PROBABILISTIC KEY-BLOCK ANALYSIS FOR SUPPORT DESIGN AND EFFECTS OF MINING-INDUCED STRESS ON KEY-BLOCK STABILITY - A CASE-STUDY

Numerical modelling has been used to determine the magnitude and direc tion of stresses around drives both adjacent to and distant from open stopes. The modelling predicted that drives near the stopes were situa ted in a stress-relaxed zone. The peripheries of drives that are sited in solid rock away from the influence of stoping are in compression. Key-block modelling was utilized to determine the typical geometries o f critical blocks and distinct-element modelling was used to determine block stability. The results indicated that the roofs of drives in th e solid rock would be stabilized by mining-induced stresses. In the st ress-relaxed zone key blocks would fail into the excavation. In the si dewalls of drives key blocks would fail into the excavation in tunnels situated both adjacent to and distant from stopes. Blocks that form i n sidewalls have shallow apex depths and large face areas. In the smal ler drives the majority of deterministic key blocks would be too large to fit into the excavation and would, thus, require no support. The c onclusions derived from modelling were seen to fit the observational d ata, indicating that key-block theory is a valid tool for analysing th e stability of roadways in the hard rock mine under investigation. Wit h the use of a risk analysis approach to key-block formation the suppo rt requirements for drives located next to stoped-out areas were formu lated. The results that were obtained with this method were compared w ith support guidelines derived by means of empirical design methods. A good agreement was found for the roofs of the drives. However, the su pport guidelines for the sidewalls did not compare well, those indicat ed by the risk analysis method fitting observational data much better than the empirical guidelines. The concept of a critical excavation wi dth was formulated for the case study. Once an excavation is larger th an the critical size the length of rockbolts need not be increased as the dimensions of the excavation increase. It is suggested that this m ay be the case in all rock masses.