Conventional and Advanced Numerical Methods of Rock Slope Stability Analysis, A Comparison Study, Gotvand Dam Right

Abutment (Iran) Case Study
A. Mahboubi, M. Aminpour, A. Noorzad
Dept. of Water Engineering, Power and Water University of Technology, Tehran, Iran
Keywords: rock slope, Conventional Methods, advanced numerical techniques, UDEC, SLOPE/W
ABSTRACT: This paper aimed to review and compare the conventional and advanced numerical techniques
used for stability analysis of rock slopes. Two important methods of probabilistic limit equilibrium analysis and
distinct element method are used to check the stability of northern abutment rock slope of Gotvand dam (iran)
and the advantages and disadvantages of each method is discussed. The results of the performed analyses
shown that the distinct element method can show the instabilities that could not be shown by the limit
equilibrium methods.
1 Introduction
Rock slope stability analyses are routinely performed and directed towards assessing the safe and functional
design of excavated slopes (e.g. open pit mining, road cuts, etc.) and/or the equilibrium conditions of natural
slopes. In general, the primary objectives of rock slope stability analyses are:
- to determine the rock slope stability conditions;
- to investigate potential failure mechanisms;
- to determine the slopes sensitivity/susceptibility to different triggering mechanisms;
- to test and compare different support and stabilization options; and
- to design optimal excavated slopes in terms of safety, reliability and economics
2 Conventional Methods of Rock Slope Analysis
Conventional methods of rock slope analysis can be generally broken down into kinematic and limit
equilibrium techniques. In addition, analytical computer-based methods have been developed to analyze
discrete rock block falls (commonly referred to as rockfall simulators)
2.1 Kinematic analysis
Kinematic methods concentrate on the feasibility of translational failures due to the formation of “daylighting”
wedges or planes. As such, these methods rely on the detailed evaluation of rock mass structure and the
geometry of existing discontinuity sets that may contribute to block instability. This assessment may be carried
out by means of stereonet plots and/or specialized computer codes which focus on planar and wedge
formation. For example, the program DIPS (Rocscience 2001a) allows for the visualization and determination
of the kinematic feasibility of rock slopes using friction cones, daylight and toppling envelopes, in addition to
graphical and statistical analysis of the discontinuity properties. . Discontinuity data and joint set intersections
defined in DIPS can though, be imported into companion limit equilibrium codes (e.g. SWEDGE - Rocscience
2001b) to assess the factor of safety against wedge failure.
2.2 Limit equilibrium analysis
Limit equilibrium techniques are routinely used in the analysis of landslides where translational or rotational
movements occur on distinct failure surfaces. Analyses are undertaken to provide either a factor of safety or,
through back-analysis, a range of shear strength parameters at failure.
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Further extensions and developments in limit equilibrium approaches include:
- increased availability of probabilistic techniques, including use of geostatistics (Pascoe et al., 1998);
- introduction of 3-D methods, some with capabilities for including support (Hungr et al., 1989; Lam et al.,
1993);
- improved searching routines for critical failure surfaces;
- integrated groundwater–stress-limit equilibrium analysis;
- incorporation of unsaturated soil mechanics;
- incorporation of surface hydrology influences (e.g., Wilkinson et al., 2000);
- integration with GIS and risk assessment.
Limit equilibrium analyses of landslides may now be undertaken using 3-D commercial software for both
wedge (e.g., SWEDGE — Rocscience, 2004) and circular/ multiplanar failure mechanisms (CLARA-W —
Hungr, 2002).
These techniques, although useful, neglect internal fracture and deformation which are arguably a prerequisite
for most 3-D failure geometries.
The increasing use of risk assessment in engineering practice, and the need to deal with parameter
uncertainty in slope analysis, has driven the development of probabilistic limit equilibrium tools. Integration of
infinite slope probabilistic analyses with Geographic Information Systems (GIS) is becoming commonplace in
landslide hazard assessment (Zhou et al., 2003; Pack et al., 2001). The quick and interactive means by which
computer-based software operates makes it ideal for incorporating probabilistic algorithms in which variations
in joint geometrical characteristics (e.g., dip, dip direction, etc.) can be assessed for its influence on the factor
of safety. Fuzzy logic routines designed to manage uncertainty in the input parameters can likewise be
incorporated (Faure and Maiolino, 2000).
2.3 Rockfall simulation
More recent developments in rockfall simulators include the use of different shaped rock ‘elements’ (Spang &
Sönser 1995) and extensions into three-dimensions (Leroi et al. 1996). In the latter, models can include the 3-
D topography based on digital elevation models, the geomechanical characteristics of the material involved
(geology of the blocks, lithology and vegetation of the ground), several common physical laws (stressdeformation
curves, hydraulic friction, Coulomb friction) and the real geometry of the blocks.
Programs such as ROCFALL (Rocscience 2001e) analyse the trajectory of falling blocks based on changes in
ve locity as rock blocks roll, slide and bounce on various materials that form the slope.
Other factors solved for include block velocity, bounce height and endpoint distance, which can be analysed
statistically over a repeated number of simulations to aid in a risk assessment.
3 Numerical methods of rock slope analysis
Conventional forms of analysis are limited to simplistic problems in their scope of application, encompassing
simple slope geometries and basic loading conditions, and as such, provide little insight into slope failure
mechanisms. Many rock slope stability problems involve complexities relating togeometry, material anisotropy,
non-linear behaviour, in situ stresses and the presenceof several coupled processes (e.g. pore pressures,
seismic loading, etc.).
Numerical methods of analysis used for rock slope stability investigations may be divided intothree
approaches:
- continuum modelling;
- discontinuum modeling;
- hybrid modelling.
Continuum modelling is best suited for the analysis of slopes that are comprised of massive, intact rock, weak
rocks, and soil-like or heavily jointed rock masses. Discontinuous modeling is appropriate for slopes
controlled by discontinuity behaviour. Hybrid codes involve the coupling of these two techniques (i.e. continuum
and discontinuum) to maximize their key advantages. Table 1 briefly summarizes the advantages and
limitations inherent in these different numerical modeling approaches.
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3.1 Continuum approaches
Continuum approaches used in slope stability analysis include the finite-difference and finite- element
methods. In both these methods the problem domain is divided (discretized) into a set of sub-domains or
elements. A solution procedure may then be based on numerical approximations of the governing equations,
i.e. the differential equations of equilibrium, the strain-displacement relations and the stress strain equations,
as in the case of the finite-difference method (FDM). Alternatively, the procedure may exploit approximations to
the connectivity of elements, and continuity of displacements and stresses between elements, as in the finiteelement
method (FEM). The salient advantages and limitations of the se two methods are discussed by Hoek
et al. (1993) and both have found widespread use in slope stability analysis.
Continuum methods are best suited for the analysis of rock slopes that are comprised of massive intact rock,
weak rocks, or heavily fractured rock masses. For the most part, earlier studies were often limited to elastic
analyses and as such were limited in their application. Most continuum codes, however, now incorporate a
facility for including discrete fractures such as faults and bedding planes. Numerous commercial codes are
available, which often offer a variety of constitutive models including elasticity, elasto-plasticity, strain-softening
and elasto- viscoplasticity (allowing for the modelling of time-dependent behaviour).




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Conventional and Advanced Numerical Methods of Rock Slope Stability Analysis, A Comparison Study, Gotvand Dam Right 2011-04-12T01:36:00-07:00 Rating: 4.5 Diposkan Oleh: Andre S