Jiping Cao

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.