Crack mechanism of buried ground fissures in Beijing's sand-clay interlayer

The Beijing Plain, characterized by a sand-clay interlayer structure, is highly susceptible to ground fissure disasters, which threaten urban construction and residents' lives. However, the characteristics of crack propagation and the influence zone of ground fissures in the sand-clay interlayer remains inadequately understood. Therefore, based on the excavation of large-scale trenches, physical simulation experiments were conducted to investigate the crack propagation of buried ground fissures within sand-clay interlayers. The results showed that two crack patterns, Ⅴ-shaped anti-dip and dip cracks, occurred during the subsidence of the hanging wall. A total of 33 cracks occurred across the entire profile, with 9 in the sand layer, 31 in the clay layer, and 7 in both types of soil. The number of cracks was significantly higher in the clay layer than in the sand layer. Sudden changes occurred as the cracks propagated to the sand-clay interface, weakening or disrupting the surface. Tensile cracking and differential settlement were observed on the surface, and the influence range of the hanging wall was 1.03 to 2.65 times that of the footwall. Additionally, FLAC3D numerical simulations were used to examine the critical displacement values required to induce cracking in the overburden soil layer due to fault movement in the bedrock. A significant positive correlation between the critical displacement (S_v,cr) and overburden thickness (H) was observed, with a correlation coefficient of 0.996. S_v,cr exhibited four stages: Increase, Stable, Increase, and Disappear. This study provides a comprehensive understanding of crack propagation in ground fissures at sand-clay interlayers, offering a scientific basis for the prevention and control of such disasters and optimizing land use in the region.