Application of Geotextiles in Civil Engineering

 

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BY MR. TANVEER MALIK, MR. T. K. SINHA
MRS. YOGITA AGRAWAL AND DR. ASHUTOSHSHUKLA

 


Abstract

Geotextiles, an emerging field in the civil engineering and other fields, offer great potential in varied areas of applications globally. Geotextiles play a significant part in modern pavement design and maintenance techniques. There is a tremendous growth in their use worldwide for transportation applications and other civil engineering applications. Geotextiles are ideal materials for infrastructural works such as roads, harbors and many others. They have a bright future, as they have multifunctional characteristics. The paper provides an overview of various natural as well as synthetic textile fibers used for application as geotextiles and use of geotextiles in Civil Engineering.

Introduction

Geotextiles were one of the first textile products in human history. Excavations of ancient Egyptian sites show the use of mats made of grass and linen. Geotextiles were used in roadway construction in the days of the Pharaohs to stabilize roadways and their edges. These early geotextiles were made of natural fibers, fabrics or vegetation mixed with soil to improve road quality, particularly when roads were made on unstable soil. Only recently have geotextiles been used and evaluated for modern road construction. Geotextiles have proven to be among the most versatile and cost-effective ground modification materials. Their use has expanded rapidly into nearly all areas of civil, geotechnical, environmental, coastal, and hydraulic engineering.


Geotextiles used in civil engineering applications are expected to carry out one or more functions over if given design life. There are five defined functions, these are; drainage, separation, filtration, protection and reinforcement. Geotextiles are normally manufactured by either woven or non-woven techniques. The functional requirements of the geotextile in a given application will determine the performance properties required, and any assessment of the products durability will be based on the degradation of these properties over a given time.
Geotextiles particularly refers to permeable fabric or synthetic material, woven or non-woven, which can be used with geotechnical engineering material. They apply to a broad range of civil engineering construction, paving, drainage and other applications. Geotextiles are extensively used with soil, rock, earth or any other geotechnical engineering-related material.

Geotextile Forming Fibres and Polymers

Different fibers from both natural as well as synthetic category can be used as geotextiles for various applications.

Natural fibers:
Natural fibers in the form of paper strips, jute nets, wood shavings or wool mulch are being used as geotextiles. In certain soil reinforcement applications, geotextiles have to serve for more than 100 years. But biodegradable natural geotextiles are deliberately manufactured to have relatively short period of life. They are generally used for prevention of soil erosion until vegetation can become properly established on the ground surface. The commonly used natural fibers are

Ramie:
These are subtropical bastfibers, which are obtained from their plants 5 to 6 times a year. The fibers have silky luster and have white appearance even in the unbleached condition. They constitute of pure cellulose and possess highest tenacity among all plant fibers.

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 bleedings. Thus, it is possible to manufacture designed biodegradable jute geotextile, having specific tenacity, porosity, permeability, and 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. 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, ligomass is formed, which increases the soil organic content, fertility, texture and also enhance vegetative growth with further consolidation and stability of soil.

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

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

Polyesters (PET):
Polyester is synthesized by polymerizing ethylene glycol with dimethyle terephthalate or with terephthalic acid. The fiber has high strength modulus, creep resistance and general chemical inertness due too which it is more suitable for geotextiles. 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. It possesses high resistance to ultraviolet radiations. However, the installation should be undertaken with care to avoid unnecessary exposure to light.

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

Polypropylene (PP):
Polypropylene is a crystalline thermoplastic produced by polymerizing propylene monomers in the presence of stereo-specific Zeigler-Natta catalytic system. Homo-polymers and co-polymers are two types of polypropylene. Homo polymers are used for fiber and yarn applications whereas co-polymers are used for varied industrial applications. Propylene is mainly available in granular form.
Both polyethylene and polypropylene fibers are creep prone due to their low glass transition temperature. These polymers are purely hydrocarbons and are chemically inert. They swell by organic solvent and have excellent resistance to diesel and lubricating oils. Soil burial studies have shown that except for low molecular weight component present, neither HDPE nor polyethylene is attacked by microorganisms.

Polyvinyl chloride (PVC):
Polyvinyl chloride is mainly used in geo membranes and as a thermo plastic coating materials. The basic raw materials utilized for production of PVC is vinyl chloride. PVC is available in free- flowing powder form.

Essential Properties of Geotextiles

The three main properties which are required and specified for a geotextile are as follows:

 Mechanical responses
 Filteration ability
 Chemical resistant

 

These are the properties that produces the required working effect. They are all developed from the combination of all the physical form of the polymer fiber, their textile construction and the polymer chemical characteristics. For example, the mechanical response of a geotextile will depend upon the orientation and regularity of the fibers as well as the type of polymer from which it is made. Also the chemical resistance of a geotextile will depend upon the size of individual component fiber in the fabric as well as their chemical composition. Fine fibers with a large specific surface area are subject to more rapid chemical attack than coarse fibers of same polymer. Mechanical responses include the ability of a textile to perform work in a stressed environment and its ability to resist damage in an arduous environment.


The filteration performance of a geotextile is governed by several factors. To understand this, it is essential to be aware that the function of the textile is not truly a filter in the literal sense. In general, filters remove particles suspended in a fluid. For example, dust filters in air-conditioning units, or water filters, which are intended to remove impurities from suspension. Quite the opposite state of affairs exists with geotextile filters. The geotextile function is to hold intact a freshly prepared soil surface, so that water may exude from the soil surface and through the textile without breaking down that surface. If water is allowed to flow between the textile and the soil interface with particles in suspension, it will tend to dug up the textile which will fail in its function. In practice, it has been found that, in conjunction with a textile, the soil will tend to filter itself, provided that the integrity of its external surface is maintained. The actual process taking place is the passage of a liquid form a solid medium that is held intact by a permeable textile. The process is not one of restraining the passage of solids that are suspended within a liquid medium.
 

Geotextile are rarely called upon to resist extremely aggressive chemical environments. Particular examples of where they are, however, include their use in the basal layers of chemical effluent containers are waste disposal sites. This can happen if and when leaks occur, permitting effluent to pass through the impermeable liner of the textiles have been incorporated directly in the leach ate disposal system above the impermeable liner. Ultraviolet light will tend to cause damage to most polymers, but the inclusion of additives, in the form of antioxidants chemicals and carbon black powder, can considerably reduce this effect. The only time when a geotextile is going to be exposed to sunlight is during the construction period.

Basic Function of Geotextile

Every textile product applied under the soil is a geotextile. Geotextiles form one of the two largest groups of geosynthetics. The mode of operation of a geotextile in any application is defined by six discrete functions: separation, filtration, drainage, reinforcement, sealing and protection. Depending on the application the geotextile performs one or more of these functions simultaneously. Depending on the required function, they are used in open-mesh versions, such as a woven or, rarely, warp-knitted structure, or with a closed fabric surface, such as a non-woven.

Separation:
Separation is function to prevent mutual mixing between 2 layers of soil having different particle sizes or different properties. It is used in all classes of roads and similar civil foundation, as the base of construction on contaminated layer is the single most cause of premature failure. The use of separator prevents pumping effect created by dynamic load and also helps the passage of water while retaining soil particles. In theses types of geotextiles, thickness and permeability are most important characteristic properties. The effect of separation is illustrated in figure given below.

Filtration:
It is defined as the establishment of a stable interface between the drain and the surrounding soil. In all soils water flow will induce the movement of fine particles. Initially a portion of this fraction will be halted at the filter interface; some will be halted within the filter itself while the rest will pass into the drain. The geotextile provides an ideal interface for the creation of a reverse filter in the soil adjacent to the geotextile. Infiltration, fabrics can be either woven or non-woven, to permit the passage of water while retaining soil particles. Porosity and permeability are the major properties of geotextiles, which involves in filtration action.

Drainage (Transmissivity):
The function of drainage is to gather water, which is not required functionally by the structure, such as rainwater or surplus water in the soil, and discharge it. This refers to the ability of thick nonwoven geotextile whose three-dimensional structure provides an avenue for flow of water through the plane of the geotextile. Above Figure also illustrates the transmissivity function of geotextile. Here the geotextile promotes a lateral flow thereby dissipating the kinetic energy of the capillary rise of ground water.

Reinforcement:
This is the synergistic improvement in the total system strength created by the introduction of a geotextile into a soil and developed primarily through the following three mechanisms i.e. lateral restraint through interfacial friction between geotextile and soil/aggregate, forcing the potential bearing surface failure plane to develop at alternate higher shear strength surface and membrane type of support of the wheel loads. In this method, the structural stability of the soil is greatly improved by the tensile strength of the geosynthetic material. Due to their high soil fabric friction coefficient and high tensile strength, heavy grades of geotextiles are used to reinforce earth structures allowing the use of local fill material. Reinforcement provided by geotextiles allow embankments and roads to be built over very weak soils and allows for steeper embankments to be built.
 


 

Sealing Function:
A non-woven geotextile performs this function when impregnated with asphalt or other polymeric mixes rendering it relatively impermeable to both cross-plane and in-plane flow. The classic application of a geotextile as a liquid barrier is paved road rehabilitation, as shown in Figure given below. Here the non-woven geotextile is placed on the existing pavement surface following the application of an asphalt tack coat. The geotextile absorbs asphalt to become a waterproofing membrane minimizing vertical flow of water into the pavement structure.

Protection:
Erosion of earth embankments by wave action, currents and repeated drawdown is a constant problem requiring the use of non-erodible protection in the form of rock beaching or mattress structures. Beneath these is placed a layer of geotextile to prevent leakage of fine material. The geotextile is easily placed, even under water. Soil waste and hazardous landfill structures are designated with impervious geo-membrane layer along with geotextile thus ensuring that no ground water contamination takes place. It also acts as drainage gallery. It is used in the thermal power stations for disposing off the fly ashes in the ash-ponds constructed with impervious geo-membrane layer along with geotextile that protects the membrane from punching and soil polluting.

 



Application Areas of Geotextiles

Modern geotextiles are usually made from synthetic polymers- polypropylenes, polyesters, polyethylene, and polyamides - which do not decay under biological and chemical processes. This makes them useful in road construction and maintenance.

Geotextiles in Road Industry
In the road industry there are four primary uses for geotextiles Separation, Drainage, Filtration and Reinforcement.

In separation, inserting a properly designed geotextile will keep layers of different sized particles separated from one another. In drainage, water is allowed to pass either downward through the geotextile into the subsoil, or laterally within the geotextile which functions as a drain. In filtration, the fabric allows water to move through the soil while restricting the movement of soil particles. In reinforcement, the geotextile can actually strengthen the earth or it can increase apparent soil support. For example, when placed on sand it distributes the load evenly to reduce rutting.
Geotextiles now are most widely used for stabilizing roads through separation and drainage. When the native soil beneath a road is very silty, or constantly wet and mucky then its natural strength may be too low to support common traffic loads, and it has a tendency to shift under those loads. Geotextiles keep the layers of subgrade and base materials separate and manage water movement through or off the roadbed.

Stabilization:
Higher strength woven and nonwoven geotextiles provide stabilization in addition to the primary function of separation. Through stabilization, a geotextile can increase the effective bearing capacity of low strength subgrade soils. A stabilization geotextile reduces subgrade pumping, over-excavating and required aggregate thickness. Stabilization geotextiles substantially reduce construction costs for paved and unpaved roads. For example, unpaved road aggregate thickness can be reduced by as much as 30% to 50% when a stabilization geotextile is used.

Geotextiles in pavement Repair:
A major contributor to roadway deterioration is water beneath a pavement, which softens subgrade soil which destroys pavement structural capacity. A pavement with a base, which becomes saturated, as little as 10% of the time will only have 50% of the life of a pavement where water is kept out of the base. Most of this water enters through cracks and pores in the pavement surface. Paving fabrics and repair membranes are engineered to reduce water infiltration and reflective cracking, thereby saving on costly repaving cycles. They have been proven to extend the life of highways, city streets, parking lots, and airport runways and taxiways. These kinds of geotextiles are used in new asphalt pavements, beneath overlays of rigid and flexible pavements, and beneath chip-seal pavements.

Geotextiles in retaining Walls:
Retaining walls help to maximize their land use. However, building a concrete gravity or crib wall is often impractical because of their high construction cost. Geotextiles are used for a wide assortment of reinforcement applications, including embankments over soft soils, levees and retaining walls. Geotextiles are well-suited to construction of walls with timber, precast panel and segmental block facing. In fact a geotextile retaining wall can be built for less than half the cost of a conventional wall. Woven geotextiles offer other significant advantages over conventional methods, such as simplified installation and construction, and the ability to use on-site backfill material. Polypropylene geotextiles cost approximately half the amount of polyester and polyethylene geogrids, and they require considerably less labor to install.

Geotextiles subsurface Drainage:
Geotextiles have replaced graded soil filters for drainage of virtually all structures, including groundwater intercept systems, pavements, building foundations, dams and walls. Compared to conventional soil filters, geotextiles offer advantages by providing a consistent and continuous filter, reduced excavation, reduced environmental impact, simplified, higher quality construction and a substantial reduction in material costs.

Geotextiles erosion Control:
Geotextiles have replaced graded granular filters used beneath riprap or other armor materials in revetments. Applications include drainage channels, shorelines, and bridge and pier scour protection systems. Without a geotextile filter, wave action and water movement erode subgrade soils from beneath the riprap or armor. Degradation of the subgrade negates the benefit of the riprap or armor, resulting in extensive repair and replacement.
The selection of geotextiles for permanent erosion control is similar to subsurface drainage. However, permanent erosion control applications usually require higher geotextile strength properties. The geotextile must survive placement of possibly very large, angular riprap, plus be able to endure severe wave action.

Geotextiles waste Containment:
Waste containment and environmental cleanup projects demand geotextiles with uncompromising physical properties and consistent product quality. In environmental applications, geotextiles must retain these critical properties while exposed to harsh chemical environments. Waste containment fabrics' serves in a variety of environmental applications, including filtration of fluid and gas collection systems, protection of geomembrane liners, waste daily covers and reinforcement. Geotextiles are specified for municipal waste and hazardous waste landfills, heap leach pads, sewage treatment lagoons, as well as waste containment ponds and other surface impoundments.

Geotextiles railroad Stabilization:
Maintaining track bed geometry is critical for efficient railroad operation. Subgrade pumping into the overlying ballast can create an uneven track bed, resulting in delayed arrivals and even derailments. Geotextiles perform multiple functions in railroad applications. Nonwoven fabrics are used to stabilize both new and rehabilitated tracks. They prevent contamination of new ballast with underlying fine-grained soils and provide a mechanism for lateral water drainage. Using nonwoven geotextiles beneath track beds ensures that the ballast can sustain the loads for which it was designed. These geotextiles are used in all track applications, including switches, turnouts and grade crossings. High-strength woven geotextiles can also be used to reinforce weak subgrade soils and reduce required embankment fill materials.
 


References

 Abdullah, a. b. m., a hand book of geotextiles particularly natural goetextiles from jute and other vegetable fibers, fao-2000
 Gregory, r. n., barry, c. r., geotextiles in transportation applications, featured short course, 1998.
 Terzaghi, k. and peck, r. b., “soil mechanics in engineering practice”, john wiley& sons, new york, 1967.
 Robert m. koerner 'designing with geosynthetics', 1998
 Geotextile: it's application to civil engineeering overview by dr. bipin j agrawalnational conference on recent trends in engineering & technology
 Astm (1994), annual books of astm standards, american society testing and materials.


 

 


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