Rock mass and rippability evaluation for a proposed open pit mine at Globe-Progress, near Reefton

摘要

Rock mass classification schemes such as the Q System and Rock Mass Rating (RMR) System have been designed for prediction of tunnel support, but these systems can be modified from stability analyses to excavatability assessments. Five methods have been used to classify the rock mass at Globe-Progress with the objective of predicting the type of equipment that may be used to excavate the open pit: •Seismic velocity determination •Size-Strength Method •RMRSystem •Weaver's (1975) Rippability Rating System •MacGregor et al's (1994) Productivity Prediction Method Seismic velocity determination and the Size-Strength Method are both easily performed during the feasibility stages of a project. Seismic velocities are influenced by the degree of fracturing, compaction, porosity, density and weathering, and they can therefore be used to provide a preliminary characterisation of the rock mass. The Size-Strength Method uses the two most important properties of a rock mass for classification, the discontinuity spacing and the strength of the rock material. Both methods, therefore, provide quick and accurate assessments of the rock mass quality. At the investigation or design stage of a project a complete rock mass characterisation method is used that involves a collection of geological and geotechnical parameters to fully characterise the rock mass. The method chosen for use at Globe-Progress was the RMR system, as this method is easily adapted from a stability prediction method to an excavatability prediction method. Most data required for calculation of the RMR Index is available from drillcore data logs. Simple analyses of drillcore log data show that drillcore data has been correctly logged except for the strength parameter. This was revised for every logged rock mass unit (RMU) based on quantitative strength determinations and the lithology of each RMU, so that more accurate excavatability analyses could be made using the RMR System, a modified version of Weaver's 1975 Rippability Rating Method, and MacGregor et al’s 1994 Productivity Prediction Method. The ratings for the three rock mass classification methods employed have been contoured on plans at 20 metre bench levels. The plans show that zones of poor rock, where digging to easy ripping should be expected, exist in the western pit wall, where the Chemist Shop Fault is located, and along the northern and eastern walls, following the Globe-Progress Shear Zone. Most of the overburden is classed as fair to poor rock, where easy to moderate ripping will be expected, and there is a zone of weaker rock in the axial fold of the Globe-Progress Shear Zone. This study indicates that the proposed open pit is geotechnically feasible to rip. The preliminary assessments suggest that 90% of the pit area is rippable or marginal and 10% is expected to non-rippable. The final assessments suggest that ripping will be very easy (>3500m³/hr) to difficult (250 - 750 m³/hr) using a Komatsu D575A-2 Bulldozer. Some areas of overburden may require blasting to further fragment the rock mass and aid productivity. But there are other factors, such as the bulldozer operator's experience in ripping similar rock masses, wear and tear on ripper blades, bulldozer maintenance time and transportation costs, and other restrictions that influence overall productivity and costs associated with ripping, and which cannot be determined until ripping actually proceeds.