Study On The Loess Tunnel Pressure Arch Effect Based On Coupled Discrete And Continuous Method

In the process of road construction in the Loess Plateau,many loess tunnels have been built.Based on the theory of pressure arch theory,continuous method and the principle of discrete-continuous coupling method,the deformation law of surrounding rock and the forming process of pressure arch in excavation stage of Loess Tunnel are preliminarily analyzed by means of continuous method,taking DuJiaPing Tunnel as a practical project.On this basis,the excavation model of loess tunnel under different surrounding rock grades and buried depth is established by using the discrete-continuous coupling method.The variation law of stress field and displacement field is further studied.The formation law of pressure arch is analyzed from a mesoscopic point of view.The main conclusions are as follows:(1)After tunnel excavation,stress redistribution occurs in surrounding rock,and obvious stress concentration occurs around the tunnel.Settlement deformation occurs at the vault position,while uplift deformation occurs at the arch foot position.The closer the tunnel is,the more obvious this phenomenon is.(2)The position of the inner boundary of the pressure arch is where the maximum principal stress occurs after excavation,and the position of the outer boundary of the pressure arch is the intersection of horizontal stress and vertical stress,but this method is not applicable to all stress paths.(3)After tunnel excavation,the contact force between arch roof and arch.foot is lower,and the contact force between arch waist is larger.Before excavation,the transmission path of force is mainly vertical,and after excavation,the direction is deflected.(4)When the depth of tunnel is shallow,the phenomenon of pressure arch is not obvious.When the burial depth reaches about 50m,the pressure arch can be observed obviously.With the burial depth increasing,the form of pressure arch basically does not change.(5)The maximum ground settlement occurs directly above the longitudinal axis of the tunnel,and its form is close to the normal distribution curve.The change of surrounding rock conditions has certain influence on the size and influence distance of surface subsidence.(6)With the gradual deterioration of surrounding rock conditions,the range and value of the corresponding compressive stress are also constantly weakening,and the influence range of the pressure arch will become smaller.When the surrounding rock condition is better,the range of pressure arch is larger.