Layered structures comprising coral sand and gravel have been observed in hydraulic filled foundations in the coral reefs in the South China Sea, leading to anisotropy in their physical and mechanical properties. However, the effect of a layered structure on the strength and deformation of the coral soil foundation remains unclear. In this study, a series of large-scale triaxial compression tests and step-loading tests were carried out on four types of samples, i.e., clean coral sand, clean coral gravel, sand-over-gravel layered sample, and gravel-over-sand layered sample, to investigate the impact of confining pressure and the layered structure on the strength and failure modes of these soils. The results indicate that the stress-strain relationships of all samples predominantly exhibit strain hardening under drained conditions. Under identical confining pressures, the peak strength of clean coral sand is the lowest, while that of coral gravel is the highest. The peak strengths of the two layered samples fall between these extremes, with the gravel-over-sand layered sample exhibiting higher strength. All four samples have similar peak friction angles, slightly exceeding 40 degrees. The difference in peak strength among the four types of samples is attributed to the variations in cohesion, with the cohesion of clean coral gravel being up to four times that of clean sand, and the cohesion of layered samples falling between these two. Both clean sand and clean gravel samples exhibit a bulging phenomenon in the middle, while the layered samples primarily exhibit bulging near the coral gravel layer. In the step-loading tests, the bearing capacity of the layered samples falls between those of clean coral sand and coral gravel, with the gravel-over-sand layered samples demonstrating higher strength. Moreover, the p-s curve of the gravel-over-sand layered samples obtained from the large-scale triaxial apparatus under a confining pressure of 400 kPa resembles that from the plate load tests on the same samples.

The red stratum soft rock, contained extensively in the deep soil-rock mixture (SRM) backfill area of southwest China, exhibits significant water-disintegrating properties that greatly impact the foundation's bearing capacity and deformation failure in this region. This study introduced the large-scale triaxial test to investigate the mechanical deformation characteristics of clay-red stratum soft rock mixture before and after wetting. Simultaneous, combined with the results of test, the law of water disintegration of red stratum soft rock was revealed, and its effects were analyzed in detail. The results show that: (1) Wetting intensified the crushing of rock blocks, resulting in the reduction of shear strength and critical strain of the samples, the decrease of critical internal friction angle and secant modulus, and the significant increase of the relative crushing rate of rock blocks; (2) The most significant increase and decrease of the content before and after the test occur in the particles with the particle size of 0.5-2 mm and 20-40 mm, respectively; (3) Wetting-induced breakage of the red stratum soft rock mainly occurs during the first two hours after encountering water; (4) An increase in confining pressure exacerbates the influence of wetting. Additionally, based on the theory of non-linear elasticity, with the assuming that the reduction of secant modulus causes the wetting deformation, a theoretical calculation model of the wetting axial strain was proposed. Through comparing the calculated results with the measured values obtained by using the double-line method and single-line method test, it is found that the calculation method can accurately predict the wetting axial strain of SRM and be used for quantitative analysis of wetting deformation.

Geogrid reinforcement has a limiting effect on the lateral deformation and thus improves the shear strength of the soil, the overall strength of the soil and the overall stability of the corresponding geotechnical structure. In this study, large-scale triaxial tests without and with geogrid reinforcement were conducted on three typical gravelly soils in Xinjiang using a large-scale triaxial apparatus. The shear strength and deformation characteristics of gravelly soils with different particle shapes and the stress-strain relations, strength characteristics, damage patterns, and reinforcement effects of gravelly soils with and without reinforcement were investigated. Geogrid reinforcement effectively enhances the strength of the soil; the internal friction angle remained relatively constant with and without reinforcement, whereas the cohesive force increased significantly. The reinforcement effects interpreted from the results obtained from the triaxial tests were discovered when a certain deformation or relative displacement with the reinforcement materials of the soil occurred. Under uniform test conditions, the volumetric strain of the samples of gravelly soil with reinforcement significantly decreased with increasing confining pressure, and the difference in volumetric strains with and without reinforcement was greater when the confining pressure was higher. The highlight of this study is its significance in explaining the reinforcement mechanism in gravelly soils and in selecting engineering design parameters.

In the construction of roadbeds in mountainous areas, crushed rock slag (CRS) generated by tunnel blasting is usually reused as road construction material to reduce environmental pollution and construction costs. A series of large-scale drained triaxial tests were conducted to investigate the mechanical behavior of CRS subjected to static and traffic loading. The static triaxial tests determined the maximum stress level that can be applied to the cyclic test. The cyclic triaxial test analyses the influence of cyclic stress amplitude and confining pressure on the cu-mulative strain of CRS material. The particle breakage of the sample under various conditions after cyclic loading was discussed, and the relationship between the relative breakage index and the final accumulated strain was analyzed. Test results indicated that with the increase in confining pressure, the peak strength of the material exhibits a continual enhancement, while the expansion behavior experiences a gradual attenuation. In the range of static failure strength, the increase of cyclic stress level will significantly increase the accumulated axial strain rate. After the cyclic loading, the particle breakage patterns are similar under different confining pressures. A good power function relationship exists between the relative breakage index and final axial strain, and further derivation of the functional expression of the relative breakage index and both cyclic stress ratio and confining pressure.

Geogrid is an important way to improve the bearing capacity of rubber gravel mixture. The dynamic characteristics and mechanism of geogrid-reinforced rubber gravel composites were investigated through large-scale triaxial tests. These tests involved graded cyclic loading with different layers of geogrids and were conducted using three representative rubber contents of gravel mixtures. The study focused on analyzing the development and evolution laws of cumulative plastic strain and hysteresis curves. Key parameters of dynamic characteristics, such as dynamic elastic modulus and damping ratio were compared. The influence mechanism of the coupling effect between geogrid reinforcement and rubber gravel mixtures was also discussed. The results showed that geogrid reinforcement could slow down the increase of cumulative plastic strain under the same dynamic stress. This effect became more pronounced with an increasing number of geogrid layers. Additionally, increasing the rubber content in the mixture improved the ductility of the specimen, but it greatly reduced the bearing energy of the reinforced composite. The shape of the hysteresis curve was primarily influenced by the rubber content, becoming more full, inclined, and its arrangement becoming sparser as the rubber content increased. Geogrids improved the dynamic elastic modulus of the specimens, showing a significant growth stage with an increasing number of geogrid layers. The rubber content had a major impact on the initial value and change trend of the damping ratio. These findings provide valuable insights into the behavior and performance of geogrid-reinforced rubber gravel composites under dynamic loading conditions. They contribute to the understanding of how geogrids can enhance the bearing capacity and improve the overall stability of engineering structures constructed with rubber gravel mixtures.