Appraisal of Shear Strength Using Joint Roughness Coefficient for Cut 8 Bench, Jwaneng Diamond Mine, Botswana

This study is concentrated on Debswana Jwaneng Diamond Mine, which is the largest
producer, by value, for Debswana Diamond Company. At the time of the study, the mining
activity was going on in two (2) cuts simultaneously. These two cuts are ‘Cut 8’ and ‘Cut 6,
with the areas of interest being the North-East (NE), South-East (SE) and South West (SW)
sides of these cuts.
Jwaneng Diamond Mine has had a history of a fatal slope failure in the Cut 8 North-East
Corner and several cases of rock falls and planar slope failures. The NE, SE, and SW walls of
Cut 8 are characterized by bedded dip slopes that daylight into the pit. Dip slopes are slopes
that have the same dip direction as that of the underlying strata.
The orientation and geotechnical characteristics of the bedding in these slopes make these
slopes highly susceptible to failure under conventional mining methods.
To determine the shear strength of the different rock units in North East, South East and
South West walls, the Joint Roughness Coefficient of bedding in the walls have been
calibrated in order to determine the factors of safety and probabilities of failure for the dip
slopes. Data collection has been done through laser scan mapping of pit walls. The collected
laser scans were analysed using ‘Maptek I-Site Studio 7.0’. The roughness data obtained
were presented in the form of waviness profiles. Subsequently probabilistic stability analysis
was carried out to determine the shear strength values and factors of safety correlating with
different roughness coefficients.
Conclusions can be drawn from this study that there is a positive correlation between JRC
value, shear strength, factor of safety (FOS) and a negative correlation with the probability of
failure (POF). For any given slope, a high JRC value would result in high shear strength and
consequently stable benches. JRC values for SW wall range from 0-20, for SE it is 0-10 and
for NE it is 0-6. Ideally, a JRC value of above 5 is correlated with higher FOS. The difference
in shear strength and FOS values for failure planes with the same mean JRC values can be
attributed to the difference in dip angles and other discontinuities of these failure planes.
The resultant shear strength, FOS and POF from probabilistic analysis using the mean failure
plane give a general idea of the slope condition. The facets on NE, SE and SW walls are not
having uniform pattern of stability. The sections with high FOS have a very low POF and
vice versa. The SW wall has a range of FOS between 0.54 to 2.77 that corresponds to POF
between 100 to 0 %. The SE wall has a FOS range between 0.39 to 3.81 and corresponding
POF between 100 to 0 %. The NE wall has FOS range between 0.47 to 2.8 and corresponding
POF between 100 to 0 %.
It is highly recommended that the JRC values should be taken along as many bedding planes
as possible in the dip slope in order to account for variability in that slope hence minimizing
chances of failure. Further, a conjugate study comprising the parameters like varying dip
angles, presence of other discontinuities along with the joint density and joint roughness
Coefficient, must be done in order to generate a detailed ‘Hazard Zonation Map’.