of the strength of the soil is compensated by the apparent cohesion.Figure 8
Mohr’s circles of stress and strength envelopes of structured clay under different stress paths. (a)ɳ = 3.0. (b) ɳ = ∞. (c) ɳ = −5.0. (d) ɳ = −5.0 (tensile path). (e) ɳ = ∞ (tensile path). (f) ɳ = 1.0 (tensile path)
(a)(b)(c)(d)(e)(f)Table 5
Strength indices of structured clay under different test conditions.
Stress path typeStress path slope/ɳTotal shear strength indicesEffective shear strength indicesBefore structural yieldingAfter structural yieldingBefore structural yieldingAfter structural yieldingc (kPa)φ (°)c (kPa)φ (°)c′ (kPa)ϕ′ (°)c′ (kPa)ϕ′ (°)Compression pathINPɳ = 3.040.647.2821.6010.1938.219.9425.4712.64KCPɳ = ∞42.618.1523.2510.7831.9113.9113.3317.93DEPɳ = −5.032.919.9032.239.5929.0612.797.9516.18DEPɳ = −1.534.499.3922.1111.37DEPɳ = −1.025.9410.3911.4711.28Tensile pathINPɳ = −5.032.938.0522.8212.53KCPɳ = ∞33.527.1728.819.04DEPɳ = 3.029.528.5719.879.65DEPɳ = 1.027.907.8218.218.08The strength parameters of the specimens under different stress paths differ, especially in terms of the difference in the total stress strength index. The unloading and loading processes of different stress paths have a greater effect on the cohesionc in the strength index, while the effects on the internal friction angle φ are smaller, and the cohesion c of the specimen under the compression path is higher than the tensile path.The cohesionc obtained from the increasing P path and the constant P path is higher than the decreasing P path, which suggests that the stress path has an effect on the shear strength of the structured clay, especially when the axial and lateral stresses are unloaded at the same time; i.e., the cohesion c at ɳ = 1.0 (compression path), ɳ = −1.0 (tensile path) is the smallest: because the excavation of the foundation pit is a typical unloading process, the stress state of the soil under unloading conditions is extremely complex; hence, the mechanical properties of the soil under the unloading state are completely different from those under the loading state.Therefore, in engineering practice, the mechanical parameters obtained from the conventional triaxial compression test are unsafe when performing design calculations for lateral excavation works of foundation pits or slopes in the area of the structured clay, especially in regions of severe unloading in the excavation of a foundation pit: because the shear strength of the unloading soil decreases, the reasonable parameters should be used for design calculation and construction according to the actual working conditions, to ensure the safety and stability of the engineering works.


## 4. Microstructural Analysis

To determine the microstructural changes of soils under different stress paths, SEM tests were conducted to investigate the microstructures of natural strong structured clay, remolded clay, and the specimens subjected to triaxial testing, and representative SEM observations were chosen to interpret the structural characterization.Figure9 provides SEM micrographs at four magnifications on the undisturbed samples of the soil: the microstructure of Zhanjiang strong structured clay with many clay mineral aggregates, is shown in Figures 9(a) and 9(b); there are many pores among soil aggregates formed which are dominated by intergranular pores and isolated pores. The natural Zhanjiang clay with an open, flocculated structure and the basic units is hollowed out. The nonconnectivity of these pores can explain the characteristics of Zhanjiang clay with a high pore ratio and low permeability at the same time. Figures 9(c) and 9(d) show that clay mineral aggregates are composed of kaolinite with its rolled-book shape, illite with its lamellar shape, an illite-smectite mixed layer with its superimposed lamellar shape, and small amounts of acicular, dispersed minerals. In the enlarged view, as shown in Figure 9(d), the particles are found not to be oriented and exhibit point-point, edge-edge, and edge-face contacts.Figure 9
SEM results from an undisturbed sample.Figure10 shows that, compared with the natural clay, the remolded soil has no distinct structural features, and the flocculated structure in the natural state disappears, replaced by face-face and edge-face contact of the large volume unit body, as illustrated in Figures 10(a) and 10(b). The remodeled soil has broken colloidal linkages, weak particle linkages, and highly structured dispersion. Figure 10(a) indicates that, due to the destruction of the structured linkage, the particles undergo displacement and cohesion processes, there are no distinct structured units in the form of flaky piles, and the intergranular pores have been reduced in both number and total volume.Figure 10
SEM results from a remolded sample.Figure11 displays SEM micrographs of the samples after triaxial tests under different stress paths at a consolidation pressure of 300 KPa. This shows that the structure of the natural soil has been damaged at 300 KPa and several small agglomerates have appeared. Moreover, the particle sizes in the shear band are broken into smaller particles with a more discrete distribution under compression paths at ɳ = −1.5, and the large pores are crushed into smaller pores, as shown in Figure 11(b). The microstructure of the specimen shows a “lamellar-mosaic” structure under tension paths at ɳ = 3.0 and ɳ = ∞, the particle arrangement is slightly oriented, and the particles change from overlapping to parallel stacking, as shown in Figures 11(c) to 11(d).Figure 11
SEM results of natural structured clay after triaxial tests. (a) Undisturbed sample. (b)ɳ = −1.5. (c) ɳ = 3.0 (tensile path). (d) ɳ = ∞ (tensile path).The SEM observations indicate that soil particles are rearranged in different directions under different stress conditions. The development of microstructure morphology in compression and tensile tests shows differences in vertical and horizontal directions, as shown in Figures11(b) to 11(d), and the contact relationship gradually transitions to a stable and substable state of a face-face mosaic. Moreover, Figures 11(a) to 11(d) show that the elliptical pores in natural soil evolve into triangular and wedge-shaped forms under different stress paths; this can explain the difference in strength of soils with the same pore ratio in terms of morphological ability to be stabilized. The intergranular forces of structured soils are macroscopically expressed as structured yield stresses. Compared to the hollow structure of natural soils, the soil will be damaged when the external forces exceed the intergranular forces, and the hollow structure will evolve into a closed “lamellar-mosaic” structure, which causes a loss of the soil structure and a tendency for its properties to tend to be those of the remolded soil.There are some limitations in the application of the results from our research, Zhanjiang strong structured clay is a typical regional soil, due to the differences in the physicomechanical properties of structured clay in different regions, and the results in the manuscript may not be applied to other structured clays.


## 5. Conclusion

A series of triaxial consolidation undrained shear tests were conducted on natural strong structured clay under different stress paths to determine the effects of the stress path on the mechanical characteristics. Based on the test results, the conclusions are drawn as follows:(1)
In general, the structured clay has strong stress path sensitivity, the variation characteristics of the stress-strain curves of the specimens under different stress paths are different, and the stress-strain relationship exhibits nonlinear characteristics and is of the strain-softening type. The failure strengths of the specimens at various stress paths are different, and the order is INP > KCP > DEP. The dependence of Zhanjiang clay on the stress path is highly related to its structure; the soil affected by the structure shows different mechanical properties under different consolidation stresses and shear conditions.(2)
The stress path is an important factor that determines the pore characteristics of the structured clay, and the trend in the pore pressure is related to the consolidation pressure and the structure of the soils. Moreover, the variation of pore water pressure reflects the shear dilation and the shear contraction characteristics of soils under different stress paths, especially in the case of both axial and lateral stress unloading such that the pore water pressure is very complicated. Therefore, the pore water pressure of soil under unloading conditions should be monitored in real time during engineering operations.(3)
The total stress strength indices of the structured clay are affected by the stress path, and the stress path affects the cohesion of the soil to a greater extent than the internal friction angle. The cohesion and the internal friction angle of the structured clay show a decreasing and increasing trend before and after the structure yields, respectively. Therefore, design calculations should be conducted according to the experimental mechanical parameters under the stress path experienced by the soil under actual engineering conditions to ensure safety and stability during construction.(4)
The natural Zhanjiang clay structured unit constitutes a directionally disordered, open, flocculated structure in the form of point-point, edge-edge, and edge-face contacts, and the remolded specimens have no distinct structured features, mostly comprising face-face and edge-face contacts. The shearing process of the structured clay is the process of continuous self-adjustment of its internal structure. The microstructures of the specimens have changed after shearing tests under different stress paths, with different degrees of adjustment in particle arrangement and contact relationships, pore sizes, and pore shapes.



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*Source: 9266206-2022-07-08.xml*