Reinforced earth structures notes.pdf

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MODULE 1
BASICS OF REINFORCED EARTH CONSTRUCTION
Definition: Reinforced earth is a material formed by combining earth and reinforcement. In actual
construction, the reinforced earth structures are composed of earth and reinforcing elements in the form of
strips disposed in horizontal layers. In these layers, the strips are set at certain intervals.
History: Reinforced Earth was invented in 1963, by the French architect-engineer Henri Vidal. It is a
construction material made of a frictional backfill material reinforced by linear flexible strips generally
placed horizontally. Since its invention, Reinforced Earth has found a wide use in many different areas of
civil engineering, notably in retaining walls, seawalls, dams, bridge abutments, and foundation slabs. This
technique has been adopted worldwide and the total number of Reinforced Earth structures built each year
has been continuously increasing.
Components of Reinforcement soil:
1. Soil and fill matrix
2. Reinforcement
3. Facing
In addition, other materials are required to cover associated elements such as foundation , drainage ,
connecting elements and capping units. In reinforced soil structure , soil constitutes most of the bulk. As a
general rule, it is almost always possible to build up a reinforced soil structure either using the soil which is
at site or using some other soil extracted from nearby place, but there is a need to know whether soil is
suitable or not for reinforced soil.
A variety of materials including steel, aluminium, rubber, concrete, glass, fibre wood and thermoplastics
have been successfully used as reinforcement in reinforces soil structures. Reinforcement may take the form
of strips, grids, sheets, ropes etc. It becomes important to select a suitable type and material considering both
durability and economy of structure.
Soil and fill matrix: The choice of fill matrix is determined from the following considerations
Cohesive frictional soil can be a convenient compromise between the technical benefits of cohesion less soil
and economic advantage of cohesive soil. The use of waste material as fill for reinforced soil structures is
desirable from an environmental as well as economic point of view. Coal based thermal power stations
produce massive quantities of coal ash. There are mainly two types of ashes which produce by burning the
coal. The lighter one goes up the chimney and collected either by mechanical or by electrostatic precipitator,
is known as fly ash. The other fraction containing coarser material is collected at the bottom of the furnace
known as bottom ash. Fly ash is about 90% of coal ash and poses serious environmental and disposal

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problems. The use of stabilized fly ash as a light weight fill in embankment construction is common. It is
usually placed with compaction by vibratory rollers or footpath compactors. Compaction is done at OMC.
This material can be used in reinforced soil structures.
Reinforcement: The most common type of reinforcement used in reinforced soil structure is
1. Strips
2. Grids and
3. Sheet
1. Strips: these are flexible linear elements having their breadth greater than their thickness. The thickness
usually ranges between 3 mm to 9 mm, while the breadth between 40 mm to 120 mm. The most common
strips are metals ( galvanized steel, Aluminium-Magnesium alloy , 17% chrome stainless steel)The strip
may be either plain or having several reinforcement and soil. Wherever metal strips are used as
reinforcement, provision should be made for loss of thickness due to corrosion. Marine sand dissolves
150-200 microns per year of strips of mild or galvanized steel, and 2-3 microns per year of strips of Al-
Mg alloy. Maximum rate of corrosion of the same metals with other soils is 15 to 20 times less. Ferric
steel with 17% chrome steel is found good corrosion resistant metal. This factor should be examined

carefully keeping in view the durability of the structure. However, metal reinforcements are used
providing an additional thickness of 0.75
steel, depending upon the nature of soil, for making up
from bamboo, polymers and glass fiber reinforced plastics.
Fig. 2.1 Manufacturing sequence for uniaxial and biaxial grids (After John, 1987)
Polymer Sheet
Punched Sheet
Hole Punching
carefully keeping in view the durability of the structure. However, metal reinforcements are used
of 0.75- 1.25 mm for galvanized steel and 0.1 to 0.2 mm for stainless
e nature of soil, for making up the loss of corrosion. Strips can also be formed
from bamboo, polymers and glass fiber reinforced plastics.
Fig. 2.2 Uniaxial and biaxial grids
Uniaxial grid
Biaxial Grid
Hole Punching
Longitudinal Stretching
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carefully keeping in view the durability of the structure. However, metal reinforcements are used by
ed steel and 0.1 to 0.2 mm for stainless
Strips can also be formed
Biaxial Grid
Longitudinal Stretching
Transverse Stretching

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2. Sheet Reinforcement: Sheet reinforcement may be formed from metal suck as galvanized steel,
fabric (textile) or expanded metal not meeting the criteria for the grid. Geofabrics are very common
sheet type reinforcement. These are porous fabrics manufactured from synthetic materials such as
polypropylene, polyester, polyethelene, polyamide and glass fibres. They come in thicknesses
ranging from 0.l25 mm to 7.5 mm with permeability comparable in range from coarse gravel to fine
sand. They can be constructed in a variety of ways, the most common methods being; (i) woven,
made from continuous monofilament fibres, and (ii) non-woven, made from continuous or staple
fibres laid down in a random pattern and then mechanically entangled into a relatively thick, felt-
like fabric by means of punching witch barbed needles. A wide variety of batch woven and non-
woven fabrics are available where the fibres are either bonded or interlocked. In India, many firms
are active in manufacturing geofabrics in the trade name of geotextile.
3. Facing elements: For vertical structures a facing is required. The purpose of the facing is to retain
the soil between the layers of reinforcements in the immediate vicinity of the facing and to provide
a suitable architectural treatment to the structure. Although the facing does not affect the overall
stability of the structure, it must be able to adopt the deformations without distortions and without
introducing stresses in the reinforced soil structures. Various materials like galvanized steel,
stainless steel, aluminium, bricks, precast concrete slabs, prestressed concrete panels, geotextiles
geogrids, plastics, glass reinforced plastics and timber may be used for this purpose. However,
facing made of either metal units or precast concrete panels are commonly used because of their
easy kindling and assembling (Scklosser, 1976).
a. Metal Facing: Metal facing elements are manufactured from mild or galvanized steel or
aluminium and have the same properties as the reinforcing strips. They are generally 333
mm high. In cross-section a metal facing element is semi-elliptical, and there is a continuous
horizontal joint along one edge (Fig. 2.5). Holes are provided for bolting the facing
elements to one another and to the reinforcing strips. This type of facing was the first to be
used in reinforced earth construction. Because of the shape in profile and the thinness in
cross-section of this type of facing, it can adapt itself to significant deformation. The
standard facing elements are straight, measure up to 10 m long, and weight 115 kg. Shorter
facing elements are available for connections at the extremities, and special units are
supplied for corners.
b. Concrete Panel Facing: The precast concrete panels are cruciform-shaped, weigh about
one ton, and are separated by a substantial joint. Vertical dowels set into the panels
assist in the assembly, and ensure the interaction between panels which makes the entire
facing behave a flexible unit, even in a situation where there are significant differential
settlements. The facing becomes a mosaic made up of units measuring 1.5 * 1.5 m.
Reinforcement
Soil
About 333 mm

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Each individual element is rigid, but in combination, the elements become a facing
whose-flexibility is equivalent to that obtained with metal facing elements
Principle of Reinforced Soil: The principle of reinforced soil is that an introduced material provides
a tensile restraining force that reduces the lateral stress required to maintain the equilibrium of a loaded soil.
As and when the soil element is compressed under vertical stress, it undergoes lateral deformation. When the
reinforcement added to the soil in the form of horizontal layers the soil element will be restrained against
lateral deformation as it is acted by a lateral force. It is important to note that the tensile force in the
reinforcing element depends on there being lateral strain. All reinforced soil structures are combinations of
suitable earth fill usually with several layers of the reinforcing elements placed on compacted fill. The
technique is used to construct
• vertical walls and abutments
• slopes of embankments steeper than would be stable (or at an acceptable degree of stability) with
unreinforced soil
• embankments on soft soils, where the foundation soil has inadequate bearing capacity to support the
height of fill
• unpaved roads
• special mattress-type foundations or stress-reducers for soft or backfilled ground
• Repairs to slipped material of earth slopes

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Advantages of reinforced soil
• Smaller quantities of earth fill are needed
• Steeper embankment slopes reduce the land take required
• Construction can be directly done on soft ground
• The technique is not a separate operation in the construction process, but is a part of the placing and
compaction of the earth fill. Hence structures can often be built more quickly than by conventional
methods.
Limitations of reinforced soil
Reinforced soil relies upon deformation for its effectiveness, which means the soil strain has to be
transferred to the reinforcement for it to develop its tensile or bearing resistance. There are some concerns
about the durability and long time performance of the reinforcing material. Severe and rapid corrosion of
steel reinforcement is possible. Polymeric materials degrade when exposed to ultra-violet rays and can be
damaged by rough handling on site or by sharp stones in fill. When constructing on soft soils care has to be
taken not to overstress the reinforcement.
Sandwich technique for clayey soil:
The sandwich technique contributes to shear-strength
improvement by increasing the friction angle of reinforced
specimens. The mobilized tensile strain and force of the
geotextile increased as the number of geotextile layers,
thicknesses of the sand layers, and confining pressure were
increased. The mobilized tensile strain and force were strongly
correlated to the strength difference between reinforced and
unreinforced soil. The experimental finding demonstrated that
mobilized tensile strain and force directly contribute to the shear
strength improvement of reinforced clay.

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Properties of geosynthetics: The properties of geosynthetics are typically grouped into those used for
quality assurance (QA) or quality control (QC) and those used for design. These two groups of
properties are sometimes referred to as index and performance properties, respectively. These names
have also taken on other meanings, such as index properties being those obtained from tests on the
geosynthetic itself as isolated from any surrounding soil, and performance properties being those
determined from tests where the geosynthetic is in contact with a subject soil.
Physical properties:
Physical properties of geosynthetics are basic properties related to the composition of the materials used to
fabricate the geosynthetic and include the type of structure, specific gravity, mass per unit area, thickness
and stiffness. The type of structure of a geosynthetic describes the physical make-up of the geosynthetic
resulting from theprocess used to manufacturethe material. Thestructureof thegeosynthetic often dictates
the application area for which the material is appropriate. For example, a uniaxial geogrid is appropriate
for applications where load is expected in one principal direction of the material, such as in a long slope
or retaining wall. The geosynthetic structure is most often described for geogrids. The structure of
geogrids of greatest importance is that associated with the manufacturing process used to form the
junctions of the geogrid, with examples including woven, integral and welded junctions. Structure can
also be described for geotextiles where the two main types of structure include woven and non-woven
geotextiles.
The specific gravity of a geosynthetic is measured on the basic polymeric material or materials used to
form the geosynthetic. The specific gravity is defined conventionally as the ratio of the material’s unit
volume weight to that of distilled, de-aerated water at a standard temperature. Ranges of values for the
specific gravity of commonly used geosynthetic polymers are listed in Table 2.1. The specific
gravity of the geosynthetic polymer is important in applications where the geosynthetic will be placed
underwater where polymers with values of specific gravityless than one will require weighting in order
to sink the material into position. Mass per unit area describes the mass (usually in units of grams) of a
material per unit area (generally in square metres) and should be measured with no tension applied
to the material. Typical values for geotextiles lie between 130 and 700 g/m2
while for geogrids the
values range from 200 to 1000 g/m2
.The thickness of a geosynthetic is measured as the distance
between the extreme upper and lower surfaces of the material. For geotextiles, this distance is measured
while a specified pressure is applied to the material. Thicknesses of geotextiles range from 0.25 to 7.5
mm. The thickness of common geomembranes used today is 0.5 mm. The physical property of
stiffness refers to the flexibility of the material and is not a description of the mechanical property of
stiffness which describes the material’s load–strain modulus. The flexibility of a geosynthetic is
determined by allowing the material to bend under its own weight as it is being slid over the edge of a

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table. The properties of flexural stiffness or rigidity describe the material’s capability of providing a
suitable working platform during installation and is an important property when installation is
performed over soft soil sites.
Mechanical properties
Mechanical properties of geosynthetics relate to applications where the geosynthetic is required to
bear a load or to undergo a deformation. During the construction of facilities containing geosynthetics,
loads perpendicular to the plane of the geosynthetic can be introduced as the material is placed on
irregular surfaces with soil compacted on top. These loads can be significant and can often dictate the
mechanical properties specified for the geosynthetic. Failure to specify appropriate mechanical
properties for the construction conditions may result in physical damage (i.e. punctures, tears and rips)
to the geosynthetic, which then may compromise the mechanical properties needed for proper
functioning of the application.
Loading can also be applied in the plane of the geosynthetic resulting in tension of the material. This
type of loading is generally associated with the function or operation of the constructed facility and
where the mechanical properties of the geosynthetic are typically used in the design of the facility.
Mechanical properties pertaining to the shearing resistance between the geosynthetic and the
surrounding soil are also important as this resistance is responsible for transferring load from the soil
into tensile load in the geosynthetic.
Mechanical properties of geosynthetics are often categorized as either index or performance
properties. Index properties refer to those determined on the geosynthetic itself in the absence of any
surrounding soil. These properties are sometimes referred to as in-isolation properties. Performance
properties involve those determined in the presence of a standard soil or the site-specific soil.
Tensile properties
Tensile properties of geosynthetics are generally the most important set of properties, particularly for
applications where reinforcement is the primary function of the geosynthetic. Tensile properties are
used for quality QC/QA and as design parameters for various applications. The tests used for QC and
QA purposes tend to be simpler and less time consuming to perform and interpret than those used to
generate design parameters. The grab tensile test is performed on geotextiles and provides QC and QA
Polymer Specific gravity
Polyamide 1.05–1.14
Polyester 1.22–1.38
Polyethylene 0.90–0.96
Polypropylene 0.91

information that can only be used comparatively between geotextiles with similar structures since each
material structure performs in a unique manner in this test. The test is performed by gripping the
specimen and applying a continuously increasing
corresponding elongation are measured and
may occur in the field because of the spreading action of two pieces of coarse aggregate in conta
with the geosynthetic.
Fig 2.1 Specimen sizes for various tensile tests: (a) grab; (b) geotextile wide width; (c) geogrid wide width,
method A; (d) geogrid wide width, method B and C.
Figure 2.1(b), Fig. 2.1(c) and Fig. 2.1(d) illustrate tension tests used to deter
properties of geotextiles and geogrids. Figure 2.1(b) shows the test specimen size for wide
tests on geotextiles (ASTM D4595). For geogrids
(Fig. 2.1(d)) may be used according to ASTM D6637. For the tests shown in Fig. 2.1(b), Fig. 2.1(c) and
Fig. 2.1(d), the ultimate strength, strain at failure and modulus are typically determined. The s
modulus are typically expressed in terms of a load per unit width of material rather than a stress since
stress requires the definition of material thickness, which is generally difficult to describe for most
specimen and applying a continuously increasing load until rupture occurs. The load at rupture and the
corresponding elongation are measured and reported. The grab tension test also represents loading that
may occur in the field because of the spreading action of two pieces of coarse aggregate in conta
Specimen sizes for various tensile tests: (a) grab; (b) geotextile wide width; (c) geogrid wide width,
method A; (d) geogrid wide width, method B and C.
Figure 2.1(b), Fig. 2.1(c) and Fig. 2.1(d) illustrate tension tests used to deter
properties of geotextiles and geogrids. Figure 2.1(b) shows the test specimen size for wide
tests on geotextiles (ASTM D4595). For geogrids, either multirib specimens (Fig. 2.1(c)) or single
Fig. 2.1(d), the ultimate strength, strain at failure and modulus are typically determined. The s
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load until rupture occurs. The load at rupture and the
reported. The grab tension test also represents loading that
may occur in the field because of the spreading action of two pieces of coarse aggregate in contact
Specimen sizes for various tensile tests: (a) grab; (b) geotextile wide width; (c) geogrid wide width,
Figure 2.1(b), Fig. 2.1(c) and Fig. 2.1(d) illustrate tension tests used to determine tensile design
properties of geotextiles and geogrids. Figure 2.1(b) shows the test specimen size for wide-width tension
, either multirib specimens (Fig. 2.1(c)) or single-rib
Fig. 2.1(d), the ultimate strength, strain at failure and modulus are typically determined. The strength and

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geosynthetics and does not remain constant during tensile loading. The modulus can be defined as an
initial modulus, a secant modulus or an offset tangent modulus. Modulus values are very dependent on
how the specimen is conditioned at the beginning of the test and standardized procedures should be
followed to ensure comparability of results. Since geosynthetics are rate and temperature dependent,
standards should also be followed with respect to these test variables. Geosynthetic materials are typically
direction dependent, meaning that tension tests should be performed in both principal material directions.
For tests where elongation or strain is measured, displacement measurement techniques become
important. If displacement is measured as movement between the grips, then slippage within the grips
should not occur. For geotextiles with strengths less than 90 kN/m, conventional clamping grips are
usually sufficient. Wedge grips may be good for materials with strengths between 90 and 180 kN/m. For
materials with strengths exceeding 180 kN/m, roller grips are typically used.
The tension tests described above are performed without any soil covering the geosynthetic and are
therefore known as in-isolation or in-air tests. Soil covering the geosynthetic provides confinement to the
material, which in general has the effect of increasing the material’s modulus and strength. Increases in
modulus and strength are most significant for non-woven geotextiles, but also noticeable for woven
geotextiles and geogrids (Elias et al., 1998) This results from internal friction between fibres or yarns,
alignment of curved fibres or yarns, and interlocking of soil within openings or apertures of geosynthetics
Compressibility
The compressibility of geosynthetics is defined as the relationship between the material thickness as a
function of applied normal stress and is a test most appropriate for geotextiles and geonets that need to
maintain a certain thickness to ensure water transmissivity. For geotextiles, non-woven needle-punched
materials tend to be the most compressible, while woven and non-woven heat bonded materials show
small levels of compressibility. Some materials, especially geonets, tend to experience small levels of
compression prior to collapse. For these materials, the compression strength is of most importance.
Burst strength
Burst strength tests are performed on geotextiles and geomembranes by causing a circular piece of
material clamped around its perimeter to stretch into the shape of a hemisphere by the application of
pressure on one side of the material. The material stretches in tension until rupture occurs. In the field,
geotextiles may experience this type of loading when used as a separator between soft subgrade and
coarse aggregate. As subgrade is squeezed upwards between voids of the coarse aggregate, the geotextile
takes on a hemispherical shape similar to that experienced in the burst strength test. Geomembranes may
experience this kind of loading in landfill applications where a void might open up beneath the
geomembrane layer.
Tear strength
During the installation of geotextiles, stresses may be imposed which cause tears to initiate and
propagate. Several types of tests have been developed to describe the tearing resistance of geotextiles.
The most common test is the trapezoidal tear test (ASTM D4533). In this test, the specimen is formed in
the shape of a trapezoid, as shown in Figure 2.2, and a 15-mm cut is made along one end of the specimen.
The two non-parallel sides of the specimen are gripped in parallel grips of a tension load frame with the
two grips aligned parallel to the cut made in the material and separated by a distance of 25 mm. This is

accomplished by allowing folds to occur in the material greater than 25 mm in width. Tension is then
applied and the cut in the material pr
are torn. Minimum values of tear strength are generally specified to control installation damage of
geotextiles
Puncture strength
In addition to the possibility of tears during
punctures from rocks, roots, sticks or other debris. A test has been developed to measure the puncture
resistance of these materials where a steel rod of 8 mm diameter is used to puncture a geo
stretched and clamped firmly over a cylinder of 45 mm inside diameter. The force necessary to cause the
rod to puncture through the material is known as the puncture resistance.
Friction
An adaptation of the direct shear test for soils is used to me
between geosynthetics and soils and between two layers of geosynthetics. Test method ASTM D5321
calls for a shear box measuring 300 mm by 300 mm. The shear box is configured to contain soil in the
bottom half and the geosynthetic clamped to the top half of the box. Within the confines of the top half of
the box and above the clamped geosynthetic, soil or a textured block may be used to transfer shear load
evenly across the face of the geosynthetic.
The test is performed similarly to a direct shear test on soil with normal confinement being applied to the
box prior to applying a horizontal shear displace
and shear relative to each other. The shear load is measure
against the horizontal shear displacement. The ultimate shearing resistance is plotted against the normal
stress confinement for several tests at different levels of confinement. A Mohr
is then obtained from these data. Values of cohesion and friction angle are compared with those obtained
for the soil itself to arrive at shear strength parameter efficiencies. Similar procedures are followed for
tests performed between two layers of ge
landfills where geosynthetic sheets are in contact with soil materials or other geosynthetic sheets on
slopes where sliding can occur
applied and the cut in the material propagates across the specimen as individual strands of the geotextile
In addition to the possibility of tears during installation, geotextiles and geomembranes can experience
where a steel rod of 8 mm diameter is used to puncture a geo
rod to puncture through the material is known as the puncture resistance.
An adaptation of the direct shear test for soils is used to measure the shearing resistance or friction
he geosynthetic clamped to the top half of the box. Within the confines of the top half of
evenly across the face of the geosynthetic.
rmed similarly to a direct shear test on soil with normal confinement being applied to the
box prior to applying a horizontal shear displacement that causes the two halves of the box to displace
and shear relative to each other. The shear load is measured and divided by the area of shear and plotted
stress confinement for several tests at different levels of confinement. A Mohr–Coulomb failure criteri
tests performed between two layers of geosynthetics. Results from these tests have applications in
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opagates across the specimen as individual strands of the geotextile
installation, geotextiles and geomembranes can experience
where a steel rod of 8 mm diameter is used to puncture a geosynthetic
asure the shearing resistance or friction
he geosynthetic clamped to the top half of the box. Within the confines of the top half of
rmed similarly to a direct shear test on soil with normal confinement being applied to the
ment that causes the two halves of the box to displace
d and divided by the area of shear and plotted
Coulomb failure criterion
osynthetics. Results from these tests have applications in

Pull-out resistance
Pull-out tests are typically performed to
capacity is important in situations such as retaining walls, slopes and bridging over voids, where the
geosynthetic is anchored into stable ground that is outside the zone of failure. The tes
assess interface shear resistance and stiffness properties for applications where soil is moving relative to
the geosynthetic, such as in reinforced roadways.
The test is performed in an apparatus described by ASTM D6706 and shown in
dimensions shown are minimum dimensions that may need to be increased depending on the structure of
the geosynthetic, particle size of the soil, and provisions for reducing side
confinement is provided by an air bag placed between the top of the soil and a reaction frame. A sleeve is
fitted to the front of the box where the geosynthetic enters and extends a minimum of 150 mm into the
box. The purpose of the sleeve is to reduce the amount of normal stress gene
the box as the geosynthetic is being pulled out. Measurements during testing typically consist of applied
pull- out load, horizontal displacement of the front of the geosynthetic and horizontal displacement of the
geosynthetic at several locations along the material’s length. The later is accomplished with the use of a
telltale, which consists of a protected wire attached to the measurement point on the geosynthetic and
extending out from the back of the box where it is attache
The pull-out resistance or anchorage capacity is calculated as a line load taken as the force necessary to
cause pull-out divided by the width of the specimen. This force is typically used to compute an
interaction coefficient, which is essentially the ratio of the friction angle of the geosynthetic
to that of the soil itself. To make the calculations described above, it is important that sufficiently large
displacement occurs along the entire embedment len
shearing resistance is fully mobilized. For long embedment lengths and large normal stress confinement,
this may not be the case and the test must then be interpreted as a boundary
methods have been proposed (Juran and Chen, 1988; Yuan and Chua, 1991; Perkins and Cuelho, 1999).
Hydraulic properties
Hydraulic properties of geosynthetics are important in applications where the material is used to
convey or prevent the flow of liquids and gases. Geotextiles, geomembranes, geonets, geosynthetic
clay liners and drainage composites are all materials that are called upon to perform these functions.
Applications include drainage materials behind walls and within slopes, roadways and
tion materials within roads and around drainage trenches, and liquid and gas containment for ponds,
for canals and within landfills.
out tests are typically performed to assess the anchorage or pull-out capacity of geosynthetics. This
geosynthetic is anchored into stable ground that is outside the zone of failure. The tes
the geosynthetic, such as in reinforced roadways.
The test is performed in an apparatus described by ASTM D6706 and shown in
the geosynthetic, particle size of the soil, and provisions for reducing side-wall friction. Normal stress
air bag placed between the top of the soil and a reaction frame. A sleeve is
box. The purpose of the sleeve is to reduce the amount of normal stress generated along the front wall of
is being pulled out. Measurements during testing typically consist of applied
out load, horizontal displacement of the front of the geosynthetic and horizontal displacement of the
at several locations along the material’s length. The later is accomplished with the use of a
extending out from the back of the box where it is attached to a displacement-sensing device.
out resistance or anchorage capacity is calculated as a line load taken as the force necessary to
out divided by the width of the specimen. This force is typically used to compute an
icient, which is essentially the ratio of the friction angle of the geosynthetic
displacement occurs along the entire embedment length of the geosynthetic such that the ultimate
this may not be the case and the test must then be interpreted as a boundary-value problem where several
liquids and gases. Geotextiles, geomembranes, geonets, geosynthetic
Applications include drainage materials behind walls and within slopes, roadways and
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out capacity of geosynthetics. This
geosynthetic is anchored into stable ground that is outside the zone of failure. The test can also be used to
The test is performed in an apparatus described by ASTM D6706 and shown in Fig. 2.3, where the
wall friction. Normal stress
air bag placed between the top of the soil and a reaction frame. A sleeve is
rated along the front wall of
is being pulled out. Measurements during testing typically consist of applied
out load, horizontal displacement of the front of the geosynthetic and horizontal displacement of the
at several locations along the material’s length. The later is accomplished with the use of a
sensing device.
out resistance or anchorage capacity is calculated as a line load taken as the force necessary to
out divided by the width of the specimen. This force is typically used to compute an
icient, which is essentially the ratio of the friction angle of the geosynthetic–soil interface
gth of the geosynthetic such that the ultimate
value problem where several
liquids and gases. Geotextiles, geomembranes, geonets, geosynthetic
Applications include drainage materials behind walls and within slopes, roadways and landfills, filtra-

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Porosity
The porosity is a convenient property in that it has the same definition (the ratio of the void volume to the
total volume) as that used for soils. The void volume, however, is difficult to measure, so the porosity has to
be calculated from other physical properties (mass per unit area, density and thickness). As a result, other
measures, including the percentage open area and apparent opening size (AOS), related to the porosity but
more easily measured and more directly related to particular applications have been developed
Percentage open area
The percentage open area is a property that is specified and measured for woven geotextiles and is a property
that describes the ratio of the open area to the total area. The open area is typically measured by shining light
through the material and projecting this light on to a screen that can be used to measure and sum the open areas.
This test is not appropriate for non-woven geotextiles since the overlap of the weaves prevents most light from
shining through even though liquid transmission is still very possible.
Apparent opening size
The AOS test was first developed for woven geotextiles but is now also used for non-woven materials. The test
is described by ASTM D4751 and consists of passing glass beads of successively larger diameter through the
material until only 5% of the beads pass through. The size of the beads in millimetres at which 5% passes is
known as O95. The corresponding size in the US sieve size is the AOS. The AOS or O95 represents the largest
particle that would effectively pass through the geotextile. The equivalent opening size (EOS) has the same
meaning as the AOS but can be specified for other percentage passing values, such as O50 or O90. The AOS is
typically specified in conjunction with requirements for filtration, with proper specification providing for soil
retention without pore space clogging
Permittivity
The permittivity describes the ability for fluid flow across the plane of the geosynthetic. It is formally defined as
the cross-plane permeability divided by the thickness of the geosynthetic. ASTM D4491 describes a constant-
head and a falling-head permeability test that is used to define permittivity under zero- normal-stress
confinement. These tests are conducted like similar tests on soils only with the apparatus sized to accommodate
the flows associated with geotextiles. Values of cross-plane permeability for geotextiles range from 0.0008 to
0.23 cm/ s with a corresponding range of permittivities ranging from 0.02 to 2.1 s–1. Non- woven needle-
punched geotextiles experience a slight to moderate decrease in permittivity as the normal stress confinement
on the material is increased. Geonets have values of permeability of the order of 1–10 cm/s. Geomembranes
have a value of 10–11 cm/s while geosynthetic clay liners have saturated values ranging from 5.0 × 10–9 to 1.0 ×
10–10 cm/s.
Transmissivity
Transmissivity describes the ability for fluid flow within the plane of the material and is defined as the in-plane
permeability multiplied by the material thickness. The test method ASTM D4716 describes a constant-head test
that can be conducted under varying normal stress confinement. Fluid is caused to flow one- dimensionally in
the plane of the material from one end to another under constant-head conditions. Values of in-plane
permeability of geotextiles range from 0.0006 to 0.04 cm/s with corresponding transmissivity values ranging
from 3.0 × 10–9 to 2.0 × 10–6 m2/s.

19
Soil retention
Geotextiles are often used as fences to retain fines as turbid water flows from disturbed areas to streams,
ponds or lakes. The ability of the geotextile to allow water flow while retaining soil particles is determined by
ASTM D5141. In this test, the site-specific soil is mixed with water to form a slurry and is poured into a flume box
set on a 8% slope with the downstream end covered by the candidate geotextile. The flow rate of the soil–water
mixture passing through the geotextile is measured together with the amount of fines. These measurements
allow for the slurry flow rate and retention efficiency to be determined. The process is repeated at least three
times to determine the degree of clogging that occurs.
Endurance properties
Endurance properties of geosynthetics focus on how short-term properties are affected by time during the
service life of the facility. Issues of endurance arise as the material is installed, while the load is sustained, and
while fluid flow is experienced.
Installation damage
The deformations and stresses experienced by geosynthetics during installation can be more severe than
the actual design stresses for the intended application and arise from the placement and compaction of
overlying fill. Damage may occurin the form of holes, tears and ruptures, which influences the mechanical
and hydraulic properties of the material. Criteria for survivability of geosynthetics have been developed by
AASHTO M288-96. These criteria consider the construction conditions of the subgrade, the contact pressure
provided by the construction equipment and the compacted base course thickness to be used. Based on the
combination of these conditions, the survivability level of the geosynthetic is assessed. The survivability
level is then expressed in terms of certain geosynthetic index properties. Field trials can also be performed
using the site-specific ground conditions, construction equipment and procedures with the installed
material exhumed immediately after placement to assess damage
Creep and stress relaxation
Creep is defined as the elongation of a material under a constant load. Stress relaxation is the reduction in
(relaxation of) stress when a material is loaded and then held at a constant level of strain. Creep is an important
consideration in design as large levels of creep can lead to excessive deformation of reinforced structures or
possible creep rupture of geosynthetics. Stress relaxation can result in more load being taken up by the soil,
which may produce unsafe conditions for situations where the soil is close to failure.Creep and stress
relaxation are interrelated and are dependent on the viscous properties of the geosynthetic. Viscous
properties of geosynthetics are dependent on the type of polymer. Creep and stress relaxation are most
significant for geosynthetics composed of polypropylene and polyethylene and less significant for polyester
and polyamide geosynthetics. The magnitude of creep and stress relaxation increase as the temperature,
magnitude of load and time increase (Greenwood and Myles, 1986). Cyclic loading can also produce creep and
stress relaxation since cyclic loading is another form of sustained loading. ASTM D5262 describes a test
method for determining elongation due to creep. The test is relatively simple to conduct and involves placing
hanging weights on a geosynthetic specimen and making periodic measurements of elongation. A stress
relaxation test is more difficult to conduct in that a fixed displacement must be applied and the load over time
must be monitored. This implies the use of a displacement controlled device typically with electronic load-
sensing devices

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Abrasion
The abrasion of geosynthetics is defined as the wearing away of any part of a material by rubbing against
another surface. Excessive abrasion can lead to a loss of properties, e.g. strength, that are needed for proper
functioning. The most pertinent ASTM specification for abrasion testing is ASTM D4886 and is used for
geotextiles. In this test, the specimen is mounted on a stationary horizontal platform and is rubbed by an
abradant (typically sandpaper) mounted on a flat block. The vertical pressure is controlled while the block
containing the abradant is moved back and forth along a uniaxial path. Resistance to abrasion is expressed as a
percentage of the original strength of the material. While this test is techni- cally valid for geogrids and
geomembranes, it has only been evaluated for geotextiles and a larger database of results are needed before
it can be used for other materials
Clogging
Clogging is an endurance property most pertinent to geotextiles. Clogging can occur over the long term as
fluid flows through the geotextile carrying with it suspended particles that become lodged within the
material. Physical tests have been devised and evaluated to match these long-term conditions and using
site specific soils. These tests suffer from the large amount of time that it takes to conduct the test. The
gradient ratio test (ASTM D5101) has been adopted to reduce the amount of testing time associated with
other more direct physical tests. The test is set-up within a vertical column with a layer of soil placed on top
of a geotextile. Vertical flow is maintained through the soil–geotextile system. The hydraulic gradient is
measured as the head loss divided by the flow length for two regions of the system. The first region contains
the geotextile and 1 in of soil above the geotextile. The second region contains 2 in of soil and extends over
a length of 1 in above the geotextile to 3 in above the geotextile. The ratio of these gradients is used to assess
the clogging potential of the system, with values of three or greater indicating the potential for clogging.
The structure of the geotextile influences the possibility for the formation of a soil cake on the upstream side
of the material. If gaps exist between the geotextile and the soil, soil fines tend to collect within these gaps and
form a soil cake. This leads to clogging of the surface of the geotextile and is referred to as blinding. Materials
with a tortuous surface, such as non-woven needle-punched materials, tend to conform more to the irregular
surfaceof asoil,formlessgaps andshow less blinding.
Degradation
Degradation of a geosynthetic results from fundamental changes of the polymer at the molecular level from
its as-fabricated state. Degradation processes leading to ageing of the polymer include molecular chain
scission, bond breaking, cross- linking and the extraction of components. Chemical fingerprinting methods
are available that detect polymer changes: however, these methods are expensive to perform. Common
and less expensive tests such as tensile strength and elongation are therefore conducted to assess the
impact of these changes.
Temperature
Increasing the temperature has the principal effect of accelerating other degradation mechanisms. When
viewed as a degradation mechanism, temperature is therefore generally associated with other mechanisms
such as those involving oxidation, hydrolysis, chemical, radioactive, biological and ultraviolet (UV) light
processes. High temperatures approaching the melting point of the polymer (165 ºC for polypropylene and
125 ºC for polyethylene) are an obvious consideration and should be avoided. Low temperatures can
influence the brittleness and impact strength of geosynthetics, which influences their workability and
potential for damage during installation.
Oxidation

21
Oxidation is a reactive process by which the elements of a material lose electrons when exposed to oxygen and
its valence is correspondingly increased. In geosynthetics, this reaction leads to a fundamental change in the
polymer and a degradation of the properties of the material. Polypropylene and polyethylene are generally the
most susceptible polymers to the oxidation process. A test method used for exposing geosynthetics to the
oxidation process is ASTM D794 specified for plastics. This test method uses an oven to apply heat with a
continuous fresh- air flow. The test is carried out to a point where there is an appreciable change in
appearance, weight, dimension or other specified properties pertinent to the application in question
Hydrolysis
Hydrolysis is a process by which a chemical compound decomposes by its reaction with water. Geotextiles can
experience hydrolysis degradation by internal or external yarn degradation (Hsuan et al., 1993), which
becomes more significant for polyester materials and for liquids with a high alkalinity. Polyamides can be
affected by liquids with very low pH values. To evaluate the effect of hydrolysis, simple tests are conducted
where a material is immersed in a liquid having a pH level of interest and at temperatures of 20 °C and 50 °C.
The strength of the material is determined after a certain amount of immersion time and compared with
initial values to detect degradation levels
Chemical degradation
Chemical degradation involves the change in material properties when the geosynthetic is immersed in various
chemicals of interest. ASTM D5322 describes a laboratory test procedure for immersing geosynthetics in
chemical liquids. Provisions are given for controlling the temperature, the pressure and the circula- tion of the
solution. ASTM D5496 describes a procedure for immersion of field specimens. These tests are most often
used in association with geosynthetics used in landfills and as liners in reservoirs, ponds and impoundments
Functions of geosynthetics:
Filtration
Unlike other methods, geosynthetics provide filtration for water passing through the soil in both
directions. Rainfall that seeps downward through the soil tends to carry along many chemicals and
other contaminants, especially on farms, around manufacturing facilities, and next to roadways.
Geotextiles and geocomposites are the two geosynthetics most widely used for this purpose. Clay
impregnated grids are also used since the bentonite layer can serve as a filter if it’s thin enough to
still allow some water to seep through. The filtration function is primarily designed to prevent soil
loss, making it essential for erosion control. Water is allowed to flow into the ground without
carrying along any valuable minerals or unwanted additions mixed in. It’s also a good option where
contamination is a concern with nearby surface waters or ground water supplies. Landfills rely on
permanent leachate systems built around filtering geosynthetics to contain every drop of
wastewater.
Separation
Separation is the most common use for geosynthetics since it’s essential for roadway construction.
Without layers of geomembranes and geotextiles to keep various materials from making direct
contact, roadways would quickly spread and crack from the constant downward forces pressing them
together. Geocomposites, geotextiles, geomembranes, geocells, and even geofoams are used to
accomplish this goal. Materials that offer separation also tend to work well for reinforcing and
improving drainage of soils as well. This means you’ll find a wide range of uses for separating

22
geosynthetics on construction and roadway projects. Agricultural and manufacturing sites also
benefit from separation membranes that isolate layers of soil that might shift if allowed to blend with
another layer.
Reinforcement
Many of the same geotextiles and grids used to separate soil layers also provide powerful
reinforcement features. This is particularly common on slopes and areas where water passes over
the surface of exposed soil. Geogrids and nets may allow for drainage as well, but they’re primarily
used for reinforcement thanks to the increased amount of contact with individual soil particles.
Reinforcement grids and membranes are also needed under roadways that will experience heavy
loads over time, to prevent splitting and spreading forces from ruining the surface. Don’t forget
about reinforcement geosynthetics for the banks and sides of holding ponds and other liquid
containment areas. Since so many uses for geosynthetics involve reinforcement or strengthening
weak soils, it’s not surprising that this function is shared by so many different materials.
Containment
Containment is similar to separation, but this function goes a step farther to contain liquids and
gases, in addition to soil particles. Geomembranes are the primary method used for containment,
followed by geocomposites and geosynthetic clay liners. In areas where filtration isn’t enough to
protect the soil and water from contamination, containment is the primary, preferred function
instead. You can also find this geosynthetic function described as the barrier method. The material
used for this process must offer a high level of impermeability. Permeable materials allow water and
other liquids to seep through, but liners and other geomembranes can keep even gases under
control. These barriers block both incoming and outgoing water from moving through a soil surface.
Containment methods are often required for farms, manufacturing facilities, mines, and other
potential sources of hazardous waste.
Drainage
When you need water to absorb into the soil below, without causing erosion or flooding, consider a
drainage geosynthetic. Thin layers of geotextiles and geosynthetic clay liners allow water to slowly
seep through in a controlled fashion. If the natural soil doesn’t have the pore capacity for steady
absorption, flooding and pooling occurs and creates extensive erosion problems. Drainage
geosynthetics encourage a better penetration rate, reducing water standing on the surface of the
ground during a heavy rainfall event. Drainage installations are generally found around roadways,
along the edges of slopes, and at the bases of retaining walls.
Geomembranes made with reinforced polyethylene (RPE) are great for containment and
reinforcement purposes. When you’re looking for a liquid and vapor barrier that won’t react to
chemicals or break down under UV exposure, turn to BTL Liners. Our RPE products are designed to
withstand the most challenging installations with minimal maintenance. Rely on our expertise to find
the right liners for your needs rather than trying to make the decision alone.

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Woven & Non Woven geotextiles Manufacturing process:
Geotextiles are split into two principal categories, nonwoven & woven. They are permeable fabrics which,
when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain. Geotextiles
are typically manufactured from polypropylene or polyester, although other materials are used. The
manufacturing method and the materials used have a significant effect on the corresponding properties of the
material and therefore the selection of the right product type is paramount.
Nonwoven Geotextiles A nonwoven geotextile is made of directionally or randomly orientated fibres which
are laid down in a web and bonded together in a variety of ways The importance of bonding is such that
specific nonwoven processes are often identified by referring to the bonding step alone .
Web Formation
The characteristics of the fibrous web are a key determinant of the physical properties of the final
product. The choice of methods for forming webs is determined by fibre length. Initially, the
methods for the formation of webs from staple-length fibres were based on the textile carding
process, whereas web formation from short fibres was based on a wet laid process similar
papermaking. These technologies are still in use, but methods based forming a web directly from
filaments immediately they exit an extruder (Spun laid) have also been developed. Fibrous webs
have little mechanical strength and a further manufacturing process is necessary to form a fabric
with useful properties. There are number of processes which are used to accomplish this as
described in next section.
Web bonding
Needle punching is a process of bonding nonwoven web structures mechanically interlocking the
fibers through the web. Barbed needles, mounted on a board, punch fibers into the web and then
are withdrawn leaving fibers entangled. The needles are spaced in a non-aligned arrangement are
designed to release the fiber as the needle board is withdrawn.
Stitch bonding is a method of consolidating fiber webs with knitting elements with or without yarn to
interlock the fibers. There are a number of different yarns that can be used. Home furnishings are a
market for these fabrics. Other uses are vacuum bags, geo-textiles, filtration and interlinings. In many
applications stitch-bonded fabrics are taking the place of woven goods because they are faster to
produce and, hence, the cost of production is considerably less.
Thermal bonding is the process of using heat to bond or stabilize a web structure that consists of a
thermoplastic fiber. All part of the fibers act as thermal binders, thus eliminating the use of latex or
resin binders. Thermal bonding is the leading method used by the cover stock industry for baby
diapers. Polypropylene has been the most suitable fiber with a low melting point of approximately
165C. It is also soft to touch. The fiber web is passed between heated calendar rollers, where the
web is bonded. In most cases point bonding by the use of embossed rolls is the most desired
method, adding softness and flexibility to the fabric. Use of smooth rolls bonds the entire surface of
the fabric increasing the strength, but reduces drape and softness.

Chemical bonding is the process of bonding a web by means of a chemical and is one of the most
common methods of bonding. The chemical binder is applied to the web and is cured. The most
commonly used binder is latex, because it is economical, easy to apply and very effective. Several
methods are used to apply the binder and include saturation bonding, spray bonding, print bonding
and foam bonding.
Hydro entanglement is a process of using fluid forces
fine water jets directed through the web, which is supported by a conveyor beit. Entanglement
occurs when the water strikes the web and the fibers are deflected. The vigorous agitation within the
web causes the fibers to become entangled.
Finishing and converting
Finishing and converting are the last operations performed on the fabric before it is delivered to the
customer. Finishing includes operations such as coating and laminating, calendaring and embossin
to impart particular surface properties, corona and plasma treatments to change the wetting
properties of the fabric, wet chemical treatments to impart anti
properties, flame retardant properties etc. Aft finishing the fa
customer specifies a rewound ready for shipment. This is known as converting.
is the process of bonding a web by means of a chemical and is one of the most
binder is latex, because it is economical, easy to apply and very effective. Several
is a process of using fluid forces to lock the fibers together. This is achieved by
the fibers to become entangled.
customer. Finishing includes operations such as coating and laminating, calendaring and embossin
properties of the fabric, wet chemical treatments to impart anti-stat" properties, anti
properties, flame retardant properties etc. Aft finishing the fabric, it is usually cut to the width the
24
is the process of bonding a web by means of a chemical and is one of the most
binder is latex, because it is economical, easy to apply and very effective. Several
to lock the fibers together. This is achieved by
customer. Finishing includes operations such as coating and laminating, calendaring and embossing
stat" properties, anti-microbial
bric, it is usually cut to the width the

25
Woven Geotextiles:
Woven geotextiles are manufactured by interlacing two parallel sets of elements
at right angles to form a coherent structure. The properties of the geotextile will
be a function of the elements used and the weave pattern.
Weaving can be summarised as a repeat of three primary actions:
Shedding: where the ends are separated by raising or lowering heald frames to
form a clear space where the pick can pass.
Picking: where the weft or pick is propelled across the loom by hand, as air-jet, a
rapier or a shuttle.
Beating-up: where the weft is pushed up against the fell of the cloth by the reed.
The elements that are used to weave a geotextile may be produced in a variety
of ways:
1. Slit Film Tape
2. Extruded Flat Tape
3. Multifilament Yarn
4. Tape Yarn
5. Monofilament
6. Combination Weaves
Slit Film Tape: A slit film tape is similar to an extruded film tape with the
principal difference being that rather than being directly extruded from a tape a
sheet is produced and then slit down into narrow strips. The same drawing
process is used to impart strength into the weaving element. It is common
place to fibrillate the tape by adding spall nicks. This fibrillation process can
allow for a tighter weave and a higher strength finished fabric.
Extruded Flat Tape: An extruded flat tape is manufactured using the screw
extrusion process previously described. The tape is drawn off the extruder by a
bank of rollers which rotate at differing speeds to apply tension. This tension
orientates the molecules in the polymer to impart strength. The drawing
process will reduce the thickness of the tape so that it can be woven into a
relatively strong fabric.
Tape Yarn A yarn uses a slit film tape which is then wound into a yarn to allow
for the production of heavier, stronger woven geotextiles. The fibrillation
process allows for the tape to be twisted and spun into a yarn which provides a
much stronger weaving element.

26
Monofilament Although, strictly speaking, the term monofilament could
encompass flat tapes, it generally refers to extruded elements with a circular
cross section. The manufacturing process is basically the same as that which is
used to produce an extruded flat tape except the die head is shaped differently.
The finished fabric would have a very different structure to that of a tape,
normally with differing filter properties
Multifilament A multifilament yarn uses similar technology to that which is
used to produce a fibre for a nonwoven geotextiles, except rather than lay the
fibres down randomly the fibres are spun into a yarn and used in the weaving
process. As with a fibre manufacturing process, the elements are drawn to
impart strength into the weaving element.
Combination Weave: The weaving process is such that the warp and weft yarns
do not necessarily need to be the same type of weaving element. It is therefore
possible to manufacture woven geotextiles which combine different
combinations to obtain a variety of finished fabrics
Major Differences Between Woven and Non-Woven Geotextiles
Woven Geotextiles
This category of geotextiles is manufactured by weaving. Individual threads, be
it monofilaments, fibrillated yarns, slit films or other material, are woven
together on a loom one large, uniform piece. This process gives woven
geotextiles a high load capacity, which makes them good for applications like
road construction. Weaving threads or films together means these geotextiles
aren’t very porous, which makes them a poor fit for projects where drainage is
important. That same characteristic does make them ideal for some erosion
control projects where water must be passed over a surface without draining
through to the soil below. Woven geotextiles will also resist corrosion and hold
up for long-term applications.
Non-woven Geotextiles
Rather than weaving together fabric on a loom, non-woven geotextiles are
manufactured by bonding materials together, either through chemical or heat,
needle punching or other methods. They’re made of synthetics and most often
used in filter or separation applications. Non-woven geotextiles typically aren’t
as good of a fit for stabilization or reinforcement projects. You’ll often find
them protecting geomembrane lining systems from interior and exterior

27
penetrations. The non-woven geotextile will break down faster than their
woven counterparts. But, for projects where pooling water is a major concern,
non-woven geotextiles are likely the right choice.
QUESTION BANK
1. Explain reinforced earth structures
2. What are the functions of geotextiles, explain with neat sketch.
3. With neat sketch explain the components of Reinforced earth structures
4. Explain the Properties of geotextiles.
5. Explain the tests conducted to examine the properties of geotextiles.
6. List the types of geotextiles, explain with neat sketch
7. Explain the manufacturing process of geotextiles
8. Explain the advantages and disadvantages of geosynthetics.
9. Explain the principle of RES.
REFERENCE
1. “Geosynthetics in civil engineering”, Edited by R. W. Sarsby
2. “Designing with geosynthetics”, Robert M Koerner

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MODULE 2
Design of Reinforced Earth Retaining Walls

43
Soil Nailing Techniques:
Soil nailing is a ground stabilisation technique that can be used on either natural
or excavated slopes. It involves drilling holes for steel bars to be inserted
into a slope face which are then grouted in place. Mesh is attached to the bar
ends to hold the slope face in position.
They are commonly used as
a remedial measure to stabilise
embankments, levees, and so on.
Other applications for soil
nailing include:
1. Temporary excavation shoring.
2. Tunnel portals.
3. Roadway cuts.
4. Under bridge abutments.
5. Repair and reconstruction of
existing retaining structures.
The main considerations for deciding whether soil nailing will be appropriate include;
the ground conditions, the suitability of other systems, such as ground anchors,
geosynthetic materials, and so on and cost.Although soil nails are versatile and can be
used for a variety of soil types and conditions, it is preferable that the soil should be
capable of standing – without supports – to a height of 1-2 m for no less than 2 days when
cut vertical or near-vertical. Soils which are particularly suited to soil

44
nailing include clays, clayey silts, silty clays, sandy clays, glacial soils, sandy silts,
sand, gravels. Soil nailing can be used on weathered rock as long as the weathering is
even (i.e. without any weakness planes) throughout the rock. Soils which are not well-
suited to soil nailing include those with a high groundwater table, cohesion-less soils, soft
fine-grained soils,highly-corrosive soils, loess, loose granular soils, and ground exposed to
repeated freeze-thaw action.
Design considerations that will inform the design include:
Strength limit: The limit state at which potential failure or collapse occurs.
Service limit: The limit state at which loss of service function occurs resulting
from excessive wall deformation.
Height and length.
Vertical and horizontal spacing of the soil nails.
Inclination of the soil nails.
Ground properties.
Nail length, diameter and maximum force.
Drainage, frost penetration, external loads due to wind and hydrostatic forces.
A drainage system may be inserted once all the nails are in place. This involves a
synthetic drainage mat placed vertically between the nail heads, which extends to the wall base and
is connected to a footing drain.
Some of the advantages of using soil nailing include:
They are good for confined spaces with restricted access.
There is less environmental impact.
They are relatively quick and easy to install.
They use less materials and shoring.
They are flexible enough to be used on new constructions, temporary structures or on
remodelling processes.
The height is not restricted.
Limitations of using soil nailing include:
They are not suitable for areas with a high water table.
In soils of low shear strength, very high soil nail density may be required.
They are not suitable for permanent use in sensitive and expansive soils.
Specialist contractors are required.
Extensive 3D modelling may be required.

45
Methods of soil nailing
1. Drilled and grouted Soil Nailing method
a. In this type of soil nailing methods, the holes are drilled in walls or slope face
b. Then, nails are inserted in the pre-drilled holes. diameter of nails ranges from 100-
200mm with spacing of 1.5m
c. After that, the hole is filled with grouting materials such as concrete, shotcrete etc.
2. Driven soil nailing method
a. It is used for temporary stabilization of soil slopes.
b. Driven soil nailing method is considerably fast.
c. However, it does not provide corrosion protection to the reinforcement steel or nails.
d. In driven soil nailing method, the nails are driven in the slope face during
excavation.
e. Diameter of the nail is around 19mm to 25mm which is comparatively small.
f. Nail spacing is 1m to 1.2m.

46
3. Self drilling Soil Nailing method
a. Hollow bars are used in self driven soil nailing method.
b. Bars are drilled into the slope surface. Grout injected simultaneously during the
drilling process.
c. It is faster than drilled grouted nailing.
d. Lastly, self-drilling soil nailing method provides more corrosion resistance to nails
than driven nails.
4. Jet Grouted Soil Nailing method
a. In jet grouted soil nailing method, jets are used for eroding the soil for creating holes
in the slope surface.
b. After that, steel bars are installed in this hole and grouted with concrete.
c. Finally, jet grouted soil nailing provides good corrosion protection for the steel bars
(nails)

47
5. Launched Soil Nail method
a. In this method of soil nailing, the steel bars are forced into the soil with a single shot
using compressed air mechanism.
b. The installation of soil nails is fast, but controls over length of bar penetrating the
ground is difficult.
c. Utilized to reinforce an unstable or potentially unstable soil mass. capacity into the
sliding soil.
d. nail diameter is 38mm and its length is around 6m
Construction Sequence of a Soil Nail Wall
The typical sequence of construction of a soil nail wall is described below and shown schematically
in the figure.
1. Excavation. The depth of the initial excavation lift (unsupported cut) may range between
2.5 and 7 ft, but is typically 3 to 5 ft and reaches slightly below the elevation where the first
row of nails will be installed. The feasibility of this step is critical because the excavation
face must have the ability to remain unsupported, until the nails and initial face are installed,
typically one to two days. The type of soil that is excavated may limit the depth of the
excavation lift. The excavated platform must be of sufficient width to provide safe
access for the soil nail installation equipment.
2. Drilling of Nail Holes. Drill holes are advanced using specialized drilling equipment
operated from the excavated platform. The drill holes typically remain unsupported.
3. A) Nail Installationand Grouting. Tendons are placed in the drilled hole. A tremie grout
pipe is inserted in the drill hole along with the tendon; and the hole is filled with grout,
placed under gravity or a nominal, low pressure (less than 5 to 10 psi). If hollow bars are
used, the drilling and grouting take place in one operation.
B) Installation of Strip Drains. Strip drains are installed on the excavation
face, continuously from the top of the excavation to slightly below the bottom of
the excavation. The strip drains are placed between adjacent nails and are unrolled down to
the next excavation lift.

48
4. Construction of Initial Shotcrete Facing. Before the next lift of soil is excavated, an
initial facing is applied to the unsupported cut. The initial facing typically consists of a
lightly reinforced 4-in. thick shotcrete layer. The reinforcement includes welded-wire mesh
(WWM), which is placed in the middle of the facing thickness (Figure 2.1). Horizontal and
vertical bars are also placed around the nail heads for bending resistance. As the shotcrete
starts to cure, a steel bearing plate is placed over the tendon that is protruding from the drill
hole. The bearing plate is lightly pressed into the fresh shotcrete. Hex nuts and washers are
then installed to engage the nail head against the bearing plate. The hex nut is wrench-
tightened within 24 hours of the placement of the initial shotcrete. Testing of some of the
installed nails to proof-load their capacity or to verify the load-specified criterion may be
performed before proceeding with the next excavation lift. The shotcrete should attain its
minimum specified 3-day compressive strength before proceeding with subsequent
excavation lifts. For planning purposes, the curing period of the shotcrete should be
considered 72 hours.
5. Construction of Subsequent Levels. Steps 1 through 4 are repeated for the remaining
excavation lifts. At each excavation lift, the strip drain is unrolled downward to the
subsequent lift. A new panel of WWM is then placed overlapping at least one full mesh cell
with the WWM panel above. The temporary shotcrete is continued with the previous
shotcrete lift.
6. Construction of Final Facing. After the bottom of the excavation is reached and nails are
installed and tested, the final facing is constructed. The final facing may consist of CIP
reinforced concrete, reinforced shotcrete, or prefabricated panels. Weepholes, a foot drain,
and drainage ditches are then installed to discharge water that may collect in the continuous
strip drain.

Components of soil Nailing
Soil-Nail Reinforcement
This is the primary element of your soil
of tensile resistance to your
high yield deformed bar made of steel. In some instances, a fiber
as the reinforcement for your soil
Soil Nail Head
In general, the soil nail head compr
pad. The main function of the
activate a tensile force. Furthermore, it promotes the local stability of the soils between the
soil nails and the ground near a sloped surface.
Corrosion Protection Measures
These will depend on the soil aggressiveness and design life of your soi
of the leading corrosion protection measures include corrugated plastic sheathing and hot
dip galvanizing. For the corrosion protection of couplers, anti
polyethylene on heat-shrinkable sleeves are the common
Slope Facing
This element protects the sloped ground to reduce erosion and the adverse effects associated
with surface water on inclined land. Slope facing can be hard, flexible or soft or a mix of all
three consistencies. A hard
solely non-structural. Structural slope facing transfers loads from the free surfaces between
soil nail heads and in so doing boosts the stability of your soil
Components of soil Nailing:
Nail Reinforcement
This is the primary element of your soil-nailed wall. Its principal function is the provision
of tensile resistance to your structure. The reinforcement used in this case is often a solid
high yield deformed bar made of steel. In some instances, a fiber-reinforced polymer is used
as the reinforcement for your soil-nailed wall.
In general, the soil nail head comprises a steel bearing plate, nuts, and a reinforced concrete
pad. The main function of the soil nail head is the provision of a reaction for soil
soil nails and the ground near a sloped surface.
Corrosion Protection Measures
These will depend on the soil aggressiveness and design life of your soi
of the leading corrosion protection measures include corrugated plastic sheathing and hot
dip galvanizing. For the corrosion protection of couplers, anti-mastic sealant material and
shrinkable sleeves are the commonly used alternatives.
three consistencies. A hard slope facing is non-structural or structural while a soft one is
structural. Structural slope facing transfers loads from the free surfaces between
soil nail heads and in so doing boosts the stability of your soil-nailed wall.
50
nailed wall. Its principal function is the provision
structure. The reinforcement used in this case is often a solid
reinforced polymer is used
ises a steel bearing plate, nuts, and a reinforced concrete
head is the provision of a reaction for soil nails to
These will depend on the soil aggressiveness and design life of your soil-nailed wall. Some
of the leading corrosion protection measures include corrugated plastic sheathing and hot-
mastic sealant material and
ly used alternatives.
structural or structural while a soft one is
structural. Structural slope facing transfers loads from the free surfaces between
nailed wall.

51
Design Considerations:
A soil-nailed system is required to fulfil fundamental requirements of stability, serviceability and
durability during construction and throughout its design life. Other issues such as cost and
environmental impact are also important design considerations.
Stability: The stability of a soil-nailed system throughout its design life should be assessed. The
design of a soil-nailed system should ensure that there is an adequate safety margin against all the
perceived potential modes of failure.
Serviceability: The performance of a soil-nailed system should not exceed a state at which the
movement of the system affects its appearance or the efficient use of nearby structures, facilities or
services.
Durability: The environmental conditions should be investigated at the design stage to assess their
significance in relation to the durability of soil nails. The durability of a steel soil-nailed system is
governed primarily by the resistance to corrosion under different soil aggressively.
Economic Considerations: The construction cost of a soil-nailed system depends on the material
cost, construction method, temporary works requirements, build ability, corrosion protection
requirements, soil-nail layout, type of facing, etc.
Environmental Considerations: The construction of a soil-nailed system may disturb the ground
ecosystem, induce nuisance and pollution during construction, and cause visual impact to the
existing environment. Appropriate pollution control measures, such as providing water sprays and
dust traps at the mouths of drill holes when drilling rocks, screening the working platform and
installing noise barriers in areas with sensitive receivers, should be provided.
Design Philosophies & precautions: After a preliminary analysis of the site, initial designs of the
soil nail wall can be begin. This begins with a selection of limit states and design approaches. The
two most common limit states used in soil nail wall design is strength limit and service limit states.
The strength limit state is the limit state that addresses potential failure mechanisms or collapse
states of the soil nail wall system. The service limit state is the limit state that addresses loss of
service function resulting from excessive wall deformation and is defined by restrictions in stress,
deformation and facing crack width under regular service conditions. The two most common design
approaches for soil nail walls are limit state design and service load design. Initial design
considerations include wall layout (wall height and length), soil nail vertical and horizontal spacing,
soil nail pattern on wall face, soil nail inclination, soil nail length and distribution, soil nail material
and relevant ground properties. With all these variables in the mind of the design engineer the next
step is to use simplified charts to preliminarily evaluate nail length and maximum nail force. Nail
length, diameter and spacing typically control external and internal stability of the wall. These
parameters can be adjusted during design until all external and internal stability requirements are
met. After the initial design is completed, final design progresses where the soil nail wall has to be
tested for external and internal failure modes, seismic considerations and aesthetic qualities.
Drainage, frost penetration and external loads such as wind and hydrostatic forces also have to be
determined and included in the final examination of the design. Soil nail walls are not ideal in

52
locations with highly plastic clay soils. Soils with high plasticity, a high liquid limit and low
undrained shear strengths are at risk of long-term deformation (creep).
The main points to be considered in determining if soil nailing would be an effective retention
technique are as follows:-
First, the existing ground conditions should be examined. Next, the advantages and disadvantages
for a soil nail wall should be assessed for the particular application being considered. Then other
systems should be considered for the particular application. Finally, cost of the soil nail wall should
be considered.
Soil nail walls can be used for a variety of soil types and conditions. The most favourable
conditions for soil nailing should be follow before starting any operation of soil nailing and they
are:
The soil should be able to stand unsupported one to two meters high for a minimum of two days
when cut vertical or nearly vertical.
Also all soil nails within a cross section should be located above the groundwater table. If the soil
nails are not located above the groundwater table, the groundwater would negatively affect the face
of the excavation i.e. the bond between the ground and the soil nail itself.
Based upon these ―favourable conditionsǁ for soil nailing stiff to hard fine-grained soils which
include stiff to hard clays, clayey silts, silty clays, sandy clays, and sandy silts are preferred soils.
Sand and gravels which are dense to very dense soils with some apparent cohesion also work well
for soil nailing. Weathered rock is also acceptable as long as the rock is weathered evenly
throughout. Finally, glacial soils work well for soil nailing. A list of ―unfavourable or difficult soil
conditionsǁ for soil nailing can include dry, poorly graded cohesion-less soils, soils with a high
groundwater table, soils with cobbles and boulders, soft to very soft fine-grained soils, highly
corrosive soils, weathered rock with unfavourable weakness planes, and loess. Other difficult
conditions include prolonged exposure to freezing temperatures, a climate that has a repeated
freeze-and-thaw cycle and granular soils that are very loose.
Comparison of soil nailing with Reinforced earth structures
Soil nailing is embraced by practicing engineers as a highly competitive well proven technique.
Soil nailing has certain similarities to both reinforced earth and anchoring, although its particular
operating principles and construction methods give it a firm and distinct identity. Similar
considerations distinguish it from allied in situ soil reinforcing techniques such as reticulated root
piles and soil dwelling. Most applications of soil nailing to date have been associated with new
construction projects such as foundation excavations and slope stabilization, for both temporary and
permanent works. The system has equal facility in a wide range of remedial projects, and indeed it
is most likely that nailing will find its wide applications in the India in this field, bearing in mind
the prevailing economic trends. It is to be hoped that the growth of the technique in India can be
fostered by practical research collaborations between industry, the universities and government, in
the manner of developed countries like France, Germany, United States of America and United
Kingdom, who are the current leaders in this field.

53
MODULE 3
Design of Reinforced Earth Foundation
Modes of failure
If layer of reinforcing strips or ties are placed in the soil under a shallow strip
foundation, the nature of failure in the soil mass similar to the pattern shown in the fig
a. Such a failure generally occurs when the first layer of reinforcement placed at a
depth d greater than about 2/3B. If the reinforcements in the first layer are strong and
are sufficiently concentrated, they may act as rigid base at a limited depth. The type
failure shown in fig b shown in the fig b could occur if d/B is less than about 2/3 and
the number of layer of reinforcement (N) is less than about 2 to 3. In this type of
failure, reinforcement tie-pullout occurs. The most beneficial effect of reinforced earth
is obtained when d/B is less than about 2/3, and the number of reinforcement layers is
greater than 4 but not more than 6 to 7. In this case the soil mass fails when the upper
ties break fig c.

64
MODULE 4
Geosynthetics for roads and slopes
Introduction
Use of geosynthetics results in significant savings, improved performance and very
good serviceability in both short term and long termGeosynthetics have made it
possible to construct roads and pavements in seemingly difficult situations such as
marshy stretches, soft and organic deposits and in expansive soil areas.
Geosynthetics have been successfully used to fulfill a number of functions that
contribute significantly to the good performance of roadways. They include the
functions of separation, filtration, reinforcement, stiffening, drainage, barrier, and
protection. One or more of these multiple functions have been used in at least six
important roadway applications. The applications include the migration of reflective
cracking in asphalt overlays, separation, stabilization of road bases, stabilization of
road soft subgrades, and lateral drainage.
Mechanism
1. Lateral restrainment of the base and subgrade through friction and interlock
between the aggregate, soil and the geosynthetic
2. Increase the system bearing capacity by forcing the potential bearing capacity
failure surface to develop along alternate, higher shear strength surfaces
3. Membrane support of the wheel loads

65
Functions
Fig. shows a paved road section with the location of possible geosynthetic layers and
the various functions that these geosynthetics can fulfill. These functions include:
Separation: The geosynthetic, placed between two dissimilar materials, maintains the
integrity and functionality of the two materials. It may also involve providing long-
term stress relief. Key design properties to perform this function include those used
to characterize the survivability of the geosynthetic during installation.
Filtration: The geosynthetic allows liquid flow across its plane, while retaining fine
particles on its upstream side.Key design properties to fulfill this function include the

66
geosynthetic permittivity (cross-plane hydraulic conductivity per unit thickness) and
measures of the geosynthetic pore-size distribution (e.g. apparent opening size).
Reinforcement: The geosynthetic develops tensile forces intended to maintain or
improve the stability of the soil-geosynthetic composite. A key design property to
carry out this function is the geosynthetic tensile strength.
Stiffening: The geosynthetic develops tensile forces intended to control the
deformations in the soil-geosyntheticcomposite. Key design properties to accomplish
this function include those used to quantify the stiffness of the soil-geosynthetic
composite.
Drainage: The geosynthetic allows liquid (or gas) flow within the plane of its
structure. A key design property toquantify this function is the geosynthetic
transmissivity (in-plane hydraulic conductivity integrated overthickness).While
comparatively less common in roadway pplications, additionalgeosynthetic functions
include:
Hydraulic/Gas Barrier: The geosynthetic minimizes the cross-plane flow, providing
containment of liquids or gasses. Key design properties to fulfill this function include
those used to characterize the long-term durability of the geosynthetic material.
Protection: The geosynthetic provides a cushion above or below othermaterial (e.g. a
geomembrane) in order to minimize damage during placement of overlying
materials. Key design properties to quantify this function include those used to
characterize the puncture resistance of the geosynthetic material.
Benefits
• Reducing the intensity of stress on the subgrade
• Preventing subgrade fines from pumping
• Preventing contamination of base materials
• Reducing the depth of excavation
• Reducing the thickness of aggregate required for stabilization of subgrade
• Reducing disturbance of subgrade during construction
• Allowing an increase in strength over time
• Reducing differential settlement in roadway and in transition areas from
cut to fill
• Reducing maintenance and extending the life of the pavement
Subgrade Conditions in which Geosynthetics are useful

• Poor soils
(USCS: SC, CL, CH, ML, MH, OL,
(AASHTO: A-5, A
• Low undrained shear strength
f = Cu < 90ka
CBR<3 {Note: CBR as determined with
ASTMD 4429 Bearing Ratio of Soils in Place} MR
• High water table
• High sensitivity
Summary Recommendation
Effectiveness of Geosynthetics as a function of subgrade strength
Design
Two main approaches
1. No reinforcing effect of the geotextiles
Conservative, applicable for thin roadway sections with relatively small live loads,
where ruts are 50 to 100mm
(USCS: SC, CL, CH, ML, MH, OL, OH, and PT)
5, A-6, A-7-5, and A-7-6)
Low undrained shear strength
CBR<3 {Note: CBR as determined with
ASTMD 4429 Bearing Ratio of Soils in Place} MR 30
High water table
High sensitivity
Summary Recommendation
No reinforcing effect of the geotextiles
where ruts are 50 to 100mm
67
30MPa

68
P/pt
1.414P/pt
2. Reinforcing effect is considered
Applicable for large live loads on thin roadways, where deep ruts (>100mm) may
occur and for thicker roadways on softer subgrade
Based on both theoretical analysis and empirical tests on geotextiles, Steward,
Williamson and Mohney (1977), reports the bearing capacity factors for different ruts
and traffic conditions both with and without geotextile separators
Design Ruts(mm) Traffic passes of
80kNaxel
equivalents
Bearing capacity
factor No
Without
geotextiles
<50
>100
>1000
<100
2.8
3.3
With geotextiles <50
>100
>1000
<100
5.0
6.0
The Giroud and Noiray approach
Normal highway vehicles including lorries B =
L = 0.707B
Heavy construction plant with wide or double tyres B =
L = 0.5B

69
For construction plant, a typical value of pt is 620 kN/m2. The stress p applied
to the cohesive formation by the axle is

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