Insights into the breaking mechanism and fragment pattern of soft rock assisted by free face under TBM wedge cutter indentation

Abstract

As an innovative and efficient rock breaking method, free face-assisted rock breaking is usually applied to tunnel excavation with TBM multi-stage cutterhead. To reveal the free face-assisted breaking mechanism of soft rock, the finite-discrete element method (FDEM) is adopted to study the mudstone fragmentation characteristics, cutter force, acoustic emission (AE) and rock breaking efficiency. Then, the numerical tests of mudstone samples with different free face conditions under TBM cutter indentation are conducted. The variations of fragmentation pattern, rock breaking angle and maximum cutter force with free face spacing and height are analyzed. Finally, a fragment extraction algorithm is used to identify rock fragment distribution. The fragment area, cutter work and specific energy (SE) are calculated. Results show that free face-assisted fragmentation has a higher rock breaking efficiency with more complete and larger-area fragments compared with traditional breaking method. There are three rock fragmentation patterns controlled by several critical values of free face spacing and height. Besides, a spacing of 75 mm and a height of 125 mm can bring the largest fragment area and the highest rock breaking efficiency. The results can provide theoretical guidance for the design of TBM multi-stage cutterhead for efficient tunneling in soft rock strata.