UPPER BOUND SOLUTIONS OF SAFETY FACTOR FOR SHALLOW TUNNELS SUBJECTED TO NONLINEAR FAILURE CRITERION AND STRENGTH REDUCTION METHOD

The frequently used safety factor is an effective index for evaluating the stability of a tunnel. To study the stability of a shallow tunnel subjected to a nonlinear failure criterion, the nonlinear failure criterion was introduced into the energy calculation of upper bound theorem by using generalized tangential technique. Based on the existing rigid multiple blocks failure mechanism, combining strength reduction technique, the objective function of the safety factor for a shallow tunnel in the limit state was established. Employing the sequential quadratic programming to optimize this objective function, the upper bound solutions of safety factor for a shallow tunnel were obtained. The changing law of safety factor for different parameters was investigated. The results of parametric analysis show that the influence of nonlinear coefficient mon the safety factor and failure surface of the shallow is significant. Moreover, the safety factor decreases with the increase of nonlinear coefficient mand the failure surface enlarges with the increase of nonlinear coefficient mwhile keeping other parameters constant.