Analysis Of Shield Tunnel Face Stability

Shield tunneling method is becoming the most primary method in underground excavation with the minimum soil disturbance, but because of the limit of geology conditions and construction technique, it is inevitable that shield constructions disturb circumjacent environment. Tunnel face instability leads to the surface collapse that destroys the surrounding environment badly, causes huge economic losses, hinders the normal operation of the traffic, and seriously affects people's lives, so shield tunnel face stability is key point and becoming an important task. However shield tunnel face pressure depends on engineering experience mostly at present in native, the study of face tunneling shields is relatively less.The paper developed the following work and research according to the related circs:(1) According to the stability theory, the limit face pressure is that when face pressure is changed very small, displacement of the center at tunnel face increases large suddenly. Through the principle of shiled progress and the datums of measurement, and so far, there is no simple and direct software in simulating and forecasting the limit support pressure of shield tunnel face stability at home and abroad. By means of VB6.0 and FLAC3D, software analysis integration system about the face limit support pressure is founded under complex geology conditions, the calculations that simplify operation of software and modeling of tunnel construction mechanics are simple and convenient, ecumenical engineers simulate and predict tunnel face stability expediently using the system.(2) On the assumption that the shape of sliding block is a trapezoid wedge and introducing the Terzaghi's effective relaxed soil pressure theory, limit support pressure of tunnel face under seepage by upper theorem are deduced. It is providing a basis for the setting of tunnel face pressure.(3) Strength reduction method is used in stability analysis of shield tunnel face and the face safety factor is defined. And the effect of soil parameters, the support pressure and ground water level on face safety factor is analyzed,all these can provide useful reference to the setting of tunnel face support pressure in design and construction. (4) Using the genetic programming method and selecting the main influencing factors of the surface settlement by shield tunneling, including tunnel overburden, shield diameter, the thrust of shield tunneling, advancement rate of shield, fill factor of grouting, cohesive, friction angle and compression modulus of the soil et al. GP program is trained using practical maximum surface settlement data, and then the maximum surface settlement model of GP is set up, a new method of prediction of the maximum surface settlement by shield tunneling is provided.