Disintegration Characteristics of Remolded Granite Residual Soil with Different Moisture Contents

Abstract

Granite residual soil (GRS) has prominent disintegration characteristics which have induced various geological disasters and engineering problems. The initial moisture content is believed to affect the disintegration of GRS significantly. To explore the effects of the initial moisture content on the soil disintegration characteristics and disintegration mechanism, disintegration tests were performed on remolded GRS with different initial moisture contents via the balance method, and the quantitative disintegration indices were corrected, considering the effects of water-absorption weight gain, in combination with a parallel water-absorption test. The disintegration characteristics and mechanism were thoroughly investigated, starting with the disintegration process curves and disintegration morphology, and combined with strength theory, X-ray diffraction (XRD) and X-ray fluorescence (XRF), the matric suction test, and the triaxial shear test. The results are as follows. (1) The corrected method improves the accuracy of the quantitative disintegration evaluation. (2) During the two disintegration stages, the forms of disintegration are dispersive fragmentation and progressive or block separation, and the soil matric suction and weakening of intergranular joining forces, respectively, are the drivers of disintegration. The first stage is usually completed within 1.5–2 min, and the disintegration ratio is usually within 20%. (3) The trends of change within the disintegration during the two stages show opposite water-content-dependent modes, and the soil samples with lower moisture contents have better water stability and slower disintegration in the second stage. The average disintegration rate of the soil with a moisture content of 24.4% in the first and second stages was approximately 1/5 and 13 times, respectively, that of the soil with a moisture content of 6.1%; these values can be rendered as 0.049%/s and 0.82%/s, respectively. The results provide some theoretical references for soil and water conservation and engineering applications in the GRS field.

1. Introduction

Granite residual soil (GRS), the product of in situ weathering of granite rocks under hot and humid climatic conditions, is widely distributed in the subtropical and tropical areas of Southeast Asia, South America, and Africa [1]. China has a large area of GRS spread, especially in Guangdong Province, Fujian Province, and the Hong Kong Special Administrative Region [2]. GRS is considered a problematic regional soil due to its unique geological origin, specific composition, and structural characteristics. It has unique geological engineering properties, such as being easy to disturb and a tendency to disintegrate when immersed in water [3,4,5]. The mechanical properties and stability of GRS are significantly affected by water. Soil disintegration, which refers to the phenomenon of soil fragmentation, breaking, and separation or weakening of its strength in hydrostatic water, is closely related to the resistance of soil to water intrusion and is a critical evaluative indicator of soil erosion [1,6,7,8,9,10,11]. The prominent disintegration behavior of GRS is responsible for the apparent collapsing erosion of the granite weathering zone, resulting in severe soil erosion and environmental damage, as well as natural geohazards such as landslide and mudslide disasters [12,13,14,15]. The disintegration behavior may also lead to several engineering problems, such as embankment slump, slope instability, pile foundation failure, and foundation pit collapse [15,16]. Therefore, research on the disintegration characteristics of GRS has theoretical and practical significance in soil and water conservation and in engineering applications in the GRS field.

However, current research on the disintegration characteristics of GRS is still relatively scarce, partially because a unified standard testing method is lacking [16]. Many scholars have conducted extensive research on rock disintegration [17,18,19,20], which has been facilitated by the standardization of the rock-durability test method (ASTM D 4644-16, 2016). However, the rock-durability test standard cannot be simply extended to soils, restricting the study of soil disintegration behavior [3,16]. Research on GRS disintegration gradually started in the early 21st century [1]. Many scholars have designed different experimental devices to conduct disintegration tests, including the introduction of the quantitative disintegration index, which mainly includes the pontoon [12] and mass methods [6,7,15]. The pontoon method has poor stability and significant reading errors. In contrast, the mass method overcomes these shortcomings, but often ignores the effects of water-absorption weight gain during the soil disintegration process, and the disintegration mass measured is less than the actual value.

Some scholars have used the quantitative testing method to evaluate GRS disintegration characteristics and its influential factors, including internal factors (e.g., dry density, initial moisture content, degrees of weathering, structure, and mineral composition) and external factors (e.g., temperature, pH, and dry–wet cycles) [1,6,7,8,9,10,11,12,15,16,21]. Scholars agree that the initial moisture content significantly affects the disintegration characteristics of GRS. Some scholars believe that the average disintegration rate of GRS increases with an increase in the initial moisture content [6], and the initial disintegration rate decreases with an increase in the initial moisture content. In contrast, the relationship between the disintegration rate in the second stage and the initial moisture content has been unclear [12]. However, the dynamic disintegration process and the GRS disintegration mechanism at different initial moisture contents have not been elucidated. Most current research on tracking the disintegration process utilizes only a qualitative description of the disintegration morphology; it does not effectively incorporate the disintegration time and rate. In addition, many fundamental questions about disintegration mechanisms remain unclear, such as the basic disintegration principles and driving forces.

Therefore, combined with a parallel water-absorption test, an improved quantitative disintegration evaluation method was utilized to conduct an in-depth study of the disintegration characteristics and mechanism of GRS at different initial moisture contents. The disintegration characteristics of GRS were thoroughly investigated, starting with the disintegration process curves and disintegration morphology. In addition, the disintegration mechanisms, including the basic disintegration principles and driving forces, were analyzed in-depth, based on the strength theory of unsaturated soil of intergranular suction, combined with X-ray diffraction (XRD) and X-ray fluorescence (XRF) tests, the matric suction test, and the triaxial shear test. The technology roadmap of this study is shown in Figure 1.

2. Materials and Methods

2.1. Soil Sampling and Determination of Its Basic Physicochemical Properties

The GRS was collected from a foundation pit in a commercial center in Shenzhen City, Guangdong Province, in southern China (22°32′34″ N, 114°04′58″ E), and the sampling depth was 8–10 m below the ground surface. The sampling location is shown in Figure 2. According to the engineering geologic investigation report, there is a large area of GRS within the foundation pit, and its parent rock is late-Yanshanian granite. The basic physical properties of the GRS were determined, and the results are summarized in

Table 1. Basic physical properties of the GRS samples.

Moisture Content /(%)	$\mathbf{Dry} \mathbf{Density} {\mathit{\rho }}_{\mathbf{d}}$/(g/cm3)	$\mathbf{Void} \mathbf{Ratio} {\mathit{e}}_0$	$\mathbf{Specific} \mathbf{Gravity} {\mathit{G}}_{\mathit{s}}$	Optimum Moisture Content /(%)	Maximum Dry Density /(g/cm3)	$\mathbf{Liquid} \mathbf{Limit} {\mathit{w}}_{\mathit{L}}$ /%	$\mathbf{Plastic} \mathbf{Limit} {\mathit{w}}_{\mathit{p}}$ /%	Particle Composition/%
								>0.25 mm	0.075–0.25 mm	<0.075 mm
19.35	1.45	0.793	2.60	20.5	1.64	45.58	22.24	31.6	10.09	58.31

. The grain size distribution curve is presented in Figure 3, demonstrating that the GRS consisted mainly of silt clay (approximately 58.31%) and middle and coarse sand (approximately 31.6%), with some fine sand (approximately 10.09%). The coefficient uniformity Cu was 100 > 5, and the coefficient of curvature Cc was 0.18 < 1. According to the Unified Soil Classification System (ASTM D 2487-00, 2000) and the GRS classification standard suggested by Wu [22], GRS can be defined as sandy lean clay and a high-liquid-limit sandy clay with discontinuous gradation, respectively. The mineral composition and chemical components of the GRS were obtained via XRD and XRF tests, respectively. The XRD spectrum is shown in Figure 4, and the dominant mineral composition of the GRS can be identified as kaolinite (77.80%), quartz (14.74%), and goethite (3.72%). The XRF test results are presented in

Table 2. Chemical components of the GRS.

Oxides	Al2O3	Fe2O3	K2O	MgO	P2O5	SiO2	TiO2	Co3O4	ZrO2	CeO2	NiO	CuO	SO3	Rb2O	LOI
Content/%	30.76	3.34	0.85	0.22	0.05	50.93	0.27	0.02	0.02	0.02	0.01	0.02	0.04	0.02	13.43

, and reveal that the main chemical components of GRS are SiO₂ (50.93%), Al₂O₃ (30.76%), and Fe₂O₃ (3.34%), which are mainly derived from quartz and kaolinite minerals, kaolinite and a certain amount of free alumina, and goethite and a certain amount of free iron oxides, respectively.

2.2. Specimen Preparation and Testing Methods

The dry density of the remolded specimens used in the study was the same as that of natural soils (1.45 g/cm³), and the degree of saturation was preset as 20%, 40%, 60%, or 80%, that is, the moisture contents were 6.1%, 12.2%, 18.3%, and 24.4%, respectively. The procedure for remolded specimen preparation was as follows: (1) A proper amount of air-dried soil was crushed and screened with a 2 mm sieve. (2) The sieved soil was put into sealed plastic bags, and the soil mass moisture contents were measured. (3) The soil was mixed with the corresponding amount of distilled water based on the preset water content, and the mixture was stored in sealed plastic bags overnight to promote uniform mixing. (4) The mixture was compacted in a mold to the preset dry density using static compaction. The cylindrical specimens with d = 39.1 mm and h = 80 mm were prepared for disintegration, water absorption, and triaxial shear tests. The cutting ring specimens, with d = 61.8 mm and h = 20 mm, were prepared according to the optimum moisture content of w = 21.5% and then saturated to conduct the soil–water characteristic curve test.

2.2.1. Disintegration and Water-Absorption Tests

For this study, disintegration tests were performed via the balance method (Figure 5). The disintegration apparatus designed by the authors comprises three parts: (1) a support system, including a support framework, balance bar, thin strings, and a metal mesh; (2) a water tank; and (3) a data acquisition system, including an electronic scale, camera, and computer. The size of the metal mesh was 100 mm × 100 mm, and the grid dimensions were set to 10 mm × 10 mm, according to the particle size distribution curves, to permit the largest coarse particles to pass through.

The disintegration test procedures were as follows: (1) Install the disintegration device and fill the tank with water. (2) Put the metal mesh, suspended on the electronic balance, through the balance bar and the four thin strings, and into the water tank without a load, and reset the reading of the electronic balance to zero. (3) Place the soil specimen on the metal mesh and record the soil sample’s mass change and dynamic disintegration morphology via the electronic balance and camera, respectively.

The water density is assumed to remain constant during the disintegration process. The disintegration ratio $p_t$ and rate $v_t$, which are essential quantitative indexes for characterizing the disintegration behavior, are calculated as follows:

$$p_t=\frac{{\rho }_0}{({\rho }_0-{\rho }_w)m_0}(T_0-T_t+m_{\mathrm{wt}})\times 100\%$$

(1)

$$v_t=\frac{p_{t_{i+1}}-p_{t_i}}{t_{i+1}-t_i}$$

(2)

where $T_0$ and $T_t$ are the balance reading when the specimen is immersed in water and at time $t$, respectively; $m_0$ is the sample weight before water immersion; ${\rho }_0$ is the specimen density; $m_{\mathrm{wt}}$ is the water-absorption weight of the sample at time $t$ during the disintegration test; $p_t$ is the cumulative disintegration ratio at time $t$; and $v_t$ is the average disintegration rate from time $t_i$ to time $t_{i+1}$.

Due to the difficulty of measuring the change in water-absorption weight $m_{\mathrm{wt}}$ during the disintegration process, previous studies often ignored the effect of weight gain due to water absorption, which may have affected the accuracy of the quantitative disintegration indices. In this study, the relevant quantitative evaluation indicators were corrected through the water-absorption test. The cylindrical bucket made of medium-speed filter paper was used for the water-absorption test (Figure 6). The cylindrical bucket is permeable to water and prevents the soil sample from disintegrating during water absorption.

The procedures of the water-absorption test were similar to those of the disintegration test. The cylindrical bucket containing the specimen was sealed and placed on the metal mesh of the disintegration tester, after which the balance-reading change was recorded. The cumulative water-absorption weight at time t during the water-absorption test was calculated as follows:

$$m_{\mathrm{wt}}^{\prime }=T_t^{\prime }-T_0^{\prime }$$

(3)

where $T_0^{\prime }$ and $T_t^{\prime }$ are the balance readings when the specimen is immersed in water and at time t, respectively.

Presumably, soil samples with the same initial conditions exhibit the same moisture content at any time during the water-absorption and disintegration tests. The following relative expression can be established:

$$m_{\mathrm{wt}}=m_{\mathrm{wt}}^{\prime }[1-p_t]$$

(4)

The corrected disintegration-ratio calculation formula is obtained by substituting Equation (4) into Equation (1):

$$p_t=\frac{{\rho }_0}{({\rho }_0-{\rho }_w)m_0+{\rho }_0{m}^{\prime }{}_{\mathrm{wt}}}(T_0-T_t+m^{\prime }{}_{\mathrm{wt}})\times 100\%$$

(5)

2.2.2. Matric Suction Measurement and Triaxial Shear Tests

To analyze the disintegration mechanisms deeply, the soil–water characteristic curve and unconsolidated-undrained triaxial shear tests were performed using the pressure membrane gauge and GDS stress path triaxial tester, respectively.

As shown in Figure 7, the pressure membrane gauge is a 1500-15-bar type, and its maximum working pressure is 1500. The soil–water characteristic curve test procedures were as follows: (1) Weigh the saturated soil samples first and then place them into the pressure cells, covering the clay plate thoroughly. (2) Seal the pressure cells and apply a series of air pressures corresponding to the required matric suction in the following order: 5, 10, 25, 50, 100, 200, 400, 800, and 1000 kPa. (3) Reweigh the soil samples after each pressure is balanced and then return them to the apparatus to apply the subsequent pressure. (4) Plot the relationship curve SWCC of matric suction and volumetric water content from the experimental data. The volumetric water content was calculated as follows:

$${\theta }_w=w\ast \frac{{\rho }_d}{{\rho }_w}$$

(6)

where ${\theta }_w$ is the volumetric water content, $w$ is the mass water content, ${\rho }_d$ is the dry density of the soil sample, and ${\rho }_w$ is the water density.

The sketch map of the GDS triaxial test system is shown in Figure 8. The unconsolidated-undrained triaxial shear tests of remolded samples with different initial moisture contents were performed under confining pressures of 75, 125, 225, and 350 kPa, and the shear rate was 0.5%/min.

3. Results

3.1. Water-Absorption Test and Test Reliability Evaluation

The water-absorption curves of the specimens with different initial water contents were obtained from the water-absorption test (Figure 9). Figure 9 shows that the water-absorption weight increases nonlinearly with the soaking duration, and the lower the initial moisture content is, the greater is the final cumulative water-absorption weight. The water absorption can be roughly divided into three stages: rapid initial increase, slow increase, and final saturation [23]. In the early stage, water quickly infiltrates the pores due to the effects of matric suction, and the lower the water content is, the faster is the water-absorption rate. This process is generally completed within 1 min. In the second stage, some pores are saturated with water; the water-absorption weight increases slowly, and the water-absorption rate is much slower than that in the first stage. The average water-absorption rate of the soil with a moisture content of 6.1% was approximately five times that of the soil with a moisture content of 24.4% for both the first and second stages, which can be described as 0.396 g/s and 0.059 g/s, respectively. In the third stage, most pores are saturated with water, the saturation degree of the soil reaches approximately 85%, and the water-absorption curve is close to the horizontal line.

The balance-reading change during the disintegration process can be roughly divided into two stages (Figure 10). In the first stage, the sample’s disintegration is less than its water absorption, resulting in the balance reading exhibiting sustainable growth. In the second stage, the sample’s disintegration is much higher than its water absorption, so the balance reading continuously decreases from point B to point C.

The balance reading comprehensively reflects the sum of the water-absorption weight and the soil mass after disintegration. The disintegration ratio of soil is often underestimated by ignoring the influence of water-absorption weight gain, resulting in the uncorrected disintegration-ratio curve lying below the corrected one, as shown in Figure 11, which takes the soil sample with an initial moisture content of 12.2% as a representative. Meanwhile, the balance reading demonstrates sustainable growth during the initial stage due to the influence of water-absorption weight gain, which may cause a “negative” disintegration ratio. The impact of water-absorption weight gain gradually weakens as the disintegration proceeds, resulting in the two curves gradually converging. In summary, the corrected method is more realistic and can improve the accuracy of the quantitative disintegration evaluation.

3.2. Disintegration Process Curves

The corrected calculation method was utilized to obtain the disintegration process curves of specimens with different initial moisture contents (Figure 12), including the disintegration-ratio and rate curves. Noticeably, the disintegration-ratio curves demonstrated a nonlinear increasing trend over time, whereas the disintegration-rate curves exhibited a fluctuating trend. The disintegration process curves can be roughly divided into two stages, which differ notably for specimens with different initial moisture contents. In the first stage (Stage 1), the disintegration-ratio curves gradually become gentler, and the disintegration rate gradually decreases with the increase in the initial water content. The average disintegration rate of the soil with a moisture content of 24.4% was approximately 1/5 of that of the soil with a moisture content of 6.1%, which is only 0.049%/s (Figure 13). This process is generally completed within 1.5–2 min, the disintegration ratio is usually within 20%, and it is mainly dependent on the water-absorption capacity of the soil. The disintegration process curves significantly changed during the second stage (Stage 2). When the initial water content was low (w₀ ≦ 12.2%), the rise in the disintegration-ratio curve was relatively gentle, the disintegration rate was low and stable, and complete disintegration occurred slowly. When the initial water content was high (w₀ ≧ 18.3%), the disintegration-ratio curve rose sharply, and the disintegration rate varied significantly, exhibiting a trend of fluctuation; complete disintegration occurred quickly. Meanwhile, the average disintegration rates increased gradually with the increase in the initial water content, which is opposite to the changing trend of the initial stage. The average disintegration rate of the soil with a moisture content of 24.4% was approximately 13 times that of the soil with a moisture content of 6.1%, which was 0.82%/s (Figure 13).

3.3. Disintegration Morphology

Disintegration morphology is a visual characterization rendered to verify the disintegration mode, and which corresponds to the disintegration process curves. It is an indispensable part of analyzing the disintegration mechanisms of rock and soil [14]. The disintegration morphology was characterized by combining the disintegration process curves. Soil samples with initial moisture contents of 12.2% and 24.4% were selected as representative examples, and the disintegration morphology is shown in Figure 14.

The disintegration process can be roughly divided into two stages, combining the disintegration curves and disintegration morphology characteristics. The first stage is the water-absorption and fragmentation stage, in which surface dispersive fragmentation mainly occurs, and is characterized by prominent particle aerosolization and bubble overflowing. Many air bubbles escape from the sample’s surface and continuously rush to the water’s surface, and the volume of bubbles gradually increases as the pressure from the surroundings decreases during the overflow process. In addition, the particle aggregates, or small particles on the outlet ends of pores and the immersion surface, quickly fragment from the parent body in smoke-like particles, and the solution becomes slightly turbid. At this stage, the lower the soil moisture content is, the more air bubbles will overflow and the more intense the fragmentation reaction will be.

The second stage is the softening and separating stage. When the initial moisture content is low, the disintegration form is mainly progressive separation. Air bubbles continue to overflow from the sample’s surface, but the quantity continuously decreases. The particle aggregates on the immersion’s surface are wetted and softened, exhibiting progressive disintegration behaviors, specifically, separation from the parent body layer by layer. The water around the soil gradually becomes more turbid, and fine soil particles rise with the bubbles and continuously float on the water. The particles at the boundary and compacted delamination rapidly separate. When $t$ = 420 s and 600 s, penetrating cracks appeared at the compacted delamination, and the sample was divided into three blocks. These blocks continued to be separated layer by layer, from the outside to the inside, in granular or lamellar form, until the soil mass utterly disintegrated. When the initial moisture content is high, the disintegration form is mainly block separation. Almost no air bubbles escape from the soil’s surface. The solution gradually becomes more turbid, fine soil particles continuously rise and float on the water, and cracks appear at the immersion surface at the compacted delamination. When t = 120 s and 140 s, through-cracks appeared at the compacted delamination, and the parent body broke into three large blocks along the penetrating cracks under the double actions of softening and gravity. These large blocks continued to break into smaller soil pieces along the surface cracks but did not wholly disaggregate before falling into the water. The soil samples with lower moisture contents have better water stability and slower disintegration in the second stage, which is opposite to the trend of changes in the initial stage.

In a word, the disintegration morphologies are consistent with the trends of changes in the disintegration process curves.

4. Discussion

4.1. Mechanical Disintegration Mechanism

Soil disintegration occurs along the direction of the minimum intergranular joining forces, without destroying the particles [24,25,26]. The soil disintegration must satisfy the stress condition of the intergranular disintegration force being greater than the intergranular joining forces. The intergranular joining forces hinder soil disintegration; they include joining-forces with direct contact, microcosmic joining-forces without direct contact, and joining-forces caused by cementation or gas-liquid shrink film [27]. In general, joining-forces with direct contact have little effect on the soil-strength characteristics, whereas the latter two types of joining-forces, including absorbed suction and structural suction, control the soil-strength characteristics. Structural suctions can be divided into intrinsic and variable structural suctions according to whether they are affected by changes in the water content. The absorbed and variable structural suctions are collectively called the generalized suction of unsaturated soil and critically affect the intergranular joining forces [28]. The shear-strength formula of unsaturated soil based on the intergranular suction was proposed by Tang [29].

$$\tau =s_i^{\prime }+(\sigma -u_a)\tan {\phi }^{\prime }+(s_a^{\prime }+s_c^{\prime })\tan {\phi }^{\prime }$$

(7)

$$c=s_i^{\prime }+(s_a^{\prime }+s_c^{\prime })\tan {\phi }^{\prime }$$

(8)

where $s_i^{\prime }$, $s_c^{\prime }$, $s_a^{\prime }$ are the intrinsic structural suction, variable structural suction, and absorbed suction, respectively; and $c$ is the apparent cohesion, which can better reflect the intergranular joining forces and critically affects the soil anti-disintegration.

4.2. Mechanism behind Disintegration Stage Differences

The disintegration’s driving forces exhibit significant differences between the two disintegration stages of GRS with different initial water contents. During the first stage, the dominant driving forces of disintegration arise from the high tensile stress generated by the sudden increase in pore pressure caused by transient infiltration and the expansive force generated by the overflow bubbles, which is closely related to the soil matric suction. Matric suction can be obtained from the soil–water characteristic-curve test. The three models of Gardener, Van Genuchten, and Fredlund and Xing [30,31,32] were used to fit the soil–water characteristic curve (Figure 15). Noticeably, the matric suction increases with the decrease in the volumetric moisture content, and the smaller the volumetric moisture content is, the more pronounced is the change in matric suction. The correlation coefficients of the three fitting curve models were all above 0.95, and the Fredlund and Xing model had the highest correlation coefficient. Accordingly, the matric suction corresponding to different initial moisture contents can be back-calculated.

Matric suction reflects the ability to absorb water or expel air from the soil. The initial average water-absorption rate and average disintegration rate are approximately proportional to the matric suction, and the greater the matric suction is, the faster are the initial average water-absorption rate and the average disintegration rate (Figure 16). After immersing the specimen in water, the water quickly infiltrates the soil pores and micro-fissures under matric suction, forcing the gas in the interconnected pores out rapidly, and forming bubbles. Under the action of the expansion thrust generated by the bubble overflow, the particles on the outlet end of pores quickly fragment from the parent body in the form of aerosolization particles. In addition, some relatively large volumes of gas in connected pores and some gas in closed pores cannot be discharged in time, causing the pore pressure $u_a$ to rise sharply and generating considerable tensile stress $\sigma =-u_a+({\mathrm{s}}_a^{\prime }+{\mathrm{s}}_c^{\prime })$, which exceeds the intergranular joining forces, resulting in the particle aggregates on the immersion surface fragmenting from the parent body. The greater the soil matric suction, the more violent the disintegration and the more visible the particle aerosolization phenomenon and overflowing bubbles.

After the first stage, some pores of the soil sample are saturated with water, and the matric suction of the soil decreases, weakening the subsequent infiltration. In other words, matric suction is no longer the dominant factor influencing soil disintegration. In the second stage, the dominant driving forces of disintegration primarily arise from the combined actions of the weakening of intergranular joining forces caused by the dissolution of intergranular cement and the floating weight of soil. The goethite contained in the main mineral composition of GRS is widely recognized as an essential source of soil-cementing force and as a significant iron-bearing mineral which plays a decisive role in the intergranular joining forces and is strongly water-sensitive [27,33]. When the initial moisture content is low, the goethite and other cementing substances exist in a quasi-crystalline or crystalline form and have high strength and high water stability, meaning they cannot quickly dissolve during the first stage. The soil sample is humidified layer by layer from the surface to the interior over time; the cementing substance gradually dissolves, weakening the intergranular joining forces, and the particle aggregates on the immersion surface separate from the soil matric layer by layer until complete disintegration occurs under the action of its floating weight. When the initial moisture content is high, the goethite and other cementing substances are amorphous and have poor strength and poor water stability, meaning they readily dissolve in the first stage, resulting in the intergranular joining forces weakening sharply. In addition, the soil sample is close to saturation after the first stage of water absorption, and its weight increases, resulting in the larger-scale soil block being separated suddenly from the soil matric in the form of a slump along the weak cementation surface under the action of its floating weight. Moreover, these soil blocks did not wholly disaggregate before falling into the water.

The disintegration in the second stage is closely related to the soil’s intergranular joining-forces. The triaxial shear tests were conducted to evaluate further the effect of the intergranular joining-forces of GRS on the second stage of disintegration. The axial stress–strain curves of GRS with different initial moisture contents are shown in Figure 17. The stress–strain curves for all the tested samples exhibited typical stress-hardening behavior: the stiffness and peak strength of the stress–strain curve gradually increased as the water content decreased, and the stress-hardening trend increased with the increase in confining pressure. The cohesion $c$ and internal friction angle $\phi$ were obtained from the total stress failure envelope. The cohesion $c$ and internal friction angle $\phi$ decreased with increasing water content (Figure 18). Cohesion $c$ can better reflect the intergranular joining forces and is an essential component of the soil disintegration resistance. The average disintegration rate at the second stage has a good power-function relationship with the cohesion $c$ (Figure 19). The more significant the cohesion $c$ is, the greater the intergranular joining forces are, and the slower is the second–stage disintegration.

5. Conclusions

The following are the main conclusions drawn.

(1)

The disintegration ratio of soil is often underestimated when the influence of water-absorption weight gain is ignored, and a “negative” disintegration ratio would then be obtained in the initial stage. However, the corrected method can overcome this shortcoming and improve the accuracy of the quantitative evaluation of the disintegration.

(2)

The disintegration characteristics and driving forces differ significantly during the two stages. In the first stage, the disintegration’s form is mainly dispersive fragmentation, and particle aerosolization and bubble overflowing are apparent. This process is generally completed within 1.5–2 min, and the disintegration ratio is usually less than 20%. The disintegration’s driving forces are primarily derived from the sizeable tensile stress generated by the sudden increase in pore pressure caused by water infiltration and the expansive force generated by the overflow bubbles, and are closely related to the soil matric suction. In the second stage, disintegration is mainly composed of progressive and block separation when the initial moisture content is low and high, respectively. The disintegration’s driving forces are mostly the results of the weakening of intergranular joining forces caused by the dissolution of intergranular cement, which can be reflected by cohesion.

(3)

The initial water content has a significant effect on the disintegration of GRS. The soil samples with lower moisture content have better water stability and disintegrate slowly in the second stage, which is opposite to the changing trend of the initial stage. The average disintegration rate of the soil with a moisture content of 24.4% in the first and second stages was approximately 1/5 and 13 times, respectively, that of the soil with a moisture content of 6.1%; these values can be rendered as 0.049%/s and 0.82%/s, respectively. The average water-absorption and disintegration rates are approximately proportional to the matric suction in the first stage. The average disintegration rate in the second stage has a positive power-function relationship with cohesion.

This study can provide a valuable theoretical reference for sustainable development in the GRS field. The research method and findings presented in this paper can provide a more reliable and accurate technical means for quantitative evaluation of soil erosion and contribute valuable theoretical insights applicable to soil and water conservation, as well as the understanding and mitigation of geological disasters and engineering issues induced by GRS. There are many factors affecting the disintegration characteristics of GRS. This study only preliminarily expounds on the effect of initial moisture content and mechanism, and research on other influencing factors can be conducted in the follow-up study.