Study On Deformation Of The Soft Surrounding Rock And Support Stability Of Tunnel Engineering

With the in-depth implementation of the strategy of China’s transportation power,the construction of a large number of tunnel projects will be inevitable,and the tunnel construction environment is increasingly complex,such as shallow buried tunnel,extra-long tunnel,super-large cross-section tunnel and soft rock tunnel construction.Due to the weak nature of the surrounding rock properties,poor stability and large excavation deformation,shallow buried soft rock tunnels are prone to cause surface settlement,vault subsidence,tunnel face collapse and tunnel support damage in the process of engineering construction.It will have a serious impact on the stability of surface structures,tunnel construction safety and personnel safety.Based on this,this paper takes the Heimaguan Tunnel of Kanglue Expressway as the engineering background to study the deformation law of surrounding rock and the stress characteristics of tunnel support during the excavation of soft rock tunnel.The main contents include:(1)The definition,engineering mechanics characteristics,classification and categorisation of soft surrounding rock are clarified,the factors affecting the stability of tunnel surrounding rock and the formulae for calculating the loosening pressure in the surrounding rock of deep and shallow buried tunnel are derived;The deformation pressure sharing values of surrounding rock and the tunnel support under different supporting stiffness and supporting timing are studied,and the optimal supporting timing is determined.(2)The time-dependent deformation characteristics of surface settlement,vault subsidence and horizontal convergence of the V-grade soft surrounding rock section in Heimaguan Tunnel were analyzed by field monitoring;Peck theory was used to study the distribution characteristicss and laws of surface settlement trough during tunnel construction,and compared with the field monitoring results;On this basis,the vault subsidence and horizontal convergence models of tunnel based on exponential function,logarithmic function and hyperbolic function were established,and the optimal timing of secondary lining was established.The results show that the surface settlement,vault subsidence and horizontal convergence of shallow buried section of soft rock tunnel tend to be stable at about 25-30d;The surface settlement of soft rock tunnel calculated by Peck theory and the field monitoring results are basically the same,the former decreased is only 0.72% less than the latter,and the vault subsidence and horizontal convergence are consistent with the predicted results of the exponential function model;The optimum time for secondary lining of the soft rock tunnel is determined to be 26-27 d.(3)A finite element numerical model was established to study the time-dependent variation patterns and characteristics of the vertical displacement,horizontal displacement,principal stress of the surrounding rock and the tunnel support at the monitored section.The results show that the displacement and stress of the monitored section increase gradually with the advance of the excavation face;The vertical displacement and horizontal displacement of each monitoring point basically tends to be stable at the 20 th excavation step(the distance between the monitoring section and the excavation face is 40m);The principal stress of the surrounding rock and the tunnel support changes obviously in the first 10 excavation steps(the distance between the monitoring section and the excavation face is about 20m),and then basically tends to be stable,and the principal stress value of the tunnel support is small,indicating that the construction plan and supporting measures can ensure the stability of the project construction.