Reinforcing cost savings because of the availability of

Reinforcing
soils with short fibers in random fashion has attracted attention in the last
few decades. Reinforcing soil with natural fibers are used frequently with the
attention of increasing peak compressive strength, shear strength, and
ductility and reducing post peak strength loss. The main advantages of using
short fibers over planar reinforcement are: absence of potential plane of
weakness, feasibility of application within a limited space compared with the
use of planar reinforcement such as geo-textiles and significant cost savings
because of the availability of fibers at lower cost and no need for a specific
design. Expansive soil deposits occur in the arid and semi-arid region of the
world and are problematic to engineering structures because of their tendency
to heave during wet season and shrink during dry season. If the soils are
expansive in nature, the problems of construction increase as these soils have
low shear strength, high compressibility, high swelling potential, and so on.

Randomly
distributed fibers in soil-termed as RDFS is among the latest technique in
which fibers of desired type and quantity are added in the soil, mixed and laid
in position. The composite material is also called ‘ply soil’. Thus the method
of preparation of RDFS is similar to conventional stabilization techniques.

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RDFS
is different from the other reinforcing methods in its orientation. In
reinforced earth, the reinforcement in the form of sheets, rods, bars, textiles
etc. is laid horizontally at specific intervals, whereas in RDFS fibers are
mixed randomly in soil thus making a homogeneous mass. While building the Great
Wall of China, the clay soil was mixed with tamarisk branches. The addition of
straw of wheat locally called “turi” to the clay-mud plaster is also very
popular in villages and is ancient.

Experimental
work done by various investigators from last many years has established beyond
doubt that addition of fibers in soil improves the overall engineering performance
of soil. Among the notable properties that improve are greater extensibility,
small loss of post peak strength, isotropy in strength and absence of planes of
weakness. Thus RDFS has been used in many civil engineering projects in various
countries including USA, Japan and India in the recent past and further
research is in progress for the many hidden aspects of it.

Mechanism
of Soil Reinforcement is explaining, in which the fibers extend and break under
tensile stress, even if the overall applied stress is isotropic. The initial
fiber shape is represented here as straight, which is simplistic. The isotropic
compression causes relative movement among particles and consequently produces
tensile stresses in the fibers located among them. There is also the possibility
of an additional mechanism of fiber breaking during testing by squeezing and
crushing of the sand particles, cutting the fibers trapped between them, as
shown in Fig.1

Fig
1: Illustration of Proposed Mechanism of Fiber Breakage Under Isotropic Loading
(Consoli et al, 2005)

 

Randomly
distributed fiber-reinforced soil (RDFS)

 

Experiment results reported by various investigators
have shown that fiber reinforcement causes significant improvement in strength
and stiffness of clay. Past research has demonstrated that inclusion of fibers
significantly improves the response of soils under both static and dynamic
loading conditions. The technique of reinforcing the soil increases the
stiffness and load carrying capacity of the soil through frictional interaction
between the soil and the reinforcement. Many studies were conducted relative to
the engineering properties of soil strengthened with randomly distributed,
small, discrete inclusions. The reinforcement may vary, either in form (strips,
sheets, grits, bars, fibers), texture (rough or smooth), or relative stiffness
(high such as steel or low such as fabrics and fibers). The distinction between
high modulus and low modulus reinforcements were pointed out and classified the
reinforcements into two major categories: (a) ideally inextensible inclusions
(i.e. metal strips and bars) and (b) ideally extensible inclusions (i.e.
natural and synthetic fibers, plant roots and polymeric fabrics). The soil
reinforced with inextensible inclusions is generally known as ‘reinforced
earth’, whereas soil reinforced with extensible inclusions is termed as ‘PLY
soil’. Fiber reinforcement comes in the category of extensible inclusions.
Synthetic fibers comprise polypropylene, polyester, nylon and plastics and have
greater resistance to attack by acids and alkalis or other chemicals. The
polypropylene fibers have high tensile strength and melting point to 165oC.
Though this category has inherent defect of getting affected by the
ultra-violet rays from sunlight but for buried material this negative point is
not there. Asbestos, glass, carbon fibers have been found to be resistant to
alkalis and other chemical’s attack, but long exposure to such adverse
environment has been found to lead to corrosion damage. Plastic wastes were
used as reinforcing materials and found an improvement in the peak and ultimate
strength of cemented and soil alone. Materials such as polyethylene
terephthalate (PET), plastic bottles, waste plastic LDPE carry bag, HDPE
packaging strips are profusely and widely produced. According to the the best
way to handle the increasing pressure of waste plastic on open dumps is to
utilize it for ground improvement after shredding. Tyre buffing, the
by-products of the tyre treated process referred to as “Rubber fibers” can be
used as reinforcing materials, the high strength, durability and availability
of scrap tyres have prompted their use, in the form of tyre particles, as light
weight aggregate in geo engineering applications. The roots of plants have a
great influence on the shear strength and stability of the natural slopes. The
degree of increased strength imparted by the roots depends on the concentration
and properties of the roots. But this category of reinforcement has been found
to have low resistance towards attack by alkalis and other chemicals. The other
natural fibers like jute, sisal, banana have tested and found to loose in
strength when subjected to alternate “wetting and drying” in environment
created by lime content present in the soil.

 

Types of reinforcements

The different types of reinforcements used for reinforcing soils
can be categorized into:

Artificial and Natural

 

 

Artificial
reinforcements

The
geo-synthetics are a term which essentially includes a family of materials that
are natural or man-made that is used in conjunction with soil, to improve the
performance of soil in specific context. Geo-synthetics are most popular and
have gained enormous momentum due to their flexibility in usage and the
financial benefits.  Geo-synthetics are
flexible, porous man made products primarily consists of polymers, woven or
nonwoven and are available in the form of geo-membrane, geo-nets, geo-grids
etc., which are used for different purposes like containment, drainage and
reinforcement. Geo-grids are high strength materials composed of polyethylene,
polypropylene or PVC coated polyester and are commonly used for reinforcement
purpose to stabilize a slope, construct a soil wall to increase the bearing
capacity of weak soil. More recently the use of geo-grids as soil
reinforcements is in vogue in construction of reinforced earth structures. The
geo-grids can be used very effectively as reinforcements to improve the factor
of safety against bearing capacity failure and to increase the bearing capacity
of granular soil, which can replace the conventional footing with reinforced
earth slabs.

Natural coir fiber reinforcement

Currently, the global annual
production of coir fiber is about 350,000 metric tons (MT). Yet, even in the
world’s top two producers, India and Sri Lanka, which account for about 90% of
global coir fiber production, combined, this renewable resource is
underutilized; local coir mills process only a fraction of the available husks,
which accrue more or less year round as a waste during coconut processing Coir
is a natural fiber produced from the husk of coconut which is abundantly
available in the coconut growing States of India like Kerala. Coir fiber is
extracted through a traditional retting process and from the untreated husk; it
is extracted through a mechanical process. Its development as an industry in
India commenced in the State of Kerala centuries ago. Two ply coir yarns were
spun either by hand or spinning wheels throughout the coastal belt of Kerala
from time immemorial. The breathtakingly beautiful lakes and lagoons of Kerala
have been natural allies of the coir sector. Coir yarn is the raw material for
the manufacture of an array of furnishing products like mats, mattings, rugs,
carpets etc. Growth of technology in the sector is resulting in the development
of a larger diversity of products. Coir is a fiber with intrinsic strength of
multi-cellular fiber giving it the toughness and brushing quality, unlike
competing fibers. Today, coir offers a range of options from live-in-style
products to geo-applications. The potential of rubberized coir products like
mattresses, furniture/vehicle upholstery and insulation pads are immense. Due
to natural edge and a skilled workforce in Kerala, Indian coir caught global
attention for its colour, texture, eco-friendliness, bio degradability etc.
earning for it a premium price in international markets. Since 2001, rising Chinese demand for coir, an expanding
market for coir-based erosion control products, and the spread of coir pith as
a peat moss substitute in horticulture resulted in higher production and
prices.

Brief History
of Coir

About A.D. 60, a Greek sailor wrote about a
coconut-producing East African village whose boats were made of planks
sewn together with coconut fibers. By the eleventh century, Arab traders
were teaching people in Sri Lanka and India how to extract and process
coconut fibers.
In 1859, James Darragh and Henry Smail
established the first coir manufacturing firm in Alleppey that led other
to follow suit, primarily from Britain and Netherlands.
1947 saw the dissolution of a number of large
factories, with laid-off workers being given machinery as part of their
retrenchment package. This was the start of the coir cottage industry that
continues to dominate the production model today.
Coir production changed little until efforts to
mechanize it began in the middle of the twentieth century. In India, a
de-fibrin machine was invented in 1950. Because mechanization would
eliminate a significant number of jobs, it is being introduced gradually.
In the past decade, coir processing in Europe and
other developed countries has largely stopped, with most manufacturing now
being undertaken in India. However, though India continues to dominate the
global coir market, competition is on the rise from South East and East
Asia.

Coir products are made from fiber from the husks of
coconuts, using for the most part production techniques that barely changed
in the past century. Traditional products have mainly revolved around the
production of mats and coir textiles.

World Scenario of Coir fiber production and its advantages.
Some of the advantages of coir fibers are;
100% Natural, Easy to install, Versatile, Coir
is a biodegradable organic fiber material which is rigid, strong and tensile
in nature, adds organic material to
soil thus reducing pollution, Reduces maintenance cost by improving
durability.

Among natural fibers, fibers obtained from
coir in proven to be most durable when used in soils because of it having high
lignin content. India is the largest coir producer in
the world accounting for more than 80 per cent of the total world production of
coir fiber. The coir sector in India is very diverse and involves households,
co-operatives, NGOs, manufacturers and exporters. The coir industry employs
more than a lakh people of whom a majority is from rural areas belonging to the
economically weaker sections of society. Nearly 80% of the coir workers in the
fibre extraction and spinning sectors are women. The development of coir
industry has all along been in areas where there is a concentration of coconut
trees and availability of coconut husk. Historically, the coir industry started
and flourished in Kerala which has a long coast line, lakes, lagoons and
backwaters providing natural conditions required for retting. However, with the
expansion of coconut cultivation, coir industry has picked up in the States of
Tamil Nadu, Karnataka, Andhra Pradesh, Orissa, West Bengal, Assam, Tripura,
Pondicherry and the Union Territories of Lakshadweep and Andaman & Nicobar
Islands through the efforts of Coir Board. Fig. 2 shows production of coir
fibers in different countries in the form of pie chart and Fig. 3 shows the
coir production in different states in India. The production and processing
methods in coir industry still continue to be mainly traditional. Coir
is abundantly available in most parts of south and costal India, Sri Lanka,
Philippines, Indonesia, Malaysia, Brazil, and others.