Effect of particle gradation and rock block content on soil-rock mixture shear strength parameters

Soil-rock mixture (SRM) is highly inhomogeneous and loose geomaterial found in the quaternary formation, composed of a certain percentage of rock blocks, fine-grained soil, and pores. In Peninsular Malaysia, SRM can commonly be found in the granitic formation covering the mountainous areas where slope failures are prone to occur due to deep tropical weathering, heavy rainfall, and steep terrain. Numerous research studies have been conducted to investigate the mechanical behaviour of SRM, especially the influence of rock block content. However, the effect of particle size distribution has been studied in limited detail. This study uses sieve analysis, specific gravity, compaction, and direct shear box tests to determine the shear strength parameters of weathered type SRM with particle size distribution. The sample collected was divided into well-graded, poorly-graded, and gap-graded types of particle size distribution with 30%, 50%, and 70% rock block content, respectively. The percentage of rock block content is crucial in controlling the shear strength parameters. The specific gravity, dry density, optimum moisture content, and estimated porosity values measured show a considerable difference when the particle size distribution (PSD) and rock block content change. The findings also show that for a well-graded SRM, cohesion and friction angle values show a non-linear relationship against the rock block content. Meanwhile, as the rock block content increases, the cohesion decreases and inversely has an increment of friction angle for poorly-graded SRM. In contrast, the cohesion of gap-graded SRM increases when the rock block content increases. Hence, by understanding the influence of particle size distribution on the shear strength parameters of SRM, a better evaluation can be made in designing slope hazard mitigation.