Model Test And Theory Study On The Urban Underground Engineering Hazards Induced By Loss Of Groundwater And Sand

In recent decades,with the acceleration of urbanization progress in China,underground engineering develops rapidly.The total length of urban railway routes under construction has already exceeded 500 kilometers.However,engineering disaster occurs frequently during the construction and operation period of underground structure,which not only brings huge economic loss and threatens human's life but also increases public nuisance.The investigation of failure cases has shown that most of disaster of underground engineering is induced by loss of water and soil.However,few studies have investigated the mechanism of ground deformation induced by loss of water and soil.Firstly,leakage of tunnel was analysed carefully by finite different method.The influence of leakage on the surrounding ground and parallel tunnel from eight different locations was researched.The results show that the lower leakage position causes wider region of soil deformation.The occurrence of parallel tunnel increases the differential settlement of the ground surface.With the development of leakage,associated ground loss will happen.A new visually experimental device was developed to research the mechanism of loss of water and soil.The sand of different gradations was tested.The evolution law of soil loss mass and ground deformation was investigated.Based on the test results,a simplified model to predict the range of ground deformation induced by loss of water and soil was proposed,and the parametric analysis based on the simplified model was conduced.Besides,numerical simulations of the tests were conducted using PFC-CFD method,and the micro parameters were studied.Two main roles of water are revealed: one is increasing the ground deformation;the other is accelerating the loss of soil.The research shows that the goodness of fit of Peck's curve to the ground settlement decreases with increasing soil loss.The loosening ellipse can be used to described the range of soil movement,and the forming and breaking of soil arch occurs repeatly during the invasion of groundwater and associated ground loss process.With the increase of the opening width on the underground structure,the processes of leakage,soil erosion and invasion of groundwater and associated ground loss will occur in order.Through tests,the critical opening widths of the joints between tunnel segments in Fujian standard sand and Tianjin fine sand were proposed.The deterioration degree of the underground structure can be evaluated by the critical value,and the influence factors of the soil erosion mass were studied.The research shows that the invasion of sand decreases the water tightness of the tunnel sealing gasket.With the increase of soil effective stress,the decrease of water pressure and segment opening width,the soil erosion mass decreases.During the construction of a tunnel using EPBS,spewing is a kind of common hazard,which can lead to loss of soil.The combination of finite difference method in the working chamber and analytical method in the screw conveyor was used to build a simplified model.The evaluation method of spewing of the screw conveyor was proposed based on the simplified model.Comparisons with the practical engineering examples show the reasonability of the proposed method.