Research On Unloading Disturbance Displacement Characteristics And Its Catastrophe By Upper Excavation Of Shield Tunneling In Soft Soil Stratum

Considering the space distribution and time history effect of unloading disturbance displacement by upper pit excavation of shield tunneling in rheological soft soil stratum,as well as the key theoretical and technical issues of construction method,parameter sensibility,structure stability,prediction and prevention of catastrophe evolution,a systematic study has been carried out based on full investigations.Comprehensive research methods,including theoretical analysis,similar model test,numerical simulation and on-site testing are employed.This study mainly focused on the basic characteristics of unloading disturbance displacement by ground excavation,the characteristics of disturbance displacement by upper soil excavation,the influence on unloading disturbance displacement by different construction methods,the applicability and optimization of strengthen methods,the influence of soil rheological properties on unloading disturbance displacement by pit excavation,the prediction to unloading disturbance displacement by excavation and its catastrophe evolution.Some innovative and practical research results are achieved as follow:(1)By numerical simulation analysis of the geometry dimension effect,parameter sensitivity of soil deformation property and strength property of the pit excavation disturbance displacement in general soft soil stratum,the existence of full unloading status of the disturbance displacement space effect during the pit excavation and the evolution tendency of its peak displacement value have been revealed;(2)According to the excavation background of typical pit in soft stratum,a systematic study on the space distribution,time effect and the phasic characteristics of construction steps under typical excavation conditions has been carried out by 3-D similar model test.The main reason of pit excavation soil disturbance displacement is demonstrated to be the instant elastic recovery by upper unloading and the soil rheological time effect.The macro constitution and mechanism are confirmed.(3)Through recursive simulations of testing data,some conclusions are obtained that the variation of soil creep displacement and accumulative displacement along time during the whole construction process can be generally expressed by a sigmoidal function,and the sedimentation displacement of arbitrary depth layer soil can be generally expressed by a Guss function.An quantitative calculation method of soil creep displacement,accumulative displacement time history and space distribution of foundation pit vertical displacement has been established.The research achievements can provide theoretical support for prediction of displacement distribution and time history of soft soil foundation pit excavation.(4)The distribution characteristics and its dynamic evolution behaviors of disturbance displacement of the pit surrounding soil and the tunnel structure below during the whole process of pit excavation are systematically studied.The Guss function relation of the space distribution of the pit horizontal section's displacement is derived.A useful method is found to quantitatively modify the sedimentation displacement of arbitrary depth by displacement depth attenuation factor and correct sedimentation displacement under creep effect by displacement time effect factor.(5)The basic property and its risk tendency of the tunnel excavation disturbance displacement under unloading effect due to upper pit is systematically investigated.The investigation shows that the floating displacement of the tunnel bottom plate and the subway line structure is unavoidable after unloading by upper fit excavation and the safety risk of subway operation will be increased.The general floating mechanism of the tunnel during the pit excavation can only be abated but not eliminated by any strengthen methods,as well as the increasement of subway operation safety risk.Targeted strengthen methods can reduce the safety risk of the lower tunnel structure.(6)Two typical and applicable prediction methods for long term displacement and risks for pit excavation are established.One is the calculation method by combining soil creep model and FEM for soil long term displacement.And the other is the prediction method based on the combination of analogous testing and on-site testing for time effect of the disturbance displacement by pit excavation.The long term displacement and technical risk probably caused by the prediction plan in the initial design or organization design phases are effectively addressed.The proposed methods also can help with quick prediction response from real time inspection,displacement control,optimization for construction methods and parameters by real time.Through the step-by-step simulation for determining the relationship of displacement along time and the inspection data of accumulated displacement,the accuracy of prediction to the displacement variation rule and maximum displacement value in follow-up construction process are effectively improved.Reliable supports are provided for the control of displacement and risks during the whole construction process.The research results have been applied to Shanghai Metro Line #10 Project,Ningbo Metro Project and Shanghai Hongqiao International Culture City Super-Pit Project,etc.The applications show that the concluded testing rules and theoretical achievements of this paper fit the on-site inspection results of the projects very well.It can provide useful references for the prediction calculation of excavation displacement of rheological soft soil stratum,the selection and optimization for envelop strengthen plan,evaluation and prediction for environment risks.