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.

Weathered granite soil (WGS) is highly water-sensitive and widely distributed across southern China, where the region's rainy climate contributes to geological hazards such as collapsing erosion, landslides, and ground subsidence. This study aims to elucidate the impact of this rainy climate on the deterioration of WGS by investigating the suffosion characteristics of granite residual soil (GRS) and completely weathered granite (CWG) at various stages of weathering. The research explores how suffosion affects their mechanical properties and microstructural features. A series of suffosion tests were conducted under controlled water pressure, followed by one-dimensional consolidation tests, cyclic triaxial tests, scanning electron microscopy, X-ray diffraction, and X-ray fluorescence analyses to analyze the deterioration mechanisms at both macro- and micro-scales. The results show that suffosion leads to the loss of fine particles and overall settlement of the soil samples. Microscopically, Mica is almost entirely lost, iron cementation is disrupted, and clay minerals, along with quartz and feldspar debris, are eroded, causing microstructural damage. The loss of minerals at the micro-scale exacerbates the formation of pores and cracks, increasing WGS porosity and promoting the progression of suffosion. On the macro-scale, suffosion alters the physical properties of WGS, with fine particle migration and loss leading to soil skeleton deformation, reduced stiffness, and decreased compressibility. Furthermore, a suffosion index is proposed, correlating microstructural changes with macroscopic mechanical parameters. This study has practical and theoretical significance for slope stability, collapsing erosion prevention, and surface subsidence mitigation in WGS in southern China.

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.

Understanding the temperature-dependent mechanical behavior and fracture characteristics of granite is crucial for many engineering projects. In this study, the real-time temperature curves of granite specimens were obtained during the heating and cooling process, and the thermal treatment tests were conducted. The physical properties of the specimen before and after thermal treatment, including mass, volume, and P-wave velocity, were measured. The acoustic emission (AE) signal in the uniaxial compression is monitored. The results indicate that the physical properties of granite deteriorate with temperature, while the mechanical properties show two effects of thermal strengthening and thermal weakening. This phenomenon is comprehensively analyzed by literature statistical data and optical microscopic observation. Furthermore, the AE characteristic is strongly dependent on temperature. High temperature induces more AE ring count to appear in the early stage of loading. As the temperature increases, the crack initiation stress decreases and the table crack propagation stage becomes longer. The attenuation of high-frequency signals and the enhancement of low-frequency signals are related to the development and interaction mechanism of thermally-induced crack and stress-induced crack. At 600 degrees C, the global b-value increases significantly. Meanwhile, the evolution of dynamic b-value helps explain the failure process of granite under axial load after thermal treatment. In addition, a new thermo-mechanical damage statistical constitutive model of granite considering temperature effects is proposed by introducing AE parameters. The main advantages of this model can well fit the nonlinear behavior of granite in the early loading stage after thermal treatment, and reflect the failure process of granite before the peak value. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

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.

Granite saprolite (GS) slope failure is a common yet catastrophic phenomenon in South China. Although the impact of subtropical climate, characterized by high temperatures and heavy rainfall, is widely recognized, the effect of the capillary imbibition and drying (CID) process, which frequently occurs during the dry season, on the hydro-mechanical properties of GS and slope stability is largely overlooked. This research examines natural GS specimens with various degrees of weathering subjected to CID cycles. The study investigates the capillary imbibition (CI) process and the evolution of the soil's hydromechanical properties across CID cycles. The results indicate that the CI process in GS is fundamentally different from that in clays and sands. The aggregated structure of GS comprising numerous fissures and large pores plays a critical role. In addition, the CID cycles cause the hydro-mechanical degradation of GS, including a finer particle composition, decreased shear strength, and increased permeability and disintegration potential, where damage to soil cementation and fissure development are identified as critical factors. This investigation reveals new insights into the mechanical properties of GS that are essential for the development of effective landslide management strategies in South China. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

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.

On May 1, 2024, a small embankment collapse occurred in the early hours of the morning on the Meida Highway in Meizhou City, Guangdong Province, resulting in 48 fatalities. The small-scale collapse caused massive casualties and garnered widespread attention. In detail, there is a significant lack of precipitation at the time of the 51 Meida collapse disaster, lagging 10 h behind the peak precipitation. The collapse occurs on a mountainous slope, with a hollow catchment area located above the embankment. Multiple potential streams converge in the area, contributing to the water flow towards the slope. Within the western zone of the Lianhua Mountain fault, the collapse area is crossed by fault lines at approximately 800 m on the upper side and 650 m on the lower side. Bedrock fractures formed by faults act as water conduits. The combination of catchment topography and potential faults enriches the water around the embankment slope, contributing to its instability. The disaster site is situated within granite formations. The refilling soil, composed of weathered granite, exhibits poor hydro-mechanical properties, making the slope particularly susceptible to failure due to the effects of multi-source water infiltration. A key insight from this research is that potentially unstable embankment slopes should be identified by considering the interaction between multi-source water and soil/rock. Greater emphasis should be placed on factors such as fault development and hollow topography above the slope, which influence the effects of multi-source water. These factors should be quantified in future studies to improve the assessment of unstable highway slopes in mountainous regions. The findings and strategies outlined in this study can serve as a valuable reference for assessing both embankment and natural slopes in mountainous areas.