Evaluating Time-Dependent Strength Gain in Tailings Using Cone Penetration Test Data from a Large Tailings Storage Facility

This paper evaluates the time-dependent evolution of basic cone penetration test parameters and shear strengths within a 33-m-thick tailings layer in a 100-m-tall tailings storage facility (TSF) in North America. The TSF has experienced several periods of inactivity since initial tailings deposition began in 1976, including one nearly decade-long pause. The studied tailings layer was deposited during the latter half of the 1980s, making it approximately 35–40 years old. Soil behavior types derived from four piezocone penetration tests conducted in 2007, 2015, 2022, and 2024 classify the tailings as contractive clay-like materials. However, laboratory index testing on samples from a supplementary boring characterizes the tailings as sandy silt to silt-like in nature. Particle size distribution curves place the tailings within the coarse tailings and hard rock tailings zones of the indicative gradation chart published by ICOLD in 2021, highlighting discrepancies among these classification approaches. Equilibrium pore pressure profiles, derived from pore pressure dissipation tests and piezometer readings, along with a compression wave velocity profile, indicate sub-hydrostatic yet saturated conditions within the studied layer. These findings reflect vertical drainage associated with the gradual dissipation of self-weight-induced excess pore pressures. This vertical drainage is also likely influenced by localized dewatering through underlying alluvial washes. Between 2007 and 2024, cone tip resistance, sleeve friction, and estimated peak and residual (liquefied) undrained shear strengths increased consistently at rates of about 2%–4% per year, driven by the ongoing consolidation of the tailings. Similarly, the median shear wave velocity increased from 284 m/s in 2015 to about 300 m/s in 2024, indicating increasing tailings stiffness over time.