Slope stability analyses of rock slopes and a comparison of limit equilibrium and continuum numerical methods.
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Date
2016
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Abstract
Open pit mines are amongst the world’s largest geotechnical structures accounting for a large
proportion of the world’s metals and minerals. The pressure of future supply of these resources
meeting the demands of a growing population leads the mining industry into precarious
environments in attempts to access deeper set resources. Increasing depths associated with surface
mining operations directly relate to slope angles where steeper slopes are economically appealing
due to a reduction in waste rock removal, but at the expense of an increasing risk of slope failure.
Rock slope instability in such environments represents a significant hazard and is the cause of
serious injuries and fatalities as well as major financial losses. It is therefore essential to rigorously
manage this hazard through regular rock slope stability analyses.
There are numerous types of slope stability methods of analysis, all of which aim to safely uphold
natural or anthropogenic structures. The Limit Equilibrium Method is particularly useful in hard
rock masses with distinctive discontinuities. It allows for the prediction of stable or unstable
conditions by calculating the stresses acting on a block and subsequently comparing them to the
shear resistance provided by the discontinuity separating the block from the rock mass. This
method however requires the user to define a potential failure surface in advance thus indicating
its usefulness to a limited range of failure modes. Open pit mines are however dynamic
environments which give rise to more complicated responses by the rock mass. The availability
of computers and the continued advancements made in computational power has encouraged the
development of sophisticated computer codes which are capable of modelling increasingly
complex problems. More complex rock slope movements can be determined by numerical
modelling techniques such as the Finite Element Method and Finite Difference Method. These
methods can analyse the stability of slopes directly through the use of the Shear Strength
Reduction technique where strength properties are reduced until failure takes place.
This study explores the aforementioned techniques applied to three critical profiles (A, B, C)
selected based on areas of known concern in an open pit mine. Structurally controlled (i.e. planar,
wedge, toppling) and non-structurally controlled (i.e. circular) failure mechanisms were assessed
via LEMs and compared with numerical models represented as pseudo-discontinuum media.
Pseudo-discontinuum media were based on a range of joint network models incorporating
kinematically feasible joints which vary in terms of orientation, length, spacing and persistence.
These included the Parallel Deterministic network of infinite length and of finite length, the
Ubiquitous Joint network model and the Veneziano Joint network model. The geotechnical model
was based on data obtained from four different consultants over a period of 15 years which spans conceptual to design levels. Materials were modelled based on the Generalized Hoek-Brown and
Equivalent Mohr-Coulomb failure criteria and results were reported in terms of safety factors,
probability of failure and shear strain.
Rock masses represented as continuum media in non-structurally controlled Limit Equilibrium
Methods and numerical methods determined stable conditions with good agreement in safety
factors between all methods and both strength criteria as well as shear strain accumulation zones
between the numerical models. Structurally controlled Limit Equilibrium Method results revealed
that profile AA’ is the most critical slope with a significant probability of planar and wedge failure
at stack angle level. Safety factors for large scale planar failure of profile BB’, although stable,
remains below the acceptance criteria for the overall slope angle. Profile CC’ produced acceptable
safety factors and was deemed stable. These results correlated with that of the Ubiquitous Joint
network model in the Finite Difference Method and the Parallel Deterministic network of infinite
length in the Finite Element Method. Shear strain accumulation of predicted failure modes is
however better modelled by the former. The introduction of rock bridges along discontinuity
planes in the Parallel Deterministic network of finite and Veneziano Joint network model
significantly contributed to stability, reaching minimum safety factors of 3.77 and depicting
approximately circular failure surfaces.
A rockfall analysis showed the significance of fall body shape on rockfall trajectories along
topographies experiencing crest loss to varying degrees. The three-dimensional analysis revealed
that good slope/cut slope topography performs well in terms of catching falling bodies at upper
benches as opposed to the moderate and poor slope/cut slope topography which permits
movement to the bottom of the slope. The two-dimensional analysis however overestimated
results where a minimum of 80% of falling rocks reached the pit floor. Approximately angular
shaped rocks achieved greatest velocities in the three-dimensional analysis as opposed to the
circular bodies doing so in the two-dimensional analysis.
Description
Master of Science in Environmental Science. University of KwaZulu-Natal, Durban 2016.