Grouting isolation and reinforcement test and measurement analysis of the parallel existing operating line of the overlapping tunnel

To verify the feasibility of ensuring the safe operation of the existing tunnel by applying the grouting reinforcement isolation wall method in the small-clearance, long parallel section between Nanjing Metro Line 6 and the existing Line 4, a representative test area was selected based on similar stratum distribution. Studies and tests were conducted on the reinforcement method, including grouting process and mix ratio optimization, in-situ deep soil grouting, coring inspection and water injection tests, saturated uniaxial compressive strength tests, as well as earth pressure and deep soil horizontal displacement monitoring. The results indicate that for the strata in this project, it is advisable to employ bentonite-cement slurry and rapid-setting high-strength cement-based slurry. The grouting operation should adopt the methods of "skip-hole drilling" and "cyclic grouting in intervals", with staged grouting for formations at different depths. The lateral reinforcement extent of the grouted zone reaches ≥2.70 m, which can serve as the control criterion for formal grouting construction. During grouting, the earth pressure induced at the equivalent horizontal distance from the existing line was 0.508 MPa, slightly exceeding the design requirement. However, the maximum horizontal displacement of the soil was 4.64 mm, satisfying the design requirements. After grouting reinforcement, the stratum changed from extremely fragmented to moderately complete to relatively complete, and from medium permeable stratum to slightly permeable stratum, and the saturated uniaxial compressive strength of the core samples taken from the inspection hole is more than 1 MPa. Therefore, it is feasible to use this grouting process and parameters for the grouting isolation and reinforcement construction near the existing operating lines. This can provide a reference for the grouting isolation and reinforcement under similar working conditions.