Analysis of Seepage Failure in Water-Rich Composite Sand Layer Caused by Shield Tunneling

In 2018, a sudden influx of water and sand in a section of Foshan Metro Line 2 caused the ground to collapse. Special formation conditions and hydraulic conditions are the key factors for the accident. Through a sand loss model test, it is found that the seepage erosion of single sand layer and composite sand layer is significantly different. The erosion cavity of single sand layer is funnel-shaped, and that of composite sand layer is bowl shaped. Under the same conditions, the seepage failure in the composite sand layer is stronger than that in the single sand layer. The incident occurred in the river network of the Pearl River Delta. The shield machine is in a composite sand layer composed of the lower medium coarse sand and the upper silty fine sand. At the same time, it is adjacent to the strong water supply of the river. The combination of the two is prone to water and sand gushing, causing rapid loss of sand water in the surrounding strata and large-area collapse. The field phenomena are consistent with the test results, revealing the formation and hydraulic reasons that are easy to cause accidents in the high water-rich composite sand layer. In the survey and design stage of similar projects, it is necessary to pay attention to the identification of composite sand layer, and take "sealing" measures for shield to prevent seepage outlet.