Geotextiles can be worth their cost in
aggregate
Installed properly, these materials
stabilize stone bases
Information for this article was
provided by Amoco Fabrics and Fibers, Atlanta, Ga.
Geotextiles perform three basic
functions in stabilizing aggregate sections: separation,
drainage and reinforcement. Some agencies hesitate to
specify geotextiles for these functions because of a
belief that the material adds to the cost of a project.
However, with a good design method for geotextile use,
and proper installation, most projects realize a 30
percent to 40 percent drop in required aggregate base
thickness. This leads to a drop in production costs
because the nonwoven geotextile only costs a fraction of
that saved from the reduction in required aggregate.
The use of a geotextile for the
separation of aggregate and the soil subbase is easily
justified to anyone who has placed an aggregate section
and has seen it lose its effectiveness over time from the
intermixing with underlying subgrade soil. Investigations
of failures in unpaved and paved surfaces generally
reveal the presence of fine-grained soils intermixed with
the aggregate base.
As an aggregate layer is loaded, the
bottom loosens with tension cracks allowing the
underlying fines, under pressure, to migrate up into the
aggregate. As little as 10 percent to 20 percent fines
can completely destroy the structural strength of the
aggregate by interfering with the hard, stone to stone
contact. As fines infiltrate a portion of the structural
section, flexure increases, fines migrate further upwards
and the section deteriorates until complete structural
section failure occurs. This process can quickly destroy
the effectiveness of several inches (millimeters) of
aggregate.
Geotextiles provide a separation layer
between the aggregate and the subgrade soil, to prevent
migration of fines and thus indefinitely preserve the
original aggregate structural thickness.
The geotextile usually costs no more
than 2 inches to 3 inches (50 mm to 75 mm) of compacted,
in-place aggregate, but can save several inches
(millimeters) of aggregate. The separate function is more
dramatic over weak subgrade soils, but is economically
practical in the long run to use even on more competent
subgrades.
Geotextiles are recommended for this
separation function because of their low cost,
coefficient of friction, elongation and drape to conform
to any surface, effective filtering even after
elongation, abrasion and puncture resistance, and their
high coefficient of permeability. Geotextiles are made of
polypropylene, and, as such are basically inert and will
last indefinitely in a buried application.
One extra benefit of using a geotextile
for separation is that almost all the aggregate over the
geotextile can be reclaimed and reused. This is
particularly economical in temporary uses such as mine
haul or logging roads or anywhere aggregate is expensive
and equipment is available to reclaim the uncontaminated
stone.
The drainage function of a geotextile
can be critical to structural section performance.
If the subgrade soil is subject to
persistent or even occasional wet conditions, the
geotextile placed over it must be highly permeable to
allow rapid drainage of water from the loaded subgrade
soils up into the free draining aggregate base.
Otherwise, under the rapid loading conditions from
traffic, water pressures in the soil can fail the
subgrade by soil liquification.
Geotextiles provide this critical
permeability as they filter or keep the fines from
migrating upward into the aggregate. Maintaining drainage
of the aggregate base and subgrade soil is very important
to prevent accelerated failure of the support system. A
geotextile also allows the use of more open, free
draining aggregates instead of those with fines, which
are weakened by moisture and are freeze-thaw sensitive.
Geotextiles are used in reinforcement
through mechanisms of restraint or confinement, friction,
membrane effect and local reinforcement.
These reinforcement mechanisms,
provided by all types of nonwoven and woven geotextiles,
are widely recognized.
This design approach is based on the
reinforcement function in general and years of experience
gained with the use of geotextiles. According to most
researchers, the reinforcement function of a geotextile
comes into effect when the subgrade soil is weak,
generally less than 12 psi shear strength, or CBR 3.
However, most of the research to date has dealt with
limited loading and the reinforcement function may well
be effective in stronger soils when designing for very
heavy wheel loads.
Fabrics are used in road construction
with locally available aggregate such as a crushed stone,
quarry or shotrock, sand, gravel, or sea shells to
develop a structural layer. In reinforcement, fabric
improves the performance of the aggregate-fabric-soil
(AFS) system under repetitive vehicular loading from
mechanisms including restraint on the aggregate and
subgrade layer, membrane effect, friction developed at
the fabric interfaces that creates a boundary layer, and
local reinforcement.
These mechanisms are often measured by
resistance to permanent deformation or rutting.
Two types of restraint should occur in
the AFS system. The first is related to the reverse curve
of the fabric outside the wheel path and the downward
pressure on the soil that results. This effect increases
the bearing capacity of the soil. A second type of
restraint effect occurs when the aggregate particles at
the soil-aggregate interface move from under the loaded
area but are restrained or given a tensile reinforcement
because of the presence of the fabric. The strength and
modulus of aggregate material benefit from this increased
confinement. The increased aggregate modulus decreases
the compressive strength on the soil under the wheel
load.
As the roadway undergoes large
deformation the fabric is stretched and develops tensile
stress, the magnitude of which depends on fabric strain
and fabric modulus. The net effect is a reduction under
the wheel load and an increase outside of the wheel path.
In order to develop fabric-induced
stress, substantial vertical deformations, proper
geometry, and fabric anchorage are required. Prestressing
the fabric to reduce the system deformation to get the
fabric in substantial tension is suggested.
Friction developed along the interface
between aggregate-fabric and friction-adhesion of the
fabric-soil interface create a boundary layer of
aggregate and soil adjacent to the fabric. The composite
material created contains more favorable properties of
ductility and tensile strength. The effectiveness of this
phenomenon is tied to the magnitude of friction-adhesion
developed at the interfaces. Fabrics should develop high
friction-adhesion.
Concentrated stresses from vehicular
loading can cause a punching at the points of contact
between the aggregate and subgrade. Use of fabric between
the aggregate and soft soil distributes the load, reduces
localized stresses, and increases resistance to vertical
displacement.
Installed properly, these materials stabilize
stone bases
Preparation affects job success. The
successful use of geotextiles in soil stabilization
requires proper intallation. The four basic steps
involved in placing geotextiles:
- subgrade preparation
- geotextile placement
- aggregate placement
- aggregate compaction
Careful planning and preparation for
each installation step speeds construction and insures
good performance and full benefit from nowoven
geotextiles.
Follow the manufacturer's guidelines to
determine the structural section thickness. The aggregate
selected should, whenever possible, be compactible and
non-moisture sensitive.
Usually, the geotextile is laid in the
direction of construction traffic. However, specific
project dimensions may alter this layout. Geotextile
panels should be overlapped both side to side and end to
end from 1.5 feet to 3 feet (0.5 to 1 meter), depending
on subgrade strength.
Adjacent fabric edges can be sewn in
the field with a portable sewing machine powered by a
generator. Field sewing typically requires three or four
laborers. Presewn panels can be supplied from the
factory, and fabric can be sewn 2 to 4 inches (50 to 100
mm) from the fabric edges. Use of the presewn fabric
minimizes the need for field sewing or overlapping.
Sewing costs can be compared to cost of the geotextile
lost in an overlap zone. Two laborers can easily handle a
roll of nonwoven geotextile fabric.
In normal construction practices,
trucks backdump aggregate onto the fabric. A tracked
bulldozer works best to spread the aggregate. Lighter
weight models are recommended for softer subgrades.
Front-end loaders and motor graders exert greater
pressure on the subgrade. Vibratory compactors can be
used, but only after reasonable compaction and rut
stability have been established by the bulldozer.
Nonwoven stabilization geotextiles can be used in most
weather and temperature conditions.
How to install geotextiles
Regardless of subgrade strength, the site should first be
cleared of all sharp objects, tree stumps, and large
stones that could puncture the fabric. Unless it is
necessary to achieve final grade, the vegetative mat need
not be removed, because it can provide extra support
during aggregate placement until final compaction. Brush
or cushion layers under the nonwoven fabric are usually
necessery, since the fabric prevents soil fines from
pumping into the aggregate layer.
Geotextiles should be rolled out onto
the subgrade by two people, beginning at a point that
allows easy access for construction equipment, yet is
consistent with the layout plan. On very soft subgrades,
the fabric layout and aggregate placement should begin on
the firmest soil on the site perimeter, as an anchor
point. From there the fabric can be rolled onto softer
sections.
Fabric overlaps and seams should be
made as specified. In windy weather, soil or rocks should
be placed on the fabric to hold it down until aggregate
is placed. Ground securing pins are sometimes used in the
overlap sections of the geotextiles.
A compactible, non-moisture sensitive
aggregate is then backdumped onto the fabric beginning on
firm soil at a point just in front of the fabric. This
should anchor the fabric firmly. The aggregate is then
spread in one lift to a thickness greater than that
needed for stabilization to allow for subsequent
compaction. If the thickness from one lift is too great
for satisfactory compaction, place more than one lift.
In any situation, the first lift should
be as thick as necessary to prevent the compaction from
overstressing the subgrade. The bulldozer must blade into
the load and slightly upward during aggregate spreading
for the same reason. This procedure is followed for each
load until the fabric is completely covered.
Over very soft subgrade, care must be
taken during aggregate placement to insure the fabric is
not moved out of position nor the subgrade overstressed.
The bulldozer operator can best determine which spots
need additional aggregate for good stability by watching
for rutting in the aggregate layer.
Over very soft soil conditions, mud
waves may appear during aggregate placement or use.
Normally, mud waves are not a problem if they do not
heave above the surface of the aggregate base. Stress on
the subgrade during fill placement causes suburface soil
to move away and up from the loaded area. If you expect
severe mud waves, contact your fabric manufacturer for
information on construction procedures to minimize their
adverse effects.
Vehicles should not be allowed to drive
directly on the fabric. If the fabric is damaged during
installation, the damaged section should be exposed and a
patch of fabric placed over it. The patch should be large
enough to overlap onto unaffected areas by 3 to 4 feet (1
to 1.25 meters). The aggregate is then replaced and
compacted by the bulldozer.
For full stability, the aggregate must
be compacted to required density for the design
thickness. The surface is initially compacted by walking
the tracked bulldozer back and forth over the aggregate
while waiting for the next aggregate load. From that
point on, construction traffic compacts the aggregate
until stability is obtained.
Final compaction is achieved with a
vibratory compactor, first without vibration for several
passes, then with full vibration. Any weak spots found
during final compaction usually indicate inadequate
aggregate thickness at those spots. Do not grade ruts
down. Instead, fill them with additional aggregate and
compact. This rule applies to any future rut maintenance
required.
It is important that the construction
process be monitored If field conditions change from the
design values, and cause a lower subgrade soil strength
value, structural section thicknesses must be
re-evaluated. Monitoring construction and early use of
the aggregate section pinpoints weak areas missed in soil
testing.
Equally important is monitoring the
quality of the structural section materials and the
placement method. The purpose is to detect changes so, if
necessary, design adjustments can be made on site before
excessive subgrade failures occur.
Final pavement construction should be
delayed as long as possible to monitor the unpaved
aggregate section's performance. If local areas require
it, additional aggregate should be used to correct any
rutting. Geotextile test sections provide a lot of
insight into how the design structural section will
perform.
After these steps are completed, the
road or area is ready for use. Stability will increase as
traffic and the confining action of the fabric continue
to densify the aggregate and consolidate the subgrade.
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