Geosynthetics and Geosynthetics and Reinforced Soil Structures

Reinforced Soil Walls

Prof K RajagopalProf K. RajagopalDepartment of Civil Engineering

IIT Madras, Chennaie-mail: gopalkr@iitm.ac.in

Outline of the LectureOutline of the Lecture

• Need for retaining wallsNeed for retaining walls

• Types of Retaining Walls

i f d il• Reinforced soil

• Reinforced Soil Walls

• Advantages

• Various configurationsVarious configurations

Reinforced Soil Walls - 1 2/22

Need for Retaining WallsS il h hi h i h b li i d• Soil has high compressive strength but limited tensile and shear strengths

• Can stand on its own only up to its own angle of• Can stand on its own only up to its own angle of repose

• Steeper slopes require lateral supportSteepe s opes equ e ate a suppo t• Purpose of the retaining walls is to provide lateral

support to the soil to make it stand at steep anglesInitial soil position with steep slope

Final slope at angle of repose

cv

3/22Reinforced Soil Walls-1

Types of Retaining Walls• Gravity Walls – masonry, crib walls, gabion walls, etc.• Reinforced Concrete walls – Cantilever, counterfort, buttressed 

walls• RC Walls with shear keys• Sheet Pile walls

A h d Sh t Pil W ll• Anchored Sheet Pile Walls• Braced sheet pile walls• Diaphragm walls• Reinforced Soil Retaining Walls• Anchored Reinforced Soil Walls• Soil Nailed walls driven nails screw nails pre stressed nailed• Soil Nailed walls – driven nails, screw nails, pre‐stressed nailed 

walls, etc.

4/22Reinforced Soil Walls - 1

• Gravity walls – easy to construct, can be constructed even in remote areas using lowconstructed even in remote areas using low technology, not suitable for retaining soils higher than about 2 to 3 m.than about 2 to 3 m.

• Reinforced Concrete Walls – can be used for moderate heights to about 6 to 10 m. Beyond thatmoderate heights to about 6 to 10 m.  Beyond that height, the size of the members increases rapidly. 

• The foundation pressures could be very highThe  foundation pressures could be very high leading to expensive foundation treatments.

• During seismic events, large inertial forces areDuring seismic events, large inertial forces are produced.  Most RC concrete structures fail during earthquakes due to lack of ductility or rigidity.

Reinforced Soil Walls - 1 5/22

Some early concepts

Pre-cast concrete ties with 

Selected stone aggregate

panels 1.5mx 0.8m

anchors

1 Ordinary soil fill

2

1

Coyne (1927) retaining wall at Brest, France - concrete panels can slide down relative to each other due to settlements, ties attached with anchors to develop passive resistance. In 1945, he built some similar

alls itho t anchors Notice that stone aggregate as sed to gi e

Reinforced Soil Walls - 1 6/22

walls without anchors. Notice that stone aggregate was used to give good resistance in anchors.

Some Early concepts

reinforcing elementselements

Front facing

Sectional view Plan ViewMunster (1925), USA Earth retaining structure made of wooden logs & planks –reinforcing elements are similar to ladders Front facing is a light thin facingreinforcing elements are similar to ladders . Front facing is a light, thin facing element of wooden planks. The concept is still in vogue now, except that we use steel reinforcing meshes instead of wooden ladders. Reinforcement has sliding attachment and hence can settle down with backfill

Reinforced Soil Walls - 1 7/22

sliding attachment and hence can settle down with backfill.

Conventional Retaining Wallsg

Masonry gravity retaining wallReinforced concrete semi‐gravity retaining wallwall

8/22Reinforced Soil Walls - 1

Secant pile wall at the bottom and nailed soil slope at the top

Reinforced Soil Walls - 1 9/22

p p pof the excavation

Applying pre‐stress to the nail

Deep excavation in built‐up areas

Surface treated with spray concrete after the nails are locked‐in

Reinforced Soil Walls - 1 10/22Deep excavation supported by locked‐in nails with pre‐stress

Basic Principle of Reinforced Soil

Response of unreinforced and reinforced soil to external ploading

1000

1200Reinforced

600

800

1000

i f d

Without reinforcement  With reinforcement  ‐ stronger and stiffer response 0

200

400unreinforced

p 00 5 10 15

11/22Reinforced Soil Walls - 1

Components of reinforced soil retaining wallsReinforcement Reinforced soil block

B kfill ilF i

Reinforcement layers

Reinforced soil block

Backfill soilFacing panels

Levelling pad

Foundation soilFoundation soil

• Thin, flexible facing panels - about 180 mm thickThin, flexible facing panels about 180 mm thick• Levelling pad is usually 150 mm thick and 300-400 mm wide• Backfill soil is select non-plastic fgranular fill• Horizontal reinforcement layers• Horizontal reinforcement layers

12/22Reinforced Soil Walls - 1

Henri Vidal’s Original Reinforced Earth Wall

Henri Vidal French Engineer & Architect

An early form of Reinforced E th i t l t iEarth using steel strip reinforcement and steel membrane facing

1963 : Patent filed for Reinforced Earth 13/22Reinforced Soil Walls - 1

Types of wall facings in Reinforced Soil Walls

Full‐height Panel wall

Segmental panel wall

Modular block faced wall

Geotextile wrapped retaining wall

14/22Reinforced Soil Walls - 1

Chronology of Reinforced Soil WallsYear EventYear Event

1963 Reinforced Earth patented by Henry Vidal

1968 First major RE structure built in France using steel strips1968 First major RE structure built in France using steel strips

1970 First geosynthetic wall using polyester (PET) straps in France

1971 First geotextile wall using wrap around concept built in Franceg g p p

1974 First RE structure built in USA

1980 Tensar geogrids (PP and PE) developed (first geogrids in the field)

1981 First geogrid reinforced soil retaining wall in UK

1985 Polyester geogrids developed (knitted variety)1985 Polyester geogrids developed (knitted variety)

1986 First reinforced soil retaining wall built in India at Ludhiana using fly ash as the backfill – height about 9m

Reinforced Soil Walls - 1 15/22

2006 First geogrid manufactured in India

Different Types of ReinforcementsGeosynthetic Reinforcement

– Geotextiles– Geogrids– Polymeric Strips– Grids made of strips

Steel ReinforcementS l S i– Steel Strips

– Welded Steel meshes– Steel strips with anchors– Steel strips with anchors– Twisted steel wire mesh

Reinforced Soil Walls - 1 16/22

Cost comparison

(FHWA 1997)

17/22Reinforced Soil Walls - 1

Differences between Reinforced soil walls and conventional walls

Item Reinforced Reinforced Soil WallsItem Reinforced Concrete Walls

Reinforced Soil Walls

Flexibility Rigid system Highly flexible

Settlements Cannot tolerate large total Can tolerate large deformations without distressSettlements Cannot tolerate large total or differential settlements

Can tolerate large deformations without distress

Maximum Height Limited to some heights Can be built to even large heights as much as 50 to 100 m

Constructability Skilled labour required Unskilled labour can be used

Pace of construction Slow because of curing requirements

As fast as the backfill soil is brought to site

Foundation requirements

Large foundation pressures – needs good foundation

Foundation is not required

Seismic response Attracts large inertial forces Highly flexible – can undergo large deformationSeismic response Attracts large inertial forces – failures due to rigidity

Highly flexible can undergo large deformation during seismic events without collapse

Variations to suit site Not possible Several varieties of configurations are possible

Reinforced Soil Walls - 1 18/22

a at o s to su t s teconditions

ot poss b e Se e a a et es o co gu at o s a e poss b e

Different types of wall configurations

Full-height wall S d W ll iFull-height wall Stepped Wall inWeak soils

Trapezoidal wall for very strong soils

Part-height wall Embedded wall(both sides not

Back to back walls

Reinforced Soil Walls - 1 19/22

connected)

Different Types of Wall Configurations

Tiered wall for very large heights

Bridge abutment wall

large heights

Reinforced Soil Walls - 1 20/222-tier wall

44m high 4-tier wall

Recap

This lecture has introduced the need for

Recap

This lecture has introduced the need for retaining walls and reinforced soil retaining wallswalls

Reinforced Soil Walls - 1 21/22