Slope Stabillity Analysis & Its Stabillisation

7/27/2019 Slope Stabillity Analysis & Its Stabillisation

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INTRODUCTIONWhat is Slope?

A slope may be an unsupported or supported, inclined surface of somemass like soil mass. Slopes can be natural or man made. These may be

above ground level as embankments or below ground level as cuttings.Analysis of stability of slopes is of utmost importance as its failure maylead to loss of lives and great economic losses. Failure of a mass locatedbelow the slope is called a slide. It involves downward and outwardmovement of entire mass of soil that participates in failure. Slides may

occur in almost nay conceivable manner slowly or suddenly, with orwithout apparent provocation.


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SLOPE STABILITYIn naturally occurring slopes like along hill slopes and river sides, the forces

of gravity tends to move soil from high levels to low levels and the forces that

resist this action are on account of the shear strength of the soil/rock.Factors affecting slope stability:

(1) Presence of water increases weight and reduces shear strength and hencedecreases stability.

(2) Weights of man made structures constructed on or near slopes tend toincrease the destabilizing forces and slope instability.

Causes of failure of Slopes:

1. Gravitational force.

2. Force due to seepage of water.

3. Erosion of the surface of slope due to flowing water.

4. The sudden lowering of water adjacent to the slope.

5. Forces due to earthquakes


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Assumptions in Analysis

The analysis of stability of soil consists of two parts:

1)The determination of the most severely stressed internal surface and

the magnitude of the shearing stress to which it is subjected.2)The determination of the shearing strength along this surface.

General Consideration and Assumptions in the Analysis :

-The stress system is assumed to be two-dimensional. The stresses indirection which is perpendicular to the section of soil mass are taken aszero.

-It is assumed that the coulomb equation for shear strength is applicableand the strength parameters c and are known.

-Depending upon the method of analysis, some additional assumptions

are made regarding the magnitude and distribution of forces along variousplanes.

-The conditions of plastic failure are assumed to be satisfied along thecritical surface. In other words, the shearing strains at all points of thecritical surface are large enough to mobilize all the available strength.


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Landslide(Soil slope failure)


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1. Fellenius method2. Swedish slip circle method3. Bishops method4. Janbus method

5. Friction circle method6. Taylors stability number method7. Culmanns method

8. Spencers method9. Morgenstern and price method10. Bells method

SOME CLASSICAL METHODS


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METHODS OF ANALYSIS OF STABILITY OF

SLOPE Theoretical Analysis

--Fellenius Method (The Ordinary Method of Slices)This method is applicable to soil slopes with both friction and cohesion.In this method, the forces on the sides of the slice are neglected. Thenormal force on the base of the slice is calculated by summing forces in adirection perpendicular to the bottom of the slice. Once the normal forceis calculated, moments are summed about the center of the circle tocompute the factor of safety.


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FINITE EQUILLIBRIUM METHOD(1)The finite element method (FEM) is more powerful than the limitequilibrium method (LEM) in slope stability analysis since the FEM

satisfies the equilibrium at any point in the soil mass (local domain) andthe surrounding soil (global domain), while the LEM satisfies the globalforce or moment equilibrium for the sliding mass only.

(2)The need for assumptions in the FEM will be only for thedefinition of the failure criteria rather than the derivation complication

and assumptions in the LEM. The derived factor of safety in the FEM isbelieved to be more accurate than the LEM in 2D and 3D analysis as well.

FEA PACKAGES:-PLAXIS,ABAQUS etc.


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PLAXIS Finite Element Program(1)PLAXIS is a finite element software for soil and rockthat has been used by geotechnical engineers andresearchers for more than two decades.

(2) Different soil models are incorporated in PLAXISwith a versatile library of structural elements.

(3) The automated mesh generation tool in the programmakes the creation of soil models easy and practical since

6-node as well as 15-node triangular elements areavailable. The program uses the c-method to calculatethe factors of safety of slopes.


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FOS from PLAXIS 2D Dry slope


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The c-Method(1)The c-method is based on the reduction of the shearstrength (c) and the tangent of the friction angle ( tan ) ofthe soil.

(2) The parameters are reduced in steps until the soil massfails. PLAXIS uses a factor to relate the reduction in theparameters during the calculation at any stage with theinput parameters according to the following

Equation:


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SLOPE STABILIZATION

INTRODUCTIONSlope stbilization methods generally reduce driving forces,increase resisting forces ,or both.Driving forces can bereduced by excavation of material from the appropriate partof the unstable ground and drainage of water to reduce thehydrostatic pressures acting on the unstable zone .Resistingforces can be increased by:

1)Drainage that increases the shear strength of the ground.2)Elimination of weak strata or other potential failure zones.3)Building of retaining structures or other supports .4)Provision of in situ reinforcement of the ground

5)Chemical treatment to increase shear strength of theground.


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VARIOUS METHODS OF SLOPE STABILIZATIONARE DISCUSSED BELOW:

1)Unloading:By Excavation and Lightweight Fill.

2)Buttressing: It is a technique used to offset or counter the drivingforces of a slope by an externally applied force system that increases theresisting force.It may consist of Soil and rock fill and Counterberms.

Rock Buttress used to control unstable slope.


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3)Drainage:Off all stabilization techniques considered ,proper water drainage is mostimportant.Drainage reduces destabilizing hydrostatic & seepage forces on a slope as well as

the risk of erosion and piping .Various drainage techniques are Surface Drainage,subsurfacedrainage.

4)Reinforcement:Soil Nailing,Stone Columns.

Soil Nailing For stability


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5) RetainingWalls:Retaining walls provide lateral support to vertical slopes of soil. Theyretain soil which would otherwise collapse into a more natural shape. The retained soil issometimes referred to as backfill.The types of retaining wall are Cantilever retaining walls

,Driven Piles,Drilled Shaft Walls ,Tieback Walls.


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Use of driven piles to stbilizeslopes.

Tieback Reinforcement


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Cantilever Type Retaining Wall


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CONCLUSIONIn practice, different slope stability methods produce different slipsurfaces and hence different minimum factors of safety Therefore, It isimportant to compare the analysis results with field measurements ratherthan comparing the factors of safety only. The most critical issue inevaluating different analysis methods is the ability of that method to

capture the most critical slip surface.For stabilization we generally go for reinforced slopes or retaining walls.These slopes have greater factor of safety than corresponding non-reinforced or unsupported slopes. Although, they decrease the amount ofearth work involved the cost is significantly increased due to the addition

of these structures.


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Slope Stabillity Analysis & Its Stabillisation

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