Abstract: When tunneling in a narrow chamber with microcracks and extremely high water pressure, water gushing or shaft flooding occurs. To address this problem, a case study is conducted on the chamber of the bottom car yard of inclined shaft No. 1 of the Gaoligongshan tunnel on the DaliRuili railway. The site conditions are complex and include water pressure as high as 7.6 MPa, large montmorillonite content, poor groutability, limited drainage capacity of the shaft bottom, difficult grouting and water plugging, and high construction requirements. Therefore, field tests are performed regarding selection and matching, equipment disassembly and hoisting, antioutburst measures for extremely high water pressure, and standards for starting and ending water plugging in advance. The following conclusions are obtained: (1) An advance water plugging and grouting scheme is proposed. This scheme includes rational step distance, water sealing, inside first and then outside, process connection, interval grouting, water source locking, increasing grouting holes, and local strengthening. (2) A highpower, narrowbody crawler hydraulic drill equipped with a stepless variablespeed ultrahighpressure, lowflow doubleliquid pump is customized for fast construction of repeated equipment disassembly and hoisting in narrow spaces. (3) The solutiontype modified ureaformaldehyde resin is adopted, with 23 times hydrostatic pressure as the final grouting pressure, improving the groutability of the formation and achieving a satisfactory water plugging effect. (4) The insitu water pressure test predicts water inflow in front, and based on the actual drainage capacity in the shaft, a scientific and reasonable water control principle of "plugging as the main and limited discharge" is determined. The permeability coefficient is calculated using the borehole water pressure test to determine the advance grouting starting standards, and the water plugging ending standard is determined based on the permeability coefficient when achieving a satisfactory grouting effect. (5) The optimal grouting cycle length is determined to be 25 m, which can achieve the highest comprehensive construction efficiency.

Key words: extremely high water pressure, deep and large vertical shaft, railway tunnel, grouting, equipment matching, highpressure water blockage