Study On Deformation And Failure Of Ground With Voids Induced By Urban Tunnelling

According to the results of the investigation and statistical analysis on the accidents of the urban shallow tunnel construction,it is found that the failure of the voids in the ground is one of the major causes that leads to the large settlement of the ground surface and even the ground collapse.This research focuses on the ground with voids influenced by shallow tunneling.The model test,numerical simulation and theoretical analysis are adopted to reveal the characteristics of the deformation and failure of the ground with void influenced by the urban shallow tunneling.A prediction model of ground surface settlement induced by shallow tunneling in urban ground with void is proposed,and a risk analysis model of the shallow tunneling safety in urban ground with void is established based on the Bayesian network.The main results are as follows:(1)The ground void is an important factor that affects the laws of the ground deformation,and the influences of the size and spatial position of the void on the ground deformation is revealed as follows: When the size of the void and the distance between the tunnel and void are fixed,the void directly above the tunnel leads to the maximal surface settlement.The surface settlement decreases with the distance between the tunnel and void.The deformation modes of the void vary with the positions of the void,and the vertical ellipsoid and horizontal ellipsoid are two of the main deformation modes.(2)The failure characteristics of the ground with void influenced by shallow tunneling are revealed by model test.Under the influence of the shallow tunneling,the ground void leads to a more complex and rapid failure process,a severer failure degree and a larger range of the ground surface collapse.During the failure process of the ground induced by the tunneling,the surface cracks appear before the failure of the surrounding rock of the tunnel.When there is no void in the ground or the only void is located directly above the tunnel,the location and number of surface cracks are symmetrically distributed around the tunnel.When the void is on the inclined top or the side of the tunnel,the first crack of the ground surface appears on the opposite side of the void,and the quantity and size of the cracks of the ground surface on the opposite side of the void are obviously larger.(3)Based on the results of a large number of the numerical simulations,a prediction formula of the settlement trough curve of the ground surface considering the impact of the ground void is proposed according to the features of ground deformation with void influenced by shallow tunneling.And the method to determine the empirical parameters of the prediction formula is put forward.The prediction formula proposed in this research is validated by comparing with other prediction methods.(4)Based on the Bayesian network theory,a risk analysis model of the safety of the shallow tunneling in urban ground with void is built.This risk analysis model mainly considers the void size,the void location and the distance between the tunnel and void as the risk factors.And the settlement of the ground surface,the vault settlement,the horizontal convergence of the haunch and plastic zone state are taken as the criteria of the risk evaluation.According to the risk analysis,risk prediction and risk diagnosis of this model,it is indicated that the void size is the sensitive factor to the ground surface settlement and the vault settlement,and the distance between the tunnel and void is the sensitive parameter to the horizontal convergence of the haunch and plastic zone state.(5)The risk level partition of the ground with void is proposed based on the risk analysis model of the safety of the shallow tunneling in urban ground with void.The treating measures such as the filling the void or grouting reinforced region for the ground void in different partitions are presented to ensure the safety of the shallow tunneling in the urban ground with void.