geo in highway
DESCRIPTION
it describes applications of Geosynthetic in highway.TRANSCRIPT
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).