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This document is a pre-design, initial study report on the final year

project of a group of four members undertaking the undergraduate

studies of Transportation Engineering, titled as Design of Reinforced

Earth retaining wall for highway embankments. The report consists of

the basic idea of what the reinforced earth is and how it is comparable

to other methods of strengthening the highway embankments. It will

be followed by other report(s) regarding the design.

UET

Group one

Group email

umairdurrani@hotmail.com

Final year Project

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Initial Study Report on Reinforced Earth Retaining Wall

for Highway Embankment

TABLE OF CONTENTS

Why highway embankment is needed to be strengthened? ___________________________ 3

What is done to strengthen the embankment? _____________________________________ 3

Which method is the best? ______________________________________________________ 4

What is a Reinforced Earth? _____________________________________________________ 5

Introduction to reinforced soil structure ___________________________________________ 6

The Reinforced Earth Retaining Wall ______________________________________________ 6

Mechanism Of Reinforced Soil ___________________________________________________ 7

Shear Box Analogy Concept _____________________________________________________ 8

What can be used as a Reinforcement? __________________________________________ 10

1. Geosynthetics _______________________________________________________________ 10

2. Reinforcing strips _____________________________________________________________ 10

Table Of Figures

Figure A External stabilization ................................................................................................................................ 6

Figure B Internally stabilized reinforced soil ........................................................................................................... 6

Figure C (a) Element of unreinforced soil, (b) Element of reinforced soil ................................................................ 7

Figure D Slopes showing failure surface ................................................................................................................. 8

Figure E shear box test on unreinforced and reinforced soil ................................................................................. 10

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PART 1 IntroductionIntroductionIntroductionIntroduction

Why highway embankment is needed to be

strengthened?

A highway embankment is a structure (usually made up of earth in

Pakistan) which has a sufficient height to separate the pavement

built over it from the ground level. The reasons why it is necessary

to make it strong are given below:

• It has to bear the forces of water during the flood.

• It has to bear huge loading of traffic with large number of

repetitions.

• To avoid the Land sliding.

• Over-burden

• Dead and Live Load Surcharge

• Earth Pressure

• Hydrostatic Pressure

• Seismic Loads

• Construction Loads etc…..

What is done to strengthen the embankment?

There are a large number of solutions to the problem of

strengthening an earthen embankment. These include:

• Giving slopes on both sides of embankment when

embankment height is sufficiently high.

• Ability of Plant Roots to Strengthen Soil

• Influence of Short Polymeric Fibers on Crack Development in

Clays

• Recycled Plastic Soil Nails Provide Slope Stabilization Project

NEW WORDS

Reinforced Earth

A composite material

having facing panels,

soil reinforcement

strips and select fill as

basic components.

Highway Embankment

A raised earthen

structure used for

increasing the level of

pavement from the

ground due to a number

of factors.

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• In-situ densification of soils.

• Ground improvement and modification.

• Reinforced soil.

• Grouting.

• Grading and other soil improvement methods

Sheet piling Sheet piling may be composed of steel, timber or concrete piles, with each pile

being linked to the next to form a continuous wall. Sheet pile walls are sufficiently

watertight for most practical purposes.

Grouting covers different injection techniques of special liquid or slurry materials called

grouts into the ground for the purpose of improving the soil or rock.

Muckshift can be described as the excavation and exchange of unsuitable soil regions by

more qualified ones. It is a kind of large scale land clearance.

Sandbagging Sandbags can be used for preventing a leach ate discharge downstream of the

embankment site.

Geosynthetical structures

Rock facing / rock riprap slope surface consists of rock or cobble fills, no special slope

surface treatment is necessary. Downstream slopes with outer sand and gravel should be

protected against erosion especially during flood.

Which method is the best?

Most of these methods work in different problems in different situations and therefore none

can be said as best. Another reason for this is that other than reinforcing the earth and making

a composite material called reinforced earth, all the above mentioned methods require having

slopes of embankment for an economical design.

There can be a situation where one wants to have an embankment with angle of repose 90

degrees. This can be achieved by employing the reinforced earth concept for the embankment.

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PART 2 Reinforced EarthReinforced EarthReinforced EarthReinforced Earth

What is a Reinforced Earth?

A Reinforced Soil System (RSS) is a composite material which has the following basic

components;

• Facing Panel (Commonly made of concrete, steel plate, wire mesh, block etc…)

• Soil Reinforcement Strips (Galvanized steel, geotextiles, etc)

• Select Fill (Cohesionless soil meeting specific defined requirements)

The frictional forces created when combining the select fill with the flexible metallic or non-

metallic reinforcing strips result in a robust structural material, commonly known as Reinforced

Earth. The strips are attached to a front facing panel, which may be manufactured from

concrete or steel. The facing material selected is generally dependent on it having sufficient

durability to accommodate the design life of the structure, and also meet the aesthetic needs

of the project. The Reinforced Earth monolithic mass acts cohesively and supports it’s own

weight and any applied loads which may include all the forces as described in part 1 of this

report. The forces induced in the steel strips can be precisely calculated and depend on ;-

• Strip geometry

• Strip frictional characteristics

• Vertical soil pressure on the strip

• Strength and stiffness characteristics on the strip

Importantly, the durability of the structure relies heavily on the ability of the soil reinforcement

strip to maintain a level of tensile strength in the operational environment for the duration of

the structure’s design life. The strip made up of steel, if used, is therefore designed to include a

sacrificial steel thickness, which predicts the amount of strip corrosion throughout the design

life of the structure. This is achieved by controlling the environment in which the strip will be

operating. The select fill, whilst having certain physical requirements that ensure it is activated

in forming part of the structural mass, is also required to have electrochemical characteristics

that also ensures that corrosion of the strip is not excessive or beyond the allowance made in

the strip design. Furthermore, the strip is coated with zinc galvanizing for further protection.

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The final length and frequency of the soil reinforcement strips is a function of the combinations

of geometric and physical properties of the structure and the applied design loads. Whilst the

facing to the Reinforced Earth wall technically does not take on a structural role in support of

the loads, it obviously forms an important part in the wall in preventing the erosion of backfill,

supporting the soil reinforcement and weathering the local environment. Typically, for roads

projects, concrete is the only economical material that can achieve the necessary 100 year

design life without the need for any continuous maintenance or repair. The facing also forms

the most visual aspect of the structure and is often required to have some aesthetic appeal,

particularly in urban areas. Concrete can lend itself readily to the provision of architectural and

aesthetic requirements. The facing panels can however, often be a complex component to

manufacture as each facing panel may have very individual characteristics with respect to its

geometry, finish or cast-in inclusions.

Introduction to reinforced soil structure

Reinforced soil structures are fundamentally

different from conventional earth retaining

systems which are externally stabilized in that

they utilize a different mechanism for support

and are internally stabilized. An externally

stabilized system uses an external structural wall

against which stabilizing forces are mobilized, for

example, gravity retaining walls and excavations

supported with strutting (fig A)

An internally stabilized system involves

reinforcements installed within & extending

beyond the potential failure mass (tied back,

reinforced soil walls, and soil-nailed

excavations). With this system, the interactions

between the reinforcements and soil (to

mobilize the tensile capacity of closely spaced

reinforcing elements) eliminate the need for a

structural wall or a support (figure B)

The Reinforced Earth Retaining Wall

According to Reinforced Earth company,

Figure A External stabilization

Figure B Internally stabilized reinforced soil

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Reinforced Earth®

retaining walls are gravity structures consisting of alternating layers of

granular backfill and reinforcing strips with a modular precast concrete facing. They are used

extensively in transportation and other civil engineering applications. Because of its high load-

carrying capacity, Reinforced Earth is ideal for very high or heavy-loaded retaining walls.

The inherent flexibility of the composite material makes it possible to build on compressible

foundation soils or unstable slopes. These performance advantages combined with low

materials volume and a rapid, predictable and easy construction process make Reinforced Earth

an extremely cost-effective solution over conventional retaining structures.

Mechanism Of Reinforced Soil

The mechanism of reinforced soil can be explained in simple terms considering an element of

cohesionless soil shown in figure C(a) . If a vertical stress is applied on the soil it deforms both

laterally and vertically and reaches a new equilibrium. If reinforcement in the form of plane

sheet is introduced in the sample before the application of vertical stress on the sample,

Figure C (a) Element of unreinforced soil, (b) Element of reinforced soil

deformations are restrained due to the interaction b/w the soil and the reinforcement to some

extent as in figure C (b). introduction of reinforcement generates inward lateral stress, which

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resists the shear stresses that are generated when a vertical stress (sigma 1) is applied. If there

is no significant deformation, the lateral stress is equal to Ko times the vertical stress and this

condition prevails at higher vertical stresses also.

The shear stress at the interface of soil and reinforcement generates strains in the

reinforcement and tensile force is mobilized in the reinforcement. If the reinforcement force

exceeds the tensile capacity of sheet reinforcement, rupture failure occurs. Secondly, it is likely

that a slip occurs b/w soil & reinforcement if deformation are high or interface is smooth. These

two conditions viz tesile failure & pullout failure need to be examined to ensure stability of

reinforced soil structures.

Shear Box Analogy Concept

These shear box test simulates the mechanism and behavior of both unreinforced and

reinforced soils.

Figure on the left shows slope of unreinforced soil and on the right shows slope of reinforced soil.

Figure D Slopes showing failure surface

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What can be used as a Reinforcement?

1. Geosynthetics

ASTM has defined a geosynthetic as a planar product manufactured from a polymeric material

used with soil, rock, earth, or other geotechnical-related material as an integral part of a civil

engineering project, structure, or system.

Geotextiles: A geotextile is a permeable geosynthetic made of textile materials Geotextile in

fabric form are being used as a basal reinforcement of embankment and fills on the ground

Geogrids: Geogrids are primarily used for reinforcement; they are formed by a regular network

of tensile elements with apertures of sufficient size to interlock with surrounding fill material.

These are made up of High Density Polyethylene (HDPE) and interconnected longitudinal and

transverse Member. These are made from sheets of polymer by punching holes and stretching

the sheets in one or two direction.

Geocomposits: Geotextiles and related products such as nets and grids can be combined with

geomembranes and other synthetics to take advantage of the best attributes of each

component. These products are called geocomposites.

2. Reinforcing strips

Reinforced members are composed of thin wide steel or aluminum strips called ties. The

flexibility of reinforcing strips and their tensile strengths are essential elements. The reinforcing

strips are made of mild galvanized steel, stainless steel or aluminum alloy. Bolts and nuts for

fixing the ties are made of the same material as that of the reinforcing strips. The durability of

the strips depends on the chemical and electro-chemical behavior of these metals when in

contact with soil particles

Works Cited

Introduction to Soil Reinforcement and Geosynthetics [Book] / auth. Babu Visakumar. - to be

added : to be added, to be added. - Vol. to be added.

Figure E shear box test on unreinforced and reinforced soil

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