Abstract:

The authors present a case study from the geotechnical investigation for the design of an open pit steelmaking coal mine. The interbedded sedimentary sequences at the project site include several major faults that have created high structural complexity that has been addressed through continuous drilling programs over the years. Progressive rock lab testing data have been compiled for a 5-year period (2018-2022). This database provided an opportunity to quantify the reduction of knowledge uncertainty and expected range in intact rock and discontinuity shear strength properties with the use of statistical analyses tools.

To unveil the true strength behavior, raw testing data was grouped by geological formation, lithology (e.g., sandstone, siltstone, mudstone) and discontinuity types (e.g., bedding, joints). In addition, point load (PLT), uniaxial compressive strength (UCS) and triaxial (TXT) tests results were filtered by valid-invalid failure modes. Linear Mohr-Coulomb and non-linear Hoek-Brown failure envelopes were then fitted to processed data and model parameters estimated and compared for each yearly campaign.

Variability in intact rock and discontinuity shear strength properties were assessed statistically by calculating confidence intervals using subsequent additional data collected over the years and spatially, by deploying rock sample locations in 3D space. Examples include the use of a reliability index as a metric that links the status of the current database to the target level of data confidence suggested by Read & Stacey (2009) for different project levels.

This technical presentation aims to provide guidance on the application statistical methods to quantify data uncertainty for intact rock and discontinuity shear strength properties in interbedded rock masses and gain insights of the true data variability due to anisotropy and heterogeneity effects which are inherent to sedimentary rock layers. It will also address the main challenges associated with laboratory data processing and interpretation by providing practical examples from the case study project.

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ANDREW BIDWELL