|
Moisture damage to rigid pavements may manifest itself as pumping,
faulting, and D-cracking. Flexible pavements can suffer from loss of
support, asphalt stripping, alligator cracking, potholes, etc. The
detrimental effects of saturation in the pavement system are significant.
AASHTO reports:
Water in the asphalt surface can lead to
moisture damage, modulus reduction, and loss of tensile strength.
Saturation can reduce the dry modulus of the asphalt by as much as 30%
or more.
Added moisture in unbound aggregate base and
subbase is anticipated to result in a loss of stiffness on the order of
50% or more.
Modulus reduction of up to 30% can be expected
for asphalt-treated base and increase erosion susceptibility of cement
or lime treated bases.
Saturated fine-grain roadbed soil could
experience modulus reductions of over 50%.
 According to AASHTO pavement design methodology, the
more free draining the aggregate base, the higher the strength assigned to
it. An AASHTO excellent draining base will have twice the structural
contribution per inch as compared to a poor draining base.
If a clean aggregate with a low fines content is
used in construction of a pavement system, the remaining source of an
increase in the fines content would be from a fine-grained subgrade layer. A
base course typically rests on soil subgrade. While a base course layer
consists of coarse aggregate, a subgrade soil may be composed of sand, silt,
or clay. There can be many orders of magnitude difference in particle size
of base course and subgrade soils. This wide gap in particle size
distributions creates conditions that are ideal for mixing of the subgrade
soil and the base course aggregate. The two can be separated from each other
to prevent this intermixing and the corresponding decrease in transmissivity
of the base course layer. Separation is accomplished easily through the use
of geotextiles at the subgrade — base course interface.
Separation function
The separation function can be described for this
purpose as the use of a geotextile to prevent the mixing of two adjacent
material layers. A geotextile is a flexible, planar, and porous structure
consisting of a network of fibers, filaments, or yarns. A geotextile’s
structure and properties are ideally suited to ensure separation of granular
materials without compromising the movement of fluids between layers. In
fact, geotextiles of various types perform separation function in many
different applications. In geotechnical engineering, the alternative to
utilizing a separation geotextile is to incorporate an additional soil
layer, which can be cost-prohibitive.
The requirements for a separation geotextile at the
subgrade-base course interface are well documented and can be listed as
follows:
Retention: The separation geotextile must
have an opening size small enough to prevent the migration of subgrade fines
into base course under dynamic vehicle loads.
Permeability: The number of openings and
their size in the separation geotextile must be large enough not to
adversely affect the flow of liquid or air in either a downward or an upward
direction.
Survivability: The geotextile must possess
adequate strength and a construction suitable to survive installation and
in-service conditions.
Retention and permeability requirements relate to
the geotextile opening size and distribution. Survivability, on the other
hand, is a function of the strength and flexibility of geotextile.
Geotextile manufacturers offer products with wide ranges of physical,
mechanical, and hydraulic properties.
Performance
A limited number of studies on the performance of
separation geotextiles have been reported. Most of the studies fall into one
of the following two categories concerning the performance of separation
geotextiles:
The ability of
geotextiles to preserve layer separation under simulated or actual
traffic loads.
The improvement in long-term pavement
performance as a result of the layer separation.
The first item above has been studied extensively
through both laboratory and field tests. A compilation of 20 laboratory
studies indicates that geotextiles are successful in preventing migration of
fines under dynamic vehicular loads provided the retention and survivability
requirements are met and the load bearing capacity of the base is
sufficient. Empirical proof of performance is also provided by over 300
million square yards of geotextile being used in the world every year in
this application. To put that number in perspective, that is equivalent to
about 40,000 lane miles of road per year.
Very few scientific studies on the improvement in
pavement performance have been reported in the literature despite the fact
that separation geotextiles are used extensively all over the world and have
been for more than 35 years. The reason certainly is the expense, time, and
effort involved. To quantify or even just demonstrate the improvement in
pavement performance resulting from separation geotextiles requires a
minimum 10 years of a controlled study of pavement sections with and without
geotextiles. Such studies demonstrate that while geotextiles do provide
performance improvement, placing an exact value in terms of percent of
improvement in design life of a pavement may be very challenging. But it
takes very little improvement in pavement performance to justify the minimal
cost of including the geotextile. Fortunately, the Geosynthetic Institute is
currently performing a very comprehensive study including sites from all
over the United States. To date, the study includes 14 sites, the oldest
being initiated in 1997. Much about the long-term performance improvement is
expected to be known as the sites approach the end of their design life of
10, 15, and 20 years.
Cost/benefit determination
The long term cost/benefit ratio calculation for
using separation geotextiles can be performed in several ways. One technique
compares the cost of geotextile to the life-cycle cost of the pavement. Such
a comparison is based on an increase in the life of a pavement structure
incorporating a separation geotextile. For example, assume that a pavement
without a geotextile requires an overlay layer within 10 years; however, the
use of a separation geotextile increases this time to 20 years. Then the
cost of installed geotextile is directly comparable to the cost of overlay
layer.
Another technique compares the cost of contaminated
base layer with the cost of geotextile used to prevent contamination. . For
example, assume that 1 inch of base layer stone gets contaminated with fine
soil from subgrade. Then the cost of geotextile can be compared to the cost
of 1 inch of lost base course layer. Such a comparison can be extended to
the whole depth of base course depending on the severity of contamination.
When comparing aggregate and geotextile costs, the cost of the installed
geotextile is very close to the cost of 1 inch of base course aggregate. As
the depth of contamination increases, however, the cost of geotextile
remains constant while the cost of aggregate increases linearly.
A common example of the effectiveness of a
geotextile used for separation is its application under an unpaved gravel
road. You must add 4 to 6 inches of gravel every few years without a
geotextile. However, with a geotextile, the road will last for many years
without any additional gravel requirement.
A separation geotextile prolongs pavement
performance by preventing the contamination of base course layer. The actual
pavement performance improvement in terms of the lifetime of a pavement will
vary and is being currently evaluated by the Geosynthetic Institute.
However, the cost of geotextiles is less than the cost of 1-inch base course
aggregate; and separation geotextiles typically prevent contamination of
several inches of base layer. Therefore, benefit significantly outweighs the
cost of using a separation geotextile in pavements.
Dhani Narejo represents GSE
Lining Technology, Houston, Texas; Mark Marienfeld represents Amoco
Fabrics and Fibers, Austell, Georgia; Bill Hawkins represents BBA
Nonwovens, Old Hickory, Tennessee; Bruce Lacina represents Mirafi,
Pendergrass, Georgia.
|
Contech’s Tenax RoaDrain is a long-term pavement
drainage system that can be engineered to quickly remove subsurface water
from pavement. It separates structural fills from the native soil and has a
high-tensile strength that allows it to provide base reinforcement and
restrain weak subgrade. It provides a positive capillary break and reduces
damage caused by frost heave and subsequent thaw. When the geocomposite
drainage layer is incorporated between the aggregate base member and the
soils, it creates a drainage path that can reduce drainage time in certain
areas from months to less than a day. Cyclic and static loading tests
indicate that the material maintains its flow capacity under very heavy
traffic conditions which exceed the typical pavement load.
Saint Gobain’s GlasGrid is a high-strength,
high-modulus, low-elongation fiberglass mesh capable of turning crack
stresses horizontally and dissipating the stress. The material is
self-adhesive which saves valuable man-hours since no messy tack coat is
required.
The Spectra Roadway Improvement System from Tensar
is typically used beneath roadways, rail yards, construction platforms, and
parking lots — structures that support vehicular traffic. The system
consists of a combination of structural biaxial geogrid and engineered fill
that enhances performance by increasing structural capacity, extending
pavement life, and increasing pavement durability. According to the company,
the system is based on the new Giroud-Han Design Methodology which supports
the use of certain geosynthetics to reduce aggregate requirements and
improve subgrade performance, thereby simplifying and speeding construction,
decreasing labor and equipment requirements, reducing undercut/overexcavation,
and improving durability.
