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Mrne Pondr Eva[ua[iart 4.4 Current State of Consolidation of Tailings • The current state of consolidation of the tailings can be evaluated using classical one- dimensional infinitesimal-strain consolidation equations. Procedures for the solution of this type of problem are detailed innumerous sources (e.g. Lambe and Whitman, 1969; Holtz and Kovacs, 1981). Since the laboratory results indicate that the tailings consolidation in the ponds are at an advanced state, the use of more complicated finite-strain consolidation equations, such as described by Gibson (Gibson et al., 1981), is not warranted. The initial state of effective stress in tailings during deposition is normally very low, due to pore pressures significantly higher than those under hydrostatic conditions. As these excess pore pressures dissipate toward the long-term, steady-state pore pressure conditions, the effective stresses in the tailings increase, causing a reduction in the size of the void spaces in the tailings. The relationship between vertical effective stress and tailings volume (as expressed by the void ratio) for samples of the Upper and Lower Pond tailings aze shown on Figures 4.1 through 4.4. By assessing the current state of effective stress relative to the initial (neaz zero) effective stress • state and the final effective stress state under long-term, steady-state pore pressure conditions, the potential for further volume reduction due to consolidation can be evaluated. Because the tailings in both the Upper and Lower Ponds aze lazgely saturated, a volume reduction would be accompanied by an equal volume of flow of pore water from the tailings. Estimation of the current degree of consolidation is complicated by the lack of detailed information concerning the long-term steady-state pore water pressure conditions within the tailings. At one extreme, it could be assumed that the pore pressure in the tailings will eventually dissipate completely. The effective stress state corresponding to this condition is indicated by the vertical lines labeled "fully drained" on Figures 4.1 through 4.4. At the other extreme, it could be assumed that internal ponding would occur and full hydrostatic pore pressures would always exist within the tailings. The effective stress state corresponding to this condition is represented by the vertical line labeled "hydrostatic ponding" on the Figures. Actual long-term pore pressure conditions aze expected to lie somewhere between these two extremes. • The current state of effective stress in the ponds can be estimated from the laboratory consolidation test results using Casagrande's procedure. These estimates are shown on Figures Homeslake M1linmg Canpany Shepherd Slifler. lrtc. yal"-JlnbrllJng.ipr 15 April l4, l99i