Reinforcement of weak soils is another application for geotextiles. Reinforcement
applications require tensioning of the geotextile and achieving sufficient tension throughout
the entire fabric is difficult. Tension may also be developed after construction is complete if
larger strains and deflections are tolerable. Current research suggests that the use of geotextile-
geogrid composites is more effective than geotextiles for reinforcement applications.
Filtration within drainage systems is also a major application of geotextiles (48). The
small aperture size will keep large particles from entering the drainage layer or pipe, while
allowing some of the small suspended particles to pass without clogging the filter.
Geotextiles are also used as a protective outer layer of geocomposites.
Geogrids, a stiff structure, differ from geotextiles in that they have large
apertures, typically 10-100 mm between ribs (50). The primary use of geogrids is soil
reinforcement. Some geogrids begin as a geomembrane with holes punched through it.
The geogrids may be run through rollers with different rotational speeds or placed in a
stretcher to elongate the polymers. Both uniaxial and biaxial elongation versions are
commercially available. The benefit of polymer elongation is that the polymer goes into a
post-yield state which increases the material strength, modulus, and resistance to creep
(50). Elongation should be in the direction of the major principal stress. If the direction of
the primary stress is unknown, it is recommended to use a biaxial grid. Many variations
of geogrids are commercially available. Choice of an appropriate type is a function of the
application and manufacturers’ specifications.
Geogrids are commonly used to improve the modulus of a granular base, by providing lateral
confinement and reducing “walk out” of the base material. Haas (50), showed that the use of
geogrids can significantly reduce deformation and improve the durability and lifespan of
paved roads. The greater resistance to failure is due primarily to an increase in stiffness and
the load spreading ability of geogrids. The increase in stiffness suggests that a decrease in the
thickness of base material or HMA is possible for some situations (51, 52, 53). A more
common approach is to consider that the increased stiffness of the standard base and HMA
thickness translates into a longer lifespan. It has been shown that the placement of geogrids at
mid-depth of a base course dissipates the magnitude of the stress transferred through the
geogrid (50). The dissipation effect of the geogrid may allow for a reduced base thickness.
Tension will need to be developed in order to realize the full capacity of the system. This can
be accomplished in two ways.
• Pre-tensioning and anchoring
• Developing tension by overburden after installation
Geonets are primarily used for drainage applications and are similar to geogrids
except that the aperture is usually about 12 x 8 mm (0.5 x 0.3 in.) (53). They are
manufactured from polyethylene. The ribs are manufactured at angles of 70° and 110°.
This diamond shaped pattern changes the amount of vertical loading that the geonet can
support. Thickness is the most influential factor on the drainage performance of a geonet,
and should be determined using ASTM D1777. A thicker net will allow better drainage.
Greater thickness can be achieved by adding a foaming agent during manufacture, which
increases the thickness up to 5-7 mm (0.2-0.3 in.) and sometimes up to 13 mm (0.5 in.).
The hydraulic properties of a geonet should be determined using ASTM D4716
Geonets are usually separated from the in situ soil, both below and above, by another
geosynthetic, such as a geotextile in pavement applications.
The long-term conditions surrounding the geonet also need to be assessed in order
to design a system that will not degrade over time. Soil may block the openings of the
geonet. Temperature can also be destructive to these systems, because the polymers will
creep faster at high temperatures. The design must account for the maximum temperature
expected. Subsurface chemicals being transported, which can damage the geonet, must be
determined. Composition of the water therefore, is important. The amount of a dissolved
chemical that the geonet and separation layers will be exposed to is much greater than in
most reinforcing situations, due to the increase flow rate of geonet systems. A high flow-
rate factor of safety must be used in order to ensure a long performance life.
These materials are almost exclusively used for drainage applications. They are
separated from the in situ soil by another geosynthetic placed on both sides of the geonet. This
separation allows for lateral drainage in embankment applications, or vertical drainage in
Geomembranes are relatively impermeable barriers used for complete separation (53).
The term impermeable layer is used because the permeability of water vapor for the
material is between 5 x 10
and 5 x 10
cm/s (1.9 x 10
and 1.9 x 10
type of geosynthetic consists of two major categories:
Modified geomembranes are impregnated with bitumen, or elastomeric materials in the
The second geomembrane type is manufactured to be waterproof. For this class of
geosynthetics, tensile strength, tear resistance, puncture resistance, and seam behavior are
more important than in other geosynthetic applications because failure or deterioration of any
type that allows increased permeability will compromise the entire system. Resistance to
chemicals must also be considered, as it may reduce the effective life of the material. To
reduce the possibility of failure, other types of geosynthetics are often used to add a protective
barrier on both sides of the geomembrane (53).
Geomembranes are used in transportation applications to stop intrusion of water
into expansive soils. This application has two variations, horizontal and vertical
depending on the direction of fluid flow. Determining if one or both are necessary
depends on groundwater flow and surface infiltration. Horizontally-installed
geomembranes vary in width depending on the application. Vertically-installed
geomembranes typically are placed to a depth of 1.5 to 2.5 m (5 to 8 ft), such as for cut
off wall applications. They must be wide enough to prevent water from vertically
infiltrating and to isolate the overlying material. In frost sensitive soils, geomembranes
will allow for the control of moisture content, reducing the effects of differential frost
heave. Geomembranes are also used for containment of runoff and contaminated fluids as
well as for waterproofing foundations, walls, and bridge abutments.
Geocells are another type of geosynthetic sometimes installed as a geocomposite.
Geocells are composed of polymer strips that are arranged to form vertical boxes, which are
then filled with sand. This soil-containment system is able to distribute large vertical forces
and compensate for weak soils. The geocell is sometimes installed with a protective geotextile
above and below (53).
Geocells are typically used for reinforcement or containment, installed in or
below the base course.
Geocomposites are a combination of two or more types of geosynthetics (53). A
geonet or geogrid with another geosynthetic on either side is a common example of a
geocomposite. A geomembrane reinforced with geotextiles is also an example of a
geocomposite. Geocomposites are often used to enhance the performance of the primary
Strip or wick drains are composites that use a large aperture geogrid or geonet middle
layer and fine aperture geotextile as a filter sandwiching the middle layer. There are many
different arrangements that can be made for various purposes. The properties of each system
are dependent on the components chosen and their interactions.
A composite is intended to create a synergistic effect where the performance of the
entire system is greater than its individual components. The primary factors in composite
selection are cost and the results achieved The construction of a temporary access road over
wetland soils was facilitated by the use of a geofabric-geogrid combination (53). The purpose
of this design was to minimize the impact on local vegetation. The use of geosynthetics
allowed for minimal disturbance to the subgrade. The use of geofabrics for separation and
geogrids to increase the friction between dissimilar layers has been effective in many
situations such as subgrade reinforcement and pavement overlays.
Applications of Geosynthetics in Minnesota
In Minnesota geosynthetics have been used primarily as separation layers between
fine-grained soils and granular bases or subbases or a geogrid reinforcement between the
embankment soil and the subbase. In this section the best practices for separation and
reinforcement applications are presented.
22.214.171.124.2. Geosyntethics as a Separation Layer
Soil separation is a primary concern for pavement sections with wet or saturated fine-
grained plastic soils. The small grain size of some soils allows the subgrade soil to infiltrate
the granular base, or the granular base to migrate into the subgrade. This mixing of subgrade
and base course material will result in contamination of the base and a decrease in stiffness
and strength of the pavement system, allowing excess deformation of the HMA surface.
Installing a separation layer will help retain the design stiffness which will help increase the
pavement life. Installation of a geosynthetic (geotextile) has been proven to be a successful
method to limit soil intrusion into a coarse aggregate (48, 49). Selection of a suitable
separation layer is dependent on the grain size of the soil. The aperture of the geosynthetic
should be smaller the smallest grains. If there is material smaller than the aperture,
migration will occur. The migration of the fines is facilitated by water and the pumping
effect caused by repeated loading.
The following section is a summary of the application of the use of geofabrics as
separation layers in Minnesota:
Purpose: Separate wet silt or clay soils from granular subbase or base materials
Conditions: Areas with high moisture content fine-grained soils near the water table and/or
where pumping action may cause infiltration of the soil into the upper layers.
• Mn/DOT Specification 3733 Type V; this is usually a slit film geofabric with a
minimum grab tensile strength of 140 MPa (200 psi).
• Mn/DOT Type VI with a minimum bi-directional strength of 210 MPa (300 psi) is
recommended for weaker, wetter conditions; Type VI is usually a woven fabric.
• Water Conductivity – minimum of 400 liters/sq m/minute (10 gal/ft
• Manufacturer certification of geofabric must be received from contractor.
• Geotextiles used under granular materials over soft wet clays can provide separation
and eliminate contamination of the granular material however,
A geotextile needs to be placed within 0.3 m (12 in.) of the surface to
mobilize tension under wheel loads at the surface.
The key to getting a good bid price on placement of a geotextile is to allow placement in
such a way as to not significantly delay the contractor’s normal operations
Geofabrics come in standard widths, typically 4, 5 and 6 m (12, 15, and 18 ft). By
specifying an overall width that fits some combination of these widths and allowing
about 0.2 m (0.5 ft) for sewing material waste will be minimized.
• Recommended Width
The recommended width of geofabric is the width of the driving surface plus about
0.7 m (2 ft) on each side.
- Gravel Surface
, an 24-ft (8-m) width would require fabric at least 28-ft (9.1-m)
wide. Two 15-ft (5-m) rolls sewn together in the factory would produce a width a
little over 29 ft (9.2 m). On gravel surface roads, the width should be as close as
possible to the shoulder-to-shoulder width.
- Bituminous Surface
, for 24-ft (8-m) lanes and 4-ft (1.3-m) shoulders a fabric
width would be 32 ft (10.9 m). A combination of a 18-ft (6-m) and 15-ft (5-m) or
three 12-ft (4-m) rolls would be appropriate. If the width is too great pre-sewing is
not practical and field sewing is required.
• Recommended Length
By specifying bi-directional grab strength, the fabric can be placed in the long
direction typically in lengths of 200 to 300 ft (60 to 100 m). This will minimize
The area of geofabric to be used for design and bidding should be the area of the
embankment covered. Overlap and the amount of fabric allowed for proper sewing
should not be used for calculating area of coverage.
The geofabric should be laid out parallel to the centerline if field stitching if
required, using a 3-ft (1-m) overlap. Use a J-stitch with a double stitch, not more than
½ in. (12 mm) apart (Figure 4.6).
If prayer stitches are used then two lines of sewing should be used. A 401 stitch is
best. All seams should be sewn “face up” for inspection.
Figure 4.6 Type V Woven Geofabric Connected Using a “Prayer seam” with 75-mm (3-in.)
Overlap and 401 Stitch.
Best: No wind, dry, warm
Okay, Slight wind, some precipitation, cool
Worst: Windy, wet, cold
• Placement Proper placement is critical
must be stable:
For normal hauling operations geofabric will not substitute for poor subgrade
Roll out and stretch out over subgrade
Provide some anchor on edges (small shovels of soil)
Minimize wrinkles (Fabric should be “Stretched” across subgrade)
Transverse Continuity (joints): near end of roll
a. Place next roll like shingles with 2-m (6-ft) overlap or
b. Sew the connection; (double or triple stitch)
Placement of Granular Material over Geofabric
1. Trucks (belly dumps) can travel directly on geofabric if extremely careful. No turns,
braking or spinning tires
2. Place material down center in a windrow
3. Spread material forward and to the sides (stretch fabric to remove wrinkles in this
4. Cover middle portion of the fabric first with a 75 to 100-mm (3 to 4-in.) layer of
granular material. This may require one or two truck dumps side by side between 20
and 30 m (70 to 100 ft) long to get the proper sized windrow. A shorter distance may
result in a windrow too high and cause the trailer to ride up on the windrow and spin
At the end of a workday the contractor should place an additional. 75 to 100-mm (3 to
4-in.) layer of granular over the fabric and complete the spreading operations over the
entire fabric width.
Typically, 1.6 km (1 mi) of roadway can be placed in this way in one working day.
Figure 4.7 Granular Material placed on Overlapped Geofabric
Figure 4.8 Typical Section Using Geofabric.
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