Same‑track coupled experimental and numerical investigation on hard rock fragmentation mechanisms and damage evolution in HPWJ‑assisted TBM cutting

High-pressure waterjet (HPWJ) precutting prior to tunnel boring machine (TBM) disc cutters has emerged as a promising technique to improve hard rock fragmentation efficiency. However, the mechanical correlation between precut kerf geometry and rock damage evolution remains insufficiently understood. This study carries out full-scale, same-track coupled experiments, in which a 17-inch disc cutter penetrates granite specimens at a constant penetration depth of 6 mm under pure disc cutting conditions and along HPWJ-prefabricated kerfs with systematically varied depths of 2, 4, 6, and 8 mm. The experimental results demonstrate that HPWJ precutting reduces the mean normal force by up to 32% and the peak normal force by 22% compared with those under conventional pure disc cutting. In particular, the specific cutting energy is decreased by 34% under the maximum kerf depth condition. With increasing kerf depth, rock cutting chips exhibit a progressive morphological transition from powdery debris to lamellar fragments. Complementary finite element simulations are performed using the JH2 constitutive model, which is calibrated by uniaxial compression and Brazilian splitting tests with good agreement in stress‑strain curves between experimental and numerical results. The simulation results further reveal that same-track HPWJ precutting nonlinearly restrains the growth of the crushed dense core zone. Notably, the projected area of the dense core under a kerf depth of 8 mm accounts for only 11.3% of that in the non-kerf baseline case. The precut kerfs also redirect rock damage patterns, shifting dispersed microcracking to lateral macroscopic fracturing, which promotes crack coalescence during the subsequent disc rolling process. These mechanical findings establish a systematic design framework for HPWJ-disc cutter coupled cutterheads, providing quantitative references for waterjet nozzle parameter optimization, cutter spacing configuration, and zoned cutting track arrangement. This study further lays a solid theoretical basis for the engineering application of waterjet technology in TBM excavation in extremely hard rock formations.