Evolution of Water Table and Pore-Water Pressure in Stopes with Submerged Hydraulic Fill

Abstract

Hydraulic fill is often used to fill large underground voids created during mining operations. It is produced and distributed as a slurry at a pulp density (P) of 65–75% (solids content). Consequently, a substantial amount of water needs to drain after placement. Inadequate drainage has been regarded as the main cause of barricade failure for stopes with hydraulic fills. Therefore, a good understanding of the evolution of total and effective stresses within such backfilled stopes is critical for barricade design. Most existing analytical solutions for assessing stresses in stopes are based on Marston’s approach by considering a fully drained (without pore-water pressure) or hydrostatic state. These conditions are not always representative of the stress state in stopes and behind barricades. When slurried hydraulic fill is placed into a stope, the self-weight consolidation or sedimentation of the fill can take place fairly rapidly. Ponding could first occur on the top of backfill if the drainage through barricades is not sufficient. Drainage through the barricade then allows the water table to descend, changing the pore-water pressures with time. In this paper, the authors present analytical solutions for estimating the evolution of water table and pore-water pressures in stopes with submerged hydraulic fills. These solutions were validated using numerical simulations. The results show that the ponding on the top of the settled fill has a negative impact on the barricade safety. Two-dimensional (2D) modeling without considering the reduced drift area tends to underestimate the pore-water pressures in stopes and behind barricades, which may render the barricade design nonconservative. Discussion follows on some particular features and limitations of the proposed analytical and numerical solutions.