Highwall stability is a
major consideration in optimising opencast mine geometry. It involves assessing
the risk of those slope failures that are large enough to have significant cost,
safety and/or operational implications. These risks are then balanced against
the value of the resource to arrive at an acceptable slope design.
Load-deformation models (commonly referred to as numerical' models) of slopes
provide a powerful tool in this decision-making process since they can be
verified well before slope failure occurs. This gives the slope designer
increased confidence in reviewing designs during excavation, potentially
allowing early revision of pit geometry to either increase or decrease safety
margins as appropriate. However, these models are not always adopted as they
depend heavily on an appropriate choice of constitutive model and material
parameters.

A
preliminary load-deformation model of a 170m highwall in Solid Energy North
(SEN) Operations' Awaroa 4 opencast coal mine in the Rotowaro Coalfield has been
prepared. This uses a strain-dependent constitutive model based on laboratory
test results, strength parameters developed by SEN and excavation monitoring
data. The laboratory testing has included triaxial compression tests on
large-diameter fractured samples to characterise the stress-strain response of
the closely-jointed soft rock masses that exist in many parts of the mine. Data
from the monitoring of a 25m cut made during excavation of a haul road has been
used to refine and verify the constitutive model. The resulting highwall model
has predicted highwall crest movements corresponding to the excavation of each
bench, for comparison with monitored values. Key model outputs are total
horizontal movements of about 0.5m near the highwall crest and shear strains of
0.5-0.7% at the locations of potential bedding-parallel shear planes underlying
the highwall. Most of the movement is expected to occur as creep rather than as
an immediate response to excavation.