Shearing characteristics and shearing resistance mechanism of volcanic soil with high vesicularity

Basaltic scoria soil with high vesicles is widely distributed in eastern Jilin, China. The vesicle content varies considerably in different grain sizes. It is insufficient to use a narrow grain size interval to evaluate volcanic soil mechanical properties. In this study, a comprehensive investigation is performed on the chemical composition and the physical and mechanical properties of volcanic soil by XRD, XRF, sieving tests, compaction tests, and shear tests. The results show that volcanic soil is low to moderately weathered gravel with fine grains. The coarse-grain volcanic soil (CGVS) exhibits angularity and high vesicularity; hence, the stress-displacement curves in the direct shear tests show softening characteristics. As for fine-grain volcanic soil (FGVS), the stress–strain curves show strain-softening at lower confining pressure levels, while strain-hardening at higher confining pressure levels. Under the same degree of compaction, the effective internal friction angle of CGVS was smaller than FGVS, while the effective cohesion presented an opposite trend. Shear tests indicated that the shear strength mechanism of CGVS was influenced by the interlocking effect among the soil grains, while the shear strength mechanism of FGVS was influenced by the friction effect among the soil grains. The shear strength of volcanic soil was determined by shear tests on different grain sizes, which avoids the limitations of narrow grain size interval tests. The results of this study can help in guiding the application of volcanic soil in road foundation engineering.