New techniques for characterising damage in rock slopes: implications for engineered slopes and open pit mines

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New techniques for characterising damage in rock slopes:
implications for engineered slopes and open pit mines
D Donati Simon Fraser University, Canada
D Stead Simon Fraser University, Canada
D Elmo University of British Columbia, Canada
E Onsel Simon Fraser University, Canada
Abstract
The stability of high rock slopes is becoming an increasingly important concern in the fields of mining and civil
engineering. The need for mineral resources due to the exponential world population growth is driving the
excavation of deeper and steeper open pit mines. Today, large open pit mines can reach depths in excess of
1 km. Maintaining and monitoring the stability of the excavation is of paramount importance to ensure the
safety of miners, equipment, and mining operations, as well as the profitability of the mine. Despite safe,
state-of-the-art mining practices being followed, pit slope deformations occur, usually controlled by
geological factors and driven by the progressive accumulation of stress within the pit walls. The deformation
of high engineered rock slopes is inevitably associated with the formation of slope damage features, such as
rock mass dilation and bulging, brittle fracture and rockfalls. The progressive accumulation of slope damage
can reduce the slope rock mass and discontinuity strength causing a decrease in stability, potentially resulting
in slope failure. Blast damage, localised at the pit wall surface, may also promote rockfalls and increase the
risk of slope instability.
In this paper, we present the results of recent slope damage research undertaken in the Engineering Geology
and Resource Geotechnics Group at Simon Fraser University. The focus of this ongoing research program
includes the definition and characterisation of slope damage, modelling, monitoring and visualisation of slope
damage. The factors and mechanisms that can promote and/or induce the accumulation of slope damage
within engineered slopes are discussed. The role of engineering geological factors, including geological
structures, rock mass quality, lithology, intact rock strength, stress magnitude and groundwater, are
addressed and a preliminary rock slope damage interaction matrix approach is presented. Examples of the
characterisation of damage using field mapping and remote sensing are presented. New methods of
quantifying slope damage are also described.
The range of numerical modelling techniques we have used in the investigation of rock slopes is outlined, with
a focus on the explicit simulation of rock slope damage accumulation. The critical inter-relationship between
slope damage and fracture connectivity is discussed with implications for pit slope design. The importance of
continuous monitoring of slope deformation (damage) is highlighted both for the purposes of early warning
systems, and as a means to constrain numerical simulations. Finally, a brief discussion on the potential
applications of innovative, immersive geo-visualisation methods, such as mixed and virtual reality, in the
interpretation of slope damage mechanisms in engineered slopes is provided.
Keywords: slope damage, brittle fracture, fracture connectivity, remote sensing, numerical modelling
1 Introduction
During the past few decades, the stability of high rock slopes has become an increasingly important focus of
geoscientists and engineers in natural slopes, engineered rock cuts and in large open pits. The excavation of
steeper and deeper large open pit mines allows reduction in the waste rock (stripping ratio) and maximises
ore recovery (Obregon & Mitri 2019). In turn, this can result in a decrease in open pit slope. An additional
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collection and interpretation (Onsel et al. 2018). The software EasyMap MR (EMMR, developed in
collaboration with SRK Consulting) has been developed to allow rock mass characterisation to be performed
using MR. EMMR exploits the Microsoft HoloLens and its inbuilt scanner to create in real time a
georeferenced 3D model of the rock outcrop. The software allows discontinuity and geological mapping to
be undertaken directly onsite, automatically storing geological data collected by the user in the form of
points, polylines, polygons, and annotations. The 3D model and the data are saved, allowing them to be
reviewed and shared, enhancing interpretation and quality control (Onsel et al. 2019). This method of
holographic mapping is being extended to allow holographic mapping of slope damage features both in the
field and on remote sensing data in the office. Microsoft HoloLens applications have also been developed to
display 3D numerical modelling of open pit-underground interaction, to measure joint surface roughness, to
visualise 3D models of rock slope remote sensing data, and to visualise 3D virtual models of rock cores for
logging, rock mass classification, and quality control (Onsel et al. 2019). Mysiorek et al. (2019) presented an
innovative application of VR and MR techniques, allowing holographic visualisation of numerical simulation
of rockfall, to be performed using Unity software (www.unity.com), based on 3D data collected in the field
using remote sensing techniques (i.e. TLS and SfM).
6 Summary and conclusion
The progressive deformation of rock slopes results in the formation and accumulation of slope damage. The
type, orientation, size, and spatial distribution of slope damage features is strictly related to the slope failure
mechanism, rock mass quality, location and morphology of the rupture surface. Thus, we suggest that slope
damage assessments should be considered as a critical procedure in slope stability investigations. In
particular, as large open pits become deeper and open pit-underground interaction more common the role
of stress-induced damage must be considered.
This paper summarises the factors and mechanisms controlling the formation and accumulation of slope
damage in natural, and engineered slopes, including open pits. It demonstrates the use of state-of-the-art
remote sensing methods and approaches that have been employed to map, describe, and classify rock slope
damage features. Remote sensing data, when coupled with traditional field data, allows for an enhanced
interpretation of the mechanisms underlying the instability and deformation behaviour of rock slopes. The
data collected can be used as input for numerical modelling analyses, from simple kinematic analyses to more
sophisticated methods, including hybrid and lattice scheme software for the analysis of brittle fracturing of
intact rock. Finally, the use of innovative geo-visualisation techniques, such as MR and VR, provides the
engineer with an immersive experience, allowing for both enhanced data interpretation and analysis.
Acknowledgement
The methods summarised in this paper are built on the research undertaken in the past 15 years by many
former and present members of the Engineering Geology and Resource Geotechnics Research Group at
Simon Fraser University. Their contribution is acknowledged. The authors would like to thank Dr Loren Lorig
and Dr John Read of CSIRO, Australia, for their assistance in the provision of the Slope Model code.
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