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.

Particle breakage is an important factor affecting the mechanical properties of granular materials. In this study, the influence of particle breakage under different fine particle content is investigated by DEM. Through 3D scanning and Voronoi tessellations, the breakable particle model with realistic shape is constructed. A series of confined cyclic loading tests were performed at different fine particle content. Then, the particle breakage characteristics, including the degree of breakage and the breakage pattern, were evaluated. In addition, the compaction deformation was analyzed according to the evolution of porosity. Finally, the influence mechanism of particle breakage is explained from two perspectives of particle contact and particle motion. On the one hand, with the increase of fine particle content, the number of contacts on the coarse particles is increasing. Hence, the coarse particles can withstand greater forces without breaking. On the other hand, the displacement of coarse particles and the porosity decrement have very similar evolution curve. This indicates that the Z-axis displacement of coarse particles can directly reflect the variation of sample porosity. In addition, particle breakage has little effect on particle rotation. The effect of particle breakage on porosity is mainly realized through the effect of particle translation rather than particle rotation.

The role of particle shape on soil mechanical response has been studied extensively especially through numerical means. The underlying micromechanics of how particle shape may affect the soil mechanical responses at element scale remains unclear. Systematic micromechanical experiments that consider in situ tracking of the evolution of fabric during the shearing process is missing. Aided by a miniaturised triaxial apparatus and X-ray computed tomography (CT), this study presents a series of triaxial compression on four granular soils with different particle shapes yet the same mineralogy, grading, and initial density. Evolution of three-dimensional soil fabric quantifiers during shearing was captured based on 192 full-field CT images. The results revealed that the initial shearing reduced the packing density without changing the particle packing pattern, followed by particle sliding and particle rotation, which redistributed the force chains and formed a new packing pattern to resist shearing, causing strain localisation, and reductions in both the contact number and concentration of contacts direction. Fabric anisotropy increased before reaching the peak and attained the maximum value as the soil approached the critical state. Particle shape, especially when quantified by overall regularity or other combinations of descriptors, displayed more significant linear correlations with critical-state parameters than by local descriptor.