Geotextiles in Road Construction, Maintenance and Erosion Control

Geotextiles in Road Construction, Maintenance and Erosion Control

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Geotextile applications are changing rapidly as research shows results and manufacturing processes improve. Geotextile and their application in road construction, maintenance and erosion control were discussed in detail. Fundamental considerations in design and installation were also discussed. India is a large producer of jute. Jute is a low cost, renewable, biodegradable and eco friendly natural product. Jute-based geosynthetics is finding increasing acceptability among geotechnical engineers primarily because of its eco concordance, facility of production of tailor-made fabrics and price competitiveness. However civil engineers are still apprehensive about its long- term effectiveness on account of its bio-degradability .Applications of Jute geotextiles in ground improvement, Improvement in Pavement Performance, Erosion Control of Denuded Slopes and in Drainage and Filtration Application presents an interesting picture. The paper also discusses the mechanism of erosion control on surface and in the riverbanks followed by a case study in a river in West Bengal. In another case study on strengthening of sub-grade in a road also in West Bengal the mechanism of functioning of Jute Geotextile has been discussed.

A geotextile is any permeable textile material used with foundation, soil, rock, earth, etc. that is an integral part of a constructed project,structure or system. It may be made of synthetic or natural fibers. In contrast; a geomembrane is a continuous membrane-type liner or barrier. It must have sufficiently low permeability to control migration of fluid in a constructed project, structure or system. A geotextile is designed to be permeable to allow the flow of fluids through it or in it, and a geomembrane is designed to restrict the fluid flow. Geotextile related materials are fabrics formed into mats, webs, nets, grids, or formed plastic sheets (Figure 1). They are considered to be different from geotextiles.

History of Geotextile Applications

Textiles were first applied to roadways in the days of the Pharaohs. Even they struggled with unstable soils which rutted or washed away. They found that natural fibers, fabrics or vegetation improved road quality when mixed with soils, particularly unstable soils. Only recently, however, have textiles been used and evaluated for modern road construction. This fact sheet clarifies the confusion over terms and definitions of geotextiles, and discusses their common roadway and erosion control applications. In the 1920’s the state of South Carolina used a cotton textile to reinforce the underlying materials on a road with poor quality soils. Evaluation several years later found the textile in good workable condition. They continued their work in the area of reinforcement and subsequently concluded that combining cotton and asphalt materials during construction reduced cracking, raveling, and failure of the pavement and the base course. When synthetic fibers became more available in the 1960’s, textiles were considered more seriously for roadway construction and maintenance. As these new synthetic fabrics evolved, there was confusion over terms and definitions. Textiles and membranes now have reasonably well-accepted definitions in the construction industry, due mostly to the work of Dr. Jean Pierre Giroud. Giroud created the original terms geotextile and geomembrane, using the Latin prefix “geo” meaning “soil”. Later the term geosynthetic came into popular use, and the three terms were used interchangeably, creating confusion.

Geotextiles and their application

Modern geotextiles are usually made from synthetic polymers – polypropylenes, polyesters, polyethylenes, and polymides – which do not decay under biological and chemical processes. This makes them useful in road construction and maintenance. Geotextiles made of materials are most commonly used. The makeup of these fabrics determines their best application, so it is important to understand their characteristics. Geotextiles can be produced as a non-woven, a knitted, or a woven fabric. We will focus on the non-woven and woven fabrics since knitted fabric is rarely used. Whether the fabric is woven or non-woven is an important characteristic in choosing a geotextile for a particular use. The non-woven fabric, which looks like a felt fabric, is an arrangement of fibers either oriented or randomly patterned in a sheet. Materials commonly made of non-woven fabric include upholstered furniture coverings and cloth interiors of automobiles. These fabrics can be manufactured in a variety of ways, bonding fibers together using chemical, thermal or mechanical processes (Figure 2). The bonding methods do not significantly change the function of the fabric. Non-woven geotextile fabric is more likely to stretch than woven geotextile. It has the ability to let water flow along the plane of the geotextile.

The woven geotextile, which looks like burlap,is a sheet made of two sets of parallel strands systematically interlaced to form a thin, flat fabric. The strands may be slit film which are flat, or monofilaments which are round (Figure 3). The way these two sets of yarns are interlaced determines the weave pattern which in turn determines the best application for that woven fabric.Weave patterns come in a virtually unlimited variety which do affect some properties of the fabric. However, a buyer will specify properties of the fabric such as porosity, strength and elongation, not weave pattern. In general a woven geotextile is less likely to stretch, and does not let water flow as freely as non-woven geotextiles.

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. How it is used depends on the drainage requirements of the application. 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, for example, its natural strength may be too low to support common traffic loads, and it has a tendency to shift under those loads. Although the subgrade may be reinforced with a base course of gravel, water moving upward carries soil fines or silt particles into the gravel, reducing its strength. Geotextiles keep the layers of subgrade and base materials separate and manage water movement through or off the roadbed. However, a layer of geotextile cannot be used as a substitute for using an adequate thickness of free draining soils (like clean sand) to reduce frost heaving.

Geotextiles for separation

In separation functions geotextiles keep fines in the subgrade from migrating into the base course. Tests show that it takes only about 20% by weight of subgrade soil mixed into the base course to reduce its bearing capacity to that of the subgrade.This problem usually is due to the movement of large amounts of water. When large loads cross the surface of the roadway they set up a pumping action which accelerates this water movement and soil particle migration, and speeds up the failure of the road. (Figure 4).

Two important criteria for selecting a geotextile for separation are permeability and strength. The geotextile used for separation must allow water to move through it while retaining the soil fines or sand particles. It should let water pass through it at the same rate or slightly faster than the adjacent soil. It must also retain the smallest soil particle size without clogging or plugging. Figure 4 is a diagram of the application of a geotextile as a separator. To select a geotextile, you will need to know the grain size distribution of the subgrade and the subbase as well as the permeability of the geotextile.

In selecting a specific geotextile for separation we must consider its basic strength properties. Be sure to take into account how its physical properties will survive the construction process as well as how it will survive the pressures of traffic on the gravel cover and enhance the life of the road. These strength properties are described in manufacturers’ literature and design manuals in a variety of terms including burst and abrasion resistance, and puncture, grab, and tearing strength (Figure 5).
Be sure when we use fabric for separation that you do not assume in the pavement design that it will also provide structural support. It will not.

Design considerations for using geotextiles in separation.

When using geotextiles, consider the following.

1.     What has been the past performance of geotextiles in similar types of soil?
2.     You will need to know solid characteristics and the permeability of the subgrade, and match them to the permeability criteria of the geotextile.
3.     Select the fabric strength requirements on the basis of constructability. More specifically, it must withstand placement and survive the construction period without puncturing, tearing,bursting, abrading, etc. Is the fabric sufficiently workable for the specific application? That is, can the geotextile support the workers and equipment during gravel placement.
4.     Use standard load guidelines for designing pavement strength with no allowance for the geotextile.
5.     In an existing roadway, check to see if additional subbase was added previously for extra structural support to counter the soil weakness and reduce rutting under construction equipment to three inches. If so, reduce that subbase by 39%-50% and include a geotextile in the design between the subgrade and subbase.
6.     Select the cover carefully. If you will be applying a surface course, you may use a cleaner aggregate with less than 15% fines. If this will be a gravel road and traffic will travel directly on the aggregate, then you must provide more fines (at least 15%) or the aggregate will whip off the fabric.

Geotextiles for erosion control

Geotextiles can be used many ways for erosion control. One of these is with riprap along stream banks, lake shores, and other bodies of water to keep finer soils beneath the riprap from eroding.(Figure 8). Geotextiles recommended for erosion control should have permeability, resistance to abrasion, and high resistance to ultraviolet rays as primary considerations.

Erosion control covers a variety of conditions from high velocity stream flow to heavy wave action, to less severe conditions.

All conditions should be considered before selecting a fabric.The following instructions describe how to install geotextiles on stream banks and similar steep slopes. These may be modified for applying geotextiles in less severe conditions such as rip rapping in ditches. Geotextile/riprap installations may also be used in specifically designed systems to protect against scouring around bridge piers and abutments,and in other water installations.To install geotextiles for any riprap system:

–    Before starting, review such design considerations as wave action, bank steepness, etc.
–    Identify soils by particle size and permeability as these will determine certain geotextile specifications.
–    Identify the size of riprap planned for this application.
–    Review past weather and climate conditions for such information as levels of ice, wave action, and amount of sunlight for their effect on riprap/geotextile installations.Ultraviolet rays in sunlight deteriorate most synthetic materials. If exposure to ultraviolet rays is anticipated, select a geotextile with high resistance to ultraviolet rays.
–    Depending on the type of installation and the care it will need, you may have to consider abrasion to ensure that the geotextile will survive installation.

Use of Jute Geotextiles in Road Construction

India is a large producer of jute. Jute is a low cost, renewable, biodegradable and eco-friendly natural product. Jute geotextiles are used in many geotechnical applications. A series of field experiments were carried out by CRRI using jute geotextiles for different functions, are described as follows: –

Jute Geotextiles as Separator to Improve Pavement Performance

The performance of pavements constructed on soft soils can be improved using jute geotextiles. Jute fabric when used as separator prevents the penetration of subgrade material into voids of granular base course. The permeability characteristic of the fabric also aids in faster dissipation of pore pressures and ensures better drainage which results in better long term performance of the pavement. Provision of fabric enables subgrade develops its full bearing capacity and thus controls rutting. Jute geotextile was used as a separator between subgrade and sub-base layers. Results showed negligible settlements of the pavement after six months under traffic and no signs of surface distress observed in the treated test section.

Jute Geotextiles for Ground Improvement

The field subsoil was soft silty clay and the water table was 0.5 m below the ground level, the whole area gets submerged during high tide. The highway constructed earlier faced problems of subsidence of the fill during construction, excessive post construction settlements and lateral spreading of fill material, etc. On the basis of settlement calculations, it was estimated that as much as 30 per cent of the fill sinks into the soft subsoil during construction itself, necessitating large quantities of costly fill material,thereby, pushing up the cost of construction. The problem was solved through the use of jute geotextiles. Monitoring of completed embankment i.e. both treated and control stretch showed better performance of road embankment constructed using jute geotextile.

Jute Geogrid for Erosion Control of Denuded Slopes

On the basis of field studies, conducted in the past by CRRI, it has been concluded that shallow sacrificial slides constitute a significant proportion of landslides in areas with moderate rainfall intensity and where soil cover is medium cohesive in nature. Surfacial slides extend to only a couple of metres below the slope surface and originate as a result of erosion from down flowing water over the denuded slopes. If erosion is allowed to proceed unchecked, there is every possibility that the damage may spread laterally thus increasing the depth of erosion, eventually resulting in a much larger damaged slope area. Vegetative turfing represents one of the most important corrective measures. In the case of freshly exposed cutting made for road construction, vegetative turfing is important, even as a preventive measure. In the case of deep-seated slides, however, vegetative turfing is only one of the techniques among available corrective measures and as such it can prove to be effective only when conjointly implemented with other corrective measures. Based on several field trials carried out by the Institute, the use of jute geogrid technique has been developed for treatment of erodible slopes as a part of landslide correction works.

Jute Geotextile for Drainage and Filtration Application

The field conditions were the stretch of hill road was located on debris slide area and debris consists of micacious sandy silt. A number of seepage points exist on the uphi ll as well as on downhill slopes. The road stretch was experiencing subsidence during the monsoon every year, including damages to the restraining structures. Breast walls constructed earlier had been damaged due to slip. To arrest the sinking of road pavement, a systematic network of roadside trench drains and cross trench drains were constructed using non-woven jute geotextiles. The trench drains were made of rubbles encapsulated in non-woven jute geotextiles to stop the finer particle entering into the voids of encapsulated rubbles, thereby preventing clogging the trench drains. About 1000 sq.m of non-woven jute fabric having 750 gsm has been used for drainage application on about 100 m length of road stretch. The monitoring of field experiments on this particular stretch of treated road has shown very encouraging and satisfactory results.There has been no further sinking and subsidence of the road at this location after three years.

Case Study with JGT for Erosion Control

Location of the Site

Left bank of the river Phulahar in the district of Malda, West Bengal, up stream of Sankharitala Ghat near Mathurapur (35 km from Malda town).

Problem

Flood during the monsoon as a result of high precipitation caused rise in water level both in the Ganga and the Phulahar connected with it. There was a heave-up of the excess water at the mouth of the narrower Phulahar that takes a bend at a distance of one and a half kilometers from its outfall in the Ganga. The concave bank was understandably subjected to heavy erosion that was accentuated due to strong protective measures undertaken on the opposite end for the stability of a big land-form (Bhutni Diara) that has  emerged within the Ganga.On earlier occasions of flood also, the same stretch was subjected to similar erosion.Irrigation & Waterways Department, Govt. of West Bengal constructed earlier an apron of loose boulders to control of the bank. The apron could not stand the erosive forces and gave in.The problem is unabated erosion at the eastern stretch during the flood, engulfing chunks of land every year.

Object

The object is to control erosion in the vulnerable stretch of 750 meters (approximately) of the river Phulahar by use of Jute Geotextile and/or other measures.

Remedial concept

Erosion at the toe of the bank can be controlled by construction of submerged repelling spurs or by construction of a toe wall. Irrigation & Waterways Department adopted the second option, presumably for avoiding flow repulsion to the opposite end that could destabilize the protective work around Bhutni Diara. The bank slope measuring 12 meters in length was given a ‘break’ after 5 meters from the bank top, forming a berm of a 1 meter.

Some features of relevance

The bank soil was made up of fine sand (average particle size – 0.175 mm). Co-efficient of soil permeability was of the order of 10-4 per sec. The monsoon discharge veers around 9330 cumec while the maximum velocity of the current was of the order of 2 meters per second.

Implementation

Implementation of the remedial concept was done in 2004 after the monsoon. A toe wall with crated boulders (900 X 1200 in size) was constructed. The bank slope behind the toe wall was prepared and dressed to 2:1 slope and bitumen-treated woven Jute Geotextile was laid over it. An armour of loose granite boulders (Rajmahal trap) in two layers having a thickness of 450 mm was placed over the fabric.

Feature of Jute Geotextile used

The JGT used had the following features—
–    Weight per unit area –760 gsm (1200 gsm after treatment with bitumen)
–    Tensile Strength (MDXCD) –minm. 20 kN/m x 20 kN/m when grey 21 kN/m X 21 kN/m after treatment
–    Elongation at break (MDX CD) –maxm.10% X 10%
–    Porometry –150 micron ± 10%
–    Flow Rate at 50 mm constant water head—14 litres/ m2/second
–    Permittivity at 50 mm constant water head—350 x 10 –5 per sec.
–    Puncture Resistance –400 kN

Performance Evaluation

The treated stretch of the affected bank is in a fine shape two years after remedial measured were taken with Jute Geotextile. The letter of the Executive Engineer, Malda Irrigation Division is confirmatory. The toe wall also played its desired role in controlling erosion at the toe.

Mechanism of Strengthening of Road Sub-grade

One of the underlying principles for separation and reinforcement function to be effective by use of geosynthetics is not only to ensure segregation and withstanding dynamic stresses respectively, but also to help generate membrane effect that exerts an upward thrust against the imposed loads. Initial tensile strength of JGT and its low extensibility certainly helps in this respect. But more important are the functions of filtration and drainage of geosynthetics that ensure natural consolidation of a road subgrade.Permittivity (flow across the geosynthetics) and transmittivity (flow along the plane of geosynthetics) are two critical parameters that help maximize soil consolidation only when soil in the sub-grade is not allowed to migrate. Roadside drainage and lateral restraint of the pavement are two other technical necessities. All the requirements as indicated could be achieved if the fabric porometry is rightly decided. Decision on pore size of JGT or, for that matter, any geosynthetic is critical.Consolidation of soil, as already mentioned, is a time-dependent process and may continue for a protracted period. In one of the field applications on roads done in Kakinada Port in Andhra Pradesh, an increase of CBR by about 3 times were observed after a lapse of 6 years of laying of woven JGT on a severely damaged road! (Rao et al 1993, Sreerama Rao 2003) This is again a pointer that long term durability of JGT is not a technical indispensability in all cases.One of the difficulties faced in placement of JGT under granular base/sub-base is the probability of puncturing. The fabric may be punctured by the sharp edges of coarse aggregates. The difficulty can be obviated by spread of a thin cushion of sand above or under the fabric as necessary. In one of the field studies, JGT was overlain by WBM comprising sharp-edged brick metal. Thickness of WBM was reduced and part of it was replaced by brick flat soling for prevention of puncturing. Admittedly there were chances of inhibition of permittivity in such an  arrangement. The difficulty was overcome by keeping a gap of 10 mm in each joint. In reality the extent of transitivity exceeds the magnitude of permittivity, the arrangement worked and the performance of the treated pavement was satisfactory.

Case Study in Road Construction

Location of the Site

Andulia-Boyratola Road is a 3.5 km long rural road located under Haroa block of the North 24 Parganas, West Bengal. It passes through three unconnected habitations and is close to agricultural fields, fishing ponds and small scale industries. The area in future is expected to show up due to developmental programmes being envisaged in the area. The proposed road is likely to take on a larger traffic in future. It rests on a ground higher than the ambient ground level by 725 mm.The average annual rainfall in the area is 1500 mm.

Soil

The soil is basically organic silty clay (IS Soil classification- OL), Average L.L. is 46% while average P.L. is 27.5% (PI is 18.5%). OMC was found to be 23.5% while MDD was observed to be 1.72 gm/cc. The average soaked CBR value of the sub-grade was found to be 3.22 % (4 days).

Traffic

Traffic census was carried out at the nearest all weather road (AWR) for the purpose of design , CVPD (Commercial Vehicles per day) has been taken as 22, considering 10% of the traffic in the nearest all-weather road (AWR).

Salient Features of the Project

The DPR was drawn up in accordance with IRC SP 20:2002. Woven Jute Geotextile (JGT) was laid on the sub-grade to improve its CBR. Pavement was designed with an enhanced CBR value of 4.93% (1.5 times the exiting CBR of the subgrade -rounded off to 5%). The total cost of the project worked out to Rs. 1.48 crores. The financial component of JGT was 4.34% of the total project cost. The thickness of the sub base,the base and the wearing coarse was 200 mm, 150 mm and 26 mm respectively. (as per the relevant curve corresponding to the assumed CBR, CVPD & average annual rainfall).

Installation of Woven JGT

Woven untreated JGT (specifications given below) was laid on the sub-grade after raising the road level to 1200 mm from G.L. (the level of the sub-grade). The top width of the road was finished to 7500 mm with 1:1.5 side slope while the actual carriage way measured 3750 mm.After the sub-grade was rolled to OMC, the specified JGT was laid in taut condition on the prepared sub-grade, taking care to ensure that there was no gap between the subgrade and the fabric. Overlaps at sides (150 mm) and ends (300 mm) were provided duly stapled at an interval of 300 mm with broad- headed nails. A cushion of a thin layer of local sand was provided on JGT to avoid puncturing during rolling the subbase and the base of the pavement.On laying the fabric, the pavement was finished as per specifications stated in the approved DPR which was vetted by the Civil engineering Department, Bengal Engineering & Science University, Sibpur,West Bengal.

Specifications of Woven JGT

The specifications adopted for the woven untreated JGT were as under—
–    Weight—– 810 gsm (untreated)
–    Thickness–2 mm
–    Width—— 76 cm
–    Tensile Strength—minm. 30 kN/meter (both in machine & cross directions)
–    Elongation at break—max. 10 % (both in MD & CD)
–    Porometry— 150 micron + 10%
–    Permittivity at 50 mm constant water head—350 x 10-5 per second
–    Puncture Resistance —0.600 kN

Inference

Field CBR was ascertained by the Civil Engineering Department of Bengal Engineering & Science University (also the accredited State Technical Agency for the PMGSY Programme). CBR was found to be 10%-a figure double the enhanced design value of the sub-grade CBR estimated after laying of JGT.The case study is a pointer that i) with JGT, the sub-grade CBR could be improved by around three times the control value even after one and half years and ii) thickness of the pavement, reduced by 45 mm, almost compensated the cost on JGT. Evidently the design was conservative and that further reduction in thickness would have been possible entailing further cost reduction.

Conclusion

–    Certain fundamental considerations are necessary for success in any application of Geotextiles . We must know the soils to select the proper geotextile in road construction and maintenance. In many installations, permeability may override concern for durability and resistance to bursting, puncturing and tearing.In other installations, such as a separator in a road where the geotextile will be subjected to severe loads, durability is of concern.
–    Permeability should also always be considered in separation uses to allow moisture to move freely through the system. This avoids excessive hydrostatic pressures which cause soil failure.
–    Most geotextile system failures result from improper installation, improper selection of fabrics, a change of conditions from the original design, or a combination of these factors.
–    In the present realm of growing global emphasis on adoption of bio-technical measures,Jute Geotextile deserves encouragement due to its several striking attributes. Of all the ingredients of natural geosynthetics, jute happens to be the best spinnable fibre that ensures making of customized fabric to meet site-specific requirements.
–    JGT is also the most drapable of all geosynthetics—a property essential for control of surficial soil erosion. Its low extensibility and high initial strength helps in enhancing the bearing capacity of soil.
–    Its environmental concordance is by far its most attractive feature along with its cost-competitiveness. When we consider the environmental price of JGT, it matches with its man-made counterpart both in respect of price and technical suitability in most of the geotechnical applications. This is because long durability of geosynthetics is not a technical necessity in majority of applications.

References

–    Chaudhury, Goswami & Sanyal–Application Of Jute Geotextile In Rural Road Construction Under PMGSY—A Case Study In West Bengal (Indian Geotechnical Conference at Ahmedabad, India,2005)
–    International Trade Centre UNCTAD/GAATT, Geneva –Jute Geotextiles-A Survey of Marketing & Distribution Systems in Selected European Countries (Prepared for IJO in 1991)
–    Ramaswami & Aziz –Jute Geotextile For Roads (Intl. Workshop on Geotextiles at Bangalore, India,1989).
–    Sanyal–Control Of Bank Erosion Naturally- A Pilot Project in Nayachara Island in the River Hugli (National Workshop on Role of Geosynthetics in Water Resources Projects at New Delhi,1992)— Sanyal & Chaudhury–Appraisal Of Performance (All India Seminar on Applications of Jute Geotextiles in Civil Engineering at Kolkata,India 2002)
–    Sanyal & Chakravarty –Application Of Jute Mattress in the Bank Protection In The Hugli Estuary (Geotextiles & Geomembranes, U K.1993)
–    Sreerama Rao –Jute Geotextile Application In Kakinada Port Area (National Seminar on Jute Geotextiles & Innovative Jute Products at New Delhi, India, 2003.

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