A New Tool for Field Characterisation of Joint Surfaces for Engineering Design

Rock Engineering 2009 - Rock Engineering in Difficult Conditions
Chris Hawkes,
Abstract Joint surface characterisation is an important component to all rock classification systems and contributes about 30% to the overall classification value. In mine design work, it is important to improve our ability to estimate rock mass properties to try and keep up with improving techniques for modelling rock mass behaviour. Current methods of assessing the contribution of joint properties to rock mass properties often consist of very subjective descriptions of general joint surface character. Improvements in joint roughness measurements were applied to rock characterisation in mining in the early 1990s. These techniques were simple and designed to improve an engineer’s ability to characterise the joint properties into the very general and often subjective rock characterisation categories used by rock classification systems. Further improvement to methods of characterising joint surfaces have not been developed for mining applications. There are no well established, commonly used design techniques which can apply improved data on joint surface conditions to justify improved data collection.

In the petroleum geomechanics field, the success of thermal enhanced recovery operations, CO2 sequestration and waste injection depends upon a tight, low permeability cap rock. The majority of any escape of reservoir fluids would, in most cases, be along discontinuities in the cap rock. Lab testing for estimating gas and liquid permeability through fractures is an important component in analyzing and predicting hydraulic integrity of cap rocks. One goal of an effective cap rock evaluation is to link laboratory test results to simple measurements of fracture roughness that can be conducted on core samples.

A simple hand held laser profilometer has been developed for recording the roughness characteristics of a joint surface. This paper presents this new field tool and discusses how data from this field instrument can be used to assist in designing stable underground openings, as well as effective steam, CO2 or waste injection programs. It is hoped that this tool will also encourage the collection of improved rock classification data for mining geomechanics design.
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