Semianalytical Analysis of Overexcavation and Critical Support Pressure for Support Design in TBM Tunneling through Squeezing Rock Condition

Abstract Support damage is a typical hazard scenario associated with tunnel boring machine (TBM) drives in the squeezing rock condition. The stress release of the rock mass and the improvement of the support capacity are two effective methods to prevent support damage. The stress in the rock mass can be released by allowing a certain amount of overexcavation. The support capacity can be evaluated by solving the required support pressures corresponding to different critical strain levels. This paper aims to achieve a better understanding of preventing support damage for TBMs in the squeezing condition by quantitatively analyzing the overexcavation and the required support pressure. A semianalytical solution of the ground reaction curve is obtained. A simplified method of solving the support pressure with specific rock displacement is proposed. Based on the two solutions, the procedures for solving for the overexcavation and the critical support pressure are presented. Corresponding to the critical strain levels of 1%, 2.5%, 5%, and 10%, the response surfaces for critical support pressures in relation to the Geological Survey Index (GSI) and in situ stress are obtained via the nonlinear regression method. The influences of GSI, in situ stress, and the mechanical parameters of the concrete lining and shield on the size of the overexcavation are comprehensively investigated. A method to guide support design in the squeezing rock mass is proposed. Three tunnel cases are presented to demonstrate the application of the method, and three case histories that encounter support failures are discussed to illustrate the appropriate design measures through the proposed method.