Geotechnical properties of coal and mine pillar design in the Greymouth and Reefton coalfields, West Coast, South Island

摘要

This project has estimated coal strength in selected parts of the Greymouth and Reefton Coalfields in order to better estimate the size of pillars needed to maintain stability of the underground workings. Coal strength in known to decrease with increasing rank, and the Greymouth Coalfield displays to a very high rank gradient increasing from west to east. The mines assessed by this study were the Bishop Block, Strongman No. 2, Spring Creek, Roa and Terrace (Reefton Coalfield) Mines. Core samples could not be obtained from all locations so 63.5mm cubes and point load tests were used, and compared to a control group of with a known UCS/cube relationship, in order to develop an equation from which a UCS equivalent value could be determined. Coal strength drops from 24 MPa in the west (Strongman No. 2 E seam) to 1.3 MPa in the east of the coalfield (Roa mine Kimbell seam). Other coal properties also shows changes corresponding to changes in coal strength including carbon, volatile matter, ash, and the degree of cleating. Ash is the only one of these which is not related to increasing rank. Cleat frequency, which increases with coal rank has the most significant effect on coal strength. The equations of Bieniawski and Salamon-Munro have been used for pillar strength calculations with panel pillars designed to a factor of safety of 1.6. Optimum pillar sizes for each of the locations in this study have been calculated, but small changes to these sizes may be necessary depending on local conditions such as faults and sheared zones. Pillar design must take into account the chance of pillar shearing once seam dip increases above 20° as the shear strength becomes greatly reduced with increasing seam dip. Coal from the Spring Creek Mine shows a high degree of anisotropy and so pillars have been designed for specific seam dips at this location. Bearing capacity of the fireclay in the Terrace Mine is greatly reduced with increasing seam dip and overburden thickness, thus increasing the chances of floor heave. Pillars need to be of adequate size so as not to transfer excess overburden load to the mine floor, which would result in floor heave.