This study aims to quantitatively elucidate the mechanisms by which principal stress axis orientation controls the mechanical behavior of rock, focusing on both macroscopic mechanical behavior and microscopic failure processes. Three types of complex stress-path tests were conducted on hollow-cylinder gray sandstone specimens using PFC3D. The study identifies that crack growth is governed by the principal stress orientation under low axial stress, while non-coaxial deformation dominates under high axial stress, leading to reduced influence of stress orientation and more complex fracture patterns. Based on acoustic emission monitoring data obtained from the numerical simulations, two novel indices, namely damage efficiency (DE) and non-coaxial damage index (NCDI), are proposed. These indices quantitatively characterize the coupling effect between stress axis rotation and damage accumulation in rock. These findings provide deeper insight into the failure mechanisms of rocks under non-conventional stress paths and support improved design and risk assessment in geotechnical engineering.

Utilizing waste materials as soil replacements offers a sustainable path for geotechnical engineering. This study explores sugarcane bagasse, an abundant agricultural waste as a partial soil replacement, focusing on its dynamic characteristics. While previous studies examined static behavior with small reinforcement percentages, dynamic characterization with partial sand replacement remains unexplored. This research fills that gap using strain-controlled cyclic triaxial and bender element tests on six mixes, with bagasse content from 0% to 50% by volume (BS0 to BS50). The effects of confining pressure, relative density, and strain amplitude (0.3%-3%) were studied, focusing on maximum shear modulus, shear modulus, and damping ratio. Bagasse-sand mixes restricted cyclic loads via interparticle networks. Increased bagasse content reduced shear modulus but enhanced the damping ratio. Stress-strain curves showed stiffness degradation, with BS0 displaying larger hysteresis loops. Excess pore pressure and strain contributed to this reduction. Shear wave velocity and maximum shear modulus declined with more bagasse, BS0 had the highest (98.7 MPa), BS50 the lowest (7.25 MPa). Mixes up to 30% bagasse retained a relatively sand-dominated response, while higher contents showed bagasse dominated behavior. Overall, these mixes offer eco-friendly, vibration-reducing solutions and promote sustainable waste management, when optimally mixed.

Sandstone widely occurs within the hydro-fluctuation belts of reservoir banks and is susceptible to progressive deterioration under repeated wetting-drying cycles, posing potential risks to long-term slope stability. To investigate this process, moderately weathered sandstone collected from a landslide hydro-fluctuation belt in the Three Gorges Reservoir area was subjected to 0-20 wetting-drying cycles. Uniaxial and conventional triaxial compression tests were conducted in combination with nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) to characterize the mechanical degradation and mesoscopic structural evolution of the rock. In addition, a two-dimensional numerical damage model incorporating mesoscopic heterogeneity was established based on continuum damage mechanics and the Weibull statistical distribution. The results show that wetting-drying cycles significantly reduce the peak strength, elastic modulus, cohesion, and internal friction angle of sandstone, exhibiting a clear nonlinear degradation trend. Meanwhile, the failure mode gradually evolves from shear-dominated brittle failure to a more ductile pattern characterized by multi-crack interaction. At the microscale, water-rock interaction and cement dissolution promote micropore development and connectivity, increasing intermediate pore volume and weakening the load-bearing skeleton of the rock. Numerical simulations reproduce the progressive evolution of microcracks into macroscopic conjugate fracture networks, providing a reliable basis for evaluating reservoir bank slope stability under cyclic wetting-drying conditions.

The soil-cement grout interfaces are commonly involved in a wide range of civil engineering, and their strength significantly influences the stability of the whole structure. This paper investigates the strength of the interface between unsaturated soil and cement grout. A series of direct shear experiments are conducted on the interface under different initial water contents. Based on the test results, the strength equation expressed in terms of the degree of saturation was proposed. The proposed expression does not include suction, thereby avoiding the need for its measurement or control. Furthermore, a constitutive model for the soil-grout interface is then established within the framework of the disturbed state concept. The critical state strength equation is adopted to represent the fully adjusted state, while an elastic model is taken as the relatively complete state. The shear modulus related to the degree of saturation is used in the elastic model. The model has a total of 8 parameters, including 4 strength parameters, 2 elastic modulus parameters, and 2 disturbance parameters. All these parameters can be predicted by direct shear tests. The model is validated against experimental data, and the corresponding model parameters are analyzed accordingly. Results show that the proposed model can satisfactorily reproduce the interface behavior. The proposed model requires only conventional direct shear tests, effectively simplifying experimental and computational procedures and shortening test duration.

Soil improvement techniques are widely used in deep excavation projects near metro stations; however, there remains a lack of reliable guidance for determining the parameters of the Hardening Soil model with small-strain stiffness (HSS model) for cement-stabilized soils. Laboratory tests on cement-stabilized soils from the Shanghai West Station project were performed at curing ages of 3, 7, 14, and 28 days to determine the corresponding HSS model parameters. By comparing these results with the parameters of in-situ soil, the strength development and interrelationships among the parameters at different curing ages were analyzed, leading to the establishment of an HSS parameter determination method based on curing-age evolution. Numerical analysis of an engineering case shows that the calculated horizontal deformation of the retaining wall agrees well with the measured values, validating the applicability of the proposed parameter determination method for excavations involving cement-stabilized soils; furthermore, the effects of curing age and related parameters on excavation behavior are analyzed to provide references for engineering design.

Controlling leakage and infiltration in subsurface is critical for maintaining underground structural stability and mitigating environmental impacts. Conventional cement-based grouts have high CO₂ emissions and the potential for internal discontinuities. This study investigated the applicability of biopolymer-based grout systems, including xanthan gum grout, agar gum grout, gelatin grout, and gelatin-tannic acid grout, as sustainable alternatives to conventional cement grout. Scaled geotechnical models featuring various structures, including H-core, Double H-core, Secant-Construction in Place (CIP), and Trapezoid-core, were subjected to high-pressure (up to 500  kPa) permeability tests to assess their impermeability and injection performance. The pre-treatment hydraulic conductivity ranged from approximately 10^-8 to 10^-5 cm/s, whereas post-treatment values after 1-7 days of curing decreased to approximately 10^-9-10^-6 cm/s for most biopolymer-treated specimens. Among the tested materials, gelatin-tannic acid grout showed the most pronounced short-term permeability reduction due to crosslinking-enhanced gel formation. In particular, crosslinked gelatin with tannic acid produced robust gel networks, ensuring favorable impermeability under high-pressure conditions. Injectability tests showed that agar gum exhibited higher initial injection volumes due to its low viscosity, while smaller amounts of xanthan gum achieved comparable permeability reductions, underscoring economic and environmental advantages. By contrast, although cement grout offers a low unit material cost, its large-scale use significantly increases CO₂ emissions and can re-initiate leakage pathways through internal discontinuities during prolonged high-pressure injection. Overall, these findings demonstrate the potential of biopolymer-based grouts for short-term permeability reduction in scaled geotechnical structures. However, long-term durability, biodegradation resistance, cyclic hydraulic loading, and field-scale validation should be further investigated before practical implementation.