An empirical assessment of the shear strength of rock discontinuities of Nchanga open pit mine
Date
2012-07-25
Authors
Mondoka, Fexon
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Abstract
An empirical assessment of the peak shear strength of rock discontinuities occuring in the hangingwall rock formations at Nchanga Open Pit is presented in this dissertation.
A brief history and geology of the open pit including its structural and geometrical setting are outlined. Some recent slope failures have been included to show the trend in the mechanism of failure.All discontinuities occuring in the rock masses have been summarized under two types. Type 1 covers all natural planes of weakness such as joints, minor faults, bedding and cleavage planes. Type 2 includes all artificial fractures such as through-going tension cracks and fractures resulting from blast damage. The description of the rock types follows local nomenclature.The shear strength behaviour of the two types of discontinuities has been defined following the empirical shear strength criterion. The criterion is expressed as an equation based on three index parameters controlling the shear behaviour of a discontinuity or joint, namely:(i ) the joint roughness coefficient (JRC),(ii)the joint wall compressive strength (JCS), and (iii) the residual angle of internal friction ($r).These index parameters were determined following experi¬mental procedures proposed by Barton and Choubey (1977).The results of the field and laboratory measurements conducted for the determination of these index parameters are presented both in tabular and graphical forms. Mean values of these index parameters have been applied into the 'empirical equation' defining the peak shear strength of a discontinuity in terms of a total friction angle (arctan T/Sn). This equation has further been interpreted into a log-tan chart. The chart has a logarithmic scale on the ordinate and a tangent scale on the abscissa. Plots of the governing stress ratios, JCS/Sn and T/6n which are the joint compressive strength and shear strength to effective normal stress ratios, are presented on the charts.
Five rock types were represented in the investigation; the Shale with Grit (SWG), Chingola Dolomite (C/DOL), Dolomitic Schist (DOLSCHIST), Upper Banded Shale (UBS), and the Feldspathic Quartzite (TFQ). For each of the five rock types, a log-tan chart has been plotted to define the peak shear strength criterion for its Type 1 and Type 2 discontinuities. The range of friction angles obtainable from a graphical interpretation of the charts is shown to compare well with values already in design use on the Mine. The added advantage resulting from the findings of this study is that, reliable predictions of the shear strength information pertaining to the five rock formations represented in the investigation can now be made.
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Keywords
Open Pit Mining , Nchanga Mine