Collapse is a severe event during metro tunnel construction, particularly in soil-sand-rock composite strata. Variations in the thickness of sand layers, owing to their high compressibility, flowability, and thixotropic properties, significantly impact the mechanical response and deformation of strata, which also directly determine the stability of tunnel excavation processes. Regarding the complexity of the engineering response of the soil-sand-rock composite strata, a series of model tests were conducted to reveal the collapse mechanism of the metro tunnel in the soil-sand-rock composite strata subjected to varying sand layer thickness in this paper. Meanwhile, the influence of sand thickness on the evolution of collapse form, the response of strata stress, and the variation of the displacement and strain fields were systematically discussed by the monitoring data from the miniature earth pressure cells and a two-dimensional full-field deformation measurement system. Moreover, the collapse evolution process was recorded by the industrial camera. The test results indicated that the horizontal displacement of the tunnel shoulders was more affected by the increase in sand layer thickness than the horizontal displacement of the haunches before the tunnel collapse. The high compressibility of the sand layer resisted the transmission of surcharge load to the rock layer. Once the overlying rock layer above the tunnel vault lost its bearing capacity, the thixotropy and flowability properties of the sand layer caused the collapse face to expand funnel-shaped to both sides. The collapse width of the sand-soil interface was proportional to the sand layer thickness. The greater the sand layer thickness, the weaker the ability to provide stable support for the foot of the temporary stratigraphic arch, further reducing the stability of the temporary stratigraphic arch and leading to a faster collapse rate of the composite strata. In general, the results of this research offer valuable guidance for preventing and controlling tunnel collapse in soil-sand-rock composite strata.

Collapse is a typical type of accident during subway tunnel construction. Complex geological conditions, particularly soil -sand -rock composite strata, significantly contribute to strata instability, posing a serious threat to tunnel safety. Indeed, the progressive failure leading to the collapse of subway tunnels in these composite strata exhibits distinct characteristics from that of single strata and warrants further research. In this paper, model tests were conducted to investigate the collapse process of the soil -sand -rock composite strata under different overlayer rock thicknesses. Soil pressure sensors monitored the mechanical response of the strata, while a twodimensional full -field deformation measurement and analysis system (XTDIC-2D) provided displacement and strain fields. Additionally, an industrial camera captured video footage of the failure process. The results demonstrated that the collapse characteristics of the soil -sand -rock composite strata were significantly impacted by overlayer rock layer thickness. As the thickness of the rock layer decreased, the collapse easily expanded to the sand layer under a slight loading. The sand layer exhibited distinct behaviors with high compressibility, thixotropy, and flowability during the collapse process. The high compressibility of the sand layer before collapse resulted in strain concentration within it, thereby resisting the deformation of the rock strata. After the collapse of the rock strata, the thixotropic and flowability properties caused the collapse surface to expand in a funnel shape towards both sides. A stabilized stratigraphic arch could not be formed due to the arch foot being located above the sand layer, which cannot provide stable support. Overall, the results of this research offer valuable guidance for the prevention and control of tunnel collapse in soil -sand -rock composite strata.