Optimized Deep Soil Mixing for Cutoff Walls in Tailings Dams: Correlating Laboratory Results with Field Performance

This study presents the concept, execution, and performance verification of a deep soil mixing (DSM) cutoff wall constructed at one of the largest copper tailings storage facilities (TSFs) in Europe, located in Bulgaria. The 500 m long wall, comprising 192 triple-panel DSM elements, was designed to mitigate seepage beneath a staged embankment raise. An integrated program combining laboratory optimization, field trials, and full-scale verification testing was implemented to ensure reliable performance under variable ground conditions. Laboratory mixing of representative tailings and gneissic soils with different binder contents identified an optimal formulation using blast furnace slag cement. Field trials confirmed that a two-cycle mixing procedure provided superior column uniformity. Verification testing of 66 boreholes across the completed barrier, including 198 unconfined compressive strength (UCS) and permeability tests, yielded hydraulic conductivities of 10-7–10-8 m/s and bulk densities of 1.8-2.0 g/cm³. Statistical evaluation using the Bartlett-Kendall method indicated vertical homogeneity of strength, while lateral variability justified subdivision into three UCS zones. The resulting characteristic and design strengths confirm that the adopted DSM approach ensures both mechanical integrity and hydraulic efficiency for the investigated site conditions. Beyond the specific case study, the paper highlights an integrated laboratory–trial–verification workflow for the design and quality assurance of DSM cutoff walls in heterogeneous tailings deposits.