Reinforcing at specific intervals, whereas in RDFS
Reinforcingsoils with short fibers in random fashion has attracted attention in the lastfew decades. Reinforcing soil with natural fibers are used frequently with theattention of increasing peak compressive strength, shear strength, andductility and reducing post peak strength loss. The main advantages of usingshort fibers over planar reinforcement are: absence of potential plane ofweakness, feasibility of application within a limited space compared with theuse of planar reinforcement such as geo-textiles and significant cost savingsbecause of the availability of fibers at lower cost and no need for a specificdesign. Expansive soil deposits occur in the arid and semi-arid region of theworld and are problematic to engineering structures because of their tendencyto heave during wet season and shrink during dry season. If the soils areexpansive in nature, the problems of construction increase as these soils havelow shear strength, high compressibility, high swelling potential, and so on.Randomlydistributed fibers in soil-termed as RDFS is among the latest technique inwhich fibers of desired type and quantity are added in the soil, mixed and laidin position. The composite material is also called ‘ply soil’.
Thus the methodof preparation of RDFS is similar to conventional stabilization techniques.RDFSis different from the other reinforcing methods in its orientation. Inreinforced earth, the reinforcement in the form of sheets, rods, bars, textilesetc. is laid horizontally at specific intervals, whereas in RDFS fibers aremixed randomly in soil thus making a homogeneous mass. While building the GreatWall of China, the clay soil was mixed with tamarisk branches.
The addition ofstraw of wheat locally called “turi” to the clay-mud plaster is also verypopular in villages and is ancient.Experimentalwork done by various investigators from last many years has established beyonddoubt that addition of fibers in soil improves the overall engineering performanceof soil. Among the notable properties that improve are greater extensibility,small loss of post peak strength, isotropy in strength and absence of planes ofweakness. Thus RDFS has been used in many civil engineering projects in variouscountries including USA, Japan and India in the recent past and furtherresearch is in progress for the many hidden aspects of it.Mechanismof Soil Reinforcement is explaining, in which the fibers extend and break undertensile stress, even if the overall applied stress is isotropic.
The initialfiber shape is represented here as straight, which is simplistic. The isotropiccompression causes relative movement among particles and consequently producestensile stresses in the fibers located among them. There is also the possibilityof an additional mechanism of fiber breaking during testing by squeezing andcrushing of the sand particles, cutting the fibers trapped between them, asshown in Fig.1 Fig1: Illustration of Proposed Mechanism of Fiber Breakage Under Isotropic Loading(Consoli et al, 2005) Randomlydistributed fiber-reinforced soil (RDFS) Experiment results reported by various investigatorshave shown that fiber reinforcement causes significant improvement in strengthand stiffness of clay. Past research has demonstrated that inclusion of fiberssignificantly improves the response of soils under both static and dynamicloading conditions. The technique of reinforcing the soil increases thestiffness and load carrying capacity of the soil through frictional interactionbetween the soil and the reinforcement.
Many studies were conducted relative tothe 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 betweenhigh modulus and low modulus reinforcements were pointed out and classified thereinforcements 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 soilreinforced with inextensible inclusions is generally known as ‘reinforcedearth’, whereas soil reinforced with extensible inclusions is termed as ‘PLYsoil’.
Fiber reinforcement comes in the category of extensible inclusions.Synthetic fibers comprise polypropylene, polyester, nylon and plastics and havegreater resistance to attack by acids and alkalis or other chemicals. Thepolypropylene fibers have high tensile strength and melting point to 165oC.Though this category has inherent defect of getting affected by theultra-violet rays from sunlight but for buried material this negative point isnot there. Asbestos, glass, carbon fibers have been found to be resistant toalkalis and other chemical’s attack, but long exposure to such adverseenvironment has been found to lead to corrosion damage. Plastic wastes wereused as reinforcing materials and found an improvement in the peak and ultimatestrength of cemented and soil alone.
Materials such as polyethyleneterephthalate (PET), plastic bottles, waste plastic LDPE carry bag, HDPEpackaging strips are profusely and widely produced. According to the the bestway to handle the increasing pressure of waste plastic on open dumps is toutilize it for ground improvement after shredding. Tyre buffing, theby-products of the tyre treated process referred to as “Rubber fibers” can beused as reinforcing materials, the high strength, durability and availabilityof scrap tyres have prompted their use, in the form of tyre particles, as lightweight aggregate in geo engineering applications. The roots of plants have agreat influence on the shear strength and stability of the natural slopes.
Thedegree of increased strength imparted by the roots depends on the concentrationand properties of the roots. But this category of reinforcement has been foundto have low resistance towards attack by alkalis and other chemicals. The othernatural fibers like jute, sisal, banana have tested and found to loose instrength when subjected to alternate “wetting and drying” in environmentcreated by lime content present in the soil. Types of reinforcementsThe different types of reinforcements used for reinforcing soilscan be categorized into:Artificial and Natural ArtificialreinforcementsThegeo-synthetics are a term which essentially includes a family of materials thatare natural or man-made that is used in conjunction with soil, to improve theperformance of soil in specific context.
Geo-synthetics are most popular andhave gained enormous momentum due to their flexibility in usage and thefinancial benefits. Geo-synthetics areflexible, porous man made products primarily consists of polymers, woven ornonwoven and are available in the form of geo-membrane, geo-nets, geo-gridsetc., which are used for different purposes like containment, drainage andreinforcement. Geo-grids are high strength materials composed of polyethylene,polypropylene or PVC coated polyester and are commonly used for reinforcementpurpose to stabilize a slope, construct a soil wall to increase the bearingcapacity of weak soil. More recently the use of geo-grids as soilreinforcements is in vogue in construction of reinforced earth structures. Thegeo-grids can be used very effectively as reinforcements to improve the factorof safety against bearing capacity failure and to increase the bearing capacityof granular soil, which can replace the conventional footing with reinforcedearth slabs.Natural coir fiber reinforcementCurrently, the global annualproduction of coir fiber is about 350,000 metric tons (MT). Yet, even in theworld’s top two producers, India and Sri Lanka, which account for about 90% ofglobal coir fiber production, combined, this renewable resource isunderutilized; local coir mills process only a fraction of the available husks,which accrue more or less year round as a waste during coconut processing Coiris a natural fiber produced from the husk of coconut which is abundantlyavailable in the coconut growing States of India like Kerala.
Coir fiber isextracted through a traditional retting process and from the untreated husk; itis extracted through a mechanical process. Its development as an industry inIndia commenced in the State of Kerala centuries ago. Two ply coir yarns werespun either by hand or spinning wheels throughout the coastal belt of Keralafrom time immemorial. The breathtakingly beautiful lakes and lagoons of Keralahave been natural allies of the coir sector. Coir yarn is the raw material forthe manufacture of an array of furnishing products like mats, mattings, rugs,carpets etc.
Growth of technology in the sector is resulting in the developmentof a larger diversity of products. Coir is a fiber with intrinsic strength ofmulti-cellular fiber giving it the toughness and brushing quality, unlikecompeting fibers. Today, coir offers a range of options from live-in-styleproducts to geo-applications. The potential of rubberized coir products likemattresses, furniture/vehicle upholstery and insulation pads are immense.
Dueto natural edge and a skilled workforce in Kerala, Indian coir caught globalattention 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 expandingmarket for coir-based erosion control products, and the spread of coir pith asa peat moss substitute in horticulture resulted in higher production andprices.Brief Historyof 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 fromcoir in proven to be most durable when used in soils because of it having highlignin content. India is the largest coir producer inthe world accounting for more than 80 per cent of the total world production ofcoir fiber. The coir sector in India is very diverse and involves households,co-operatives, NGOs, manufacturers and exporters. The coir industry employsmore than a lakh people of whom a majority is from rural areas belonging to theeconomically weaker sections of society.
Nearly 80% of the coir workers in thefibre extraction and spinning sectors are women. The development of coirindustry has all along been in areas where there is a concentration of coconuttrees and availability of coconut husk. Historically, the coir industry startedand flourished in Kerala which has a long coast line, lakes, lagoons andbackwaters providing natural conditions required for retting. However, with theexpansion of coconut cultivation, coir industry has picked up in the States ofTamil Nadu, Karnataka, Andhra Pradesh, Orissa, West Bengal, Assam, Tripura,Pondicherry and the Union Territories of Lakshadweep and Andaman & NicobarIslands through the efforts of Coir Board. Fig. 2 shows production of coirfibers in different countries in the form of pie chart and Fig.
3 shows thecoir production in different states in India. The production and processingmethods in coir industry still continue to be mainly traditional. Coiris abundantly available in most parts of south and costal India, Sri Lanka,Philippines, Indonesia, Malaysia, Brazil, and others.