Application of block theory for evaluating face stability under disc cutters loading of TBM, case study of a water-conveyance tunnel project

Abstract

Block theory has been widely used in analyzing the stability of rock blocks during engineering activities in jointed rock masses due to its simplicity and computational efficiency. This paper extended the traditional block theory to tunnels constructed using tunnel boring machine (TBM) by incorporating the disc cutter-block interaction mechanism. The rock blocks were divided into front, corner and rear blocks according to their spatial positions with respect to the cutterhead. The interaction between different blocks and the disc cutters mounted on the cutterhead were examined by estimating the forces exerted by disc cutters on the blocks relative to the location of the blocks in the face, direction of movement of the discs, and the depth of penetration. The estimated forces were then used in a subsequent analysis to calculate factor of safety for individual blocks and probability of blocks dislodging from its location. Two new evaluation parameters, eA and eη, were proposed to account for the probability of block instability. eA is the ratio of exposed area of unstable blocks to the face area and quantifies the size of unstable blocks relative to the face, and eη is the ratio of cutterhead rotation angles making a block unstable to the total rotation angles in a revolution which reflects the possibility of encountering an unstable block during a single round of revolution. This paper discussed the implementation of the idea of using fundamental concepts of block theory to determine block stability at the face and related analysis for assessment of face stability. The proposed method was used in evaluation of the stability conditions for a water-conveyance tunnel in western China that is currently under construction using a hard rock TBM.