Optimization of synchronous grout properties and construction parameters for shield tunnels in soft soil

Improper synchronous grouting materials and construction parameters may cause the shield segments to float, resulting in tunnel dislocation, open joints, uneven deformation, and water leakage. This paper conducts tests on shield synchronous grout performance and presents a multi-objective optimization method for grout performance. This method considers initial setting time, shear yield strength, early compressive strength, and density as optimization performance indicators. An optimized grouting ratio suitable for tunnel anti-floating is recommended and verified. The corresponding construction parameters matching the grout performance are also discussed using numerical simulation methods. Results indicate that the performance of synchronous grout and grouting pressure, distribution, and shield advancing speed have the most significant impact on tunnel stability. A synchronous grout with ratios of water-cement, glue-sand, bentonite-water, cement-fly ash, and additive-glue of 0.602, 0.613, 0.267, 0.733, and 0.010 can obtain a shorter initial setting time and higher shear and compressive strength. Accordingly, the shield tunneling speed should not exceed 83.28 min/ring, ensuring the liquid grout length is regulated at 5 rings. Alternatively, grouting should be limited to the top portion of the segment, or additional grouting holes at high positions should be installed. The related studies can offer recommendations for shield tunnel construction in soft strata.