The emergence of rock breaking technology combined with high-pressure water jet and mechanical disc cutter has changed the operation mode of traditional tunnel boring machine (TBM). The research object is the rock-breaking mechanism of the high pressure water jet cutting grooves into the rock mass on both sides of the disc cutter. We carry out tests and numerical simulations of white sandstone slab samples with constant cross-section indenters penetrating into different pre-cutting groove depths. We use the DIC method to analyze the post-fracture images. The results show that: (1) The existence of the cutting groove blocks the expansion of the penetration crack of the indenter

so that the energy can be more concentrated on the local rock block

which is conducive to the formation of the splayed cracks and promotes the fracture of the rock. (2) With the increase of groove depth

the stress state and mechanical response zones inside the rock under the indenter are changed gradually. When the groove is deep

the mechanical response zone under the indenter further presents a failure transition zone

which locates between the original crack propagation zone and the elastic zone. In this area

micro-cracks are compacted

and the sample shows obvious deformation

but no obvious failures. (3) The use of grooving changes the rock failure mechanism under the indenter. Without grooving

the mechanism shows the radial crack growth dominated by extrusion shear of the complete specimen

whereas after grooving

it shows the main crack propagation that dominated by tensile shear controlled by both the indenter and the grooves.