Three-Dimensional Continuous Upper Limit Analysis For Shallow Shield Tunneling In Undrained Clayey Ground

In urban rail transit planning,shallow-buried shield tunnels have been widely put into construction.The tunnel face instability which causes surrounding soil deformation and failure attracts broad attentions.Therefore,the reasonable support pressure that is applied at the tunnel face must be provided to ensure the safe of shield tunneling.With regard to the stability of shallow-buried shield tunneling in pure clay under undrained condition,this paper uses a three-dimensional formation loss-displacement model to study the most unfavorable failure mode,the critical supporting pressure,and some key parameters under the extent of damage to the tunnel face and the local load.The content of the research and conclusion is divided into 3 parts as the following:(1)By comparing the loss-displacement model of three-dimensional strata such as Pinto with the displacement model of Peck empirical formula,the characteristics of different continuous displacement fields are studied.The formation loss-displacement model is improved by the practical engineering significance.(2)The motion equation of point is employed to find the instability range in front of the excavation by iterating the motion trajectory of the points under the kinematically admissible velocity field.Furthermore,introducing the failure parameters to derive a new mechanism of three-dimensional formation instability.(3)On the basis of different failure parameters and tunnel depth-diameter ratio,it is found that,the extent of plastic zone increases with the increase of failure parameters and tunnel depth-diameter ratio.The necessary supporting force of the tunnel is larger under the local instability of the tunnel excavation surface.(4)On the basis of the failure parameters,the effects of soil physical-mechanical parameters and failure parameters on the tunnel support pressure is studied.It is found that,while keeping other parameters constant,the necessary support pressure decreases with the increase of the shear strength of soil,and is linearly related to the depth-diameter ratio,and increases linearly with the increase of the unit weight of soil.(5)On the basis of the failure parameters,local load applied on the three-dimensional formation loss-displacement model is introduced into upper bound analysis.Consequently,the local building load is applied to study the effects of local uniform load,and pile foundation loads on the necessary tunnel support pressure.Furthermore,the study of local uniform load is investigated by using the finite element method.