In deep roadway mining, high water pressure causes rock mass cracking and weakens the overall strength, affecting the stability of underground metal mine mining roadways. Therefore, using a numerical simulation method, we analyzed the strain softening characteristics of rocks after the inflexion point of elasticity and studied the strain distribution and the minimum support resistance under high‐pressure groundwater conditions. Considering the groundwater pressure and effective porosity on the strain softening characteristics of the surrounding rocks, we investigated the critical groundwater pressure under which the surrounding rocks would remain stable. Actual engineering verification helps to obtain the supporting characteristic curves under different influencing factors. We found that water pressure and effective porosity are the significant factors that decide the development scope of the plastic zone. The more significant the increase of the plastic zone, the more notable the changes in the support curve. Moreover, the plastic zone is likely to occur when the hydraulic head is between 30 and 50 m; when the hydraulic head exceeds 50 m, it is likely to produce a relaxation zone.

The compression modulus is a key parameter governing the settlement of bridge foundations. To accurately predict pile-group settlement, this study draws on a high-speed railway bridge project crossing the Haihe Alluvial Plain. Combining field monitoring with theoretical analysis, the compression modulus of clay-bearing strata deeper than 50 m is investigated and state-specific correction factors together with their relation to the liquidity index are proposed. The in-situ compression modulus back-calculated from measured settlement of deep clay is 10.47–22.97 times the laboratory modulus E₀. The correction factor β is closely related to soil consistency: mean β values of 0.585, 0.633, 0.648 and 0.798 are obtained for stiff, firm-to-stiff, plastic and soft clays, respectively. β increases linearly with the liquidity index, showing a strong positive correlation. The sensitivity analysis shows that a deviation of 0.05 in β leads to an error of approximately −17% to +24% in the calculated total settlement, and underestimating β yields unsafe predictions. When the measurement resolution is combined, the overall uncertainty band of β is approximately ±0.06, corresponding to an uncertainty band of approximately −21% to +28% in the calculated total settlement. Recommended values are 0.53–0.65 (stiff), 0.57–0.69 (firm-to-stiff), 0.59–0.71 (plastic) and 0.74–0.86 (soft). Within the 0.5 m disturbed zone below the pile tip, the modulus may be taken as 1.2 E₀ for soft clay, 1.6 E₀ for medium-dense sand and 2.0 E₀ for firm-to-stiff clay. The results provide a parameter basis for the settlement prediction of deep-long pile groups in the Haihe Alluvial Plain.