Temporal And Spatial Effects On The Effective Stress In The Surrounding Subsoil Of Shield Working Shaft Under Very Heavy Short-period Ground Load

During the process of shield tunneling,shield working shafts are expected to be excavated at the ends of a shield tunnel for shield launch and arrival,where the shield tunneling machine is assembled from and taken apart into many parts to be hoisted out from the shaft,respectively.The shield tunneling machine’s increasing diameter requires a deeper excavation of shield working shaft,which can cause a loss of bearing capacity of the shaft’s surrounding subsoil due to lateral unloading process.However,some key parts of shield tunneling machine become heavier so that the hoisting load applied on the top of the shaft shall be increasing.This hoisting load cannot be considered as a common live load on the ground because it has a short duration and is quasi-static,which cannot be described by means of creep models.Because the time and magnitude effects of such very heavy short-period hoisting load on the subsoil and the structure of working shaft are featured by subsoil’s time-dependent consolidation,the load’s progressive effect on the horizontal displacement of shield working shaft’s supporting structure can be studied from the point of view of the evolution of excess pore pressure in the shaft’s surrounding subsoil.Based on the project of a shield working shaft of very large diameter shield tunnel in Jinan,Shandong,China,is the study of this paper:1)Terzaghi’s one-dimension consolidation model is invoked to describe the time and magnitude effects of excess pore pressure induced by very heavy hoisting load.This is combined with the model of horizontal additional stress’ s distribution on the shaft’s supporting structure from applicable standards,which yields the time-and magnitude-dependent analytic solution of Terzaghi’s one-dimension consolidation differential equation with shape function based on Fourier expansion,by means of variable separation.Taking account of soil’s plastic deformation after effective additional stress,a Python code is programmed to calculate the time-dependent increment of horizontal displacement of shield working shaft’s supporting structure under such stress by means of total-strain elastic fulcrum method.2)A fluid-mechanical coupling numerical simulation of the key process of hoisting shield tunneling machine parts at the working shaft of large diameter shield tunnel by means of three-dimensional finite difference method numerical code FLAC3 D is conducted to discuss the very heavy short-period hoisting load’s time and magnitude effects on the horizontal displacement of shield working shaft’s supporting structure caused by subsoil’s consolidation.Numerical orthogonal tests based on this is designed by varying the load’s position,magnitude and duration period,and the temporal and spatial responses of subsoil and supporting structure are thus discussed for further details.3)The in-situ monitoring data of supporting structure’s horizontal displacement,ground settlement,and the bending moment of closed framed girder in the shaft,is used to validate and verify the results from theoretical and numerical calculations to adapt to the in-situ cases.The results show that due to the very heavy short-period hoisting load’s limited width and area of distribution,its effect on subsoil’s consolidation and the working shaft’s supporting structure is also expected to be limited.Stress dispersion weakens the hoisting load effect on deeper subsoil while it remains significant only within a limited depth range.In any case the subsoil’s consolidation always reaches the steady state before the time point of hoisting load’s removal,and so does,therefore,the displacement of supporting structure.One-dimensional consolidation can be found in the later period of hoisting process.After the hoisting load is removed,limited resilience and permanent increment of displacement of supporting structure can be observed,and the latter is caused by subsoil’s plastic deformation after effective additional stress.The in-situ very heavy hoisting load is applied on the top of diaphragm wall and concrete-hardened ground layer which can disperse the load effect so that the subsoil is nearly free from that,and the diaphragm wall’s displacement is mainly generated by P-Δsecondary effect triggered by load pressure at the top and lateral soil pressure.