Prof. BVS Viswanadham, Department of Civil Engineering, IIT Bombay

Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay

23


Module 5:

Lecture -5 on Stability of Slopes


Slope stabilization methods


Slope stabilization methods generally reduce driving forces, increase resisting forces, or both.Driving forces can be reduced by excavation ofmaterial from the approximate part of the unstableground and drainage of water to reduce hydrostaticpressure acting on the unstable zone.

Resisting forces can be increased by:

(1) Drainage that increases the shear strength of the ground(2) Elimination of weak strata or the potential failure zone(3) Building of retaining structures or other supports(4) Chemical treatment to increase shear strength of the

ground.


Unloading

Unloading is a type of slope stabilization technique to reduce the driving forces within a slide mass.

Excavation is a common method for increasing stability of a slope by reducing the driving forces that contribute to movements: This can include:

(1) Removing weight from the upper part of the slope(2) Removing all unstable or potentially unstable materials(3) Flattening slopes(4) Benching


Flattening slopes

1 2

L1 L2

Flattening theslope not onlyreduces drivingforces, but tendsto force thefailure surfacedeeper.


Rock-fill buttresses A simple method to increase slope stability is to

increase the weight of the material at the toe,which creates a counterforce that resists failure.

Riprap instead of soil ispreferable, however,because it has a greaterfrictional resistance toshear forces and is alsofree draining…


Major components of an EPS-block geofoam slope system

EPS-Block Geofoam in Slope Stabilization

(After Arellano et al. 2009)

Failure Modes External instability Internal instability Pavement system failure


Static and Seismic Slope Stability (Existing Soil Slope Material Only)

Static and Seismic Slope Stability (Both Fill Mass and Existing Soil Slope Material)



External Seismic Stability Failure Involving Horizontal Sliding of the Entire Embankment

External Seismic Stability Failure Involving Overturning of an Entire Vertical Embankment about the Toe of the Embankment



Load Bearing Failure of the Blocks

Internal Seismic Stability FailureHorizontal Sliding



Drainage Techniques Adequate drainage of water is the most important element

of a slope stabilization scheme, for both existing andpotential slopes prone to failure.

Drainage is effective because it increases the stability of thesoil and reduces the weight of the sliding mass. Drainage canbe either surface or subsurface.

Surface drainage can be through either surface ditches orshallow subsurface drains. Surface drainage is especiallyimportant at the head of the slide, where a system of cutoffditches that cross the headwall of the slide, and lateral drainsto lead runoff around the edge of the slide are effective.


Drainage Techniques The FOS against on any potential slip surface that passes

below the phreatic surface can be improved by subsurfacedrainage.

Methods that can be used to accomplish subsurface drainageare:

a) Drainage blanketsb) Trenchesc) Cut-off drainsd) Horizontal drains

e) Relief wells to lower the water pressures in layers that aredeep down in the subsoil (can’t be reachedby open excavation)

a) Drainage tunnels or galleries when there is arequirement of substantial number of horizontal drains


Slope stabilization using retaining walls

Soil-tyre retaining wallGabion retaining wall




After Grundbau und Bodenmechanik (2001)



A row of stabilizing piles embedded within a slope that is prone to failure

Slope stabilization using vertical piles


Discrete pile row used to stabilise a potentially unstable slope

After Yoon and Ellis (2009)



Driving force induced by displaced soil mass above the sliding surface

After Ashour and Ardalan (2012)



The permanent grouted anchors have beenextensively used to provide vertical and lateralsupport for natural and engineered structures duringthe past 6 decades.

The end type of anchorage, where the tendon isgrouted below the potential slip surface, has beenused to stabilize dangerous slopes to a specifiedsafety factor because of its significant technicaladvantages resulting in substantial cost savings andreduced construction period.

Slope stabilization using anchors



After Cai et al. 2003)Safety factor of slopes with anchors


The safety factor of slopes stabilized with anchors canbe calculated by the following two approaches:

(1) a vertical effect approach conventionally used inpractice, and

(2) a normal effect. Thus the safety factor for the vertical effect

approach is given by

where P is the axial tension per unit width, and θ is an angle to indicate the orientation of Anchors.



Slope stabilization using anchors Safety factor for the normal effect approach can

be obtained by dissolving axial tension in theanchor into two components, namely normal andtangential to the base of the slice, where the slipsurface intersects the anchor. The tangential component of the axial

tension was assumed to have noinfluence on the normal force at thebase of the slice where the slip surfaceintersects the anchor;


The reinforcing mechanism of anchors in slopes canbe explained using the additional shearingresistance, induced by the axial tension, on the slipsurface. The additional shearing resistance wasgiven more rationally by the normal approach thanthe conventional vertical approach


The stabilizing effect was optimal in a range of theangle θ from 7.5° to 22.5° and at the anchor position2m horizontally from the crest for 1V:1H slope.

(Cai and Ugai, 2003)



After Vaciago (2012)


Slope stabilization using stone columns

Stone columns to stabilize an unstable slope (After Abramson et al. 2002)




Slope stabilization using stone columns Stability calculations are carried-out by using

conventional slope stability analysis methods. Thebasic equations for developing average shearstrength values are:




Slope Stabilization for Berths at Gangavaram Port, India

After Keller Ground Engineering India Pvt Ltd (2007)


Slope stabilization using injected lime slurry

(After National Lime Association, 1985)

Documents

Prof. BVS Viswanadham, Department of Civil Engineering, IIT Bombay