In order to improve the utilization rate of coal gangue as aggregate in concrete, based on the orthogonal experimental design method, the L16 (4⁵) orthogonal table was used to initially prepare the mix ratio, and the concrete compressive specimens were poured, and their compressive strength was tested after standard curing for 7 days. The significance of the three factors of coal gangue ratio, coal gangue-sand ratio and water-binder ratio on strength was preliminarily understood. Then adjust the factor levels, use the L9 (3⁴) orthogonal table to arrange the factor levels, formulate the mix ratio, test its strength after curing under the same conditions, and finally analyze that the optimal mix ratio of high-volume coal gangue concrete is 90% of coal gangue. Coal gangue sand accounts for 60% of the total sand, the water-binder ratio is 0.4, the sand rate is 35%, and the amount of cement per cubic meter is 350 kg. This mix ratio can use a large amount of coal gangue as a concrete cushion or sub-base in road engineering. Keywords Coal gangue concrete Orthogonal test design Mix ratio Compressive strength

Ensuring face stability is a pressing concern in slurry shield tunneling under high water pressure. Although slurry infiltration and filter cake formation are known to affect stability, the governing role of seepage forces in the failure mechanisms remains insufficiently understood, and existing models often oversimplify the regulating effect of filter cake permeability. To address this gap, a combined numerical–theoretical approach is developed that explicitly incorporates seepage effects into the failure analysis. A three-dimensional seepage model was developed to simulate transient pore water pressure distribution ahead of the tunnel face, considering filter cake properties, stratum permeability, and applied slurry pressure. Based on the computed seepage field and a wedge-prism composite failure mechanism, an upper-bound limit analysis model was formulated that accounts for the work done by seepage forces. Results reveal a filter cake permeability threshold of 1.0 × 10⁻⁷ m/s, below which further reduction yields negligible stability improvement. Parametric studies quantify the influences of internal friction angle, cohesion, depth-to-diameter ratio, and permeability contrast between soil and filter cake. Validation against field data from the Maliuzhou Tunnel demonstrated that the calculated limit pressures consistently lie below the field-measured slurry pressures, confirming the model’s reliability and its practical utility for guiding slurry pressure selection in complex ground conditions.