Evolution of the Small-Strain Shear Modulus in the Slip Zone Soil of Accumulated-Layer Landslides as Influenced by Gravel Content

Most accumulated-layer landslides in the China Three Gorges Reservoir Area (TGRA) exhibit slow sliding behavior, and the coarse grain content in the slip zone soil exceeds one-third. Therefore, this study focuses on the gravel-containing slip zone soils that are exposed in the No. 3 Tunnel of the Huangtupo landslide, an accumulated-layer landslide in the TGRA. Using resonant column apparatus, bender element tests, and finite-element numerical simulations, we investigate the effects of varying consolidation pressures, loading and unloading paths, gravel contents, and the particle size distribution of gravels on the shear modulus of the slip zone soils. The results show that with an increase in gravel content, the Gmax of the sample first decreases and then increases under low consolidation pressure; in contrast, it gradually increases under high consolidation pressure. As the gravel content and particle size increase, the G/Gmax–γ curve decays faster, affecting the sample’s curvature coefficient (α). Moreover, the magnitudes of effective consolidation pressure (p′) and overconsolidation ratio have little impact on the shape of the G/Gmax–γ curve. The numerical simulation results further reveal that the presence of gravel leads to uneven stress distribution in the sample, with more low-stress areas appearing in the soil that surrounds the gravel. When the gravel content exceeds a certain threshold, continuous stress zones form between the gravel particles, with the advantageous particles predominantly being gravel. Finally, an empirical Gmax formula is established reflecting gravel content and size in a slip zone soil. This formula enables rapid estimation of Gmax for gravel-containing slip zone soils, providing a basis for calculating the strength and deformation of the soil and the stability of landslides comprised of such soils.