Whittall, J & Danielson, J 2021, 'Preliminary identification of persistent or pervasive
discontinuities from borehole data', in PM Dight (ed.), SSIM 2021: Second International Slope Stability in Mining
, Australian Centre for Geomechanics, Perth, pp. 149-158, https://doi.org/10.36487/ACG_repo/2135_07
Open pit slope designs are often controlled by persistent or pervasive discontinuities in a rock mass. Whether a discontinuity will be important for inter-ramp and overall scale assessments is difficult to ascertain, especially for development projects without exposed outcrops. Such assessments often rely on observations from individual boreholes grouped on stereonets without consideration of how discontinuities might link between boreholes.
This paper provides a preliminary methodology to identify persistent or pervasive discontinuity sets from boreholes with televiewer or oriented core data. If at least three boreholes are drilled proximal to each other, a plane can be fit through discontinuities from each borehole. The pole vector from each measured discontinuity can be compared with each other, and to the fitted plane, to connect similarly oriented discontinuities across the boreholes. The angular distance between the mean of the measured pole vectors and the fit plane pole vector can be filtered to remove poorly matched discontinuities, leaving a set of discontinuities that are either persistent across the boreholes or sufficiently pervasive that non-persistent planes link up (e.g. bedding planes).
The methodology can be extended to any georeferenced discontinuity orientation data to connect borehole and mapping measurements. This may be useful to connect mapped discontinuities across benches, identify through-going structures and improve persistence estimates.
The approach is useful for set ranking, interrogation of structural domain boundaries, and preliminary justifications for some degree of rock bridging. Discontinuity orientations that do not intersect at least three boreholes cannot be assessed with this methodology, especially if the boreholes are widely spaced, short, or have large differences in collar elevations.
This paper illustrates the methods and potential applications of this approach using a demonstrated example from three boreholes through a pit wall with photogrammetry data.
Keywords: persistence, set ranking, borehole data, censoring bias
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