In highway construction across the southeastern coastal regions of China, granite residual soil is widely used as subgrade fill material in pavement engineering. Its mechanical behaviour under dynamic loads warrants in-depth investigation. Dynamic events such as vehicular traffic and earthquakes are complex, involving multidirectional loads. The dynamic behaviour of soil under bidirectional cyclic loading differs significantly from that under cyclic loading in one direction. A large-scale bidirectional cyclic direct shear apparatus was utilised to carry on a series of horizontal cyclic direct shear tests on granite residual soil with water contents of 14% and 24% at different normal stress amplitudes (sigma a) (0, 100, 200 kPa). Based on these tests, discrete element method (DEM) models were developed to simulate the laboratory tests. The test results revealed that cyclic normal stress increases dynamic shear strength during forward shear but reduces it during reverse shear. The energy dissipation capacity increases with rising sigma a. The dynamic behaviour of granite residual soil is more significantly affected by cyclic normal stress when the water content is higher. The DEM simulation results indicated that as cyclic shearing progresses, the location of the maximum principal stress (sigma 1) shifts from the top of the specimen toward the shear interface. The distribution of the angle between sigma 1 and the x-axis, as well as sigma 1 and the z-axis, transitions from 'M' distribution to 'Arch' distribution. With increasing sigma a, during forward shear, the magnitude of the maximum principal stress increases, and the orientation of sigma 1 rotates toward the normal direction. Conversely, during reverse shear, the magnitude of the maximum principal stress decreases, and its orientation shifts toward the horizontal shear direction. The material fabric anisotropy coefficient decreases with increasing sigma a, while the anisotropy orientation increases with increasing normal stress.

Granite residual soils (GRS) are often encountered in geotechnical projects in the Guangdong-Hong Kong-Macao Greater Bay Area (briefly written as the Greater Bay Area, or abbreviated as GBA). The rea experiences frequent rainfall, leading to wetting-drying cycles that progressively diminish the shear strength of GRS. This weakening effect is not only significant but also accumulates, exhibiting a direct positive correlation with the number of cycles. Current studies on the soil strength attenuation due to wetting-drying cycles are typically limited to no more than 10 cycles, which is rather insufficient to uncover the long-term water-weakening behaviors and their accumulative impacts on GRS. To address this gap, typical GRS samples were first taken from the GBA and then prepared by making them go through a certain number of wetting-drying cycles (maximum of up to 100). Next, a total of 552 small- and large-scale direct shear tests were conducted to investigate the mechanisms of water-weakening effects on soil internal friction angle, cohesion, and shear strength. The degree of saturation and number of cycles were also examined to see their effects on the cumulation of water weakening. Based on results from the small-scale direct shear tests, a model was developed for assessing the weakening impact of water on soil strength. The accuracy of the model prediction was statistically evaluated. Last, the effectiveness and efficiency of the proposed model were demonstrated by validating against the results from the large-scale direct shear tests.

This paper presents an experimental investigation into the interaction mechanism between aqueous foam and unsaturated granite residual soil during conditioning. Contact filter paper tests and undrained shear tests were used to analyze foam's effects on soil water retention and shear behavior, while surface tension tests, capillary rise tests, and microscopic observations examined the role of soil particles in foam stability. The findings demonstrate that foam-conditioned granite residual soils exhibit three distinct saturation- dependent phases (soil-only, transition, and soil-foam mixture) governed by foam's gas-liquid biphasic nature, with foam injection effectively reducing matric suction in unsaturated conditions. Increasing foam injection ratio reduces shear stress while enhancing pore water pressure, with vertical displacement transitioning from contractive to expansive behavior at low shearing rate. Effective cohesion stress varies with gravimetric water content via a rational function, while other effective cohesion stress and friction angles with respect to foam injection ratio, shearing rate, and gravimetric water content obey exponential relationships. The probability distribution function, cumulative distribution function, and decay pattern of bubbles in foam-only systems and soil-foam mixtures all exhibit exponential relationships with elapsed time. Furthermore, a new water-meniscus interaction model was established to characterize rupture and stabilization mechanisms of foam in unsaturated granite residual soils, with particular emphasis on capillary-dominated behavior. Saturation-dependent particle contact modes were identified for foam-conditioned unsaturated granite residual soils, offering valuable guidance for enhancing soil conditioning protocols in earth pressure balance shield tunneling operations.

Granite residual soil exhibits a tendency to collapse and disintegrate upon exposure to water, displaying highly unstable mechanical properties. This makes it susceptible to landslides, mudslides, and other geological hazards. In this study, three common biopolymers, i.e., xanthan gum (XG), locust bean gum (LBG), and guar gum (GG), are employed to improve the strength and stability of granite residual soil. A series of experiments were conducted on biopolymer-modified granite residual soil, varying the types of biopolymers, their concentrations, and curing times, to examine their effects on the soil's strength properties and failure characteristics. The microscopic structure and interaction mechanisms between the soil and biopolymers were analyzed using scanning electron microscopy and X-ray diffraction. The results indicate that guar gum-treated granite residual soil exhibited the highest unconfined compressive strength and shear strength. After adding 2.0% guar gum, the unconfined compressive strength and shear strength of the modified soil are 1.6 times and 1.58 times that of the untreated granite residual soil, respectively. Optimal strength improvements were observed when the biopolymer concentration ranged from 1.5% to 2%, with a curing time of 14 days. After treatment with xanthan gum, locust bean gum, and guar gum, the cohesion of the soil is 1.36 times, 1.34 times, and 1.55 times that of the untreated soil, respectively. The biopolymers enhanced soil bonding through cross-linking, thereby improving the soil's mechanical properties. The gel-like substances formed by the reaction of biopolymers with water adhered to encapsulated soil particles, significantly altering the soil's deformation behavior, toughness, and failure modes. Furthermore, interactions between soil minerals and functional groups of the biopolymers contributed to further enhancement of the soil's mechanical properties. This study demonstrates the feasibility of using biopolymers to improve granite residual soil, offering theoretical insights into the underlying microscopic mechanisms that govern this improvement.

To investigate the impact of rainfall on the stability of granite residual soil slopes, indoor model box tests were conducted at three rainfall intensities (30, 60, 90 mm/h) and two rainfall durations (3. 12 h). The variations in wetting front and vertical displacement were monitored. PFC discrete element software was used to simulate direct shear tests of granite residual soil, calibrate the mesoscopic parameters of granite residual soil for varying moisture contents, and develop a discrete element slope model. The analysis concentrated on the displacement and rotation fields, instability indicators, force chains, and fabric anisotropy to reveal the mesoscopic deformation and mechanical mechanisms underlying slope instability in the model box tests. The results show that when the rainfall intensity reaches 60 mm/h or above, the slip and disturbance range of the slope expand significantly, and the slip body exhibits a circular are shape along the slope face. The slip loss rate of the slope initially decreases and then increases with prolonged rainfall; short-term low-intensity rainfall can stabilize the slope, but continuous rainfall significantly increases the slip loss rate. After 9 hours of rainfall, the displacement and rotation angle of soil particles in the slope increase markedly, forming a distinct circular are slip failure surface. Furthermore, after 9 hours of rainfall, the distributions of force chains and contact force anisotropy within the slope change significantly, with force chains on the slip surface breaking and densely concentrating in stable regions.

The long-term stability of compacted soil liners in landfill barriers depends on maintaining extremely low water permeability and resisting cracking induced by wet-dry cycles. This study investigated the potential of biochar as an amendment to improve the characteristics of granite residual soil, a commonly used material in barrier construction. Laboratory experiments were conducted on soil-biochar blends at different compaction levels (60% and 80%) and biochar concentrations (0%, 5%, 10%, and 20% by mass). The results showed that biochar addition gradually reduced saturated soil water permeability by up to one order of magnitude. Alterations in pore size distributions indicated a shift towards smaller diameters, suggesting the role of biochar in blocking macropores. The crack experiments demonstrated that biochar lowered surface crack ratios by 75% compared with untreated soil. Moreover, biochar affected the drying behaviour of residual granite soils, prolonging the evaporation period from 10 to 12 days and increasing the residual moisture content from 5% to 8%. In conclusion, biochar exhibited the potential to diminish soil permeability coefficients and alleviate soil cracking, providing valuable insights for enhancing the long-term performance of landfill containment barriers.

Granite residual soil is widely used as a subgrade filler in highway construction. Dynamic loads induced by vehicles and earthquakes are complex and involve multidirectional loads, and the dynamic behavior of soil under multidirectional cyclic loading differs significantly from that under unidirectional cyclic loading. A series of horizontal cyclic direct shear tests under cyclic normal loading were conducted using a large-scale cyclic direct shear apparatus at different shear displacement amplitudes (1, 3, 6, and 9 mm) and normal stress amplitudes (0, 100, and 200 kPa). The test results indicate that under cyclic normal stress, the dynamic shear strength of granite residual soil increased during the forward shear process but decreased during the reverse shear process. The damping ratio increases with increasing shear displacement amplitude and normal stress amplitude. This behavior is associated with higher excess pore water pressure induced by greater normal stress amplitude and larger shear displacement, which drive the soil into the yielding phase. The Granite residual soil exhibited significant asymmetric hysteretic characteristics under bidirectional dynamic loading. However, no model has yet been found to describe the asymmetric hysteretic behavior of soil under bidirectional dynamic loading. To obtain the asymmetric hysteretic curve of granite residual soil under bidirectional cyclic loading conditions in the laboratory without the instruments for bidirectional cyclic direct shear tests, the Hardin-Drnevich model and the second Masing rule were extended to propose two asymmetric hysteretic curve models under bidirectional cyclic loading based on the tests. Both models fit with the test results well.

Rainfall-induced landslides are a significant hazard in areas covered by granite residual soil in northern Guangdong Province. To study the response of granite residual soil landslides to rainfall, the most severely affected area during the floods in June 2022 and April 2024 was chosen as the study area. Geological investigations and field artificial rainfall tests were conducted to explore the deformation evolution characteristics of granite residual soil slopes under continuous heavy rainfall and to reveal the failure mechanism of rainfall-induced landsliding events. The results indicate that the granite residual soil can be divided into two layers, and the slope structure can be subdivided into three models from the geological point of view. Given that the deformation and failure characteristics of the surficial landslides are highly similar across the three models, the three models can be consolidated into a single model composed of granite residual soil and weathered granite. The intensity and persistence of rainfall are the main triggering factors of landslides in this area. The landslides are primarily characterized by surficial sliding with a traction sliding failure mode, mainly involving a granite residual soil layer thickness of about 1-3 m. The increased rate of water content and the range of pore water pressure can be used as primary indicators for slope deformation and failure. Additionally, shear dilatancy deformation during slope movement effectively mitigates deformation rates. Furthermore, debris flow is identified as a secondary disaster resulting from landslides, with landslide deposits serving as potential sources for debris flow.

Desiccation crack patterns are commonly observed in natural and engineered soils and provides preferential pathways for moisture infiltrating into the soil. Cracks occur easily in soil when moisture is lost due to desiccation. Crack formation and development are closely related to moisture content and have a marked impact on the soil deformation characteristics and hydraulic properties. However, the critical moisture content below which desiccation cracks appear in the soil is usually determined by experiment because there is a lack of research on theoretical calculation models. Therefore, a theoretical calculation model is proposed to calculate the critical moisture content, and a parameter, lambda\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\lambda$$\end{document}, based on the following relationships: between soil grain size and suction, between suction and tensile strength, and between soil cracking and tensile strength. The critical moisture content values of different grain compositions were calculated and compared with laboratory experiments of desiccation crack. The critical moisture content of the granite residual soil is between 20% (50% liquid limit) and 30% (75% liquid limit). The presented model provides a means to estimate the critical moisture content of crack formation from soil desiccation using basic soil properties. This method can estimate the characteristics of soil desiccation cracks under extreme weather condition.

Excavation of foundation pits induces stress release in the soil, leading to deformation driven by the redistribution of internal stresses and particle adjustment. Rainfall infiltration further increases soil water content, weakening particle bonding through the dissolution of cementing agents, and inducing additional wetting deformation. However, there has only been limited experimental research examining the deformation behavior of soil under the coupled effects of unloading and wetting, especially in water-rich excavation conditions, where these factors interact dynamically. This study systematically investigates the coupled effects of unloading and wetting on the deformation behavior of natural granite residual soil (GRS) through triaxial tests. The results reveal that the interaction between unloading and wetting amplifies soil deformation, with significant non-linear dependencies on confining pressure and saturation levels. The stress-strain curves of natural GRS under unloading path exhibit strain-hardening behavior, and the vertical wetting deformation decreases with increasing saturation. Furthermore, the study identifies pronounced anisotropic wetting deformation, with tensile wetting deformation significantly exceeding compressive wetting deformation under equivalent stress states. This anisotropy diminishes with increasing confining pressure, highlighting the stress-dependency of wetting deformation behavior. The hyperbolic model shows a larger wetting deformation than the linear model, underscoring its practical significance in designing safer excavation strategies under coupled unloading and wetting conditions. These findings provide a foundation for improving deformation prediction and risk management in geotechnical engineering.