Comparison of in Situ State of a Tailing Deposit with Reconstituted Laboratory Specimen States

Since a series of significant failures of tailing deposits has occurred, the importance of accurately characterizing the tailings has taken a great deal of attention over the past decade. The initial steps of tailing characterization involve using cone penetration testing (CPT) to estimate the in situ state and loosely reconstituted laboratory specimens to infer the critical state line (CSL). However, the preparation of laboratory specimens to mimic the in situ state in terms of density and fabric is considered challenging. Despite such concerns, the use of moist tamping (MT) currently dominates as the industry practice for the identification of CSL, whereas employing non-segregating slurries is also not rare. In this study, piezocone (CPTu) data and tube samples collected next to CPTu locations within the saturated regions of a tailing deposit were utilized. The in situ state was estimated using two independent methods: calculating in situ void ratios using the gravimetric water content (GWC) determined from tube samples and using CPTu screening methods where void ratios are calculated based on the state parameter (ψ) values obtained for the target depth range. Simultaneously, a series of triaxial tests were conducted using MT and SD specimens to obtain their consolidated states at varying stress levels and to infer the CSL. Results indicated that SD tests achieved slightly denser states than those obtained from MT specimens, but interestingly not states well below the CSL as commonly cited in previous studies. The CPTu inferred ψ span over a wide range, and hence the in situ states are either looser than or in agreement with the densities obtained from MT tests. The GWC measurements based on in situ states overlap with the looser end of the CPTu inferred range. This raises the question as to which degree the homogeneously prepared denser laboratory specimens have the capability to mimic the behavior of loosely deposited tailings in the tailing’s storage facility.