Behavior of Geosynthetic-Reinforced Earth

Presented by

Kousik Deb

Assistant ProfessorDepartment of Civil Engineering

IIT Kharagpur

Possible Solutions

• Geosynthetic/fiber reinforcement• Metallic strip• Stone columns• Soil-cement columns• Lime columns• Grout columns• Vibro concrete columns• Micro piles

Columnar systems

Problems for Foundations on Weak Soils

• Experience excessive settlement• Possible bearing capacity failure under load

Placement of Geosynthetic Reinforcement

Method of Installation – Stone Columns

Typical Applications of Columnar Support

Tank foundations Highway embankments

Reinforced earth wallsRailway embankments

Others

• Industrial structures

• Bridge approaches

• RunwaysIndividual footings

Combined Use of Geosynthetics and Pile/Columnar System

Widening of existing roads

Subgrade improvement

Retaining walls

Storage tank

Other Ground Improvement Techniques

preloading with or without the use of sand drains removal and replacement compaction by means of explosion dynamic compaction grouting thermal stabilization ground freezing chemical stabilization

Major Applications of Geosynthetics

Transportation and Geotechnical Geoenvironmental Hydraulic Engineering Private Development

Major Functions of Geosynthetics

Reinforcement Separation Filtration Drainage Moisture barrier

Geosynthetic (GS) Materials

geotextiles (GT) geogrids (GG) geonets (GN) geomembranes (GM) geosynthetic clay liners (GCL) geopipe (GP) geofoam (GF) geocomposites (G C)

Geotextiles (GT)

majority are made from polypropylene, polyester, polyethylenes and polyamides fibers

standard textile manufacturing woven nonwoven very versatile in their primary

function

GEOPIPE

GEOFOAM

Function vs. Geosynthetic Type

Type of Geosynthetic

Separation Reinforcement Filtration Drainage Containment

geotextile

geogrid

geonet

geomembrane

geosynthetic clay liner

geopipe

geofoam

geocomposite

Design Methods

(a) “Cost”-based on experience/availability(b) “Specification” – for common applications(c) “Function” – for specialty, critical and/or

permanent applications

Application Areas

Transportation/GeotechnicalGeoenvironmentalHydraulic Systems Private Development

Transportation and Geotechnical Applications

GTs as filters GTs and GGs as wall reinforcement GTs and GGs as slope reinforcement GC Wick Drains (also called PVDs) GC Erosion Control Systems

Geotextile Filtration

GT is acting as a filter not as a drain three design requirements:

1. adequate flow2. proper soil retention3. long-term flow equilibrium

many applications, e.g., behind retaining walls under erosion control systems around pavement underdrains (follows)

PavementStonebase

Soil subgrade

Topsoil

450 mm

400 mm

300 mm

Crushed stone/perforated pipe

GT

(GT Filter in Excavated Trench) (Crushed Stone & Perforated Pipe)

Geotextile Filtration

Wall Reinforcement Design Concepts

internal design results in:• spacing of GT or GG• length of GT or GG

external design used to assess:• overturning stability• sliding stability• bearing capacity

reduction factors on reinforcement• put on laboratory values for allowable strength

factor-of-safety• on each design aspect to resist the “unknown”

Various Types of Reinforced Retaining Wall

Elements of a GT or GG Wall Design

L0L

H

zLELR

sv45+/2

P2(live loads)P1

DSurcharge

+hs

Soil pressure

+hq

Surcharge pressure

=ht

Live load pressure

h

Total lateral pressure

(With Concrete Facing) (Green Wall with Vegetated Facing)

Reinforcement for Soil Slopes

most soil slopes become unstable steeper than 2(H)-to-1(V) (26.5°)

use GT or GG reinforcement to increase either the slope angle or height

essentially no limit, except for erosion various placement patterns are possible

Placement patterns for reinforcement

(a) Even spaced-even length (b) Uneven spaced-even length

(c) Even spaced-even lengthwith short facing layers

(d) Even spaced-uneven lengthwith short facing layers

(One that Failed)! (With Reinforcement-Steep & Stable)

Geocomposite Wick Drains also called prefabricated vertical drains (PVDs) used for rapid consolidation of saturated fine grained

soils consists of a drainage core with a GT filter/separator

wrapped completely around it typically 100 mm wide, by 2 to 10 mm thick, by 100 m

long (in roll or coil form)

(Driving Wick Drains) (Ready for Surcharge Fill)

Geocomposite Erosion Control Systems

huge array of products slope protection channel protection – increase shear stress

Nature of Waste Problem

moisture within and precipitation on the waste generates leachate

leachate takes the characteristics of the waste thus leachate is very variable and is site-specific flows gravitationally downward enters groundwater unless a suitable barrier layer

and collection system is provided

Double Liner System(with leak detection layer)

GT

GG

GN

GCLGM perforated pipe

(Geonet Leak Detection)

(Nine Layers of Geosynthetics)

Final Cover System

Possible Geosynthetic Layersin a Waste Containment System

in Final Cover - 7

in Waste Itself - 2

in Base Liner - 9

18 Layers!

(Sequential Placement of GSs)

(Seven Layers of Geosynthetics)

Hydraulic Engineering Applications

Waterproofing of Dams Waterproofing of Canals Reservoir Liners Tunnel Waterproofing & Rehabilitation Pipe Rehabilitation & Remediation

Waterproofing of Dams

masonry, concrete, earth and RCC dams GM is not a structural element, its

waterproofing many dams over 50-years old often have

leakage; sometimes excessive leakage

(Concrete Dam Leaking!)

(Completed Concrete Dam Lining) (Lined Earth Dam: Before Rip-Rap)

(Lining aConcrete Dam)

Waterproofing of Canals

conveyance of all liquids; however, water is the most common

distances and quantities vary greatly fundamental issue is leakage (i.e., how much, if any, is

allowable) some type of liner (GM or GCL) is necessary

(Lining a Canal: Before Soil Covering) (GCL Lining of a Canal)

(GM Canal 18 years after GM Lined)

Reservoir Liners

potable water industrial waters process waste waters sewage sludge industrial sludge agricultural wastes hazardous liquids

used to contain all types of liquids

Common Characteristics

generally shallow liquid depths typically 2 to 7 m side slopes from 4(H)-to-1(V) to 1(H)-to-1(V),

i.e., = 14 to 45 both exposed and covered exposed – GM durability issue covered – soil stability issue

(Lined Potable Water Reservoir)

(Huge GM Bag Transporting Potable Water)

Private Development Applications

various dwellings industrial buildings storage areas parks and

playgrounds pools and lakes

sport fields golf courses airfields agriculture aquaculture liquid transportation

Selected Areas of Focus

New Tunnel Waterproofing

many old tunnels without GMs are leaking key is to use a GT and GM behind the permanent

concrete surfacing in turn, this requires a GP drainage system

Pools, Ponds and Lakes

sites vary from small-to-huge usually access is limited liners required for leakage control covers sometimes required for

contamination control and for safety

Soil-Geosynthetics Interaction

• Confinement Effect• String Effect

Design of Geosynthetic-Reinforced Earth

Failure above top reinforcement layeru > 0.5B

Failure between reinforcement layersh > 0.5B

Failure on a two-layer soil system

Failure within reinforced zone

Types of Failure of Reinforced Earth under Footing Load

Sharma et. al., 2009

Improvement in Settlement and BearingDas et. al., 1998

Optimum Thickness of Sand Bed

Optimum Width of Geosynthetic LayerLee et. al., 1999

Failures of Reinforced Embankment

Bearing

Rotational

Internal Spreading

Reinforced Retaining Wall

Types of FailureCai and Bathurst, 1996

Design of Geosynthetic-Reinforced Wall

L0L

H

zLELR

sv45+/2

P2(live loads)P1

DSurcharge

+hs

Soil pressure

+hq

Surcharge pressure

=ht

Live load pressure

h

Total lateral pressure

Internal Stability:Step 1Determine the active pressure distribution (a) on the walla = Ka v = Ka γ zWhere Ka = Rankine earth oressure coefficient = tan2 (45◦- /2)

Step 2Select a geotextile that has an allowable strength of g

Step 3Determine the vertical spacing of the layer at any depth zSv = g/(Ka γ z FS)Where FS is the factor safety. The magnitude of FS is generally 1.3 to 1.5

Step 4Determine the length of each layer of geotextileL = LR + LE

LR = (H –z)/tan(45◦+ /2)andLE = (Sv a FS)/(2 v tan F) ≥ 1mWhere F = 0.67

Step 5 Determine the lap length, L0 = (Sv a FS)/(4 v tan F)The minimum lap length should be 1m

Design ExampleA geotextile-reinforced retaining wall 6 m high. For the backfill, γ = 19 KN/m3 and = 36 ◦. For the Geotextile, g = 16 KN/m.

Ka = tan2 (45◦- 36 ◦/2) = 0.26

At 2 m depthSv = g/(Ka γ z FS)

= 16/(0.26 x 19 x 2 x 1.5) = 1.08 m

At 4 m depthSv = 16/(0.26 x 19 x 4 x 1.5) = 0.54 m

At 6 m depthSv = 16/(0.26 x 19 x 6 x 1.5) = 0.36 m

So, use Sv = 0.5 m for z = 0 to 4 m and Sv = 0.33 for z > 4m

Determination of L

L = (H –z)/tan(45◦+ /2) + (Sv a FS)/(2 v tan F) = 0.51 (H –z) + 0.435 Sv

At z = 0.5, L = (2.8 + 1) = 3.80 mAt z = 1.0m, L = (2.55 + 1) = 3.55 mAt z = 1.5m, L = (2.3 + 1) = 3.3 mAt z = 2.0m, L = (2.0 + 1) = 3.0 mAt z = 2.5m, L = (1.8 + 1) = 2.8 mAt z = 3.0m, L = (1.5 + 1) = 2.5 mAt z = 3.5m, L = (1.3 + 1) = 2.3 mAt z = 4.0m, L = (1.0 + 1) = 2.0 mAt z = 4.33m, L = (0.85 + 1) = 1.85 mAt z = 4.67m, L = (0.68 + 1) = 1.68 mAt z = 5.0m, L = (0.51 + 1) = 1.51 mAt z = 5.33m, L = (0.34 + 1) = 1.34 mAt z = 5.67m, L = (0.17 + 1) = 1.17 mAt z = 6.0m, L = (0 + 1) = 1.0 m

Determination of L0

L0 = (Sv a FS)/(4 v tan F)

= 0.218 Sv= 0.218 x 0.5 = 0.1 m < 1 m

Take L0 = 1 m

L = 4 m

L = 3 m

L = 2 m

External Stability:

(i) Check for overturning F.S > 3

(ii) Check for SlidingF. S > 3

(i) Check for bearing capacityF.S > 3 to 5

• The Reinforced Earth walls were designed for a groundacceleration of 0.10 g.

• This resulted in increase in the amount of reinforcing strips compared to static design.

• The actual ground acceleration was measured at 0.4 g.

Izmit Earthquake, Turkey, 1999

Performance of Reinforced Retaining Wall under Seismic Condition

Bridge has been collapsed

Nimbalkar et. al., 2006

Forces acting on single horizontal elemental slice containing reinforcement

Cai and Bathurst, 1996

Effect of Earthquake loading on Factor of Safety

Choudhury et. al., 2008

Effect of Earthquake loading on Required reinforcement Length

At present, the available codes for seismic design of reinforced soil wall systems (e.g., FHWA 1996, AASHTO 1998

Summary

Geosynthetics are bona fide engineering materials Test methods and designs are available Basic advantage of geosynthetics is quality control

of factory manufactured products Field performance has been excellent Geosynthetics potential is awesome!