Figure 1: Rotational failure of slope on circular slip surface Slope stability analysis is performed to assess the safe design of a human-made or natural (e.g., excavations, etc.) and the equilibrium conditions. Is the resistance of inclined surface to by or collapsing.
The main objectives of slope stability analysis are finding endangered areas, investigation of potential failure mechanisms, determination of the slope sensitivity to different triggering mechanisms, designing of optimal slopes with regard to, and, designing possible remedial measures, e.g. Barriers and. Successful of the slope requires information and site characteristics, e.g. Properties of / mass, slope, conditions, alternation of materials by, or systems, movements and in joints, earthquake activity etc. The presence of water has a detrimental effect on slope stability.
Water pressure acting in the pore spaces, fractures or other discontinuities in the materials that make up the pit slope will reduce the strength of those materials. Choice of correct analysis technique depends on both site conditions and the potential mode of failure, with careful consideration being given to the varying, weaknesses and limitations inherent in each. Before the stability analysis was performed graphically or by using a hand-held calculator. Today have a lot of possibilities to use analysis, ranges from simple limit equilibrium techniques through to computational limit analysis approaches (e.g., ) to complex and sophisticated (/-element codes). The engineer must fully understand limitations of each technique. For example, limit equilibrium is most commonly used and simple solution method, but it can become inadequate if the slope fails by complex mechanisms (e.g.
Internal and, progressive, of weaker soil layers, etc.). In these cases more sophisticated techniques should be utilised.
Also, even for very simple slopes, the results obtained with typical limit equilibrium methods currently in use (Bishop, Spencer, etc.) may differ considerably. In addition, the use of the concept is increasing today. Risk assessment is concerned with both the consequence of slope failure and the of failure (both require an understanding of the failure mechanism). Within the last decade (2003) has been developed to remotely scan a rock slope to monitor the spatial deformation of the face. Small movements of a rough wall can be detected with sub-millimeter accuracy by using interferometry techniques.
Main article: The, proposed by of is a technique used to assess the stability of slopes under seismic conditions. It may also be used for static conditions if the value of the horizontal load is taken as zero.
The method can analyse a wide range of slope failures as it may accommodate a multi-wedge failure mechanism and therefore it is not restricted to planar or circular failure surfaces. It may provide information about the factor of safety or about the critical acceleration required to cause collapse. Comparisons The assumptions made by a number of limit equilibrium methods are listed in the table below. Method Assumption Ordinary method of cells Interslice forces are neglected Bishop's simplified/modified Resultant interslice forces are horizontal. There are no interslice shear forces. Janbu's simplified Resultant interslice forces are horizontal.
An empirical correction factor is used to account for interslice shear forces. Janbu's generalized An assumed is used to define the location of the interslice normal force. Spencer The resultant interslice forces have constant slope throughout the sliding mass.
Chugh Same as Spencer's method but with a constant acceleration force on each slice. Morgenstern-Price The direction of the resultant interslice forces is defined using an arbitrary function. The fractions of the function value needed for force and moment balance is computed. Fredlund-Krahn (GLE) Similar to Morgenstern-Price. Corps of Engineers The resultant interslice force is either parallel to the ground surface or equal to the average slope from the beginning to the end of the slip surface. Lowe and Karafiath The direction of the resultant interslice force is equal to the average of the ground surface and the slope of the base of each slice.
Sarma The shear strength criterion is applied to the shears on the sides and bottom of each slice. The inclinations of the slice interfaces are varied until a critical criterion is met.
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The table below shows the statical equilibrium conditions satisfied by some of the popular limit equilibrium methods. See also: A more rigorous approach to slope stability analysis is. Unlike limit equilibrium analysis which makes ad-hoc though often reasonable assumptions, limit analysis is based on rigorous plasticity theory. This enables, among other things, the computation of upper and lower bounds on the true factor of safety.
Programs based on limit analysis include:. OptumG2 (2014-) General purpose software for geotechnical applications including slope stability. LimitState:GEO (2008-) General purpose geotechnical software application based on for plane strain problems including slope stability. GEO5 Slope Stability (1989-) Program is used to perform slope stability analysis of embankments, earth cuts, anchored retaining structures and MSE walls. Stereographic and kinematic analysis.
See also: Kinematic analysis examines which modes of failure can possibly occur in the rock mass. Analysis requires the detailed evaluation of rock mass structure and the geometry of existing discontinuities contributing to block. Representation of the planes and lines is used. Stereonets are useful for analyzing discontinuous rock blocks. Program DIPS allows for visualization structural data using stereonets, determination of the kinematic feasibility of rock mass and statistical analysis of the discontinuity properties. Rockfall simulators Rock slope stability analysis may design protective measures near or around structures endangered by the falling blocks.
Simulators determine travel paths and trajectories of unstable blocks separated from a rock slope face. Method described by Hungr & Evans assumes rock block as a point with mass and moving on a ballistic trajectory with regard to potential contact with slope surface.
Calculation requires two restitution coefficients that depend on fragment shape, slope surface roughness, momentum and deformational properties and on the chance of certain conditions in a given impact. Program ROCFALL provides a statistical analysis of trajectory of falling blocks. Method rely on changes as a rock blocks roll, slide or bounce on various materials., bounce height and location of rock endpoints are determined and may be analyzed statistically. The program can assist in determining remedial measures by computing and location of impact on a barrier. This can help determine the capacity, size and location of barriers. Numerical methods of analysis Numerical modelling techniques provide an approximate solution to problems which otherwise cannot be solved by conventional methods, e.g. Complex geometry, material, non-linear behaviour, in situ stresses.
Allows for material and failure, modelling of, dynamic loading, assessing effects of parameter variations etc. However, numerical modelling is restricted by some limitations.
For example, input parameters are not usually measured and availability of these data is generally poor. Analysis must be executed by well trained user with good modelling practise. User also should be aware of boundary effects, meshing errors, hardware memory and time restrictions.
Used for slope stability analysis can be divided into three main groups:, discontinuum and hybrid modelling. Continuum modelling. Figure 3: Finite element mesh Modelling of the is suitable for the analysis of soil slopes, massive intact rock or heavily jointed rock masses. This approach includes the and finite element methods that the whole mass to finite number of elements with the help of generated mesh (Fig.
In method (FDM) equilibrium equations (i.e. Strain-displacement and ) are solved. Finite element method (FEM) uses the approximations to the connectivity of elements, continuity of and stresses between elements.
Most of numerical codes allows modelling of discrete, e.g.,. Several constitutive models are usually available, e.g., elasto-plasticity, strain-softening, etc. Discontinuum modelling. See also: and Discontinuum approach is useful for rock slopes controlled by discontinuity behaviour.
Rock mass is considered as an aggregation of distinct, interacting blocks subjected to external loads and assumed to undergo motion with time. This methodology is collectively called the method (DEM).
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Discontinuum modelling allows for sliding between the blocks or particles. The DEM is based on solution of dynamic equation of equilibrium for each block repeatedly until the boundary conditions and laws of contact and are satisfied. Discontinuum modelling belongs to the most commonly applied numerical approach to rock slope analysis and following variations of the DEM exist:. distinct-element method. (DDA).
particle flow codes The approach describes mechanical behaviour of both, the discontinuities and the solid material. This methodology is based on a force-displacement law (specifying the interaction between the deformable rock blocks) and a (determining displacements caused in the blocks by out-of-balance forces).
Are treated as boundary conditions. Deformable blocks are discretized into internal constant-strain elements. Discontinuum program UDEC (Universal distinct element code) is suitable for high jointed rock slopes subjected to static or dynamic loading. Two-dimensional analysis of translational failure mechanism allows for simulating large displacements, modelling deformation or material yielding. Three-dimensional discontinuum code 3DEC contains modelling of multiple intersecting discontinuities and therefore it is suitable for analysis of wedge instabilities or influence of rock support (e.g. Rockbolts, cables). In (DDA) displacements are unknowns and equilibrium equations are then solved analogous to finite element method.
Each unit of finite element type mesh represents an isolated block bounded by discontinuities. Advantage of this methodology is possibility to model large deformations, rigid body movements, coupling or failure states between rock blocks. Discontinuous rock mass can be modelled with the help of methodology in the form of particle flow code, e.g. Program PFC2D/3D. Spherical particles interact through frictional sliding contacts. Simulation of joint bounded blocks may be realized through specified bond strengths.
Law of motion is repeatedly applied to each particle and force-displacement law to each contact. Particle flow methodology enables modelling of granular flow, fracture of intact rock, transitional block movements, dynamic response to blasting or seismicity, deformation between particles caused by shear or tensile forces.
These codes also allow to model subsequent failure processes of rock slope, e.g. Simulation of rock Hybrid/coupled modelling Hybrid codes involve the coupling of various methodologies to maximize their key advantages, e.g. Limit equilibrium analysis combined with finite element groundwater flow and stress analysis adopted in the SVOFFICE or GEO-STUDIO suites of software; coupled particle flow and analyses used in PF3D and FLAC3D. Hybrid techniques allows investigation of piping slope failures and the influence of high groundwater pressures on the failure of weak rock slope. Coupled finite-/ codes, e.g.
ELFEN, provide for the modelling of both intact rock behaviour and the development and behaviour of fractures. Rock mass classification Various systems exist for the design of slopes and to assess the stability of slopes. The systems are based on empirical relations between rock mass parameters and various slope parameters such as height and slope dip.
The method for rock slope engineering and rock mass classification developed by Barton and Bar is one such method. It expresses the quality of the rock mass for slope stability using the Q-slope value, from which long-term stable, reinforcement-free slope angles can be derived. See also. References.