Uncertainty in ground support design and implementation in underground mining

Abstract

In underground mining, there are a number of uncertainties in the ground support design process and during implementation of ground support designs. The minimisation of geological uncertainty is one of the rock engineering principles outlined by Bieniawski (1992) and by Stacey (2004, 2009). Generally, there are two main types of uncertainty; uncertainty due to naturally variable phenomena in time or space and that due to lack of knowledge or understanding (Baecher and Christian, 2003). McMahon (1985) listed six types of geotechnical uncertainty: risk of encountering unknown geological conditions; risk of using incorrect geotechnical criteria; bias and/or variation in estimated design parameters is greater than anticipated; human error; design changes and excessive conservatism. All of these types of uncertainty are encountered in the design of ground support. They occur in the form of uncertainties around: the design block size; loading conditions; spatial variability of ground conditions; rock mass strength; shear strength; discontinuity spacing; discontinuity orientations, etc. Uncertainty is not only present in the design of ground support but also in the implementation phase. This occurs in the form of variations in installation quality; adherence to patterns and spacing; human error in the application of the correct ground support standard; not identifying a change in ground conditions, etc. Conventional deterministic and empirical design methods do not adequately cater for uncertainties in the design of ground support. Probabilistic design methods such as the Point Estimate Method and Monte Carlo Simulation can be applied to better understand the uncertainties in the design process. Limiting uncertainties in ground support implementation can be achieved by: training of operators and supervisors; good supervision of implementation; and a well-considered and managed quality control programme. It is important to feed the results from the quality control and monitoring programmes back into the design. In this paper, the various uncertainties with ground support design and implementation will be reviewed and discussed in context of McMahon (1985) and Baecher and Christian (2003). The application of probabilistic design methods in ground support design and feedback of quality control data will be discussed.