Stabilising an underground void: monolithic construction using self-consolidating concrete

Abstract

In 2007, during a planned water level raise at the Giant Mine, which had recently been placed into care and maintenance, a large movement of fill occurred in a series of connected vertically stacked cut-and-fill stopes. The movement mobilised over 170,000 metric tonnes of previously placed unconsolidated rock and sand fill and left a 70,000 m 3 void separated from arsenic trioxide storage vaults, by a thin sill pillar. Review of historical mine plans and investigations onsite suggested that timber sill mats and bulkheads which had likely destabilised when the area became variably saturated during flooding, created a connection between remnant voids in the upper and lower mining horizons resulting in the void. Furthermore, it was determined that if the water level was changed again, with either a rise or drawdown that the rock and fill within the stope below the void could once again be mobilised, further reducing the stability of the void. The mine is to be remediated and closed and a permanent solution to stabilising this void, which is approximately 85 to 125 m below ground surface, was required. Given the lack of future underground access and a desire by the project proponent to reduce long-term monitoring and care requirements, a unique solution was required. While stabilising mine voids during active production mining or during mine closure activities, practitioners would typically utilise a cemented mine waste product such as cemented rockfill (CRF), cemented paste backfill (CPB) or cemented sand fill. Because of the potential for the previously placed sand and rockfill in the void to move deeper into the mine void and possibly destabilise the workings, a manufactured sill pillar that would stay in place should the material below it move was required. In this case, it was determined that the void span was sufficiently large that these types of materials would not be self-supporting should such a movement occur. Therefore, a self-consolidating concrete (SCC), also known as self-levelling concrete was used to manufacture a 16,600 m 3 (70 x 24 x 10 m) monolithic concrete sill pillar to be installed.