Slope stability analysis

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Method of slices Pett bish.gif
Method of slices

Slope stability analysis is a static or dynamic, analytical or empirical method to evaluate the stability of slopes of soil- and rock-fill dams, embankments, excavated slopes, and natural slopes in soil and rock. It 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. [1] [2] Slope stability is the resistance of inclined surface to failure by sliding or collapsing. [3] 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, and designing possible remedial measures, e.g. barriers and stabilization. [1] [2]

Contents

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. [4] [5] 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. [6] 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. [7]

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). [1] 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). [8] [9]

Limit equilibrium analysis

A typical cross-section of a slope used in two-dimensional analyses. Slope 2d plain.svg
A typical cross-section of a slope used in two-dimensional analyses.

Conventional methods of slope stability analysis can be divided into three groups: kinematic analysis, limit equilibrium analysis, and rock fall simulators. [10] Most slope stability analysis computer programs are based on the limit equilibrium concept for a two- or three-dimensional model. [11] [12] 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. [13] In rock slope engineering, methods may be highly significant to simple block failure along distinct discontinuities. [10] 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. [13] The most commonly used variation is Terzaghi's theory of shear strength which states that

where is the shear strength of the interface, is the effective stress ( is the total stress normal to the interface and is the pore water pressure on the interface), is the effective friction angle, and 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. [12] [14] 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. [13] [15]

The location of the interface is typically unknown but can be found using numerical optimization methods. [16] 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. Pett bish.gif
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. [14] [15] 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., . 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

Division of the slope mass in the method of slices. Slope 2d mos plain.svg
Division of the slope mass in the 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. [17]

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 () and shear () 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
{\displaystyle b}
. The slices on the left and right exert normal forces
E
l
,
E
r
{\displaystyle E_{l},E_{r}}
and shear forces
S
l
,
s
r
{\displaystyle S_{l},s_{r}}
, the weight of the slice causes the force
W
{\displaystyle W}
. These forces are balanced by the pore pressure and reactions of the base
N
,
T
{\displaystyle N,T}
. Slope 2d equil mos plain.svg
Force equilibrium for a slice in the method of slices. The block is assumed to have thickness . The slices on the left and right exert normal forces and shear forces , the weight of the slice causes the force . These forces are balanced by the pore pressure and reactions of the base .

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

where represents a linear factor that determines the increase in horizontal force with the depth of the slice. Solving for gives

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

where is the slice index, 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:

Using Terzaghi's strength theory and converting the stresses into moments, we have

where is the pore pressure. The factor of safety is the ratio of the maximum moment from Terzaghi's theory to the estimated moment,

Modified Bishop’s Method of Analysis

The Modified Bishop's method [18] 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

where

where, as before, is the slice index, is the effective cohesion, is the effective internal angle of internal friction, is the width of each slice, is the weight of each slice, and is the water pressure at the base of each slice. An iterative method has to be used to solve for 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 [19] 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. [19]

Sarma method

The Sarma method, [20] 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. [21]

MethodAssumption
Ordinary method of slicesInterslice forces are neglected
Bishop's simplified/modified [18] Resultant interslice forces are horizontal. There are no interslice shear forces.
Janbu's simplified [22] Resultant interslice forces are horizontal. An empirical correction factor is used to account for interslice shear forces.
Janbu's generalized [22] An assumed line of thrust is used to define the location of the interslice normal force.
Spencer [19] The resultant interslice forces have constant slope throughout the sliding mass. The line of thrust is a degree of freedom.
Chugh [23] Same as Spencer's method but with a constant acceleration force on each slice.
Morgenstern-Price [24] 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) [17] Similar to Morgenstern-Price.
Corps of Engineers [25] 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 [26] 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 [20] 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. [21]

MethodForce balance (vertical)Force balance (horizontal)Moment balance
Ordinary MSYesNoYes
Bishop's simplifiedYesNoYes
Janbu's simplifiedYesYesNo
Janbu's generalizedYesYesUsed to compute interslice shear forces
SpencerYesYesYes
ChughYesYesYes
Morgenstern-PriceYesYesYes
Fredlund-KrahnYesYesYes
Corps of EngineersYesYesNo
Lowe and KarafiathYesYesNo
SarmaYesYesYes

Rock slope stability analysis

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

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:

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. [32] [33] Stereographic representation (stereonets) of the planes and lines is used. [34] Stereonets are useful for analyzing discontinuous rock blocks. [35] Program DIPS allows for visualization structural data using stereonets, determination of the kinematic feasibility of rock mass and statistical analysis of the discontinuity properties. [32]

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. [36] Analytical solution method described by Hungr & Evans [37] 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. [38]

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. [39]

Continuum modelling

Figure 3: Finite element mesh Mesh fem.jpg
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. [40] 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. [39]

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: [39]

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. [39]

Discontinuum program UDEC [41] (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. [41] Three-dimensional discontinuum code 3DEC [42] 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). [39]

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. [39]

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. [43] [44] 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 [39]

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; hydro-mechanically coupled finite element and material point methods for simulating the entire process of rainfall-induced landslides. [45] 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.

See also

Related Research Articles

<span class="mw-page-title-main">Geotechnical engineering</span> Scientific study of earth materials in engineering problems

Geotechnical engineering, also known as geotechnics, is the branch of civil engineering and Geological engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics to solve its engineering problems. It also relies on knowledge of geology, hydrology, geophysics, and other related sciences.

Structural analysis is a branch of solid mechanics which uses simplified models for solids like bars, beams and shells for engineering decision making. Its main objective is to determine the effect of loads on the physical structures and their components. In contrast to theory of elasticity, the models used in structure analysis are often differential equations in one spatial variable. Structures subject to this type of analysis include all that must withstand loads, such as buildings, bridges, aircraft and ships. Structural analysis uses ideas from applied mechanics, materials science and applied mathematics to compute a structure's deformations, internal forces, stresses, support reactions, velocity, accelerations, and stability. The results of the analysis are used to verify a structure's fitness for use, often precluding physical tests. Structural analysis is thus a key part of the engineering design of structures.

<span class="mw-page-title-main">Soil mechanics</span> Branch of soil physics and applied mechanics that describes the behavior of soils

Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids and particles but soil may also contain organic solids and other matter. Along with rock mechanics, soil mechanics provides the theoretical basis for analysis in geotechnical engineering, a subdiscipline of civil engineering, and engineering geology, a subdiscipline of geology. Soil mechanics is used to analyze the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils. Example applications are building and bridge foundations, retaining walls, dams, and buried pipeline systems. Principles of soil mechanics are also used in related disciplines such as geophysical engineering, coastal engineering, agricultural engineering, hydrology and soil physics.

<span class="mw-page-title-main">Slope stability</span> Stability of soil or rock slopes

Slope stability refers to the condition of inclined soil or rock slopes to withstand or undergo movement; the opposite condition is called slope instability or slope failure. 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 finite element method (FEM) is a powerful technique originally developed for numerical solution of complex problems in structural mechanics, and it remains the method of choice for complex systems. In the FEM, the structural system is modeled by a set of appropriate finite elements interconnected at discrete points called nodes. Elements may have physical properties such as thickness, coefficient of thermal expansion, density, Young's modulus, shear modulus and Poisson's ratio.

Discontinuous deformation analysis (DDA) is a type of discrete element method (DEM) originally proposed by Shi in 1988. DDA is somewhat similar to the finite element method for solving stress-displacement problems, but accounts for the interaction of independent particles (blocks) along discontinuities in fractured and jointed rock masses. DDA is typically formulated as a work-energy method, and can be derived using the principle of minimum potential energy or by using Hamilton's principle. Once the equations of motion are discretized, a step-wise linear time marching scheme in the Newmark family is used for the solution of the equations of motion. The relation between adjacent blocks is governed by equations of contact interpenetration and accounts for friction. DDA adopts a stepwise approach to solve for the large displacements that accompany discontinuous movements between blocks. The blocks are said to be "simply deformable". Since the method accounts for the inertial forces of the blocks' mass, it can be used to solve the full dynamic problem of block motion.

<span class="mw-page-title-main">Shear band</span>

A shear band is a narrow zone of intense shearing strain, usually of plastic nature, developing during severe deformation of ductile materials. As an example, a soil specimen is shown in Fig. 1, after an axialsymmetric compression test. Initially the sample was cylindrical in shape and, since symmetry was tried to be preserved during the test, the cylindrical shape was maintained for a while during the test and the deformation was homogeneous, but at extreme loading two X-shaped shear bands had formed and the subsequent deformation was strongly localized.

There have been known various classifications of landslides. Broad definitions include forms of mass movement that narrower definitions exclude. For example, the McGraw-Hill Encyclopedia of Science and Technology distinguishes the following types of landslides:

<span class="mw-page-title-main">UTEXAS</span> Computer program

UTEXAS is a slope stability analysis program written by Stephen G. Wright of the University of Texas at Austin. The program is used in the field of civil engineering to analyze levees, earth dams, natural slopes, and anywhere there is concern for mass wasting. UTEXAS finds the factor of safety for the slope and the critical failure surface. Recently the software was used to help determine the reasons behind the failure of I-walls during Hurricane Katrina.

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<span class="mw-page-title-main">Structural engineering theory</span>

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SVSLOPE is a slope stability analysis program developed by SoilVision Systems Ltd.. The software is designed to analyze slopes using both the classic "method of slices" as well as newer stress-based methods. The program is used in the field of civil engineering to analyze levees, earth dams, natural slopes, tailings dams, heap leach piles, waste rock piles, and anywhere there is concern for mass wasting. SVSLOPE finds the factor of safety or the probability of failure for the slope. The software makes use of advanced searching methods to determine the critical failure surface.

<span class="mw-page-title-main">Discontinuity layout optimization</span>

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Sarada Kanta Sarma is a geotechnical engineer, emeritus reader of engineering seismology and senior research investigator at Imperial College London. He has developed a method of seismic slope stability analysis which is named after him, the Sarma method.

The shear strength of a discontinuity in a soil or rock mass may have a strong impact on the mechanical behavior of a soil or rock mass. The shear strength of a discontinuity is often considerably lower than the shear strength of the blocks of intact material in between the discontinuities, and therefore influences, for example, tunnel, foundation, or slope engineering, but also the stability of natural slopes. Many slopes, natural and man-made, fail due to a low shear strength of discontinuities in the soil or rock mass in the slope. The deformation characteristics of a soil or rock mass are also influenced by the shear strength of the discontinuities. For example, the modulus of deformation is reduced, and the deformation becomes plastic rather than elastic. This may cause, for example, larger settlement of foundations, which is also permanent even if the load is only temporary. Furthermore, the shear strength of discontinuities influences the stress distribution in a soil or rock mass.

The sliding criterion (discontinuity) is a tool to estimate easily the shear strength properties of a discontinuity in a rock mass based on visual and tactile characterization of the discontinuity. The shear strength of a discontinuity is important in, for example, tunnel, foundation, or slope engineering, but also stability of natural slopes is often governed by the shear strength along discontinuities.

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The Sarma method is a method used primarily to assess the stability of soil slopes under seismic conditions. Using appropriate assumptions the method can also be employed for static slope stability analysis. It was proposed by Sarada K. Sarma in the early 1970s as an improvement over the other conventional methods of analysis which had adopted numerous simplifying assumptions.

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In geology, numerical modeling is a widely applied technique to tackle complex geological problems by computational simulation of geological scenarios.

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Further reading