Discussion on the mechanism and critical condition of ground collapse of clay layer induced by karst groundwater pumping in underground engineering construction

In the process of underground engineering construction in karst area, pumping groundwater can easily induce ground collapse and thus leads to engineering accidents. Many researches on the issue are mainly focused on sandy soil, with limited on clay layer which usually has low permeability for groundwater flow. However, a large number of cases show that some serious collapse events are caused by pumping occurred in the clay layer. This paper discusses the mechanism and critical conditions of the collapse caused by groundwater pumping in the clay layer. Firstly, we analyses the influence of construction dewatering on the clay properties and groundwater dynamic conditions on the interface, and the affected area are divided into three areas of I, II and III. Secondly, based on the coupling effect of soil-water interaction, the mechanism of ground collapse induced by pumping water is established. In I and II area, the mechanism is disintegration, and its criterion parameter is soil water content. In area III, the mechanism is the potential erosion, and its criterion parameter is the velocity of groundwater. Thirdly, in order to establish the relationship between pumping volume and criterion parameters, the paper derives the formulas of pumping volume under two conditions, pumping phreatic water and confined water in partially penetration well, Q_cr=(V_cr lr_0)/0.16 and Q_cr=(2.33V_cr lr_0)/(0.16(arsh l/r_0 -arsh l/(3.33r_0 ))). Then, taking Nanning karst foundation pit as an example, it is calculated that the critical pumping volume of ground collapse at the site is 17.97 m3?d-1. Furthermore, the field pumping verification test was carried out in the site to verify the correctness of the value. The results show that the theoretically calculated critical pumping volume has certain engineering application and reference significance in this paper. However, more engineering examples and research are still needed to continuously improve the calculation method.