Study on mechanical properties and failure modes of soil-rock mixture with a special structure before and after grouting

In western China, a unique soil-rock mixture composed of unconsolidated or weakly consolidated gravel and soil layers is prone to instability under rainstorms or earthquakes. Grouting has proven effective for reinforcement. Indoor direct shear tests were conducted on both ungrouted and grouted samples with gravel sizes of 7.5–15 mm, 15–30 mm, and a mix of both, to investigate the influence of grouting amount and composition on reinforcement effect. The results indicated that under low normal stress, the shear strength of ungrouted samples, ranked from highest to lowest, was as follows: mixed-size samples, 15–30 mm samples, and 7.5–15 mm samples. Under high normal stress, the ungrouted 15–30 mm samples performed the best. Slurry Volume Proportion (SVP) is the ratio of slurry volume to void volume, used to control the amount of grout in samples. Grouting significantly increased shear strength, with higher SVP further enhancing the effect. Among grouted specimens, those with 15–30 mm gravel exhibited the greatest shear strength, whereas 7.5–15 mm gravel samples showed the lowest shear strength. Grouting had little effect on the internal friction angle but markedly increased cohesion. The failure modes of both grouted and ungrouted specimens were categorized. There are two types of failure modes for ungrouted specimens: failure caused by gravel rotation and failure caused by gravel breakage. For grouted samples, two distinct failure modes were observed: one is friction failure between grouting consolidated body and soil layer and another is internal failure of grouting consolidated body. Variations in gravel size and grouting amount influenced these failure modes, thereby affecting the overall shear strength of the mixtures.