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Milds # Slope stability analysis

Slope stability analysis is a static or dynamic, analytical or empirical method to evaluate the stability of earth and rock-fill dams, embankments, excavated slopes, and natural slopes in soil and rock. Slope stability refers to the condition of inclined soil or rock slopes to withstand or undergo movement. The stability condition of slopes is a subject of study and research in soil mechanics, geotechnical engineering and engineering geology. Analyses are generally aimed at understanding the causes of an occurred slope failure, or the factors that can potentially trigger a slope movement, resulting in a landslide, as well as at preventing the initiation of such movement, slowing it down or arresting it through mitigation countermeasures.

The stability of a slope is essentially controlled by the ratio between the available shear strength and the acting shear stress, which can be expressed in terms of a safety factor if these quantities are integrated over a potential (or actual) sliding surface. A slope can be globally stable if the safety factor, computed along any potential sliding surface running from the top of the slope to its toe, is always larger than 1. The smallest value of the safety factor will be taken as representing the global stability condition of the slope. Similarly, a slope can be locally stable if a safety factor larger than 1 is computed along any potential sliding surface running through a limited portion of the slope (for instance only within its toe). Values of the global or local safety factors close to 1 (typically comprised between 1 and 1.3, depending on regulations) indicate marginally stable slopes that require attention, monitoring and/or an engineering intervention (slope stabilization) to increase the safety factor and reduce the probability of a slope movement.

A previously stable slope can be affected by a number of predisposing factors or processes that make the safety factor decrease - either by increasing the shear stress or by decreasing the shear strength - and can ultimately result in slope failure. Factors that can trigger slope failure include hydrologic events (such as intense or prolonged rainfall, rapid snowmelt, progressive soil saturation, increase of water pressure within the slope), earthquakes (including aftershocks), internal erosion (piping), surface or toe erosion, artificial slope loading (for instance due to the construction of a building), slope cutting (for instance to make space for roadways, railways or buildings), or slope flooding (for instance by filling an artificial lake after damming a river).

## Examples

Figure 1: Simple slope slip section

As seen in Figure 1, earthen slopes can develop a cut-spherical weakness area. The probability of this happening can be calculated in advance using a simple 2-D circular analysis package. A primary difficulty with analysis is locating the most-probable slip plane for any given situation. Many landslides have only been analyzed after the fact. More recently slope stability radar technology has been employed, particularly in the mining industry, to gather real time data and assist in determining the likelihood of slope failure.

Figure 2: Real life landslide on a slope

Real life failures in naturally deposited mixed soils are not necessarily circular, but prior to computers, it was far easier to analyze such a simplified geometry. Nevertheless, failures in 'pure' clay can be quite close to circular. Such slips often occur after a period of heavy rain, when the pore water pressure at the slip surface increases, reducing the effective normal stress and thus diminishing the restraining friction along the slip line. This is combined with increased soil weight due to the added groundwater. A 'shrinkage' crack (formed during prior dry weather) at the top of the slip may also fill with rain water, pushing the slip forward. At the other extreme, slab-shaped slips on hillsides can remove a layer of soil from the top of the underlying bedrock. Again, this is usually initiated by heavy rain, sometimes combined with increased loading from new buildings or removal of support at the toe (resulting from road widening or other construction work). Stability can thus be significantly improved by installing drainage paths to reduce the destabilizing forces. Once the slip has occurred, however, a weakness along the slip circle remains, which may then recur at the next monsoon.

Slope stability issues can be seen with almost any walk down a ravine in an urban setting. An example is shown in Figure 3, where a river is eroding the toe of a slope, and there is a swimming pool near the top of the slope. If the toe is eroded too far, or the swimming pool begins to leak, the forces driving a slope failure will exceed those resisting failure, and a landslide will develop, possibly quite suddenly.

## Measuring the Angle of Repose

The angle of repose is defined as the steepest angle of granular unconfined material measured from the horizontal plane on which the granular material can be heaped on without collapsing, ranging between 0-90°. For granular material, the angle of repose is the main factor that influences the slope's stability under different conditions in relation to the cohesiveness/friction of the material, the grain size, and the particle shape.

### Theoretical Measurement

Figure 3: This free body diagram demonstrates the relationship between angle of repose and material on the slope.

A simple free body diagram can be used to understand the relationship between the angle of repose and the stability of the material on the slope. For the heaped material to collapse, the frictional forces must be equivalent to the horizontal component of the gravitational force $mg\sin \theta$ , where $m$ is the mass of the material, $g$ is the gravitational acceleration and $\theta$ is the slope angle:

$mg\sin \theta =f$ The frictional force $f$ is equivalent to the multiplication product of the coefficient of static friction $\mu$ and the Normal Force $N$ or $mgcos\theta$ :

$mg\sin \theta =N\mu$ $mg\sin \theta =\mu mg\cos \theta$ $\left({\frac {\sin \theta }{cos\theta }}\right)=\mu$ $\theta _{R}=\arctan(\mu )$ Where $\theta _{R}$ is the angle of repose, or the angle at which the slope fails under regular conditions, and $\mu$ is the coefficient of static friction of the material on the slope.

### Experimental Measurement

#### Tilting Box Method

This method is particularly well suited for finer grained material < 10mm in diameter with relatively no cohesion. The material is placed in a base of a box, which is gradually tilted at a rate of 18°/min. The angle of repose is then measured to be the angle at which the material begins to slide.

#### Fixed Funnel Method

In this method, the material is poured down a funnel from a certain height into a horizontal base. The material is then allowed to heap until the pile reaches a predetermined height and width. The angle of repose is then measured by observing the height and radius of the cone and applying the arctangent rule.

## Angle of Repose and Slope Stability

The angle of repose and the stability of a slope are impacted by climatic and non-climatic factors.

### Water Content

Water content is an important parameter that could change the angle of repose. reportedly, a higher water content can stabilize a slope and increase the angle of repose. However, water saturation can result in a decrease in the slope's stability since it acts as a lubricant and creates a detachment where mass wasting can occur.

Water content is dependent on soil properties such as grain size, which can impact infiltration rate, runoff, and water retention. Generally, finer-grained soils rich in clay and silt retain more water than coarser sandy soils. This effect is mainly due to capillary action, where the adhesive forces between the fluid, particle, and the cohesive forces of the fluid itself counteract gravitational pull. Therefore, smaller grain size results in a smaller surface area on which gravitational forces can act. Smaller surface area also leads to more capillary action, more water retention, more infiltration, and less runoff.

### Vegetation

The presence of vegetation does not directly impact the angle of repose, but it acts as a stabilizing factor in a hillslope, where the tree roots anchor into deeper soil layers and form a fiber‐reinforced soil composite with a higher shear resistance (mechanical cohesion).

### Roundness of Grains

The shape of the grain can have an impact on the angle of repose and the stability of the slope. The more rounded the grain is, the lower the angle of repose. A decrease in roundness, or an increase in angularity, results in interlocking via particle contact. This linear relationship between the angle of repose and the roundness of grain can also be used as a predictor of the angle of repose if the roundness of the grain is measured.

## Applications of Angle of Repose in Science and Engineering

The angle of repose is related to the shear strength of geologic materials, which is relevant in construction and engineering contexts. For granular materials, the size and shape of grains can impact angle of repose significantly. As the roundness of materials increases, the angle of repose decreases since there is less friction between the soil grains.

When the angle of repose is exceeded, mass wasting and rockfall can occur. It is important for many civil and geotechnical engineers to know the angle of repose to avoid structural and natural disasters. As a result, the application of retaining walls can help to retain soil so that the angle of repose is not exceeded.

## Slope stabilization

Since the stability of the slope can be impacted by external events such as precipitation, an important concern in civil/geotechnical engineering is the stabilization of slopes.

### Application of Vegetation

The application of vegetation to increase the slope stability against erosion and landslide is a form of bioengineering that is widely used in areas where the landslide depth is shallow. Vegetation increases the stability of the slope mechanically, by reinforcing the soils through plant roots, which stabilize the upper part of the soil. Vegetation also stabilizes the slope via hydrologic processes, by the reduction of soil moisture content through the interception of precipitation and transpiration. This results in a drier soil that is less susceptible to mass wasting.

Stability of slopes can also be improved by:

• Flattening of slopes results in reduction in weight which makes the slope more stable
• Soil stabilization
• Providing lateral supports by piles or retaining walls
• Grouting or cement injections into special places
• Consolidation by surcharging or electro osmosis increases the stability of slope
Figure 1: Rotational failure of slope on circular slip surface

## Analysis methods

Figure 3: Slope with eroding river and swimming pool
Figure 4: Method of slices

If the forces available to resist movement are greater than the forces driving movement, the slope is considered stable. A factor of safety is calculated by dividing the forces resisting movement by the forces driving movement. In earthquake-prone areas, the analysis is typically run for static conditions and pseudo-static conditions, where the seismic forces from an earthquake are assumed to add static loads to the analysis.

Slope stability analysis is performed to assess the safe design of a human-made or natural slopes (e.g. embankments, road cuts, open-pit mining, excavations, landfills etc.) and the equilibrium conditions. Slope stability is the resistance of inclined surface to failure by sliding 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 safety, reliability and economics, designing possible remedial measures, e.g. barriers and stabilization.

Successful design of the slope requires geological information and site characteristics, e.g. properties of soil/rock mass, slope geometry, groundwater conditions, alternation of materials by faulting, joint or discontinuity systems, movements and tension 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 strengths, weaknesses and limitations inherent in each methodology.

Before the computer age stability analysis was performed graphically or by using a hand-held calculator. Today engineers have a lot of possibilities to use analysis software, ranges from simple limit equilibrium techniques through to computational limit analysis approaches (e.g. Finite element limit analysis, Discontinuity layout optimization) to complex and sophisticated numerical solutions (finite-/distinct-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 deformation and brittle fracture, progressive creep, liquefaction of weaker soil layers, etc.). In these cases more sophisticated numerical modelling 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 risk assessment concept is increasing today. Risk assessment is concerned with both the consequence of slope failure and the probability of failure (both require an understanding of the failure mechanism).

Within the last decade (2003) Slope Stability Radar 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.

## Limit equilibrium analysis

Conventional methods of slope stability analysis can be divided into three groups: kinematic analysis, limit equilibrium analysis, and rock fall simulators. Most slope stability analysis computer programs are based on the limit equilibrium concept for a two- or three-dimensional model. Two-dimensional sections are analyzed assuming plane strain conditions. Stability analyses of two-dimensional slope geometries using simple analytical approaches can provide important insights into the initial design and risk assessment of slopes.

Limit equilibrium methods investigate the equilibrium of a soil mass tending to slide down under the influence of gravity. Translational or rotational movement is considered on an assumed or known potential slip surface below the soil or rock mass. In rock slope engineering, methods may be highly significant to simple block failure along distinct discontinuities. All these methods are based on the comparison of forces, moments, or stresses resisting movement of the mass with those that can cause unstable motion (disturbing forces). The output of the analysis is a factor of safety, defined as the ratio of the shear strength (or, alternatively, an equivalent measure of shear resistance or capacity) to the shear stress (or other equivalent measure) required for equilibrium. If the value of factor of safety is less than 1.0, the slope is unstable.

All limit equilibrium methods assume that the shear strengths of the materials along the potential failure surface are governed by linear (Mohr-Coulomb) or non-linear relationships between shear strength and the normal stress on the failure surface. The most commonly used variation is Terzaghi's theory of shear strength which states that

$\tau =\sigma '\tan \phi '+c'$ where $\tau$ is the shear strength of the interface, $\sigma '=\sigma -u$ is the effective stress ($\sigma$ is the total stress normal to the interface and $u$ is the pore water pressure on the interface), $\phi '$ is the effective friction angle, and $c'$ is the effective cohesion.

The methods of slices is the most popular limit equilibrium technique. In this approach, the soil mass is discretized into vertical slices. Several versions of the method are in use. These variations can produce different results (factor of safety) because of different assumptions and inter-slice boundary conditions.

The location of the interface is typically unknown but can be found using numerical optimization methods. For example, functional slope design considers the critical slip surface to be the location where that has the lowest value of factor of safety from a range of possible surfaces. A wide variety of slope stability software use the limit equilibrium concept with automatic critical slip surface determination.

Typical slope stability software can analyze the stability of generally layered soil slopes, embankments, earth cuts, and anchored sheeting structures. Earthquake effects, external loading, groundwater conditions, stabilization forces (i.e., anchors, geo-reinforcements etc.) can also be included.

### Analytical techniques: Method of slices

Schematic of the method of slices showing rotation center.

Many slope stability analysis tools use various versions of the methods of slices such as Bishop simplified, Ordinary method of slices (Swedish circle method/Petterson/Fellenius), Spencer, Sarma etc. Sarma and Spencer are called rigorous methods because they satisfy all three conditions of equilibrium: force equilibrium in horizontal and vertical direction and moment equilibrium condition. Rigorous methods can provide more accurate results than non-rigorous methods. Bishop simplified or Fellenius are non-rigorous methods satisfying only some of the equilibrium conditions and making some simplifying assumptions. Some of these approaches are discussed below.

#### Swedish Slip Circle Method of Analysis

The Swedish Slip Circle method assumes that the friction angle of the soil or rock is equal to zero, i.e., $\tau =c'$ . In other words, when friction angle is considered to be zero, the effective stress term goes to zero, thus equating the shear strength to the cohesion parameter of the given soil. The Swedish slip circle method assumes a circular failure interface, and analyzes stress and strength parameters using circular geometry and statics. The moment caused by the internal driving forces of a slope is compared to the moment caused by forces resisting slope failure. If resisting forces are greater than driving forces, the slope is assumed stable.

#### Ordinary Method of Slices

In the method of slices, also called OMS or the Fellenius method, the sliding mass above the failure surface is divided into a number of slices. The forces acting on each slice are obtained by considering the mechanical (force and moment) equilibrium for the slices. Each slice is considered on its own and interactions between slices are neglected because the resultant forces are parallel to the base of each slice. However, Newton's third law is not satisfied by this method because, in general, the resultants on the left and right of a slice do not have the same magnitude and are not collinear.

This allows for a simple static equilibrium calculation, considering only soil weight, along with shear and normal stresses along the failure plane. Both the friction angle and cohesion can be considered for each slice. In the general case of the method of slices, the forces acting on a slice are shown in the figure below. The normal ($E_{r},E_{l}$ ) and shear ($S_{r},S_{l}$ ) forces between adjacent slices constrain each slice and make the problem statically indeterminate when they are included in the computation. Force equilibrium for a slice in the method of slices. The block is assumed to have thickness $b$ . The slices on the left and right exert normal forces $E_{l},E_{r}$ and shear forces $S_{l},s_{r}$ , the weight of the slice causes the force $W$ . These forces are balanced by the pore pressure and reactions of the base $N,T$ .

For the ordinary method of slices, the resultant vertical and horizontal forces are

{\begin{aligned}\sum F_{v}=0&=W-N\cos \alpha -T\sin \alpha \\\sum F_{h}=0&=kW+N\sin \alpha -T\cos \alpha \end{aligned}} where $k$ represents a linear factor that determines the increase in horizontal force with the depth of the slice. Solving for $N$ gives

$N=W\cos \alpha -kW\sin \alpha \,.$ Next, the method assumes that each slice can rotate about a center of rotation and that moment balance about this point is also needed for equilibrium. A balance of moments for all the slices taken together gives

$\sum M=0=\sum _{j}(W_{j}x_{j}-T_{j}R_{j}-N_{j}f_{j}-kW_{j}e_{j})$ where $j$ is the slice index, $x_{j},R_{j},f_{j},e_{j}$ are the moment arms, and loads on the surface have been ignored. The moment equation can be used to solve for the shear forces at the interface after substituting the expression for the normal force:

$\sum _{j}T_{j}R_{j}=\sum _{j}[W_{j}x_{j}-(W_{j}\cos \alpha _{j}-kW_{j}\sin \alpha _{j})f_{j}-kW_{j}e_{j}]$ Using Terzaghi's strength theory and converting the stresses into moments, we have

$\sum _{j}\tau l_{j}R_{j}=l_{j}R_{j}\sigma _{j}'\tan \phi '+l_{j}R_{j}c'=R_{j}(N_{j}-u_{j}l_{j})\tan \phi '+l_{j}R_{j}c'$ where $u_{j}$ is the pore pressure. The factor of safety is the ratio of the maximum moment from Terzaghi's theory to the estimated moment,

${\text{Factor of safety}}={\frac {\sum _{j}\tau l_{j}R_{j}}{\sum _{j}T_{j}R_{j}}}\,.$ #### Modified Bishop’s Method of Analysis

The Modified Bishop's method is slightly different from the ordinary method of slices in that normal interaction forces between adjacent slices are assumed to be collinear and the resultant interslice shear force is zero. The approach was proposed by Alan W. Bishop of Imperial College. The constraint introduced by the normal forces between slices makes the problem statically indeterminate. As a result, iterative methods have to be used to solve for the factor of safety. The method has been shown to produce factor of safety values within a few percent of the "correct" values.

The factor of safety for moment equilibrium in Bishop's method can be expressed as

$F={\cfrac {\sum _{j}{\cfrac {\left[c'l_{j}+(W_{j}-u_{j}l_{j})\tan \phi '\right]}{\psi _{j}}}}{\sum _{j}W_{j}\sin \alpha _{j}}}$ where

$\psi _{j}=\cos \alpha _{j}+{\frac {\sin \alpha _{j}\tan \phi '}{F}}$ where, as before, $j$ is the slice index, $c'$ is the effective cohesion, $\phi '$ is the effective internal angle of internal friction, $l$ is the width of each slice, $W$ is the weight of each slice, and $u$ is the water pressure at the base of each slice. An iterative method has to be used to solve for $F$ because the factor of safety appears both on the left and right hand sides of the equation.

#### Lorimer's method

Lorimer's Method is a technique for evaluating slope stability in cohesive soils. It differs from Bishop's Method in that it uses a clothoid slip surface in place of a circle. This mode of failure was determined experimentally to account for effects of particle cementation. The method was developed in the 1930s by Gerhardt Lorimer (Dec 20, 1894-Oct 19, 1961), a student of geotechnical pioneer Karl von Terzaghi.

#### Spencer’s Method

Spencer's Method of analysis requires a computer program capable of cyclic algorithms, but makes slope stability analysis easier. Spencer's algorithm satisfies all equilibria (horizontal, vertical and driving moment) on each slice. The method allows for unconstrained slip plains and can therefore determine the factor of safety along any slip surface. The rigid equilibrium and unconstrained slip surface result in more precise safety factors than, for example, Bishop's Method or the Ordinary Method of Slices.

#### Sarma method

The Sarma method, proposed by Sarada K. Sarma of Imperial College is a Limit equilibrium 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 slices 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 line of thrust is used to define the location of the interslice normal force.
Spencer  The resultant interslice forces have constant slope throughout the sliding mass. The line of thrust is a degree of freedom.
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.

The table below shows the statical equilibrium conditions satisfied by some of the popular limit equilibrium methods.

Method Force balance (vertical) Force balance (horizontal) Moment balance
Ordinary MS Yes No Yes
Bishop's simplified Yes No Yes
Janbu's simplified Yes Yes No
Janbu's generalized Yes Yes Used to compute interslice shear forces
Spencer Yes Yes Yes
Chugh Yes Yes Yes
Morgenstern-Price Yes Yes Yes
Fredlund-Krahn Yes Yes Yes
Corps of Engineers Yes Yes No
Lowe and Karafiath Yes Yes No
Sarma Yes Yes Yes

### Rock slope stability analysis

Rock slope stability analysis based on limit equilibrium techniques may consider following modes of failures:

• Planar failure -> case of rock mass sliding on a single surface (special case of general wedge type of failure); two-dimensional analysis may be used according to the concept of a block resisting on an inclined plane at limit equilibrium
• Polygonal failure -> sliding of a nature rock usually takes place on polygonally-shaped surfaces; calculation is based on a certain assumptions (e.g. sliding on a polygonal surface which is composed from N parts is kinematically possible only in case of development at least (N - 1) internal shear surfaces; rock mass is divided into blocks by internal shear surfaces; blocks are considered to be rigid; no tensile strength is permitted etc.)
• Wedge failure -> three-dimensional analysis enables modelling of the wedge sliding on two planes in a direction along the line of intersection
• Toppling failure -> long thin rock columns formed by the steeply dipping discontinuities may rotate about a pivot point located at the lowest corner of the block; the sum of the moments causing toppling of a block (i.e. horizontal weight component of the block and the sum of the driving forces from adjacent blocks behind the block under consideration) is compared to the sum of the moments resisting toppling (i.e. vertical weight component of the block and the sum of the resisting forces from adjacent blocks in front of the block under consideration); toppling occur if driving moments exceed resisting moments

## Limit analysis

A more rigorous approach to slope stability analysis is limit analysis. 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 (also includes elastoplasticity, seepage, consolidation, staged construction, tunneling, and other relevant geotechnical analysis types).
• LimitState:GEO (2008-) General purpose geotechnical software application based on Discontinuity layout optimization for plane strain problems including slope stability.

## Stereographic and kinematic analysis

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 instability. Stereographic representation (stereonets) 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. Rockfall simulators determine travel paths and trajectories of unstable blocks separated from a rock slope face. Analytical solution method described by Hungr & Evans assumes rock block as a point with mass and velocity 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.

## 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 anisotropy, non-linear behavior, in situ stresses. Numerical analysis allows for material deformation and failure, modelling of pore pressures, creep deformation, 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. User also should be aware of boundary effects, meshing errors, hardware memory and time restrictions. Numerical methods used for slope stability analysis can be divided into three main groups: continuum, discontinuum and hybrid modelling.

### Continuum modelling

Figure 3: Finite element mesh

Modelling of the continuum is suitable for the analysis of soil slopes, massive intact rock or heavily jointed rock masses. This approach includes the finite-difference and finite element methods that discretize the whole mass to finite number of elements with the help of generated mesh (Fig. 3). In finite-difference method (FDM) differential equilibrium equations (i.e. strain-displacement and stress-strain relations) are solved. finite element method (FEM) uses the approximations to the connectivity of elements, continuity of displacements and stresses between elements. Most of numerical codes allows modelling of discrete fractures, e.g. bedding planes, faults. Several constitutive models are usually available, e.g. elasticity, elasto-plasticity, strain-softening, elasto-viscoplasticity etc.

### Discontinuum modelling

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 discrete-element method (DEM). 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 motion are satisfied. Discontinuum modelling belongs to the most commonly applied numerical approach to rock slope analysis and following variations of the DEM exist:

The distinct-element 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 law of motion (determining displacements caused in the blocks by out-of-balance forces). Joints 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 discontinuous deformation analysis (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 distinct-element 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 ; coupled particle flow and finite-difference analyses. Hybrid techniques allows investigation of piping slope failures and the influence of high groundwater pressures on the failure of weak rock slope. Coupled finite-/distinct-element codes provide for the modelling of both intact rock behavior and the development and behavior of fractures.

## Rock mass classification

Various rock mass classification 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 Q-slope method for rock slope engineering and rock mass classification developed by Barton and Bar expresses the quality of the rock mass for assessing slope stability using the Q-slope value, from which long-term stable, reinforcement-free slope angles can be derived.

## Probability classification

The slope stability probability classification (SSPC) system is a rock mass classification system for slope engineering and slope stability assessment. The system is a three-step classification: ‘exposure’, ‘reference’, and ‘slope’ rock mass classification with conversion factors between the three steps depending on existing and future weathering and damage due to method of excavation. The stability of a slope is expressed as probability for different failure mechanisms.

A rock mass is classified following a standardized set of criteria in one or more exposures (‘exposure’ classification). These values are converted per exposure to a ‘reference’ rock mass by compensating for the degree of weathering in the exposure and the method of excavation that was used to make the exposure, i.e. the ‘reference’ rock mass values are not influenced by local influences such as weathering and method of excavation. A new slope can then be designed in the ‘reference’ rock mass with compensation for the damage due to the method of excavation to be used for making the new slope and compensation for deterioration of the rock mass due to future weathering (the ‘slope’ rock mass). If the stability of an already existing slope is assessed the ‘exposure’ and ‘slope’ rock mass values are the same.

The failure mechanisms are divided in orientation dependent and orientation independent. Orientation dependent failure mechanisms depend on the orientation of the slope with respect to the orientation of the discontinuities in the rock mass, i.e. sliding (plane and wedge sliding) and toppling failure. Orientation independent relates to the possibility that a slope fails independently from its orientation, e.g. circular failure completely through newly formed discontinuities in intact rock blocks, or failing partially following existing discontinuities and partially new discontinuities.

In addition the shear strength along a discontinuity ('sliding criterion') and 'rock mass cohesion' and 'rock mass friction' can be determined. The system has been used directly or modified in various geology and climate environments throughout the world. The system has been modified for slope stability assessment in open pit coal mining.