Failure mechanisms of ground collapse caused by shield tunnelling in water-rich composite sandy stratum: A case study

•Leakage in water-rich composite sandy stratum is an important consideration during tunnel construction.•A laboratory modelling is conducted to investigate progression rate of seepage erosion.•Experimental observations identify four stages of erosion development.•Potential preventive treatment to avoid similar failure is proposed.•Critical influential factors to cause the differences between two sandy stratums were analysed under leakage condition. When shield tunnels are built in saturated sandy layers, seepage erosion frequently occurs due to unexpected leakage. Observations from most accidents indicate that erosion does not lead to a large area of ground collapse; however, the study case of a shield tunnel of the Guangdong Metro, China, is unique because the erosion process between leakage initiation and final ground collapse took less than 3 h, which had not been observed before. This study evaluated the progression rate of seepage erosion in the study case and between the study case and other similar cases through analyses of small-scale flume experiments. The flow paths of the eroded particles were investigated during the erosion process, and modeling observations identified four stages in erosion development: erosion initiation, cavity initiation, cavity development, and soil collapse. The experimental results indicate that the difference in seepage erosion development is primarily due to the distance between the leakage point and the underlying high-permeability layer but is also influenced by the differential water pressure head across the sample. Finally, the findings of this study identify a potential treatment that can be applied to prevent such failure in the field: adding a seepage wall structure similar to that investigated in the laboratory experiments.