Shaimerden in Kazakhstan has one of the world's largest deposits of zinc oxide. The mine, on a remote Kazakh steppe, is planned to reach down to a depth of 200m below ground level, through extensive sandy and clayey drift deposits, into underlying limestone.
Fine grained soils above the solid limestone create stability problems. The British arm of Canadian engineers Golder Associates was appointed by the mine owner to review the stability of the slopes at the mine, and provide advice on the security of access ramps.
Over the site's lifetime, the pit slopes cutting through fine alluvial sands overlying stiff Cretaceous multi-coloured clays above strong limestone have become significantly unstable.
This instability is due to twin complications of groundwater and ground conditions. Golder provided advice on: safe slope angles through the overburden; the extent to which the slope might regress should no action be taken; and the optimum location for access ramps to ensure continued production.
Golder's technical development director, geotechnical services, Richard Elmer, conducted site visits in March and June.
"An analysis of the current stability of the pit slopes adjacent to the south access ramp was undertaken, together with an estimation of the potential area that could be affected by regression of the pit crest," he says.
Elmer developed a 3D digital terrain model of the mine, so that sections could be analysed using limit equilibrium slope stability methods.
These could be drawn anywhere around the pit.
The main access ramp is on the south side of the pit and has been undergoing repair. The pit slope walls had failed to the east and west of the ramp. But the area around the ramp was considered safe, as the 25m wide ramp itself acts as a stabilising influence as it is made from good quality material.
A second access ramp entered the mine from the east.
Elmer decided that if there was a risk of failure it was above the access ramp where the slope is at its highest.
"This can be mitigated by reduction of the slope height in this area by cutting the slope back.
"Continued removal of material from the toe of the failed slopes has been encouraging the failed material to move down the slope.
" This has recently been seen with the continued excavation of the toe of the failed material below the upper part of the access ramp and consequential regression of the pit line, which now threatens the ramp."
Elmer made a series of recommendations – the first was to cease excavation at the toe of the failed slopes, until the consequential retreat of the pit perimeter is understood and is acceptable.
He also suggested realigning the southern access ramp, to curve away from the pit edge to pass through the perimeter embankment. This involved designing cutting slopes and drainage measures for the new access ramp.
The upper slope needed re-grading above the eastern access ramp; while the slope below needed to be assessed to determine an appropriate set back and slope angle.
Elmer also proposed cutting back the crest of the slopes to 15° elsewhere around the pit perimeter. The slopes within the Cretaceous clays were also a little too steep so gradients needed to be made shallower.
One of the principal drivers of the ground instability is the high water table. Additional piezometers were recommended to be installed around the pit to measure the groundwater level.
An existing programme of sand drain installation, designed by Golder to reduce pore water pressures in the overburden, is to be continued and extended.