A sinkhole in the dam at the the HB mine site south of Salmo has been determined as the primary cause of the slough that threatened the stability of the tailings pond last week.
Heavy rainfall throughout the month of June was a contributing factor to some seepage and the initial slough.
In the process of relieving pressure on the dam, the sinkhole was discovered on the inside of the face approximately 1.8 metres below the normal high water mark.
Geo-technicians and engineers from the Regional District of Central Kootenay have been working since the slough last Tuesday using pumps to draw the water level behind the dam down to a safe level.
The pumps are running strictly to match inflow levels.
The sinkhole was revealed as the level of the pond was reduced.
Using tracer dye, it was confirmed that water flows from the sinkhole were a significant factor in the partial slope failure. Those flows have now been stopped.
Regular monthly and annual inspections would not have revealed the sinkhole given its location and depth in the pond.
Emergency remediation work is ongoing to build an additional supporting berm on the dam face to ensure stability and retention of tailings.
Crews are using excavators to place rock and fill materials to remediate the failed area and buttress the slope.
The area is much drier than when the slough occurred and engineers have determined it is safe to do the emergency work.
Engineers are determining the best method to permanently remediate the sinkhole and in doing so further improve the stability of the dam.
The Regional District of Central Kootenay assumed ownership of the six hectare tailings area in 1998 as part of their central landfill area.