Strong pier support technology for small coal pillar roadways in isolated-island working faces with hard roofs

To control the stability of small coal pillar roadways under the pressure of hard-roof isolated-island working faces, this study employed theoretical analysis, laboratory tests, numerical simulation, and field experiments. We determined the optimal coal pillar width and designed a high-strength, easy-to-cut pier column to work in synergy with the coal pillar. The pier column consists of easy-to-cut high-water material, internally reinforced with stirrups and externally confined by a cuttable bellow. Test results showed that the 28-day strength of a standard specimen made with a 1:1 high-water material ratio reached 17.2 MPa. Large specimens without reinforcement had a strength of only 2 MPa, which increased to 4 MPa with internal stirrups and further to 10.2 MPa with external bellow confinement. Numerical simulation indicated that the full composite structure achieved a strength of 15.6 MPa, equivalent to 90% of the standard specimen, successfully balancing high strength and cuttability. Field monitoring revealed an advance influence range of 60-75 m for the isolated-island face. The maximum load on the pier column was 16.2 MPa, and its use reduced roadway width deformation by 50% and height deformation by 87%, demonstrating effective control of roadway deformation.