Raiseboring technology has significantly reduced the disturbance caused to the surrounding rock mass compared to historical drilling and blasting techniques. However, the ground through which the shaft is established must be of sufficient quality to allow a free-standing and unsupported excavation. Empirical rock mass classification schemes have been widely and successfully used to quantify the risks attached to a shaft prior to excavation. However, the risk of raisebored excavation failures due to unforeseen ground conditions is still a concern, hence the need for improved stability assessments. This paper outlines a combination of the empirical techniques employed to identify potential geotechnical stability problems in a raisebored ventilation shaft, the numerical approach adopted for determining the effects of jointing, as well as an evaluation of the effectiveness of these methods once the excavation has been established.
The literature suggests that raises more than 4.0 m in diameter would be considered stable if established in a rock mass of at least fair quality, and that raises up to 4.0 m in diameter would exhibit only isolated incidents of failure when intersecting poor quality ground. The geotechnical risk of a 3.0–4.0 m diameter raisebored ventilation shaft was assessed by applying a combination of empirical techniques supplemented with a numerical assessment of wall stability, due to the importance of this ventilation shaft to the operation. The investigation consisted of the following aspects.