Abstract: In response to the limited applicability of the currently accepted high-geostress criterion in soft surrounding rocks prone to large deformations and within a rock mass strength range of 15－60 MPa, the authors integrate in-situ testing, case history investigations, theoretical analyses, and finite-element modeling to develop an improved criterion applicable across the entire rock mass strength spectrum. The main conclusions are as follows: (1) The failure pattern of high-geostress tunnels is strongly controlled by the mechanical properties of the associated rock mass; within the 15－60 MPa strength range, relaxation-induced fracturing and excessive convergence predominate. (2) Based on energy principles, nonlinear energy conversion accompanying rock mass fracturing is revealed: energy dissipation during fragmentation initially increases and subsequently decreases with increasing rock mass strength and brittle-ductile transition index. This finding suggests an optimal rock mass strength and brittleness index. (3) A novel high-geostress criterion is proposed that is valid for all rock mass strengths, thereby providing a theoretical basis for hazard prevention and control in tunnel engineering under diverse geological conditions. These findings effectively overcome the limitations of existing criteria and provide clear engineering guidance.

Key words: tunnel, high-geostress identification, large deformation of surrounding rock, loose and broken rocks, optimal brittleness index, strength-stress ratio