Study On The Foundation Vertical Compressive Stress Of Shallow-buried Closely-spaced Loess Tunnel

Collapsible loess has the characteristics of large pore,vertical joint development and low natural water content,the most prominent engineering feature is the occurrence of collapsing after flooding,it is also one of the most prominent geotechnical problems for the impact of loess on tunnel engineering,when shallow closely-spaced tunnels are built in collapsible loess areas,it is necessary to consider the interaction between the left and right holes,but also consider the impact of loess collapse deformation.According to the current loess norms,the use of indoor test method to determine the type of loess and the thickness of the wet soil layer is significantly different from the actual value,and this will directly affects the tunnel engineering design,construction and project investment.This paper is based on the fourth subway in Xi'an to study the vertical compressive stress for samples in loess collapsible laboratory test by using theoretical derivation and three-dimensional numerical simulation,the main research work as follows:(1)This paper compares the applicability of different division methods of deep-and shallow-buried tunnels to loess tunnels by relying on the measured data of several loess tunnels and deduces the definition formula of the deep-and shallow-buried loess tunnel;(2)This paper deduces the analytical formula of foundation vertical compressive stress of the shallow-buried loess tunnel by basing on the Flamant elasticity,which provides a theoretical formula for determining the vertical compressive stress of indoor tests;(3)Relying on the fourth subway in Xi'an,the stress characteristics are studied by comparing the foundation compressive stress of closely-spaced tunnel with the foundation self-stress.The results show that the excavation of the tunnel will cause the vertical compressive stress unloading and the thickness of the collapsible loess layer to decrease.Therefore,the tunnel engineering should be treated differently from the ground construction;(4)Basing on the calculation assumptions of the foundation vertical compressive stress formula and the indoor soil test,this paper analyzes the reasons for the difference between the calculated value of vertical compressive stress and the actual value;(5)In this paper,the characteristics of foundation vertical compressive stress of shallow-buried closely-spaced loess tunnel are studied under different depth,different horizontal clearance and different immersion conditions.