Stability Of Underground Excavation In Layered Ground With Confined Aquifer

At present,the impact of confined water on the underground excavation is more and more significant.In high water level and high permeability stratum or the stratum with confined water,due to the head difference between the tunnel face and the phreatic water or confined water level,when the support pressure exerted by the earth pressure balance shield tunnel face is insufficient,it will lead to seepage.Improper treatment can further cause ground collapse and settlement of adjacent buildings(structures),resulting in huge losses.Therefore,in the process of shield tunneling,it is very important to estimate the reasonable support pressure of the tunnel face.Most of the existing methods target the analysis of homogeneous permeable ground,and lack of the research considering the influence of confined water stratum.For the excavation in confined water stratum,the thickness of the overlying soil layer of the confined aquifer at the bottom of the pit decreases,and the high confined water will cause the uplift of the bottom of the excavation,which is very easy to cause the basal instability.In current design codes,there is a lack of the method that can reasonably consider the influence of confined water in the analysis of excavation basal stability.On the other hand,the classical pressure balance method used for the analysis of the stability of water gushing in excavation does not consider the influence of shear strength of soil layer at the bottom of excavation and size effect of excavation.This can cause excessive precipitation and significant increase of project cost.For the stability analysis in underground engineering,the limit equilibrium method has clear concept and simple calculation procedures,and its accuracy can also meet the needs of engineering evaluation,and thus being widely used.Therefore,this paper adopts the limit equilibrium method to study the shield tunnel face stability and excavation stability in confined water stratum,and proposes the corresponding analysis methods.The main research works and conclusions are listed as follows:(1)Based on the method of slices,the multilayered wedge limit equilibrium model which considers seepage is established,and the method is verified by comparing against elasto-plastic finite element method.Compared with the physical model test results in the literature,the reliability of the three-dimensional elasto-plastic finite element model is verified;The influence of different soil parameters and hydraulic head on the limit support pressure of tunnel face in homogeneous ground and layered ground is analyzed;Based on the method of slices,the method in this paper does not need to adopt an a priori assumption on soil stress distribution for considering the horizontal arching.The results of the proposed method are in good agreement with the finite element results,which can reasonably reflect the influence of layered ground and seepage on the limit support pressure.(2)The multilayered wedge model for stability of tunnel face in layered ground with confined aquifer is established by incorporating shield tunneling-induced hydraulic head distribution in ground with confined aquifers,and the model is verified by comparing with elasto-plastic finite element method.Based on the three-dimensional elasto-plastic finite element method,the distribution of hydraulic head induced by shield tunneling in layered ground with confined aquifer is analyzed,and the analytical model of the hydraulic head distribution is proposed;The proposed method is evaluated by comparing against the results of finite element method.(3)The basal stability analysis method of the excavation in layered ground with confined aquifer are established for undrained clay and the drained soil,respectively,and the method is verified by comparing with the upper bound method and the shear strength reduction finite element method.Based on the arc sliding method,the basal stability of the excavation in undrained clay is analyzed.Through the example analysis,the applicability of the method is discussed;The pore pressure distribution of the excavation in layered ground with confined aquifer is analyzed by finite element analysis,and the analytical model of pore pressure distribution is established;Based on the arc sliding method and the analytical model of pore pressure distribution,the basal stability of the excavation in layered ground with confined aquifer in drained soil is analyzed.The proposed method is verified by comparing with the shear strength reduction finite element method in the example analysis.(4)The physical model test of hydraulic uplift failure of clay is carried out,and the instability mechanism and influencing factors are revealed.Based on elastic plate theory,the method for analyzing water gushing in excavation involving undrained clay is established.The proposed method is verified by comparing with model test results.The effects of different clay layer thickness,consolidation pressure and hydraulic uplift action area on the stability of clay are analyzed from the model test;The shear failure analysis based on elastic plate can reasonably reflect the influence of clay thickness and shear strength on critical water pressure,displaying good agreement with experimental results.