Abstract: To elucidate the surrounding rock pressure mechanism in deep soft rock tunnels and develop effective deformation control technologies, a case study is conducted on the Milin tunnel. The analytical formula for the expansion pressure of the surrounding rock is derived based on the principles of elastoplastic mechanics applied to rock masses. Based on the Kastner formula, the functional relationship between the radial displacement u of a cross-section and plastic zone radius Rp is introduced. Further, the ground reaction curve (GRC) of the expansion and deformation pressures is established, thus exploring the evolution law of these pressures during radial displacement of the crosssection. Based on the convergenceconstraint method and supporting confining curve (SCC), the variation trends of the intersection of GRC and SCC under conditions of expansion pressure are analyzed to design rational control parameters for large deformations in soft rocks. The key findings are as follows: (1) The supporting structure designed only considering the deformation pressure does not meet the stability requirements of the surrounding rock. When the radial displacement of the surrounding rock reaches 530 mm, the pressure of the surrounding rock exerted on the supporting structure reaches 0.82 MPa when considering only the deformation pressure 〖WT５《TNR#I》〗p〖WT５《TNR》〗i; however, the pressure increases to 1.37 MPa when expansion pressure 〖WT５《TNR#I》〗p〖WT５《TNR》〗r is considered, accounting for 40.1% increased pressure. (2) A strategic approach involving primary support comprising a combination of "steel frame + anchor spray" is utilized to counter the displacement of the surrounding rock to 〖WT５《TNR#I》〗u〖WT５《TNR》〗peak=0.325 m. Incorporating an additional I25B steel frame with a space of 〖WT５《TNR#I》〗d〖WT５《TNR》〗=0.7 m effectively moderates the deformation pressure and mitigates excessive expansion pressure. (3) After introducing the secondlayer steel frame over 23 days, the surrounding rock deformation converged to 56.1 mm. The feasibility of the proposed support scheme was validated through theoretical calculation, numerical analyses, and engineering applications.

Key words: deep tunnel engineering, high geo-stress, large deformation of soft rock, deformation pressure, expansion pressure, support optimization design