Design of rock bolting systems for underground excavations

Stability of underground structures is critical in underground mining. Every effort must be made to preserve stability for the safety of mining operations. There are a number of techniques available for reinforcing unstable rock masses, of which rock bolting is the most widely used. D u e to the complexity of geological conditions and the variety of mining methods employed, reinforcement methods also vary widely. Rock bolting systems are designed to effectively utilise the capacity of the support elements and secure the safety of miners and the mining operation. The focus of this research was a study of discontinuities and their influence on stability of structures in a rock mass. T o determine the jointing system in a rock mass, a three dimensional scanline technique was used to collect the necessary data. This technique was used to eliminate the bias that occurs when using horizontal scanlines. The data were processed statistically and the modified K-mean method is used to cluster the data into groups. The data points in the groups are the elements of the clusters. A n assessment system treating the frequency, persistence, type and various mechanical properties of discontinuities was established to determine the major discontinuity sets which have a dominant influence on rock mass behaviour. When the major discontinuity sets were determined, the stability of the rock mass was then analysed based on an assessment of the competence of the rock mass using a modified geomechanics classification system. The magnitude and direction of the local stress field and the geometry of the opening were taken into consideration by introducing a correction to the rock mass rating. The initial design for the rock bolting system was then carried out. Due to the presence of the discontinuities, the rocks are separated into blocks weathering and infilling can occur between the walls of the discontinuities. The behaviour of a jointed rock mass may be governed by the discontinuities in sets and wedge failure m a y occur if the discontinuities intersect unfavourably. It is therefore necessary to identify potentially unstable wedges. The traditional stereographic projection technique is one of the most c o m m o n methods used and the major limitation of which is that it can only provide a graphical solution for those wedges defined by three discontinuity sets. It is also time consuming and tedious. In this project a computer aided stereographic projection technique was developed. The geometry of either tetrahedral or polyhedral wedges can now be determined. The removability of the wedges is then evaluated. If the wedge is removable the number of bolts required m a y then be calculated. Based on the initial design and the support requirements of potentially unstable wedges, a final decision on the rock bolting system to stabilise the area of interest can be made. The system developed here assists on-site designers by providing a reliable and on-going method of arriving at the primary roof bolting scheme. The applicability of the system has been tested and demonstrated in underground excavations at Pasminco Mining Broken Hill Operation and at C S A Mine, Cobar. The results obtained indicated that an enhanced rock bolting system should be adopted in the stopes in the N B H C Mine. The results agreed broadly with the current empirical design used at C S A Mine, Cobar. This research has shown that the discontinuities in a rock mass have a significant bearing on the rock mass behaviour. The rock bolting design is directly relevant to the safety and effectiveness of an underground mining operation. The design approach resulting from this research will provide a reliable rock bolting system for underground structures in a jointed rock mass