The cutterhead and rock mass characteristics affect the excavation efficiency of full face rock tunnel boring machine (TBM) strongly. The numerical models are made by the particle flow code method (PFC2D) to simulate the rock fragmentation under TBM disc cutters. Numerical simulation tests show that some factors influence the rock fragmentation process strongly. In the relative weak rocks

the regular tension cracks and the big mucks form more easily

when the disc cutters penetrate rocks alternately; in hard rocks

the formation of small disc shape mucks depends on the lateral extrusion of cutter rings. Under the double disc cutters

the rocks crush more easily because of the tension stress which comes form strong compression of cutters

and the disc shape mucks only form in very hard rocks. There is optimal cutter spacing for different strength rocks

which increase with rock strength decreases. In the process of rock fragmentation

discontinuities control the propagation process of cracks and obstruct the cracks spread to the underside of discontinuities. The discontinuities will guide the cracks spread deep in rocks

when the angle of cut-in direction and discontinuity decreases. And the discontinuities will guide the cracks spread transversely when the angle increases