GEOSYNTHETICS IN HIGHWAY ENGINEERING

INTRODUCTION

'Geosynthetic' is the term used to describe any synthetic material

(often plastic) used in combination with soil, rock and/or other natural material

applied to enhance the works intended . The application of geosynthetics to

engineering problems has grown rapidly in the last few decades.

GEOSYNTHETIC FUNCTION

The selection of a highway application geosynthetic first requires the identification

of its primary function. Typical functions of geosynthetics are shown in Figure 1

and include:

Separation: The geosynthetic acts to separate two

layers of soil that have different size distributions.

Filtration: The geosynthetic acts similar to a sand

filter by allowing water to move through the soil

while retaining all upstream soil particles.

Drainage: The geosynthetic acts as a drain to

carry fluid flows through Less permeable soils.

Reinforcement: The geosynthetic acts as a

reinforcement element within a soil mass or in

combination with the soil to produce a composite

that has improved strength and deformation

properties over unreinforced soil.

Protection: Preventing or limiting localized damage

to an adjacent material, usually a geo membrane

used to line a lagoon or a landfill. Thick geotextiles

prevent puncture or excessive strain in the

membrane.

Fluid/Gas Containment: The geosynthetic

(geomembrane) acts as a relatively impermeable barrier to fluids and gases.

GEOSYNTHETICS

The most used types of geosynthetics are:

Geotextiles: are continuous sheets of woven, nonwoven or knitted fibres. The

sheets are flexible and permeable and generally have the appearance of a fabric.

Geotextiles are used for separation, filtration, drainage and reinforcement.

Geogrids: are high strength geosynthetic materials that have an open grid-like thin

sheet appearance. The principal application for geogrids is in reinforcement of

soils.

Geonets: are low strength geosynthetic materials that have an open grid-like

appearance formed by two sets of course, parallel, extruded strands of material

intersecting at constant angles. The network forms a thick sheet with in-plane

porosity that is used to carry relatively large fluid flows.

Geocomposites: are geosynthetics that combine two or more of the above

materials.

Geocells: are relatively thick, three-dimensional networks constructed from strips

of polymeric material. The strips are joined together to form interconnected cells

that are in filled with soil.

Geomembranes: are continuous sheets that are relatively impermeable and are

used as liners for fluid containment and as vapour barriers. Occasionally used are

numerous other closely related products such as Geomats and the like. Of the

various types of geosynthetics the most used in highway engineering is the

geotextile.

TYPES OF GEOTEXTILES

Three types of geotextiles are there and they are distinguished by their method of

manufacture.

Woven: geotextiles are manufactured in a similar way to conventional textiles by a

weaving process that interlaces the fibres, usually at right angles to each other

Nonwoven: geotextiles are manufactured by boning fibres or filaments in a

random orientation. The most common method of bonding is mechanical bonding

by needle punching. Other methods such as thermal bonding and chemical bonding

are available. Nonwoven geotextilesare use for filtration applications because of

the nonwoven geotextile's winding drainage paths and wide distribution of pore

opening sizes.

Knitted: geotextiles are manufactured by knitting together threads in

a continuous manner.

PHYSICAL PROPERTIES

All specifications require physical properties

to meet prescribed limits. Some typical types

of tests use to determine a geosynthetics

physical properties are shown in Figure 2. A

geosynthetics physical properties depend

both on the geosynthetics method of

manufacture and on the properties of the

constituent polymers. In order to economize

the time for the Quality Control of

geosynthetics prior to construction is mainly

confined to the following tests:

Filtration Opening Size (FOS): The FOS is determined by wet sieving a graded

mixture of rounded glass beads (rounded sand is an alternative) and determining

the maximum size that passes through the geotextile . The value is used in

conjunction with a soil's D85 (sieve size for 85% passing) with an upper limit of

600 μm.

Strength: Strength is determined by grabbing a test sample 100 mm wide by 150

mm long central to its width using 25 mm square jaws so as to rupture a 100 mm

free length . For soil reinforcement applications more accurate testing is required

along with an appropriate engineered design. Otherwise two class of minimum

strength are specified (Class I > 330 N, and Class II > 660 N). Since manufactures

quote roll average values these strength values should be increased 20% for

general selection from trade catalogues. The Mullen diaphragm burst strengt is

used for knitted sock. Burst strength must exceed 0.6 MPa .

Permeability: Permeability is determined by measuring the flow of water through

and normal to the plane of the geotextile under a head difference of 100 mm .

Minimum coefficient of permeability of 10 μm/s is normally achieved by all

geotextiles.

Polymer Type: Generally not determined directly. Polyamide is not permitted

since, in the ground, hydrolysis has been observed causing deterioration. Polyester

nonwoven is used where a geotextile is used for filtration or erosion control

involving water.

The writer has found it desirable to require needling of at least 80 penetrations/cm2

to ensure mechanical interlock between fibres. Knowledge of Specific Gravity

(SG) is sometimes used. Polyester (SG=1.36) when saturated sinks in water.

Polypropylene (SG=0.91) and polyethylene (SG=0.94) float on water.

Thickness: Thickness is determined at a pressure of 2 kPa in accordance with

CGSB 148.1 Method 3. Minimum thickness of 1 mm is required for all filtration

applications.

Ultra Violet Light Resistance: Where geosynthetics are exposed to sunlight, such

as when used for silt fences (in air) and silt curtains (in water) UV treatment is

specified. For other applications exposure to light is limited to a maximum of 3

days. Geotextiles arriving on sit with damaged shipping protection may not be

used.

Other Tests: Special applications require special tests. Geocomposite edge drains

should be tested for core crushing both across its thickness and its the width.

HIGHWAY GEOSYNTHETICS APPLICATIONS

The main applications of geosynthetics in Highways are

(a) for use around crushed rock drains.

(b) for use around perforated sub drain pipe in non-granular material.

(c) for use in slope drains.

(d) for erosion control such as beneath rip rap, rock protection, gabion mats.

(e) for erosion control such as ditch invert protection below granular blankets.

(f) for erosion control beneath gabion baskets for low retaining walls.

(g) for separation of sub base and earth sub grade.

(h) for separation of sub base and organic soil sub grade.

(i) for separation of an open graded drainage layer from sub base, sub grade, or

When uesd to sandwich clear stone so as to form a drainage layer.

(j) for silt fences and silt curtains.

(k) for geocomposite edge drains and

(l) for reinforcements in numerous applications.

Geosynthetics are never used as a substitute for good drainage practice.

Engineered drainage is essential when using geosynthetics. Jointing or overlaps

used in non reinforcement applications on gently sloping ground permit three

alternatives (a) overlapped a minimum of 1 m, (b) overlapped 0.5 m provided both

edges are pegged, or (c) seamed with a minimum strength of the class requirement.

The up slope end is required to be placed above the down slope end. On

slopes steeper than 2H:1V the ends of the geosynthetic must be anchored.

Geosynthetic embedment of 500 mm in a soil filled trench is normal.

SPECIFICATION REQUIREMENTS

In order to ensure minimum standards that have

been found (from exhuming geosynthetics) to

give satisfactory performance certain standards

have established design requirements for geo-

textiles used in non reinforcement applications

in Highways. Table 1 is based on these require-

ments with modifications .Where geotextiles are

laid horizontally on soils containing less than

15% by weight of material greater than twice the

geotextiles filtration opening size a filter

material should be placed above or below the

geotextile. Typically this should at least be a

clean sand similar to that of concrete sand . A

possible grading is shown in Table -2. Where

a geosynthetic is used for reinforcement sizing

and strength are based on engg. design

methodology.

EXAMPLES OF APPLICATIONS

Typical examples of the applications of geotextiles in highway right of ways usage

are:

Geotextile Wrapped Aggregate Drains (and Hidden Ditches):

Geotextile wrapped open graded aggregate drains (often termed French drains

when capped with impermeable soil) and hidden ditches may be constructed with

or without a perforated pipe. Typical examples are illustrated in Figures 3, 4 and

5. Drain lengths greater than 25 m are provided with pipe bedded on a minimum

50 mm of aggregate so as to trap sediment passing through the geotextile. All pipes

should be either laid with the minimum gradients that result in the pipes being self

cleaning or installed with a geotextile or sock wrap. Minimum self cleaning

gradients are given in Table 3. Discharge outlets are required every 100 m and at

all low (minimum) elevations. Where it is not possible to provide outlets, cleanout

inlets should be provided at the same 100 m spacing.

Subdrains:

Perforated plastic pipe subdrains (generally wrapped in a knitted geotextile sock)

are installed in an excavated trench and backfilled with very clean sand (i.e.

grading as per ASTM C33 with the addition of < 2% wash sieved through the 75

μm sieve size). In addition a 50 mm thickness of bedding of the same sand is

required. Typical examples are illustrated in Figures 6 and 7. Experience with

pipe sub drains ploughed into place or installed by excavation and boot has shown

that the sub base or sub grade material mixes with the granular 'A' shoulder

material resulting in a soil of low permeability around the pipe. In addition, on flat

highways a zero pipe gradient results.

Slope Drains:

Slope drains are similar to either geotextile

wrapped aggregate drains or sock covered

sub drains as illustrated in Figure -8. They

are installed as shallow trenches intercepting

the seepage that causes piping on exit. The

layout is often installed in a herring bone

pattern.

Erosion Control for Rip Rap, Rock Protection and Gabion Mats:

Often used with a geotextiles to prevent scour

And wash out. A variety of interlocking erosion

control blocks are also available as an alternative

to rock to weight the geotextile. The purpose of

the geotextiles is to prevent moving water from

eroding the underlying soil. On river and lake

front projects any rock protection must be well

graded to avoid large voids. Wave action generally

results in detrimental differential pressures across

the geotextile if large voids are present. The most common problem with

geotextiles in erosion control applications is undermining of the geotextile. For this

reason the ends of the geotextile must be properly embedded in trenches 500 mm

deep as illustrated in Figure 9 or weighted down (e.g.the end under water).

Granular Blanket of Gravel Sheeting:

Placed on cut slopes or at the base of sloping

ditches to provide drainage and prevent erosion.

The alternative is an open graded stone placed on

a nonwoven geotextile as illustrated in Figure 10.

Because the geotextile option is generally more

expensive the geotextile option is used on slopes

greater than 2H:1V or where large flows are

anticipated. The design must take account of seepage, geotextiles placement, and

prevention of undermining. Unlined ditches may erode at slopes as low as 1%.

Erosion of ditch inverts with up to 8% slopes may be controlled by planting

Bermudagrass, Kentucky Bluegrass or other grass. Steeper ditch slopes require a

liner (generally a geotextile plus granular or rock blanket).

Gabion Baskets as Retaining Walls:

Geotextiles should be used below gabion

baskets if the gabion is placed (a) in a

drainage channel,(b) where is anticipated,

or (c) the retained soil is likely to erode.

Typical examples are shown in Figure 11.

Erosion is common at the top of the gabion

And causes slope failure or settlement.

Silt Fences and Silt Curtains:

Installed because of environmental concerns. Silt fences are used to contain soil

run off from construction areas. Silt curtains have a similar function in water rather

than air. Maintenance and periodic removal of sediment is essential.

Separation of Granular and Sub grade:

Nonwoven geotextiles are used to prevent contamination of granular by fine

material and to prevent penetration of the granular into the sub grade. A class I

geotextile is used to separate relatively smooth earth and granular while a class II

geotextile is used if the sub grade has roots and stumps present or if on an organic

soil. Nonwoven geotextiles are required to allow for greater elongation on rough

ground.

Drainage Blankets (and Clear Stone):

Drainage Blankets consist of clear stone

Sandwiched between a lower and upper

geotextile as illustrated in Figure 12. They

are used to collect and drain both ground

and surface water. They should not be

used directly on soils containing 25%

passing a 75 μm sieve and subjected to

repeated loadings. A 100 mm thick layer

of capping sand with less than 12% passing

a 75 μm sieve should first be used. The

lower geotextile should be laid on a

crowned surface free of rutting. The edges

should be directly connected to a ditch or

subdrain.

Geocomposite Drains:

Geocomposite Drains consist of an open plastic core wrapped in a geotextile.

Commonly used as edge drains to drain materials below roadway surfaces as

illustrated in Figures 13, 14 and 15. The relative ease of installation, especially for

deep drains, and versatility of the product provide clear economics and advantages

for there use. Like subdrains experience has shown that geocomposite drains

should have a very clean sand bedding and very clean sand surrounding to be most

effective. Filtration application requires a used for reinforcement require a site

specific design using methods such as given by CFEM (1992).

CONSTRUCTION CONSIDERATIONS

In drainage applications it is necessary to ensure that the granular aggregate

contained by the geosynthetic within the drainage trench is sufficiently permeable

to carry the anticipated flow. The aggregate should have a permeability sufficiently

greater than the geotextiles (generally at least 10 times) to allow the drainage

system to perform under gravity without hydraulic backup. When the drainage

distances are large or the grade small a perforated pipe or other conduit should be

placed in the aggregate drain with outlets or cleanouts located every 100 metres

and at all lowest elevation locations. Standard practice is to locate pipes or geo-

composites conduits on a 50 mm permeable bedding. This locates the conduits

invert 50 mm above the trench invert. At all outlets care must be exercised to

ensure that the bedding material has an easy ability to drain. This may require the

conduit's invert to be rapidly dropped to the trench invert level over a short limited

length of the outlet location tee or other junction. In order to minimize the

migration of fines from the soil to be filtered the geotextile should be placed as

tightly as possible against the soil. Care must be taken to ensure that there are no

gaps between the geosynthetic and the trench sides. This is particularly important

for retrofit highway pre manufactured geosynthetic edge drain systems which are

often placed against the pavement side trench walls. If there are voids or loosened

soils in the gap zone, then the flow of water towards the geotextile will inevitable

initiate the transport of fines resulting in plugging of the drainage system or

reduction of the permeability of the geotextile due to cake formation on the filter. It

is recommended that such drainage systems be placed in the centre of such

trenches or against the shoulder trench side and the trench filled with extra clean

filter graded aggregate (e.g. ASTM C-33 sand grading plus the limit of < 2%

passing the 75 μm sieve).

QUALITY CONTROL

Quality control is maintained by Designated Sources Manual (DSM). Only listed

produces may be used unless cleared by Head Office. A product is first tested by

an independent laboratory and the results presented to the Ministry of

Transportation (MOT). MOT confirms the results in its own laboratory and where

agreement exists the produce is listed in the DSM. A contractor may use any

produce listed in the DSM. At delivery to the construction site a sample of the

supplied geosynthetic is tested for consistency with the DSM. Materials that have

changed (do not conform) are removed from the DSM and the contractor and

supplier so informed. In this way MOT ensures the quality of products used in

Highways. It should be noted that, although less common today (1994),

manufacturers have altered product properties while maintaining trade names.

SPECIFICATION

Specification documents should state the following:

1. The application of the geotextile for the given specification (particularly where

two or more applications/geotextiles exist in the same contract).

2. Maximum permitted Filtration Opening Size is specified as the smaller of the

d85 of the soil or 600 μm. Where the d85 is less than any suitable geotextile's

FOS a granular filter gravel/sand must be placed adjacent the geotextile. In such

a case the geotextile's function is that of separation.

3. Type of geotextile (i.e. nonwoven for filtration).

4. Class of geotextile (class I or class II).

5. Special provisions such as, if applicable, 1 mm thickness, polyester for under

water application, polypropylene or polyethylene for floatation

SUMMARY AND CONCLUSIONS

Presented is a brief overview of geosynthetic functions and their applications to

highway engineering. Suggestions of tests and usage has been summarized in

Table 1. Where drainage is of concern, whether minor or major, it is essential to

design "so as to get the water out faster than it can get in!". Remember decreasing

a soils water content increases its strength and stiffness. Consistent with MOT

requirements the writer's experience has indicated that where geosynthetics are

used for drainage purposes a nonwoven polyester fibre geotextile should be

specified along with a minimum thickness of 1 mm. In addition the writer would

require the fibres to be well interlocked (needled).