Stability Analysis And Reliability Research On The Shallow Tunnel Under Unsymmetrical Pressure

As the dependence on the comprehensive transportation system and urban construction increases with the economic development, shallow tunnel which is an important type of structure is often an inevitable choice in engineering constructions, such as the constructions of highways and railways on slopes in hilly and mountainous areas, the development of urban under-ground space. Stability and deformation of shallow tunnels have a great significance for designing, construction and theoretical research of the tunnel because the stress and displacement of surrounding rocks caused by the excavation of shallow tunnels are more complex than deep tunnels. It is universally accepted in the academia and engineering field that multiple theories must be used to analyze the tunnels in various geological conditions because it is impossible to solve the problems thoroughly just by one theory. In hope to provide a scientific and theoretic guidance for the designing and construction of shallow tunnels, we enrich the classical theories and extend the application area of shallow tunneling. This paper mainly includes the following contents:(1) Based on the upper bound theorem of limit analysis and tangential technique of nonlinear failure criterion, the failure mechanism of surrounding rock of shallow tunnels is established considering the inclination of earth surface and the uplift at the bottom of the tunnel, the formula for calculating the surrounding rock pressure of shallow tunnels is proposed and the objective function is optimized by applying the nonlinear programming method. Through analyzing the sensibility of typical factors, the impact of excavation at the slope foot on the stability of surrounding rocks is studied. Then in combination with the comprehensive strength reduction technique which assigns different reduction factors to c and<p, an entirely new method for the estimation of surrounding rocks stability in shallow tunnels is provided.(2) Based on the engineering example of the Longtanping tunnel, regarding the cut-through of plastic zones as the failure criteria, the numerical simulation software of geotechnical is applied with the orthogonal test to analysis the main factors which impact on the surrounding rocks stability in shallow tunnels. In combination of the upper bound theorem of limit analysis, the comprehensive strength reduction technique and reliability theory, the Monte Carlo method is applied to calculate the failure probability and reliability of the surrounding rocks in shallow tunnels. Then the impact of different probability distribution and the correlation of c and cp on the reliability of the tunnel is analyzed.(3) Based on the upper bound theorem of limit analysis, Hoek-Brown failure criterion and non-associated flow rule, the three-dimensional multiple slide failure mechanism of shallow tunnel face can be structured and the formula for calculating the support pressure is proposed. The objective function is optimized by applying the nonlinear programming method and the impact of different factors on the support pressure of shallow tunnel face is analyzed. Then the stability of excavation of shallow tunnel face under the condition of pre-support with shed-pipe grouting is studied.(4) Based on the classic elastic-plastic theory, considering the strain softening effect and using the tri-linear stress-strain softening model (elastic-plastic softening-plastic residual) and the Hoek-Brown yield criterion, the approximate analytical solutions of the stress field and displacement field in the elastic zone, plastic zone and plastic residual area and the radius of plastic area of surrounding rock in circular tunnel under non-symmetrical load can be deducted by stepwise solution.(5) Aim at the special working condition that the excavation of slope and tunnel coexist in the construction, using the radial displacement of surrounding rock as initial parameters, the stochastic medium theory model of non-uniform convergence of slopes and oval tunnels is built, and the formulas for calculating the surface movement and deformation caused by excavation of tunnel and slope engineering are provided and verified by the field monitoring data. The results can extend the engineering application areas of the stochastic medium theory.