Underground design and deformation based on surface geometry

This thesis presents a method for improving underground excavation design by relating geometry and deformation to the stability of hanging wall surfaces of open stopes. Instability on a stope hanging wall occurs when the opening geometry increases past a stable limit. There are no realistic methods of predicting surface deformation or projecting future deformation with continued mining. Computer models do exist which calculate deformation, however, no methods exist for obtaining realistic input parameters for the rock mass. Both stability and deformation, for a given mining situation, are dependent on the geometry of the surface opening. Two terms have been introduced in this thesis, radius factor and effective radius factor, which are based on surface geometry. Radius factor, (RF), is related to the overall stability and maximum deformation of a surface. Effective radius factor, (ERF), is related to the local stability and deformation of a point on the stope surface. By introducing a term related to both stability and deformation, maximum allowable stable deformations can be designed for. Support practices can be tailored to provide the optimum support for the expected deformation. Deformation has been linked to geometry with instrumented case histories from several mines. Several mechanisms driving deformation have been recognized including elastic relaxation, non-elastic fracture dilation and voussoir arch deflection. With monitoring, rates of deformation accompanying mining have been determined which allow the prediction of future movement with continued mining. The modified stability graph design technique, which is an existing empirical design technique modified from the Mathews design method, has formed the basis for stability assessment. With this method hydraulic radius is used to assess surface geometry. The data base behind the design method has been re-analysed using the RF term which more accurately reflects surface stability. Case histories of stope backs, where the modified stability graph design was unsuccessful in estimating stability, have been reassessed using RF values resulting in more accurate stability assessments. This thesis successfully links hanging wall geometry with measured deformation and the onset of instability. Instrumented field movement data can be used to design support to match the predicted surface movements, as well as indicate the approach of failure. More complex geometries can now be assessed with commonly used empirical design tools.