The Wallaby Gold Mine was undergoing a significant ventilation upgrade and this required the installation of 3 new vent shafts. An extensive geotechnical and hydrological investigation program was undertaken prior to awarding the shaft construction contracts. While most of the risks were identified, several proved to be much more challenging than expected.
The first shaft to be constructed was called the VR7 vent shaft. VR7 was to be a 750m deep x 5.0m∅ intake shaft to be constructed by a combination of blind sink and line to around 90m below surface and the remaining to be by raisebore. The hydrological investigation work anticipated that there was likely to be approximately 2L/sec of water encountered during the shaft sink section. Much to the dismay of the shaft sinking contractor and Goldfields, they actually encountered 26L/sec of water. This caused significant delays to the project as it was completely unexpected. Sovereign Hydroseal were engaged to mobilise to site to seal this water. This water was quickly and efficiently reduced to less than 1L/sec.
The next shaft was the VR4 vent shaft. VR4 was to be a 250m deep x 4.5m∅ exhaust shaft and as this shaft was only some 200m from VR7 it was anticipated that it will likely strike significant water during the shaft sink as it did in VR7. The shaft collar for VR4 was down on a bench in the side of the open pit and therefor the shaft collar was around 50m below the surface level of VR7 shaft. Sovereign were engaged to work with the shaft sink contractor to grout in advance of the shaft sink face to minimize the effects of blind sinking with water. Significant water of around 20L/sec was encountered during drilling but this was successfully sealed from entering the shaft sink.
The 3rd shaft to be constructed was the VR8 vent shaft. VR8 was to be a 250m deep x 4.5m∅ exhaust shaft and was the first leg of an overall deeper shaft. This shaft was only approximately 30m away from VR4 and ran parallel on the same pit bench. For this shaft, it was recommended to pre-grout the strata between 25m and 40m below the surface prior to commencement of any shaft construction. A series of 8 holes were drilled around the outside of the final shaft excavation profile and the zone between 25m and 40m was pressure grouted. This reduced the water make inside the shaft excavation profile from around 20L/sec to approximately 1L/sec. It was then decided to drill a second series of 8 holes inbetween the pervious holes. Again the pressure grouting took place and the test holes indicated a final water make of approximately 0.2L/sec remaining. The client deemed this a great success and Sovereign Hydroseal demobilized from site.
Sovereign Hydroseal are very pleased to have been of service to Goldfield’s over the past 3 shafts and the fact that Goldfields retained Sovereign Hydroseal for all 3 shafts must be an indication of their approval and commitment in Sovereign’s capability and technology.
