Dynamic behavior and deformation mechanisms of sand-silt stratified soil under intermittent cyclic loading

Subway subgrades typically consist of alternating deposits of soil layers with significantly different physical and mechanical properties. However, the overall dynamic characteristics and the evolution of micro‐porous structures in stratified soils is often overlooked in current studies. In this study, cyclic triaxial tests were conducted on homogeneous sand, silt and stratified soils with different height ratios, and nuclear magnetic resonance (NMR) was used to investigate the changes in pore structure and moisture content. The dynamic behavior and macroscopic deformation mechanisms were systematically investigated in terms of stress amplitude, confining pressure, and layer height ratio (the ratio of sand to silt height). The results show that as the sand height ratio increases, the axial strain and pore water pressure first increase and then decrease, reaching the maximum when h_Sand: h_Silt = 2:1. When the confining pressure is 100 kPa, the axial strain of h_Sand: h_Silt = 2:1 is 181.08 % higher than that of silt. Under the dynamic loading, the stratified soils form a dense skeletal structure near the stratification plane, which hinders the flow and dissipation of pore water, so that the pore water agglomeration phenomenon occurs near the stratification plane, which aggravates the accumulation of residual pore pressure and reduces the deformation resistance. However, when h_Sand: h_Silt = 4:1, the influence of the stratification planes is significantly reduced, and the deformation characteristics approach homogeneity. This study reveals the dynamic characteristics of stratified soils by comparing and analysing homogeneous samples.