24 - NPTEL

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

24

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

Module 5:

Lecture -6 on Stability of Slopes

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

Slope stabilization methods

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

Typical embankment failures

(After Ariema and Butler 1990)

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

Effect of berm for slope stabilization

(After Abramson et al. 2002)

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

Slope stabilization with rammed aggregate piers (or stone columns)

Slope stabilization using stone columns

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

Slope stabilization using injected lime slurry/lime columns Slope stability can be improved by injection of lime slurry

to increase shear strength of clayey and silty soils.

A rotating disk auger penetrates into the ground to adepth below the slip surface, and the stabilizing agent isinjected into the resulting kneaded lime column.

One demerit of this method is that at least 80 days mustelapse before columns of stabilized soil can be subjectedto loading.

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

Slope stabilization using lime piles

Lime Column Installation Lime Column Machine

(After Broms, 1991)

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

Slope stabilization using injected lime slurry

(After National Lime Association, 1985)

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

Slope stabilization using lime piles

(After Handy and Williams, 1967)

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

Slope stabilization using lime piles

The increase in soil shear strength due to limecolumns can be expressed by estimating theaverage shear strength along a potential failure orrupture surface in the soil:

Where cu = Undrained cohesion of the soil

Scol = Average shear strength of stabilized clay within lime pilesa = relative column area = πD2/4S2 [For square pattern]

D, S = Diameter and C/C spacing of lime columns Scol is f (overburden pressure, relative stiffness of lime piles with respect to the surrounding unstabilized soil).

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

Lime treatment as a mix-in-place technique hasbeen widely and successfully used around theworld to improve weak soils. Novel techniquesusing lime are now being developed. One suchtechnique is the use of lime piles to stabilize shallowslopes.

Recent research has investigated the reactionmechanisms associated with lime pile stabilization.

Slope stabilization using lime piles

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

Slope stabilization using soil nailing technique

Passive anchors: Soilnails; dowels, rockbolts; (with orwithout facingconsisting of plates,nets, reinforcedshotcrete)

Soil nailing is a technique in which slopes, excavations or retaining walls arepassively reinforced by insertion of relatively slender elements- normally steelreinforcing bars.

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

Slope stabilization using soil nailing technique

Top-Down construction technique; Nails are not pre-stressed (i.e. passive)

(After Vaciago, 2012)

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

(After Prashant and Mukherjee, 2010)

Various types of soil nails

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

Two-zone Model of a Soil-nailed System Resist tensile stresses, shear stresses and bending

moments imposed by slope movements.

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

Schematic Distribution of Tensile Forces alongSoil Nails

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

External Failure. External failure refers to thedevelopment of potential failure surfaces essentiallyoutside the soil-nailed ground mass. The failure can bein the form of sliding, rotation, bearing, or other forms ofloss of overall stability.

Internal Failure. Internal failure refers to failures withinthe soil-nailed ground mass. Internal failures can occurin the active zone, passive zone, or in both of the twozones of a soil-nailed system.

Potential Internal Failure Modes of a Soil-nailed System

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

In the active zone, internal failure modes include:(a) failure of the ground mass, i.e., the ground disintegrates and

‘flows’ around the soil nails and soil-nail heads,(b) bearing failure underneath soil-nail heads,(c) structural failure of the soil nail under combined actions oftension, shear and bending,(d) structural failure of the soil-nail head or facing, i.e., bending orpunching shear failure, or failure at head-reinforcement or facing-reinforcement connection, and(e) surface failure between soil-nail heads, i.e., washout, erosion,or local sliding failure.

Potential Internal Failure Modes of a Soil-nailed System

In the passive zone, pullout failure at ground-grout interface or grout-reinforcement interface should be considered.

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

Potential Internal Failure Modes of a Soil-nailed System

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

Potential Internal Failure Modes of a Soil-nailed System

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

Potential Internal Failure Modes of a Soil-nailed System

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

After Ultraco (2001)

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

Raise of Groundwater table andinfiltration ofrain water createseepage pressurewithin the slope

β = 60°-85°

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

Soil nailing in railway constructionfor laying new tracks adjoining anexisting one

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

Typical cross-section of a drilled soil nailed slope

(After Prashant and Mukherjee, 2010)

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

a) Preparation of slope surface b) Mobile drilling rig alignment

c) Steel bar installation d) Grouting process

Construction of soil nailing

(After Prashant and Mukherjee, 2010)

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

Nail head

Soil-nail

Slope facing

H

α

Back slope

δ

z

u = γwz

Firm foundation

Base layer

Sv

Sh

Resistant zone

Typical distribution of axial forces

Active zone

β

Ln

Nail head

Soil-nail

Slope facing

H

α

Back slope

δ

z

u = γwz

Firm foundation

Base layer

Sv

Sh

Resistant zone

Typical distribution of axial forces

Active zone

β

Ln

Soil nailing techniquePassive inclusion, closely spaced

Increase the overall shear strength of the in-situ soil

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

FOS vs Nail inclination for different slope inclinations

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

FOS vs Nail inclination for different facing thicknesses

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

FOS vs horizontal spacing for different vertical spacings

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

FOS vs Nail inclination for different nail diameters

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

Developed slope failure surfaces for the gravity-grouted and pressure-grouted soil nails from the maximum plastic strain distribution plots

The slope reinforced with pressure-grouted soil nails exhibits expandedfailure surface from the slope surface compared with that for thegravity-grouted reinforced slope.

the groutingpressure mayincrease thestiffness of thereinforcedslope system

After Kim et al. (2013)

Reinforcing effects of pressure-grouted soils on slope stability

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

Slope stabilization using soil nails at Meenadu - WTP

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

Biotechnical slope stabilization

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

Biotechnical slope stabilization

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

Biotechnical slope stabilization

A Vetiver grass system