The Rasp Mine processing plant is situated over a waste rock dump associated with historic mining operations on the south-eastern margin of Broken Hill.
The crushing mills were constructed on 1m thick reinforced concrete raft foundations excavated into the fill.
The sag mill was commissioned in April 2012 and during May and June 2012 large amounts of water were expelled from the mills and flooded the areas surrounding the foundations. In August 2012, settlement monitoring began.
Observations on site suggested that excessive settlement of the sag mill raft foundation was occurring.
A limited field investigation was carried out by CMW in September 2012, which comprised:
- 6 dynamically driven push tube boreholes to between 3.7m and 7.2m depth (BH2);
- 6 Dynamic Cone Penetrometer (DCP) tests adjacent to the boreholes to depths of between 0.6m and 3.15m
Five boreholes were undertaken at various locations around and as close as possible to the existing mill. The sixth borehole was undertaken away from the mill. DCP tests were undertaken adjacent to the borehole positions to provide data relating to the soil density profiles.
The investigation results showed:
- The observed settlement was attributed to the absorption of this water into the uncontrolled fill fines, which are assessed to have contained a high voids content, which in turn led to a loss in shear strength and subsequent consolidation settlement of the uncontrolled fill;
- Remedial recommendations included appropriate management of surface water, to immediately halt any further water ingress; undertake necessary structural repairs to continue operating the mill; continue settlement monitoring; and undertake grout injection to re-level the raft foundation when settlements have reduced to an acceptable level.