Lithological units have been
classified geotechnically with mechanical tests and discontinuity measurements
from the GRD Macraes Limited, Macraes Gold Project and Rotowaro coal mine open
pits, New Zealand. Using these measurements slope stability probability
classifications (SSPC) have been quantified based on an adaption of Hack's
(1998) SSPC system which places less influence on rock quality designation and
unconfined compressive strength than previous slope/rock mass rating systems.
The Hack weathering
susceptibility rating has been modified by using the chemical index of
alteration values from XRF major element analysis. Slaking and weathering is an
important parameter in slope stability in the Waikato Coal Measures and Otago
schist lithologies and hence a non-subjective method of assessing
slaking/weathering in relation to the chemical index of alteration has been
introduced. Other major components of this adapted SSPC system are the inclusion
of rock moisture content and rock strength anisotropy effects on slope
stability. This paper explains the systematic approach of using the adapted SSPC
system to classify slope stability probability.
The main modifications of Hack's
SSPC system are the introduction of rock intact strength derived from
Mohr-Coulomb failure criteria which has been adapted for varying moisture
content, weathering state and confining pressure. It is suggested that the
subjectivity in assessing intact rock strength within broad bands in the initial
SSPC system is a major weakness of the initial system.
Initial comparison of rock mass
friction angles, cohesion and strengths derived from the adapted SSPC are
compared with results derived from modified Hoek-Brown criteria, MRMR and SMR
classification systems. The advantage of the modified SSPC system is that slope
stability probability, maximum batter and overall slope height and overall slope
angle for open pit walls can be predicted in addition to orientation dependent
and orientation independent stability. Examples are provided, using the modified
SSPC system to predict initial optimum pit slope designs in proposed greenfield
mine sites.