The Collapse Mechanism Failure Model Of Bearing Layer At The Pile Tip That The Shield Tunnel Down-traversing The Pile Foundation

According to the guideline of promoting low-carbon development of translation,instituting priority of public transportation and strengthening the construction of rail transit in the 13th Five-Year Plan of China,urban rail transit which based on subway has become the main force for infrastructure construction in major cities.When metros construct under the skyscraper,the engineer must face the phenomenon which metro tunnel have to crossing underneath the foundation of existing building.When the proposed tunnel is too close to the pile foundation of the existing buildings,the bearing layer between tunnel and pile foundation is easy to be damaged by punching under the action of pile foundation load.Not only posing a threat to construction safety of newly create tunnel,but cause the subsidence of the surface building.Aiming this article at this phenomenon,based on Hoek-Brown failure criterion and upper limit theorem of limit analysis,this dissertation studies the stability problems of failure mode of bearing layer at pile tip and ultimate bearing capacity when the shield tunnel passes through the pile foundation of the existing building.The main content of research is as follows:(1)By combining the process of existing buildings pile foundation down-traversing by new tunnel,the mechanics feature of hard-bearing punching failure between the bottom of pile foundation and tunnel,the upper limit failure mechanism for this failure form is constructed.Using the Hoek-Brown failure criterion and upper limit theorem of limit analysis,during the existing buildings pile foundation down-traversing new tunnel,the analytic expression of failure surface of bearing layer at pile tip and ultimate bearing capacity is derived;(2)Based on the analytic equation of the failure surface of bearing layer between bottom of pile foundation and tunnel during existing buildings pile foundation overlapped by new tunnel,the shape of failure surface of bearing layer in different parameters are derived,and the effect of different parameters on the shape and scope of collapse surface are studied;(3)According to the analytic expression of ultimate bearing capacity,the influence of ultimate bearing capacity of pile tip during existing buildings pile foundation down-traversing new tunnel of different parameters is analyzed;(4)Using finite difference software FLAC3D,the numerical calculation of shield tunnel is constructed by pile foundation load.By simulated the excavation process of shield tunnel,the numerical solution of the failure surface of bearing layer of pile tip is obtained,contrasted with solution obtained by upper limit method at the same time.The effectiveness of the solution which calculated by upper limit method is verified;(5)Based on the project which the tunnel of a subway down-traversing pile foundation of a teaching building of middle school,the failure process of the surrounding rock at the top of the shield tunnel is calculated by the method presented in this dissertation.The results prove that the method in this dissertation can be used in the analysis of the influence of shield machine construction on the pile foundation of existing buildings in practical engineering;...