Numerical modeling of crack propagation in rocks under TBM disc cutters

Abstract

The mechanism of rock fragmentation underneath disc cutters is not fully understood although a number of experimental and numerical investigations have been carried out in this field. Linear elastic fracture mechanics is widely applied for the analysis of crack problems in rock mechanics. In this study, the higher order displacement discontinuity method is modified for the analysis of crack problems using the cubic variations of displacement discontinuities and three special crack tip elements. It is shown how a new formulation of the indirect boundary element method known as the displacement discontinuity method (DDM) can be used to determine the stress intensity factors of the cracks produced in rocks underneath disc cutters of tunnel boring machines (TBMs). Crack initiation angles and propagation paths in the rock can also be predicted using this numerical procedure and a mixed mode fracture criterion (for example, the maximum tensile stress criterion). In this numerical approach, three special crack tip elements are used to increase the accuracy of the displacement discontinuities near the crack tips. This method has been used to find approximately the effect of the specific disc parameters (except speed) on the thrust force, the rolling force, and the specific energy. Crack propagation in rocks under disc cutters is numerically modeled and an optimum ratio of disc spacing to penetration depth of about 10 is obtained, in good agreement with the theoretical and experimental results in the literature.