Study On Mechanism Of Stratum Pre-reinforced And Its Application For Shallow Tunnels Construction Method In Urban Subway

Based on vast literatures and extensive practices, the mechanism of earth pre-reinforced by shallow tunnel construction method (STCM) in urban subway was investigated, by local experimentation, theory analysis, numerical simulation and parameters test at laboratory. Its application was carried throughout, and a series of results were obtained.Firstly, by a series of sites test, the earth responded law which tunneling face of SCTM passed through observation main-transect during its frontage, backside and same position were studied systematically. Distribution characters of the soil stress and its distortion were analyzed. The interaction relation among the character, and earth parameters and timbering parameters were clarified, and properties of region and structure of overlay soil for tunneling face was posted. Its three distortion areas: frontage effect, loosing and backside distortion stability were given along head face advancing, and its five distortion belts: bend subsidence, pressing dense, loosing, face and tunnel floor effect were given along its vertical section too.Secondly, the stratum structure model for analyzing overlay soil stability of tunneling was founded, and its formula for calculating level thrust force was given. To analysis its instability, a ellipsoid concept was put forward, which could interpret its structure evolvement shape and its mechanism. That non-support space was a key factor resulting in instability was posted. The concept of big structure for stratum and small structure for pre-reinforcement was demonstrated. Four effects reflecting pre-reinforcement mechanism: earth-arch stability, girder-arch, pull pole and retaining wall were given.Thirdly, the structure mechanics model for pre-reinforcement of heading were founded, and the upper limit solution model analyzing soil stability by the plastic limit theory were erected. Based on the above-mentioned models, the soil pre-reinforcement effects for the arch and frontispiece part of tunnel heading, and a gradually instability concept of face was explained. It was necessary that the frontispiece of face was pre-supported in the especial stratum.Fourthly, by the model analytic value and the FEM numerical simulation, the advance reinforcement parameters were analyzed roundly, and its results explained that the parameters had optimal values. A series of formulations conforming loads bom up for pre-reinforcement structure were given, and five rules for its design and choice were put forward. Finally, a dynamic design method confirming the stratum pre-reinforcement parameters of the STCM face was established.Finally, to aim at Shenzhen subway engineering, the results of paper have been applied at the 6 and the 3A sections. The application indicated its fruits could supervise successfully the design of pre-reinforcement parameters, and the method was good for dynamic design of the pre-reinforcement.