Slope stability probability classification, Waikato Coal Measures, New Zealand

Ferm classified lithological units have been identified and described in th e Waikato Coal Measures in open pits in the Waikato coal region. These lith ological units have been classified geotechnically by mechanical tests and discontinuity measurements. Using these measurements slope stability probab ility classifications. (SSPC) have been quantified based on an adaptation o f Hack's [Slope Stability Probability Classification, ITC Delft Publication , Enschede, Netherlands, vol. 43, 1998, 273 pp.] SSPC system, which places less influence on rock quality designation and unconfined compressive stren gth than previous slope/rock mass rating systems. The Hack weathering susceptibility rating has been modified by using chemic al index of alteration values determined from XRF major element analyses. S laking is an important parameter in slope stability in the Waikato Coal Mea sures lithologies and hence, a non-subjective method of assessing slaking i n relation to the chemical index of alteration has been introduced. Another major component of this adapted SSPC system is the inclusion of rock moist ure content effects on slope stability. The main modifications of Hack's SSPC system are the introduction of rock i ntact strength derived from the modified Mohr-Coulomb failure criterion, wh ich has been adapted fur varying moisture content, weathering state and con fining pressure. It is suggested that the subjectivity in assessing intact rock strength within broad bands in the initial SSPC system is a major weak ness of the initial system. Initial results indicate a close relationship between rock mass strength va lues, calculated from rock mass friction angles and rock mass cohesion valu es derived from two established rock moss classification methods (modified Hoeh-Brown failure criteria and MRMR) and the adapted SSPC system. The adva ntage of the modified SSPC system is that slope stability probabilities bas ed on discontinuity-independent and discontinuity-dependent data and a maxi mum slope height are predicted. The moditied SSPC system may be useful in p redicting initial optimum pit slope designs in proposed greenfield mine sit es. XRF major clement and chemical index of alteration (CIA) results obtained f ur lithologies in the Waikato coal region may be a useful mine management t ool to quantify stratigraphic thickness and palaeoweathering from wash dril l cuttings. This paper explains the systematic approach of using the adapted SSPC syste m to classify slope stability in the Waikato open pit coal mines. (C) 2001 Elsevier Science B.V. All rights reserved.