Indian Journa l of Fibre & Textile Research Vol. 22, December 1997, pp. 318-336

Geosynthetics: Recent developments G Venkatappa Rao & P K Banerjee Indi an Institute of Technology, Ha uz Khas, New Delhi 11 00 16, India

A wide variety of geotextiles and related products available to civil engineers to solve a wide range of problems are' briefly presented along with their functions and poss ible applications. The extensive work being carried out at the Indian Institute of Technology, Delhi, is high lighted. The testing required for hydraulic applications is presented in detail. The work carried out on design of needle-punched fabrics for hydraulic applications is described . A case study on use of edge drains for rural roads is presented . The work in progress on natural fibre geotextiles with jute and coir is included.

Keywords: Co.r, Edge drains, Erosion control, Geotextiles, Hydraulic behaviour, Jute, Natural fibre geotexti les, Polyester, Polypropylene, Roads

I Introduction India is a vast country with widely varying climatic and terrain conditions. The resulting di verse nature of sub-soil conditions creates a spectrum of problems for the construction engineers. During the last two decades, there has been an increase in demand for construction of civil engineering structures in a variety of topographical conditions. To improve the poor sub-soil conditions in adverse locations, civil engineers have been traditionally depending on conventional raw materials like bricks, cement, steel, etc. But situations arise where there may be non-uniform quality and/or non-availability of desired soils at and around the construction site, leading to high material transportation cost. These and other associated technical limitations of building conventional structures on weak soils lead the civil engineers to search for alternate solutions. One such alternative which has emerged as a popular material/technique in recent years is geotextiles and related products now commonly described as geosynthetics. In the majority of cases, synthetics are produced from man-made fibres; however, they can also be from natural fibres. In fact, geosynthetics are synthesized for use in civil engineering projects to facilitate construction, ensure better performance of the structures and reduce maintenance in the long run. They have

wide applications in almost all geotechnical and hydraulic engineering projects. Typically, such projects include airport and highway pavements, railways, canals, coastal erosion works, dams, embankments, retaining walls, etc. During the last decade, there has been a phenomenal growth in use of geotextiles all over the world. The expected growth in consumption of geotextiles in North America is shown in Fig.l. The Indian engineering community is however still in the process of getting acquainted with thi s family of new materials through laboratory and field research works, field tri als, conferences and workshops. The actual use ha s been very limited in comparison with the perceived need and potential. Recent trends in the use of geotex tile s in the Indian context ha ve been summarised by Dey c/ a/. 1 and Kaniraj and Venkatappa Rao ~ . With the confidence gained with the limited Indian experiences. the 1995 version of spec ifications lor Road and Bridge Works brought out by the Ministry of Surface Transport (Roads Wing) ha s included geosynthetics for use In sub-surface drains. highway pavements. slope protection works and reinforced soil structures. In this paper. an attempt has been made to present the wide spec trum of geotextiles and related products that arc currentl y available. the range of app li ca tion s and the hydralll ic propert y

RAO & BANERJEE: GEOSYNTHETICS

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m

SptCloli ty products

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319

~ ErosIon contr~ products

1996

1997

Ye ar

Fig. I-IFAI market survey projections (1997)

Fig. 2-Scanning electron pholUmicrographs of: (a) monofilamen t/tape woven fabric, (b) multifilament woven fabric, (c) intersection detailing ofa woven fabric, and (d) tape (s lit film) woven fabric

characterization of geotextiles along with a focus on natural fibre geotextiles .

Geosynthetics are currently being defined as civil engineering materials that are synthesized for use with geological materials like soil, rock or any other geotechnical engineering related material to 2 Geosynthetics As already indicated, there is a whole range of Improve or modify the behaviour of civil new products grouped under "Geosynthetics." engineering works. It is a generic term which

320

INDIAN 1. FIBRE TEXT. RES ., DECEMBER 1997

Fig. 3- Scan ning electron photomicrographs of: (a) Needlepunched fabric, (b) Needle-punched fabric-closer view, and (c) Thermally-bonded fabric

includes: • • • • •

Geotextiles Geogrids Geomembranes Geocomposites Geonets and other products, geomeshes, geowebs, etc.

geomats,

GEOTEXTILES are permeable textile materials and may be woven, nonwoven or knitted. Figs 2 and 3 show the scanning electron microscopic views of typical geotextiles. Depending on the weaving technology and the fibres used (polymer used, and the technology of drawing) the strength of woven fabrics can be as high as 1100 kN/m at 5% elongation. On the other hand, the nonwoven geotextiles are better known for their filtration and drainage in view of their high porosity. Even though they are thin and of low strength they can act as separators. The filtration characteristics emanate from their opening size distribution as distinct from that of the wovens (Fig. 2c and Figs 3a & 3b). Whereas the wovens may have uniform size/shape openings, the nonwovens are characterized by a wide range of pore sizes/shapes. A GEOGRID is a planar structure formed by a regular network of tensile elements with apertures of sl.)fficient size to allow interlocking with surrounding soil or aggregate. They are also characterized by high dimensional stability, high strength and high tensile modulus at very low elongation (achieved by patented processes of orientation of polymer molecules). They are of two varieties, viz. uniaxially-oriented (mono-oriented)

Fig. 4-(a) Uniaxially-oriented (mono-oriented) geogrid; and (b) Biaxially-oriented (bi-oriented) geogrid [Courtesy: Tenax Geosynthetics]

and biaxially-oriented (bi-oriented), as shown in Fig. 4, with enhanced strength in one or both the directions. They are primarily used for soil reinforcement. A GEOMEMBRANE is a continuous membrane type linerlbarrier composed of materials of low permeability to control flllid migration. The

RAO & BANERJEE: GEOSYNTHETICS

materials could be asphaltic or polymeric or a combination thereof. When geotextiles/geogrids/geomembranes are combined with woven or nonwoven geotextiles or geogrids for specific applications like drainage, erosion control, bank protection, etc., they are designated as GEOCOMPOSITES. One typical example is bentonite geocomposites or geosynthetic clay liners (GCL) which consist of geotextiles with bentonite filled pores (Fig. 5). When in contact with water, the bentonite causes the pores to swell, thereby forming a water tight sheet or offers protection to geomembranes. There are

. Woven geotextile

Needlepunched fibres

Nonlt'oven geOtextile

~.

xxxxx

Woven geotextile xxxxxxxxxx

321

also geotextiles wherein glass fibres are knitted on to the. nonwoven geotextiles to enable better use in highway applications. GEONETS/GEOMESHES are extruded polymer meshes and look like geogrids (but of substantially lower strength) (Fig. 6). Their function is hydraulic as they are used to drain water in a horizontal plane. They also provide space between two nonwoven geotextiles to minimise clogging or between two geomembranes to recover any possible leakage from either of the membranes, as shown in Fig. 7, as a geocomposite. GEOMATS and GEOWEBS could be coarse woven or joints obtained by partial melting, made of strips, rigid filaments or extruded (Fig. 8). They are generally flexible and junctions of overlapping strands not firmly connected. They can be synthetic or natural. Fig. 9 shows woven jute and coir mattings. There are also three-dimensional mattresses (Fig. lOa) commonly used in erosion control as well as staple fibres, continuous filaments or microgrids used as admixture to strengthen soil. There are. also double layered fabrics with spacer yams (Fig. lOb) that could be used in erosion control in running canals with a sand-cement in fill. 3 Functions By virtue of the variety of physical and mechanical properties, geosynthetics serve the following five basic functions .

Open-weave geotextile -----

3.1 Fluid Transmission

, . / Woven geotextile

..... . ,............... .,., ....... ", .... ,.,.,. ..... .,.". ....... ..... ............ ,. . .".." ...., ... ~.

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.

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A geosynthetic can collect a liquid or a gas and convey it along its own plane (Fig. 11), thus providing fluid transmission. This is conventionally termed as drainage function and is useful in strip drains and chimney drains. 3.2 Filtration

.,....... ,,.". ...... ,.,., ,.,.... ......... ....... ,.". .... , .... ..": .......: ........ ;~,

Polyethylene geomembrane.~ · Fig. 5- Typical commercially available geosynthetic clay liners

A geosynthetic acts as a filter when it allows liquid to flow perpendicular to its own plane, while preventing most soil particles from being carried away by the liquid current, as illustrated in Fig.12. This function comes into picture in strip drains (Fig:· D) •. chimney drains, French drains and erosion control. 3.3 Separation

When placed between a fine soil and a coarse

322

INDIAN 1. FIBRE TEXT. RES ., DECEMBER 1997 ---,

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Fig. 6- T ypi cal view of a geonet

Fi g. IO- (a) View of 3-dimensional mattress [Courtesy : Tenax Geosyntheti cs]; and (b) Doubl e layered fa bric with spacer ya rn used in erosion control

Fig. 7-Typical geocomposites

------------Fig. I I- Drainage fun cti on of a geosynthetic

material (gravel, stones, etc.), a geosynthetic prevents the fine soil and the coarse material from moving under the action of repeated applied loads as shown in Fig. 14. Fig. 8-Extrud ed geomat [Courtesy: Tenax Geosynthetics)

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as tensioned membrane when placed between two materials with different pressures, say in a pavement, as shown in Fig. 15 , and as tensile member in a reinforced soil structure to provide tensile modulus and strength through interface fricti on (Fig. 16).

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(b)

Fig. 9- (a) Coir matting; and (b) Jute mattin g

3.4 Reinforcement

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A geosynthetic protects a material when it alleviates or distributes stresses and strains transmitted to the protectc!d material. This can be

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323

RAO & BANERJEE: GEOSYNTHETICS

01

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Fig. 14- Separation function of a geosynthetic

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Fig. IS- Tensioned membrane function of a geosynthetic

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3.5 Moisture Barrier

A geosynthetic (geomembrane) may act as a barrier for flow of water or any other fluidhazardous or otherwise. The simpTest example for this is canal lining (Fig.I7). Geosynthetics may also serve other functions such as 'cushion' and 'protection' (Fig. 18).

Fig. 17- Geomembrane in canal lining

Water level

Filtrating geotextile

4

Raw Materials--Their Durability And Ageing Because of ease of manufacture, range of properties applicable to civil engineering structures and economic considerations, the most commonly

S ubsoil

Fig. 18- Geotextile as "protection" and filtration in erosion control of river banks

324

INDIAN J. FIBRE TEXT. RES., DECEMBER 1997

used polymers in the manufacture of geosynthetics are: • • • • •

Polypropylene (Polyolefin) Polyethylene (Polyolefin) Polyester Polyvinyl chloride Elastomers.

Polyolefins have proven to be one of the most attractive of the fibre forming polymers owing to their low cost and inert nature . A large proportion of geosynthetics today In the market IS manufactured from polypropylene. However, polypropylene fibres are highly susceptible to creep and UV degradation .

Polyester fibres have high modulus, low susceptibility to creep, and no vanatIon in mechanical properties at temperatures near 180200°C. They are also inert in sea water and weak acids. For better understanding of durability and ageing behaviour of geosynthetics, it is necessary to consider photo, thermal, chemical and microbial degradation as well as creep behaviour. Horrocks and O'Suza 3 detail the most commonly identifiable separate and associate agencies that affect geosynthetics both during installation and in use (Table 1). It must be recognised that prior to and during installation, exposure to the environment and human activity can have serious implications

Table I- Principal degrading agencies and accelerating - ageing proc;;dures for geosynthetics' Source

Effect

Accelerated-agei ng procedureivariables

Stress Pressure

Installationli n use

Rupture, creep

Tensile, grab, penetration, and creep tests/temperature, relative humidity

Wind Water

Installation In stallationli n use

Solvents/ Hydrocarbons

Installation : Diesel Minera Oils Hot Bitumen In use: Bitumen Installation/in use:Bird~ , animals, insects

Removal of volatiles Removal of additives and plasticisers Removal of additives, swelling and embrittlement

Agency Physical

Biological

Leaching tests/temperature Solvent-exposure tests/temperature

Localised damage

Chemical

Heat{+ oxygen)

Light (+ oxygen)

Installation : Hot bitumen In use: ambient environment temperature Installation: uv exposure

Weather

Storage/Installation

Water (pH)

In use: hydrolysis in acid, neutral and alkaline soils In use: exposure to natural soils and waste deposits In use: bacterial and fungal attack in soils

General chemicals

Micro-organisms

Radiation

In use: containment of low-level radioacti ve waste

Chain scission and oxidation ; loss in tensile properties Chain scission and oxidation ; loss in tensile properties Combined effects of heat, light, wind and water Chain scission; loss in tensi Ie properties General degradation of pol ymer structure Hydrolytic and oxidativc polymer-chain degradation ; loss in tensile properties Chain scission and crosslinking; loss in tensile properties

Hot-air/nitrogen oven exposure typicall y 90-ISO°C; oxygen absorption; thermal analysis (DTA, DSC; TGA) Xenon-arc exposure/temperature, rel ati ve humidit y Programmed exposure to Xenon arc in light and dark water-spray cycles Hot-water and sh:am ex posures: acid and alkali exposures/temperature . concentrations Ex posure to identified salts, waste chemicals and pollutants/temperature. concentration Incubated so il tests/tem peratu re. pH

Exposure to defined doses of a-, /1-. rand ex-radiat ion/chemical ell\'ironment

RAO & BANERJEE: GEOSYNTHETICS

325

with regard to the life. Venkatappa Rao et al. 4 and ConstructabilitylSurvivability Properties Baja/ provide a detailed review of the various • strength and stiffness aspects concerning durability. Baja/ summarises • puncture resistance that most polymers are susceptible to direct • tear resistance sunlight degradation (UV light and heat • cutting resistance combination) which can cause chain scission or • inflammability crosslinking in the polymer, leading to the polymer • temperature stability enbrittlement and failure. Polypropylene is more • ultra-violet stability susceptible to UV degradation. Carbon black and • absorption antioxidants are therefore added to melt prior to fibre extrusion to delay the onset of degradation. Durability (Longevity) By these processes, geosynthetics with as much as • abrasion resistance 120 years service life are available in the market. • temperature stability Furthermore, resistance to permanent • chemical stability deformation (creep) under long-term loading must • bioiogical stability be assessed. In this context, polyester geotextiles • wetting and drying stability. perform better. For predicting the actual life time of a buried Extensive work has been carried on the above at geotextile material, it is necessary to know the lIT-Delhi. A summary of the work carried out has nature of the surrounding soil, site specific pore been recently reported by Venkatappa Ra0 6 • The water, the ambient temperature and the stress. following presents the highlights concerning the hydraulic behaviour. 5 Testing and Evaluation In order to successfully design and con.struct 5.2 Hydraulic Behaviour of Geotextiles with geosynthetics, it is essential to test and Since the first ever use of geotextile as an evaluate them for a given application to serve the alternative to a granular filter in the reconstruction specific function. In addition, the properties are of the storm-lashed coast of Florida, USA in 1958, required for quality control, both during geotextiles are being increasingly used as a production and installation. replacement of graded filters. It has been found that geotextiles provide technical improvement, 5.1 Property Grouping The desired properties can be grouped as lowering either the construction costs or the maintenance costs or both. Their relatively small follows: pore sizes and good mechanical properties have Basic Physical Pr~perties allowed geotextiles to be used as a substitute for • constituent and method of manufacture several granular layers (Fig. 19). Some specific • mass per unit area applications of geotexti1es as filter are: behind • thickness retaining walls, in earthfill dams, beneath erosion • openmg size control works, and as silt fences and French drains. • roll width, roll length A general and broad overview of the hydraulic Mechanical Properties behaviour of geotextiles indicates that: • tensile strength a geotextile is normally chosen for filtration • tensile modulus and drainage applications depending on its • interface friction characteristic properties such as opening size • fatigue resistance and permeability. In addition, geotextile • creep resistance should function smoothly without having a • seam strength significant number of openings clogged by soil particles in the long run. Hydraulic Properties geotextiles' porometry determines the • permittivity adequacy of its ability to retain soil particles. • transmissivity

326

INDIAN J. FIBRE TEXT. RES., DECEMBER 1997

S.2.1 Hydrodynamic Sieving Test

Different experimental methods are available for the evaluation of geotextiles' pore sizes. A well defined simple method of short duration that Primary armour -

Secondary armour

,",d'!:lJI;lil"l'".:~.- C.oarse 'Jlter

.' ./~ .. -

Fine filter

Graded fi Iter -

Pri mary armour

Geotext i Ie Bose soil

allows the determination of the porometry of all types of geotextiles is preferred which simulates the field condition and presents reproducible results. Though several methods of pore size determination exist, no procedure has yet been identified as a satisfactory standard in yielding, a correct value. In the hydrodynamic sieving test apparatus developed at lIT-Delhi (Fig. 20), the geotextile specimen, loaded with a certain quantity of glass bead fraction, is continuously rotated in a' water trough forcing the glass beads to pass through the geotextile openings. After a test period, long enough to ensure that all fine particles had passed, the percentage of passing of different fractions determ ines the porometry of the geotextile tested. By conducting a preliminary investigation, the optimal working conditions of the apparatus for a

Sutatituting for fine granular layer

I

Primary armour

10&

I____ ~.

~7~0____~

G.otulit. O-Ring

~::.:::.~-:~~~~~~:.~:~~

Substituting for fin. and coarse granular Ia-,.rs

r--:==.:--::-~----

... ~::o

~~-~-:-~~~~~=~~~~~i F'~

Woter icvel

Primary armoU'

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22&

Fig.19-Graded aggregate filter and geotextile filter systems under rip-rap slope protection

------I

Fig.20-Principal features of hydrodynamic sieve apparatus

Table 2-Summary of the 0 95 values of geotextiles obtained from dry sieving and hydrodynamic sieving test methods Geotextiles

NW I NW 2 NW J NW. NWs WI

Thickness mm

Mass per unit area g/m2

2,13 4.15 2.07 2.02 2.06 0.58

NW-Nonwoven Fabric; and W-Woven Fabric.

290 470 195 240 205 206

0 95 Dry sieving test (ASTM)

0 95 Hydrodynamic sieving test

Jim

Jim

84 97 147 117 147

80 87 140

\02

106

103 135

RAO & BANERJEE: GEOSYNTHETICS

fractioned spherical glass beads of 50 g were selected. These are: (i) a cycle speed of 20 rpm, and (ii) a test duration of 1500 cycles. Table 2 presents the 0 95 values with dry sieve test as compared to hydrodynamic sieve test which is attributed to the non-renewal of the geotextile

327

specimens for different glass beads . In case of dry sieving method, the glass beads entrapped in the geotextile fabric structure are possibly released when the consecutive larger fractions are sieved in the same geotextile specimen. On the other hand, a new geotextile specimen used for each glass bead fraction in the hydrodynamic sieve test provides more representative 0 95 value because of the larger surface tested. 5.2.2 Permittivity Test

Silicon. gr'OSf

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Porou slon.

(j.otutil. ~rtorol.d

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Fig.2 1- Permittivity apparatus

~assembl y

drawing 4

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--r----- --

5.2.3 Transmissivity Test

The transmissivity test is necessary for drainage applications .. The permeameters can be of parallel or radial flow type. In either case, flow occurs along the plane of the permeameters. In the apparatus developed (Fig. 22), flow occurs radially outward from a central hole to the periphery of a circular specimen. This permeameter measures the in-plane perme.ability .of all types of geotextiles under various normal stresses. 5.2.4 Geotextile-Soil Filtration Test

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o

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!:!-+---

Both constant head and falling head permeameters are used for measuring normal permeability. ASTM : D4491-85 specifies permittivity test using constant head and falling head permeameters. The constant head test is carried out using a head of 50 mm of water. The apparatus developed at liT-Delhi, shown in Fig. 21, can be used to determine the .coefficient of permeability and the permittivity of a circular geotextile specimen (20 mm thick) under a given stress at a given head.

a.

(jroow

Fig.22- Transmissivity apparatus-lassembly drawing'

In order to carry out geotextile-soil filtration test, constant head permeameter was designed and fabricated. Fig. 23 shows the schematic design of the permeameter developed 7 in which the longterm flow test as described by Koerner and K0 8 and the gradient ratio as specified by the US Army Corps of Engineers (1977) can be conducted simultaneously for different soils. The mould and the base are provided with piezometers to facilitate measurements of the piezol!letric heads of the soil-geotextile system. Piezometer 1 measures the tailwater elevation whereas piezometers 2-4 measure the hydraulic head in the soil. Soil is directly compacted to a specified density in the mould with a Proctor

328

INDIAN 1. FIBRE TEXT. RES., DECEMBER 1997

J

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6 Applications Table 3 indicates the general and now widely accepted applications of geosynthetics. Some of these applications are discussed in the following in reference to the work carried out at lIT-Delhi.

i.mm¢> Inlfl

6.1 Geotextile French Drains in Indian Rural Roads

O-Ring

T ..,Il~ ~I

i.mmA. 'Y

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I.•r '112 6~1 L_-101

( Dimfnsions

In

mm )

Fig.23- Sch.ematic diagram of long-term permeameter9

Rural roads in India are typically constructed by laying a single course of brick on edge over the compacted subgrade. Irrespective of the season, rural roads in India have invariably been observed to have puddles of water standing on the surface, regardless of whether or not they have been provided with a side trench. They are usually in a deplorable state, caused not only by rainfall/poor drainage but also by sewage accumulating on the surface. It is widely known that a major cause of damage to rural roads is the accumulation of water on the surface and inadequate drainage of the road system. Such accumulations make the road surface not only impassable but also hazardous from the environmental viewpoint. An internally flooded road surface allows wheel loads to be transmitted directly to the subgrade, with little or no reduction in intensity. It is widely known that good surface and subsurface drainages are important in maintaining the integrity and performance of the road structure in unsurfaced rural roads where rainwater can penetrate the base very easily. The principal functions of a road-edge drainage system are to drain off the surface runoff and intercept groundwater inflows, thereby prevent weakening of the subgrade.

hammer. The geotextile specimen (supported by a Designing road-edge drains involves defining screen) is then placed securely in between the all sources of water and drainage methods capable mould and base. A filter paper and a metallic of intercepting and removing runoff to prevent its screen are placed over the specimen before accumulation on the road surface. Conventional assembling the water column. A constant ~ater methods of edge-drain construction for low-cost head of 375 mm above the soil is maintained in the rural roads usually include either a longitudinal water column so that accurately measurable open ditch (Fig. 24) or a gravel-filled trench drain volume of flow can be obtained. In order to study (Fig. 25) along the road edge. The open ditch may the worst effect of geotextile-soil clogging, slurry collapse and cease to function during heavy flow, scil samples were also tested. while gravel-filled trench drains are prone to From the various studies conducted, it was clogging owing-'to migration of fine particles into . concluded that the clogging potential of geotextile- the interstices of the gravel. A conT~entional trench soil system diminishes with the increase in soil drain requires a graded aggregate filter containing density and a long-term stabilized GR be comparatively small aggregates. Consequently, a determined for soils containing fine soil particles large drain cross-sectional area is required to (: