Study On The Mechanism And Prevention Of Progressive Failure Of Shield Tunnel Induced By Local Failure

Both in China and abroad,disasters which caused a large-scale failure and large ground surface subsidence in tunneling engineering are not very rare,not only bringing huge economic loss but also increasing public nuisance.The investigations of failure cases had shown that most of the disasters were induced by local failure of tunnel lining.However,few studies have investigated the mechanism and prevention of progressive failure of shield tunnel induced by local failure.Soil loss and unloading on the side of shield tunnel would cause failure of the tunnel lining.Firstly,particle flow codes were adopted to investigate the alteration of soil stresses around tunnel lining caused by soil loss.The results indicate that soil loss around tunnel may cause a looseing zone and an arching zone in the order from inside to outside;meanwhile,with the increase of soil loss,the loosening zone continues to extend outward.Then,based on the finite element numerical method,the deformation pattern and failure characteristic of shield tunnel were studied under different loosening extents and degrees on the side of the tunnel.The changing regularities of the segment concrete stress,the blot stress and the opening of the joints were summarized.The results show that,the loosening of soil results in the spalling of concrete near the tunnel joints at the springline and finally the joint bolts at the crown yield and fracture with the increase of the top load.A model tunnel was designed to simulate a general subway tunnel in China,whose outside diameter is 6.2m.Firstly,model tests were designed to study the effective ratio ? of the transverse bending rigidity of shield tunnel.In addition to tests conducted using a concentrated load pattern on the experimental bench,tests using a realistic load pattern in sandy soil in a rigid steel tank were conducted.Furthermore,as catastrophic tunnel collapses usually started with the partial failure of tunnel linings,a series of customized model tests were conducted to investigate the mechanism underlying tunnel collapse due to partial failure of tunnel linings.The test results show that the inrush of the sand due to the removal of a certain segment at the springline may lead to the loss of soil resistance around the openings,resulting in a sudden increase in the bending moments of the adjacent linings.In particular,an increase in the cover-depth-to-diameter ratio(C/D)can aggravate the development of the maximum bending moment and increase the risk of progressive failure of shield tunnel induced by local failure.To address the load sustained by the segments and joints during collapse that the model tests fail to show,3D numerical analyses based on the load-structure model were performed using the measured earth pressure values in the model tests.Via comparison with existing failure criteria for both segments and joints,the safety of the adjacent tunnel linings can be evaluated.To verify the progressive failure mechanism of shield tunnel induced by local failure and put forward preliminary prevention measures,the discrete element software PFC was adopted to simulate the process of progressive failure.Furthermore,the effects of prevention measures were analyzed.Strengthening the reinforcement strength of segments in weak rings,installing transverse steel skeleton and longitudinal structural steel among several rings could enhance the ability of shield tunnel to resist destruction and prevent the progressive failure of shield tunnel effectively.