GEOSYNTHETICS AND REINFORCED SOIL STRUCTURESREINFORCED SOIL STRUCTURES

Prof K. RajagopalDepartment of Civil EngineeringDepartment of Civil Engineering

IIT Madras, Chennai 600 036e-mail: gopalkr@iitm.ac.in

OUTLINE OF THE LECTUREOUTLINE OF THE LECTURE

• Introductory remarks• Introductory remarks• Historical background• Types and functions of geosynthetics

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What is a geosynthetic ?What is a geosynthetic ?

• Natural or artificial product that is used alongNatural or artificial product that is used along with soil in geotechnical constructions.

• Natural: coir, jute, hemp, etc.

• Artificial: polymeric or metallicp y

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Why geosynthetics ?

• Geosynthetics have entirely changed the way geotechnical engineering is practicedgeotechnical engineering is practiced.

• Innovative solutions to solve difficult problems economically and expedientlyeconomically and expediently

• Enables the use of local materials – sustainable solutions

• Unskilled labour can be employed• Installation does not require heavy machineryInstallation does not require heavy machinery

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Use of coir mat to promote vegetation growth to prevent surface erosion

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Use of stone filled rope net gabions for shoreline stabilization at S i N i T l Tith l G j tSwami Narain Temple, Tithal, Gujarat

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Irrigation canal lining byIrrigation canal lining by using grout filled geocells

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Narrow stretch of road

22m high geosynthetic soil retaining wall to widen a ghat road at Vijayawada

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22m high geosynthetic soil retaining wall to widen a ghat road at Vijayawada

Landfill construction using geosyntheticsat HZL Visakhapatnam to contain jerosite wasteat HZL, Visakhapatnam to contain jerosite waste

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Flexible Break water unit made of th ti b h d fill dgeosynthetics – beach sand filled

bags in rope net gabions10/48Geosynthetics - Introduction

REINFORCED SOIL

• Soil + reinforcement = reinforced soil• Reinforcement:

Ancient: Tree branches, grass reeds, straw, roots of vegetation bamboo tree trunksvegetation, bamboo, tree trunks

Modern: Steel, polymeric, natural materials• Soil is strong in compression & reinforcement is strongSoil is strong in compression & reinforcement is strong

in tension• Combined product has much better engineering

properties than the individual constituentsproperties than the individual constituents• Reinforced soil concept is similar to that of reinforced

concrete

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HISTORICAL APPLICATIONS OF REINFORCED SOILREINFORCED SOIL

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IS BEAVER THE FIRST CIVIL ENGINEER ?

• Semi-aquatic rodentE pert b ilder of Under ater d ellings/dams

ENGINEER ?• Expert builder of Underwater dwellings/dams• Its dwellings use natural reinforcements

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Ziggurats of MesopotamiaA Q f i t • Constructed of clay• Agar-Quf ziggurat, near Baghdad – 3000 years old

Constructed of clay bricks 130-400 mm high, reinforced with woven

t f d l idyea s o d• Originally believed to

be 80 m tall, now it is 0

mats of reed laid horizontally on a layer of sand and gravel at

40 m highsand and gravel at vertical spacing of 0.5 to 2 m

• Reeds were also used to form plaited ropes approximately 100 mmapproximately 100 mm in diameter which pass through the structure

d t i f tand act as reinforcement 14/48Geosynthetics - Introduction

Ziggurats of Mesopotamia(source from Reco web site)( )

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Great Wall of China7th century BC to about 17th century ADy y

Sections in eastern China were mainly made with bricks and chiseled stones, those in western China were made with less durablewestern China were made with less durable materials (often with clay or pounded earth reinforced with tree branches).

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Adobe Bricks

Adobe is derived from the Arab word, "at-tub," referring to the earth bricks Arabs made as early as 7000 BC.

These early builders realised was that if straw, which has a

d il hgood tensile strength was embedded in a block of mud, which has good compressive strength and left to dry the

A man making adobe bricks in the Draa Valley town of Tamgroute, southern Morocco,

g yresulting brick would resist both tearing and squeezing. These composite bricks made excellent building materialsexcellent building materials. Similar bricks were also manufactured by Incas and

Aztecs in American continents

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Aztecs in American continents

Applications of reinforced soil in R l I di t tiRural Indian constructions

In rural India mud walls are traditionally builtIn rural India, mud walls are traditionally builtwith bamboo mats – another fine example ofreinforced soil application from early daysreinforced soil application from early days

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Early applications in last century

• Corduroy mats for forest access roads in South Carolina 1920’sSouth Carolina 1920 s

• Filter fabrics used by Terzaghi• Polyvinyl bags for sea walls in Japan

instead of straw bags• Geotextile tubes for dykes in Netherlands • Polymer meshes/membranes in EuropePolymer meshes/membranes in Europe

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Henri Vidal (re)invents Reinforced Earth in 1963

The concept of

Henri Vidal French Engineer & Architect

reinforced soil was accidentally thought about by Mr. Vidal while playing with his children

An early form of Reinforced Earth using steel strip

i f t d t l

playing with his children on a beach

1963 : Patent filed for

reinforcement and steel membrane facing

1963 : Patent filed for Reinforced Earth

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Basic principle of reinforced earth

Without reinforcement With reinforcementWithout reinforcement With reinforcement

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Two identical pyramids made of dry sand with and without reinforcement layersreinforcement layers

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Response of Unreinforced Soil Pyramid

Immediate collapse under load from a student

Close-up of foot printsLarge deformations even under small load

Immediate collapse of unreinforced soil pyramid under loading due to lateral flow of dry sand

student

g y

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Response of Reinforced soil pyramid

Final loading by the same student t di it i ibl d f ti !!!

Careful initial loading on reinforced pyramid through deadweights standing on it – no visible deformations !!!

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pyramid through deadweights

Geotextiles or Geosynthetics ?

• Originally, most of the modern applications used geotextiles. Hence, all products were erroneously referred to as geotextiles.

• Even the international society was originally called as International Geotextile Society. Name of society changed later to International G th ti S i tGeosynthetics Society.

• Useful information can be found at website www.geosyntheticssociety.org

• Society Membership is free for students

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Graphical symbols proposed by IGSletterSymbol symbol productSymbol symbol product

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Letter Symbols for

Different Functions of Geosynthetics

B Barrier (fluid)D DrainageE Surficial Erosion ControlF FiltrationP Protection (of geomembranes)R ReinforcementS Separation

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Shallow unreinforced slope

Extra road space gained

Steep reinforced slope

Geosynthetic reinforcement layersGeosynthetic reinforcement layers30/48Geosynthetics – Introduction

aggregate layer geotextile separator

subgrade soil

Prevents the intermixing, prevents piping, strength of

Separation Function in a pavement layer

aggregate is preserved

Separation Function in a pavement layer31/48Geosynthetics - Introduction

Filtration Function

Geotextile layer acting as a filter

Water coming out without fine soil particles

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Surface Erosion protection

E i t tiErosion protection layer

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Drainage function of a geotextile layer

ingress of water into pavement layersg p yflow of waterflow of water

drain draindrain drain

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TYPES OF GEOSYNTHETICS

• Geotextiles• Geogrids• GeonetsGeonets• Geomembranes

P f b i t d ti l d i (PVD)• Pre-fabricated vertical drains (PVD)• Geosynthetic Clay Liner (GCL)• Geocells (3-d confinement)• Geocomposites & Geo-othersGeocomposites & Geo others

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GeotextilesGeotextiles

• Engineered sheet like products made of• Engineered sheet like products made of natural or synthetic materials

• Woven and non-woven types• Used for separation, drainage, filtration,Used for separation, drainage, filtration,

erosion control and reinforcement

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Woven fabric – weave tt i i iblpattern is visible

Non woven fabric Non-woven fabric –fibres are randomly oriented

Rao (1995)Two Types of Geotextiles

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yp

Some pictures of geotextilesThin geotextile – used as separator, filtration

Thick geotextile used Thick geotextile used for cushion and drainage 38/48

woven non-woven

Natural geotextiles made of juteNatural geotextiles made of jute

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A woven geotextile fabricg40/48Geosynthetics - Introduction

Geotextile layer being applied below railway trackbelow railway track

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Geogrids• Sheet like products with open apertures. Excellent

interlocking with soil. High strength products, used for g g g p ,reinforcement.

• The geogrids are of several varieties. The extruded id h l h ( N l I di d )grids have low strength (e.g. Netlon India products).

Stretched grids (e.g. Tensar products) are made by stretching process More recently several types arestretching process. More recently several types are made by knitting, welding process, etc.

• Uniaxial products used as reinforcement layers in retaining walls and embankments

• Biaxial products used are used in road bases, below rail tracks ground reinforcementrail tracks, ground reinforcement

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Geogrids manufactured by stretching process

Orientation of polymers in preferential directions

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p y p

St t h d i i l idStretched uniaxial geogrid

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Geogrids used at at Hindustan Zink Limited, Visakhapatnam

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Geogrid reinforcement in pavements

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Innovative use of geogrids for shore protection at Navi Mumbai

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