Analysis of an instrumented tight-filling-induced paste containment wall failure and lessons learned for safe backfilling

Tight backfilling is recognised as the major contributing factor of backfill containment wall (also termed barricade, bulkhead, fill-fence, etc.) failure, which can result in a dangerous outflow of fluid backfill. The mining industry has developed a set of operating procedures that reduce risk of such occurrences and mitigate the consequences of resultant outflows of paste. This case study from the Jabal Sayid Mine is presented, demonstrating how such procedures, and particularly a well-designed exclusion zone with catchment berm, successfully mitigated the consequences of a wall failure. The case study details how highresolution cameras quickly identified the failure and enabled an immediate halt in backfilling, limiting subsequent clean-up costs. Real-time total earth pressure monitoring is used to ensure backfilling remains within safe pressure limits. In this case, the increase in pressure past the mine’s standard 150 kPa pressure threshold was not acted on, contributing to the wall failure event. In review, better visualisation of pressure data and a ‘stop backfilling first, ask questions second’ emphasis to operators is required to improve safety. The wall failed at a measured pressure of 800 kPa, which provides important calibration of the 200 mm thick (minimum thickness at centre, flaring out to wider abutments) 20 MPa unconfined compressive strength shotcrete arched wall. High-resolution images show vertical and lateral fractures propagated in the 1–2 seconds prior to a localised failure zone (<1 m2) developing close to the rock abutment contact, allowing escape of fluid paste. It is emphasised that the wall had far exceeded design loads by this point. This event shows a useful example of where the weakest points of the asconstructed wall were. Finally, the pressure increase occurred over a ~ 1 hour period, providing useful insights into the mechanics of how tight filling induced wall failure events occur.