Geotechnical Issues In Construction Of A Large Diameter Raise-Bore Shaft

Ruth Stephenson and Megan Walske

Major issues were encountered during drilling of a large diameter raise-bored shaft, constructed as part of an underground mine ventilation network. The raise-bored shaft was developed through generally competent, high strength metamorphosed basalt in a high stress environment at depths of +1500 m below ground. Back-reaming was delayed due to blocky ground conditions, leading to ten cutter changes and associated increased costs.

This paper presents details of a geotechnical review of this shaft construction. Analysis of acoustic televiewer (ATV) structural data and seismic data was used to establish the presence of a thin shear causing localized blocky ground as the source of the difficult drilling conditions. Analysis of seismic data collected during drilling is presented as a means to predict difficult back-remaining conditions.