Ground Improvement Technique: Earth Reinforcement

1. Ground Improvement
Technique
By: Dr. D. N. Mudgal

2. Introduction
Ground improvement is the procedure typically defined
as using mechanical means to improve poor ground
conditions. Ground improvement methods improve the
engineering properties of the soil mass which is treated
to meet project performance requirements.

3. Objectives
1. The most common traditional objectives include
improvement of the soil and ground for use as a
foundation or construction material.
2. Increasing shear strength, durability, stiffness and
stability
3. Reducing undesirable properties (eg. Shrink/ swell
potential, compressibility, liquefability)
4. Modifying permeability, the rate of fluid to flow
through a medium; and
5. Improving efficiency and productivity by using
methods that save time and expense

4. Factors affecting choice of Improvement Method
1. Soil type: This is one of the most important parameters that will
control what approach or materials will be applicable to only
certain types of soil types and grain sizes
2. Depth and location of treatment required: Many ground
improvement methods have depth limitations that render them
unsuitable for applications for deeper soil horizons.
3. Desired/required soil properties: obviously, different methods
are use to achieve different engineering properties, and certain
methods will provide various levels of uniformity to improved sites.
4.Availability of materials: Depending on the location of the
project and materials required for each feasible ground
improvements approach.

5. To be conti……
5. Availability of skills, local experience, and local
preferences: While the engineer ma possess the knowledge
and understanding of a preferred method.
6.Environmental concerns: With a better understanding and
a greater awareness of effects on the natural environment,
more attention have been placed on methods that assure less
environmental impacts.
7.Economics: when all else has been considered, the final
decision on choice of improvement method will often come
down to the ultimate cost of a proposed method, or cost will
be the deciding factor in choosing between two or more
otherwise suitable methods.

6. Ground Improvement Techniques
1. Mechanical modification
2. Hydraulic modification
3. Physical and chemical modification
4. Modification by inclusion and Confinement

7. Soil density is increased by the application of short-term
external mechanical forces, including Compaction of
surface layers by:
1. Static,
2. Vibratory,
3. Impact rollers,
4. Plate vibrators.
 Deep compaction by heavy tamping at the surface or
vibration at depth
Mechanical Modification

8. Hydraulic Modification
Free –pore water is forced out of the soil via (by
means of) drains of wells. –
1. In coarse grained soils, this is achieved by lowering
the ground water level through pumping from
boreholes or trenches.
2. In fine-grained soils, the long term application of
external loads (preloading) or electrical forces
(electro kinetic stabilization) is required

9. Improvement of cohesive soils:
1. Cohesive soils such as soft clay with large void ratio
and higher water content have a
2. necessity to improve their characteristics. To reduce
void ratio and water content, to increase
3. strength for which increases bearing capacity of soil.
4. Generally following methods are in practice:
a. Precompression or Preloading
b. Sand Drains
c. Wick Drains
d. Stone columns

10. Physical and Chemical Modification
1. Additives include: - natural soils - industrial by-products or waste materials
(fly ash, slag), - Cementations and other chemicals (lime, cement) which react
with each other and the
ground.
2. When additives are injected via boreholes under pressure into the voids
within the ground or between it and a structure, the process is called
GROUTING. Rigs with multiple injectors deliver the stabilizing fluid into the
soil. The fluid will prefer to travel into cracks and fissures.
3. Soil stabilization by heating the ground and by freezing the ground come
under, Thermal Methods of Modification.
a. Heating evaporates water and causes permanent changes in the
mineral structure of soils.
b. Freezing solidifies part or all of the water and bonds individual
particles together

11. Modification by Inclusion and Confinement Reinforcement
by
1. Fibers, Strips, Bars, Meshes and
Fabrics.
2. In-situ reinforcement is achieved by
nails and anchors.

12. Principle of Reinforced
Earth
 Soil has an inherently low tensile strength but a high
compressive strength.
 An objective of incorporating soil reinforcement is to
absorb tensile loads or shear stresses within the
structure.
 In absence of the reinforcement, structure my fail in
shear or by excess of the deformation.
 When an axial load is applied to the reinforced soil, it
generates an axial compressive strain and lateral
tensile strain

13. To be continued….
 If the reinforcement has an axial tensile stiffness greater than
that of the soil, then lateral movements of the soil will only occur
if soil can move relative to the reinforcement.
 Movement of the soil, relative to the reinforcement, will generate
shear stresses at the soil/ reinforcement interface, these shear
stresses are redistributed back into the soil in the form of
internal confining stress.
 Due to this, the strain within the reinforced soil mass is less than
the strain in unreinforced soil for the same amount of stresses

14. TYPES OF FIBRES FOR GEOTEXTILES
 Natural Fibers
 Synthetic Fibers

15. Natural fibers
1. Natural fibers in the form of paper strips, jute nets, wood shavings or
wool mulch are being used as geotextiles.
2. In certain soil reinforcement applications, geotextiles have to serve for
more than 100 years.
3. Bio-degradable natural geotextiles are deliberately manufactured to have
relatively short period of life.
4. They are generally used for prevention of soil erosion until vegetation
can become properly established on the ground surface.
The commonly used natural fibres are –
a. Ramie
b. Jute

16. To be continued….
a) These are subtropical bast fibres, which are obtained
from their plants 5 to 6 times a year.
b) The fibres have silky luster and have white
appearance even in the unbleached condition.
c) They constitute of pure cellulose and possess
highest tenacity among all plant fibres

17. Jute
 This is a versatile vegetable fiber which is biodegradable
and has the ability to mix with the soil and serve as a
nutrient for vegetation.
 Their quick biodegradability becomes weakness for their
use as a geotextile. However, their life span can be extended
even up to 20 years through different treatments and
blendings.
 Thus, it is possible to manufacture designed biodegradable
jute geotextile, having specific tenacity, porosity,
permeability, transmissibility according to need and
location specificity.
 Soil, soil composition, water, water quality, water flow,
landscape etc. physical situation determines the
application and choice of what kind of jute geotextiles
should be used.

18. To be continued….
 In contrast to synthetic geotextiles, though jute geotextiles are less
durable but they also have some advantages in certain area to be
used particularly in agro-mulching and similar area to where quick
consolidation are to take place.
 For erosion control and rural road considerations, soil protection
from natural and seasonal degradation caused by rain, water,
monsoon, wind and cold weather are very important parameters.
 Jute geotextiles, as separator, reinforcing and drainage activities,
along with topsoil erosion in shoulder and cracking are used quite
satisfactorily.
 Furthermore, after degradation of jute geotextiles, lignomass is
formed, which increases the soil organic content, fertility, texture
and also enhance vegetative growth with further consolidation and
stability of soil.

19. Synthetic Fibers
1. The four main synthetic polymers most widely used as the raw
material for geotextiles are –polyester, polyamide,
polyethylene and polypropylene.
2. The oldest of these is polyethylene which was discovered in
1931 by ICI.
3. Another group of polymers with a long production history is
the polyamide family, the first of which was discovered in 1935.
4. The next oldest of the four main polymer families relevant to
geotextile manufacture is polyester, which was announced in
1941.
5. The most recent polymer family relevant to geotextiles to be
developed was polypropylene, which was discovered in 1954.

20. Polyesters (PET)
1. Polyester is synthesized by polymerizing ethylene
glycol with dimethyl terephthalate or with
terephthalic acid.
2. The fiber has high strength modulus, creep resistance
and general chemical inertness due too which it is
more suitable for geotextiles.
3. It is attacked by polar solvent like benzyl alcohol,
phenol, and meta-cresol. At pH range of 7 to 10, its
life span is about 50 years.
4. It possesses high resistance to ultraviolet radiations.
However, the installation should be undertaken with
care to avoid unnecessary exposure to light.

21. Polyamides (PA):
1. There are two most important types of polyamides, namely
Nylon 6 and Nylon 66 but they are used very little in
geotextiles.
2. The first one an aliphatic polyamide obtained by the
polymerization of petroleum derivative caprolactam.
3. The second type is also an aliphatic polyamide obtained by the
polymerization of a salt of adipic acid and hexamethylene
diamine.
4. These are manufactured in the form of threads which are cut
into granules.
5. They have more strength but less moduli than polypropylene
and polyester
6. They are also readily prone to hydrolysis

22. Polyethylene (PE):
1. Polyethylene can be produced in a highly crystalline
form, which is an extremely important characteristic in
fiber forming polymer.
2. Three main groups of polyethylene are –
a. Low density polyethylene (LDPE, density 9.2-9.3
g/cc),
b. Linear low density polyethylene (LLDPE, density
9.20-9.45 g/cc)
c. High density polyethylene (HDPE, density 9.40-
9.6 g/cc)

23. To be continued….
 Polypropylene (PP): Polypropylene is a crystalline
thermoplastic produced by polymerizing propylene
monomers in the presence of stereo-specific
ZeiglerNatta catalytic system.
 Homo-polymers and copolymers are two types of
polypropylene.
 Homopolymers are used for fibre and yarn
applications whereas co-polymers are used for
varied industria applications.
 Propylene is mainly available in granularform

24. Forms of Geo-textile
 Woven geotextiles
 Knitted geotextiles
 Nonwoven geotextiles
 Stitch-bonded geotextiles
 Geogrids
 Geonets
 Geomembranes
 Geocomposites

25. Woven geotextiles
 Woven geotextiles are
produced with the
interlacement of two sets
of yarns at right angles in
the weaving process.
 Woven geotextiles have
high strengths and
modulus in the warp and
weft directions and low
elongations at rupture.

26. Knitted geotextiles
 Knitted geotextiles are
produced with the
interloping of one or
more yarns in the
knitting process.
 These geotextiles are
highly extensible and
have relatively low
strength compared to
woven geotextiles, which
limits its usage.

27. Nonwoven geotextiles
 Nonwoven geotextiles are
thicker than woven and are
made either from
continuous filaments or
from staple fibers.
 They are produced in the
following bonding
techniques:
 Needle punching
 Thermal bonding
 Chemical bonding

28. Stitch-bonded geotextiles
 Stitch-bonded geotextiles
are produced by
interlocking fibers or yarns
or both, bonded by
stitching or sewing.
 Even strong, heavyweight
geotextiles can be
produced rapidly.
 Tubular geotextiles are
manufactured in a tubular
or cylindrical fashion
without longitudinal seam.

29. Geogrids
 Geogrids are materials
that have an open grid-
like appearance.
 The principal application
for Geogrids is the
reinforcement of soil.

30. Geonets
 Geonets are open grid-like
materials formed by two
sets of coarse, parallel,
extruded polymeric
strands intersecting at a
constant acute angle.
 The network forms a sheet
with in-plane porosity that
is used to carry relatively
large fluid or gas flows.

31. Geomembranes
 Geomembranes are
continuous flexible
sheets manufactured
from one or more
synthetic materials.
 They are relatively
impermeable and are
used as liners for fluid or
gas containment and as
vapor barriers

32. Geocomposites
 Geocomposites are
made from a
combination of two or
more geosynthetic types.
 Examples include
geotextile-geonet;
geotextile-geogrid;
geonet-geomembrane; or
a geosynthetic clay liner
(GCL).

33. Functions of Geotextile
 Separation
 Filtration
 Drainage
 Reinforcement
 Moisture and liquid barrier

34. Separation
 Separation is the process of
preventing undesirable mix-up
of two dissimilar materials.
 The geotextile acts as a
separating layer between fine
aggregates and coarse aggregates
or soils that have different
particle size distributions to
avoid undesirable mix-up.
 Separators also help to prevent
fine-grained subgrade soils from
being pumped into permeable
granular road bases thereby
keeping the structural integrity
and functioning of both
materials intact.

35. Filtration
 Geotextile is placed in contact
with and down gradient of soil
to be drained. The plane of the
geotextile is positioned normal
to the expected direction of
water flow.
 To perform this function the
geotextile needs to satisfy two
conflicting requirements: the
filter’s pore size must be small
enough to retain fine soil
particles while the geotextile
should permit relatively
unimpeded flow of water into
the drainage media.

36. Drainage
 The geotextile acts as a
drain to carry fluid flows
through less permeable
soils.
 The application of
geotextiles in drainage
applications has improved
the economical usage of
blanket and trench drains
under and adjacent to the
pavement structure,
respectively.

37. Reinforcement
 The geotextiles act as a
reinforcement element
within a soil mass or in
combination with the soil
to produce a composite
that has improved strength
and deformation
properties over the unrein-
forced soil.
 The geotextile interacts
with soil through frictional
or adhesion forces to resist
tensile or shear forces.

38. Moisture and liquid barrier
 The protection of civil
structures from the
effects of seeping water
is a common need.
 The geotextiles acts as a
relatively impermeable
barrier to prevent the
penetration of liquids or
moisture over a
projected service period.

39. Erosion control
1. Erosion is the process by which soil
and rock are removed from the
earth’s surface by exogenetic
processes such as wind or water
flow, and then transported and
deposited in other locations.
2. The geotextile anchored in steep
slope protects soil surfaces from the
tractive forces of moving water or
wind and rainfall erosion.

40. Applications
 Road Construction: Geotextiles are used to reinforce the base of roads,
preventing soil erosion and improving the stability of the roadbed.
 Railway Embankments: Similar to road construction, geotextiles are used to
stabilize railway embankments, reducing soil erosion and improving load
distribution.
 Landfill Liners and Covers: Geotextiles are used as part of landfill liners and
covers to prevent the leakage of contaminants into the surrounding
environment.
 Erosion Control: Geotextiles are used to stabilize slopes and prevent soil
erosion in areas prone to erosion, such as riverbanks, shorelines, and steep
hillsides.
 Retaining Walls: Geotextiles are used behind retaining walls to improve
drainage and soil stability, reducing the pressure on the wall and increasing its
lifespan.
 Stormwater Management: Geotextiles are used in stormwater management
systems to filter pollutants and control the flow of water, reducing runoff and
preventing soil erosion.

41. To be continued…
 Reinforced Earth Structures: Geotextiles are used in reinforced earth
structures, such as reinforced soil slopes and walls, to improve stability and
reduce construction costs.
 Pavement Overlay: Geotextiles are used as a separation layer between old and
new pavement layers to prevent the intermixing of materials and improve the
performance of the pavement.
 Subsurface Drainage: Geotextiles are used in subsurface drainage systems to
filter water and prevent the clogging of drainage pipes, improving the
efficiency of the system.
 Coastal Protection: Geotextile tubes and bags are used for coastal protection
and beach nourishment projects, helping to stabilize shorelines and protect
against erosion caused by waves and currents.
 Agricultural Applications: Geotextiles are used in agriculture for weed control,
soil stabilization, and erosion prevention in areas such as crop fields, nurseries,
and orchards.
 Geotextile Tubes: Geotextile tubes are used for dewatering sludge, sediment,
and other waste materials in various industries, including wastewater
treatment, dredging, and mining.