Abstract: [Objective] Solidified soil is widely used in seepage control engineering. However, the significant spatial heterogeneity caused by mixing uniformity, soil variability, and curing condition differences during construction is often overlooked, leading to considerable uncertainty in seepage control design. [Methods] This study, focusing on the solidified soil cutoff wall project for a coal slag yard, conducted field casting and curing tests. Electrical resistivity data at different spatial positions of the solidified soil were obtained using the high-density electrical method, and combined with permeability coefficients determined from field borehole sampling, an Archie and Kozeny-Carman (KC) coupled model with resistivity as input parameter was constructed to evaluate the permeability coefficient of solidified soil. Based on this, the uncertainties of resistivity data and coupled model predictions were quantified. Transient seepage fields of the cutoff wall were calculated using stochastic numerical simulation methods, and optimal design parameters and operational recommendations for the coal slag yard cutoff wall were proposed based on the statistical characteristics of breakthrough time. [Results] The results show that: resistivity test data follow a log-normal distribution, Archie-KC coupled model errors follow a normal distribution, and breakthrough times of the solidified soil cutoff wall follow a log-normal distribution; the mean and standard deviation of breakthrough time both increase with wall thickness. [Conclusion] The optimal cutoff wall thickness is 3.0 m, and the breakthrough failure probabilities at 10, 15, and 20 days are 0.07%, 3.92%, and 77.15%, respectively.