soil reinforcement 1

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ENGINEERING ASPECT OF SOIL REINFORCEMENT

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Chapter 1

Introduction

1.1 GENERAL

The use of reinforcement in improving the strength parameters of geo-

materials has taken momentum due to the availability of variety of synthetic

materials at cheaper rates. The basic principles involved in earth reinforcement

techniques are simple and have been used by mankind for centuries. One of the

essential characteristics of reinforced soil is that it is made with two types of

elements, soil grains and reinforcements. The basic mechanism of reinforced earth

involves the generation of frictional forces between the soil and reinforcement. By

means of friction the soil transfers the forces developed in earth mass to the

reinforcement thus developing tension. The earth develops pseudo cohesion in the

direction in which reinforcement is placed and the cohesion is proportional to

tension developed in reinforcement.

1.2 HISTORICAL BACKGROUND

1.2.1 Early Practices

Soil specially cohesion less material like gravel, sand and coarse silt cannot

take even low stress in tension and fails instantaneously. The early man has known

this phenomenon from intuition. Men used woven reeds in making sun dried bricks

in ancient times even prior to Christian era. Fibrous materials like vines and

papyrus are used in earth structures and mud walls in Egypt and Babylon. In the

construction of the Great Wall of China where are used extensively, branches of

trees were used as reinforcement in the construction of Agar-Quif ziggurat near

Baghdad. Romans who developed a high degree of engineering skills in

construction to meet the civic needs and military requirements built reed reinforced


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earth leaves along the river Tiber. Wharf walls in England also were constructed

by Romans using wooden scantling as earth reinforcement. In the last century Col.

Palsey introduced reinforced earth for military construction in British army. The

Dutch used reinforced earth by faggoting for sea protective works.

Fig. Papyrus used for soil reinforcement

1.2.2 Modern Development

The modern approach to reinforced earth techniques was first introduced inFrance and USA. In 1925, the concept was first introduced by Monster. The

structure built was retaining wall with reinforced earth, wood was used as

reinforcement. In the early fifties, the French constructed retaining walls

constructed of granular fill with membrane. This cladding membrane was anchored

with flexible ties. The first major work on reinforced earth was introduced in large

scale from 1964 onwards both in USA and Europe and this was followed by

detailed experimental and theoretical investigation to study the mechanism of the

reinforced earth in France. This programmed was introduced by Henry Vidal and

Franois Schlosser and the scientific approach to the study of reinforced earth

structures can be said to have opened up since then.


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Steel was used as reinforcement in the form of stripes which when exposed

to aggressive environment like humidity, access to oxygen and exposure to

corrosive agents rusts rapidly. But with the introduction of such manmade fibers

like nylon, propylene and other forms of organic stable polymers which can

withstand ultra-violet light rays and resistant to acid in industrial applications, the

deficiency suffered by steel has greatly been overcome. With the introduction of

such manmade fibers which are found to be superior to natural fibers and steel it is

now feasible to build reinforced earth structure even in soil and environment

aggressive to steel reinforcement.

Fig. Reinforced Soil Wall

1.3 PRINCIPLES OF REINFORCED EARTH

Soil mass is generally a discrete system consisting of soil grains and is

unable to withstand tensile stresses and this is particularly true in the case of

cohesion less soil like sand. Such soils cannot be stable on steep slopes and


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relatively large strains will be caused when external loads are imposed on them.

Reinforced earth is a composite material, a combination of soil and reinforcement

suitably placed to withstand the development of tensile stresses and also to

improve the resistance of soil in the direction of greatest stress. The presence of

reinforcement modifies the stress filed giving a restraint mostly in the form of

friction or adhesion so that less strains are induced and tension is avoided.

Inclusions like discrete short fibers placed random or in different layers will also

impart additional resistance by way of cohesion and friction, but these are not

included in the Vidals concept of reinforced earth.

1.4EFFECT OF REINFORCEMENT ON SOIL

1.4.1 Force transfer from soil to reinforcement

Fig. 1.1 shows cohesion less soil mass reinforced by a flat strip. The force at

the two ends of the strip is not same when there is transference of force by friction

to the soil mass (Vidal, 1969).

This restraint on the soil mass increases the resistance of the soil to failure

under applied stresses and the result interpreted in two related ways.

Figure 1.1: Stress Transfer By Soil Reinforcement


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1.4.2 Equivalent confining stress concept

Fig 1.2 (a) shows the comparison of failure stresses on two soils, one

unreinforced and the other reinforced. The increase in the deviator stress is seen to

be 3times Kp, where Kp is the coefficient of passive earth pressure equal to tan

(45 + /2) and 3 is the equivalent confining stress on sand imposed by the

reinforcement (Yang, 1972).

Figure 1.2 a: Differentiating Stress Pattern among Unreinforced and

Reinforced soil


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1.4.3 PseudoCohesion Concept

This concept (Schlosser and Long, 1974) proposes that the reinforcement

induces an anisotropic or pseudo-cohesion to the soil which depends on the

spacing and strength of the reinforcement. Fig. 1.2 (b) shows the approach.

Figure 1.2b: Equivalent confining Stress Concept

It is necessary that the reinforcement layer must be close enough so that

there is effective transfer of stress by friction or adhesion as the case may be and

hence the granular soils of high relative density are particularly suitable for use in

reinforced earth. The concept outlined above can also hold good for cohesive soils

to a very limited extent only since the adhesion of the clay to the reinforcement is

small and its effect on reinforcement is small and its effect on restraint doesnt

have a multiplying effect as in granular material.


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CHAPTER 2

REINFORCING TECHNIQUES AND MATERIAL

2.1 GENERAL

A number of materials have been reported to be successfully used as

reinforcements such as steels, geofabrics, geogrids, aluminum, glass fiber, wood,

rubber and concrete. In developed countries polypropylene based synthetic fibers

and grids are now preferred due to their available with desired properties and

durability. However, they are yet to be used widely in India as they are more

costly. The reinforcement may take the form of strips, grids, sheet materials, ropeand other combinations. The major requirements of the reinforcing materials are

strength, durability, ease of handling, high adhesion or friction with soil and

availability at low-cost.

The man made polymers are highly restraint to bacteria, alkalis and acid.

Polyamides have a very good mechanical characteristic including excellent

resistance to abrasion and absolute imperviousness to rotting. It can withstand high

temperature without its performance being affected. However, their performance

deteriorates on wetting.

Polyesters have very good resistance to abrasion and its behaviour in water

is satisfactory. It has high modulus of elasticity and has only negligible creep. It

can also withstand considerable temperature increase. Polypropylene is also rot-

proof, water and most chemical reagents do not affect its performance. It has only

fair resistance to abrasion and is affected by temperature increase. It has only a

tendency to creep. However, a majority of geo-fabrics is manufactured from

polypropylene. For use as a reinforcing material, the geo-fabrics should possess a

high modulus elasticity, low elongation and satisfactory puncture strength. For use

as an asphalted overlay material, adsorption qualities may also be essential.


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Resistance to ultraviolet radiations and surface conformity should be considered

for all jobs.

Soil has been used as a construction material from times immortal. Being

poor in mechanical properties, it has been putting challenge to civil engineers to

improve its properties depending upon the requirement which varies from site to

site and economic constraints.

Fig: Steel reinforcement for soil Fig: Geofabrics, geogrids and geonet used for soil

Fig: Glass fiber for soil reinforcement


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2.2 REINFORCING TECHNIQUES

There are many available techniques for improving the mechanical

properties of soil. The technique employed to improve the engineering and

mechanical properties of soil, can be put into five major categories. They are as

following:-

2.2.1 SOIL STABILISATION2.2.2 REINFORCED EARTH

2.2.3 SOIL NAILING

2.2.4 TEXSOL

2.2.5 FIBRE REINFORCED SOIL

(2.2.1) Soil Stabilisation: There are two primary methods of soil stabilization;

Mechanical and chemical or additive. The soil stabilization means the

improvement of stability or bearing power of the soil by the use of controlled

compaction, proportioning and/or the addition of suitable admixture or stabilizers.

Basic Principles of Soil Stabilization are as following:-

Evaluating the properties of given soil. Deciding the lacking property of soil and choose effective and

economical method of soil stabilization.

Designing the Stabilized soil mix for intended stability and durabilityvalues.


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Fig: SOIL STABILISATION

(2.2.2) Reinforced earth: Reinforced soil is composite material which is formed

by the associating of frictional soil and tension resistant elements in the form of

sheets, strips, nets or mats of metal and arranged in the soil mass in such a way as

to reduce or suppress the tensile strain which might develop

Under gravity and boundary forces.

Fig:

REINFORCED EARTH


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(2.2.3) Soil Nailing: Soil nailing ( shown in figure 2.1 and 2.2) consists of the

passive reinforcement (i.e., no post-tensioning) of existing ground by installing

closely spaced steel bars (i.e., nails), which are subsequently encased in grout. As

construction proceeds from the top to bottom, shotcrete or concrete is also applied

on the excavation face to provide continuity. Soil nailing is typically used to

stabilize existing slopes or excavations where top-to-bottom construction is

advantageous compared to other retaining wall systems.

Fig : Soil Nailing on larger area Fig : Soil Nailing on smaller area

(2.2.4) Texsol: Texsol is a new geotechnical material in which soil is reinforced by

continuous threads. It can be considered to be a composite made of sand and

continuous threads of synthetic fibers. To obtain such a material, a number of

threads are pneumatically of hydraulically projected on sand in a movement at the

extremity of conveyer belt or vent of a pipe used to build a hydraulic fill.


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Fig: Texsol fiber reinforced soil retaining wall

(2.2.5) Fiber Reinforced Soil (Ply Soil) : Randomly distributed fiber reinforced

soil termed as RDFS is among the latest ground improvement techniques in which

fibers of desired type and quantity are added in the soil, mixed randomly and laid

in position after compaction. Thus the method of preparation of RDFS is similar to

conventional stabilization techniques. RDFS is different from the other soil

reinforcing method in its orientation. In Reinforced earth the reinforcement in the

form of strips, sheets etc is laid horizontally at specific intervals whereas in RDFS

fibers are mixed randomly in soil thus making a homogeneous mass and maintain

the isotropy in strength. Modern geotechnical engineering has focused on the use

of planar reinforcement ( eg. Metal strips, sheets of synthetic fabrics ). However

reinforcing of soil with discrete fibers is still a relatively new technique in

geotechnical projects.

Concepts involving the reinforcement of soils using fibers have been used

since ancient times. For example, early civilizations added straws and plant roots

to soil bricks to improve their properties, although the reinforcing mechanism may

have not been fully understood. While building the Great Wall of China, the clay


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soil was mixed with tamarisk branches. The ancient method of addition of straw of

wheat locally called Turi to the clay mud plaster is still very popular in villages.

Improvement of soil by trees roots is similar to the work fibers. Synthetic fibers

have been used since the late 1980s, when the initial studies using polymeric fibers

were conducted. Specially, triaxial compression tests, unconfined compression

tests, direct shear tests and CBR tests had been conducted to study the effect of

fiber reinforcement on strength characteristics and other engineering properties of

RDFS. During last twenty five years, much work has been done on strength

deformation behavior of RDFS and it has been established beyond doubt that

addition of fiber in soil improves the overall engineering performance of soil.

Among the notable properties that improve are greater extensibility, small loss of

post peak strength, isotropy in strength and absence of planes of weakness.

Fig: Polypropylene strands to be mixed with soil


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Fig: Geofibers mixed with subgrade soil to raise its density

2.3 BASIC MECHANISM OF FIBER REINFORCED SOIL

Randomly oriented discrete inclusion incorporated into soil improves its

load deformation behavior by interacting with the soil particles mechanically

through surface friction and also by interlocking. The function of the bond or

interlock is to transfer the stress from soil to the discrete inclusion by mobilizing

the tensile strength of discrete inclusions. Thus,fiber reinforcement works as

frictional and tension resistance element.

2.4 ADVANTAGES OF FIBER REINFORCED SOIL

Randomly distributed fiber reinforced soil (RDFS ) offers many

advantages as listed below:-

Increased shear strength with maintenance of strength isotropy. Beneficial for all types of soil ( i.e. sand, silt and clay ). Reduced post peak strength loss. Increased ductility.


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Increased seismic performance. No catastrophic failure. Great potential to use natural or waste material such as coir fibers, shredded

tire and recycled waste plastic strips and fibers.

Provide erosion control and facilitate vegetation development. Reduce shrinkage and swell pressure of expansive soil. No appreciable change in permeability. Unlike lime, cement and other chemical stabilization method the

construction using fiber reinforcement is not significantly affected by

weather conditions.

Fiber reinforcement has been reported to be helpful in eliminating theshallow failure on the slope face and thus reducing the cost of maintenance.

2.5 TYPES OF FIBERS

Fibers can be classifiedbroadly in two categories; Synthetic fiber and

Natural fiber. Some commonly used fibers are; Coconut fiber, Jute fiber, Cotton

fiber, Wool fiber, Asbestos fiber, Polyester fiber, Polyamide fiber, Polypropylene

fiber, Rubber fiber, Metallic fiber and Glass fiber.

2.5.2 Synthetic Fibers: Various types of synthetic fibers are polypropylene,

nylon, plastic, glass, asbestos etc. They are generally prefer over natural

fiber due to higher strength and resistance. They are resistant to acidic,

alkaline, chemicals, sea water and have high melting point ( 165

o

C ). Theyalso show a great biological resistance and some are prone to fire. Important

properties of them are versatility, excellent, chemical resistance, low density,

high melting point and moderate cost.


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weakness surface in the structure of soil. The process that it play the role on

soil is physical, so the basic chemical properties and physical structure do

not change significantly, without affecting the soil environment. In recent

years, fiber reinforced soil is used in geotechnical engineering frequently

and it was widely used in the dam projects, road works, waste sanitary

landfill liner and so on.

2.6 DIRECTION OF PLACEMENT

Fibers can be oriented or randomly mixed in soil. In oriented category, the

inclusions are placed within the soil at specific positions and direction where as in

random category, inclusions, are mixed with soil and placed within the probable

shear zone. The concept of randomly reinforced soil is comparatively new in the

geotechnical field. French ministry of public works uses Texsol as RDFS. In the

field placing the fibers at some orientation is a tedious job. In reinforced soil the

added material (the Geo synthetic sheet, etc) is layered at specific direction and

position, which may keep the soil weaken in some other direction. Whereas in ply

soil, the isotropy in strength is maintained.

Random reinforcement have been provided to different type of soils in form

of mesh elements, discrete fibers, continuous yarn/filament (Texsol) metallic

power, waste tire chips, waste plastic strips, etc by various investigators.


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CHAPTER 3

STRENGTH PROPERTIES OF FIBER REINFORCED SOIL

3.1 GENERAL

Fibers mixed with the soil particles uniformly, the strength of the composite

soil is increased. Polypropylene fibers in a random distribution put in fly ash

studied through dynamic triaxial tests with different fiber content, load and

confining pressure. Taesoon Park analyzed strength properties and stability of

polypropylene staple fiber for the backfill body and the retaining wall. The effect

of the type, length and content of fiber reinforcement strengthening on soil. The

shear test, tensile test, fracture toughness test and determination of hydraulic

fracturing were also studied on polypropylene fiber reinforced clay. Certain efforts

were put on to research the resistance capacity of tensile cracks of polypropylene

fiber reinforced clay soil under static and dynamic load. Some research came up

with the assumption that mixing distributed polypropylene fibers into the lime soil,

in order to solve the problem of soil filling Lime-induced brittle failure. And

unconfined compression test, shear test, swelling test and shrinkage test were

carried out with kinds of soil samples under different fiber and lime content.

Strength properties and reinforcement mechanism were studied through the

conventional triaxial test under the condition of untrained and unconsolidation.

Triaxial tests were carried out with different length and content of the

polypropylene fiber reinforced on red clay. From analysis of existing research

results, it is concluded that the unconfined compressive strength, dynamic strength

and the ability of resistance to liquefaction were improved obviously when the soil

mixed with fibers. Synthetic soil significantly increased the cohesion of the soil,

improved soil shear strength, and the mechanical properties of reinforced soil are


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isotropic. At the same time the fiber reinforced soil would not present a typical pull

damage and adhesion destruction .It has the characteristic of plastic failure.

The mechanism of continuous fiber randomly distributed on the

soil summarized as "bending mechanism" and " interleaving mechanism."

Bending mechanism is that fiber in the distribution of the soil is composed of

numerous curved transitions. When the soil withstood external force, the fiber is in

tension, the pressure and friction of soil particles are produced by the earlier

concave side of curved fiber, which play the role on soil (Figure 3.1). Interleaving

mechanism is that there are numerous interwoven fibers points due to random

distribution fibers in the soil. When the fibers are forced at the intersection, they

will have the trend of displacement, at that moment the trend will be meet by the

other fiber to prevent such displacement. That is any deformation of the fiber will

affect the fibers which were interwoven in all directions and form the force area

(Figure 3.2).

Fig: Bending Mechanism Fig: Interleaving Mechanism


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Fig: Relationship of Stress-Strain Fig: The Relationship of Fiber

on fiber reinforced soil Content strength on fiber reinforced soil

3.2 FACTORS AFFECTING THE STRENGTHCHARACTERISTICS OF ENGINEERING PROPERTIES

OF RDFS.

The factors on which the strength characteristics and other engineering properties

of RFDS depend are as following:

(i) Type of soil it includes soil gradation expressed in terms of mean grain

size (D50) and uniformity coefficient (Cu).

(ii) Type of fiber: Monofilament or fibrillated

(iii) Denier of fiber: It is the weight (in gm) of 9000 m long fiber.

(iv) Fiber length

(v) Aspect ratio: It is defined as the ratio of the length of fiber to its diameter

(vi) Fiber soil surface friction.


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3.3 THE CONSTITUTIVE OF FIBER REINFORCED SOIL

The methods of calculation and analysis on the reinforced soil is divided into

two categories currently. One approach is separeted model. The one-dimensional

linear or a thin layer of rectangular element were used in simulation, so the

constitutive model could be used with the no-reinforced soil. Another approach is

that establishing composite model of the reinforced soil. The core is that composite

material was put on macro. The idea that considering the constitutive relationship

is more intuitive and applied widely in reinforced soil. But the fiber reinforced soil

is three dimensional, and the interface between the fiber and soil could not be

isolated. It was proposed that the reinforcement effect can be considered as an

additional compressive stress on the soil skeleton. The method is called equivalent-

additional stress. The equivalent additional stress method does not require the

introduction of new models, also reflect the anisotropy of the fiber reinforced soil.

But in some cases, the scale factor is difficult to fix. Nowadays the study of

constitutive on the fiber of reinforced soil is not enough. So how to choose a

reasonable model needs further exploration.

3.4 FACTORS AFFECTING MECHANICAL CHARATERSTICS OF

THE FIBER REINFORCED SOIL

The particularly factors affecting characteristics of the fiber reinforced soil is

important to study so as to take full advantage of fiber reinforcing effect. Certain

experiments were carried out to study the effect of fiber parameters reflecting the

characteristics of the fiber reinforced soil. The factors of strength properties of

fiber reinforced soil could be considered from the following aspects:-


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(1) Physical and mechanical properties of fiber itself. The friction generated among

the contact area is greater and the effect of fiber reinforced is better when the

contact area between fiber and soil particles is greater. The surface area can be

changed by changing the shape of the fiber and nowadays the shapes of fiber on

the market are "C", "J", "I", "Y", "+" types, etc. The problem is how to select the

shape of fiber. At the same time, its studies found that the toughness of fiber has a

certain relationship with the strength of the fiber reinforced soil. A reasonable

choice of fiber shape and toughness is critical on properties.

(2) Differences of filling on the reinforcing effect is significant, the friction bite

force between different types of soil particle and the continuous distribution fiber

is different, thereby the effectiveness of reinforcement is different so it is important

to choose the filling.

(3) Fiber content is closely related to strength and stability of the composite soil.

With the increasing of fiber content, strength and stability of the composite soil

will improve ,then peak and eventually decline. At the same time the best amount

of reinforcement directly influence the economy of the fiber soil.

(4) The length of fiber affects the properties of fiber reinforced soil. the current

research is lack of uniform understanding on the mechanism of fiber length so it

needs to be explored in depth.

(5) The moisture of the fiber reinforced soil has influence on characteristics. The

reinforced soil lubrication increased between the soil particles and the fiber when

the water content is too high, the further step is that cohesion and internal friction

angle drop lower. At the same time, compaction of soil is difficult, resulting in


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"spring" effect. On opposition, when water content is too low, the bond among the

filling will be affected. When the fiber reinforced soil used in engineering practice,

a reasonable choice of the construction environment is directly related to project

quality.


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ENGINEERING ASPECT OF SOIL REINFORCEMENTREFERENCE

1)WWW.GOOGLE.COM

2)WWW.WIKIPEDIA.COM

3) Soil Mechanics (BY B C PUNMIA)
http://www.google.com/http://www.google.com/http://www.google.com/http://www.wikipedia.com/http://www.wikipedia.com/http://www.wikipedia.com/http://www.wikipedia.com/http://www.google.com/

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