Estimating the in situ state of a thickened tailings deposit

Estimating the in situ state that will be achieved within a thickened tailings storage facility is often challenging, as laboratory-prepared specimens are known to not necessarily reproduce the fabric and density of the tailings when they settle under full-scale high energy deposition. Samples prepared in the laboratory from non-segregating slurries often result in quite dense states, which may not occur in full-scale deposition. Some of these issues were investigated during a recent geotechnical investigation of a thickened tailings deposit. The investigation consisted of cone penetration test (CPT) probes followed by piston samples at locations directly adjacent to the probes within saturated regions near the base of the tailings. The piston samples were used to estimate in situ density by means of gravimetric water content (GWC) of the entire sample, and to provide material for preparing loose, moist tamped, reconstituted samples for measuring the critical state line (CSL) of the recovered tailings. The resulting state indicated by comparing the in situ GWCs and the CSLs agreed well with that inferred by CPT-based methods, providing additional confidence in both techniques. A reconstituted sample prepared as a slurry in the lab exhibited a denser state than that indicated from in situ GWCs, raising questions about the accuracy of laboratory-prepared slurries for investigating the density likely to be achieved in situ.