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The material properties included in the original work of TF 25 were based on experience in the use of these materials at the time. The resulting work was published in the AASHTO Specification Book as
Specification M-288 on Geotextiles.
Because of the use of geotextiles in the United States was new and there were very few, if any, accepted design methodologies for geotextiles, the task force took a
conservative approach to its work. The specifications did not cover or encourage designing for the use of these materials and did not promote the performance and economic benefits of following detailed design
procedures.
Following their initial review, the task force continued to develop guide specifications for geotextiles used in paving, subsurface drainage, erosion control, sediment control (silt fences) and
separation applications. In 1986, the task force approved proposed specifications, including material property values and notes on construction and installation procedures, for each of the five
applications. Between 1986 and 1990, the five individual specifications were merged into a single material specification, which no longer included the construction and installation notes that had appeared in
the individual documents.
In 1989, this single document was submitted for an AASHTO Subcommittee on Materials ballot as a revision to the existing AASHTO M288 Specification for Geotextiles Used for Subsurface
Drainage Purposes. As indicated by the title, the existing specification applied to drainage fabrics alone. The revision represented an enlargement of the applications covered by M288.
The revisions
were approved, and the revised specification first appeared in the Standard Specifications for Transportation Materials and Methods of Sampling and Testing, 15th edition (AASHTO 1990) as AASHTO Specification
M288-90 on Geotextiles. A formal report of TF 25's work was published separately in 1990.
By 1992, geotextile usage had grown at a rapid rate, and the work of Task Force 25 was nearly 10 years old.
As such, AASHTO Subcommittee on Materials, Technical Section 4e approved a formal review of M288-90. A preliminary review was done with a small task force in early 1992, and a formal Joint AASHTO-Industrial
Fabrics Association International (IFAI) Task Force was established later that year. The task force was composed of academic, industrial, and state and federal transportation departmental representation.
When published in the AASHTO Specification Book, M288-90 had not included the construction and installation notes developed by TF 25. One of the new task force's first responsibilities was to approach AASHTO
and work out the logistics of including some form of these notes in M288-92. Once this was accomplished, the task force divided into two sub-groups, one to work on material properties, and the other work on
the construction and installation notes. Both were included when M288-92 was published in 1993.
Revisions to specifications: M288-96
As experience was gained and
design methodologies developed, the need to revisit Specification M288-92 was realized. A joint AASHTO-IFAI Task Force was formed in 1994 to review and revise the specification, and the resulting work was
adopted by AASHTO in 1996 and published in 1997. The latest specifications are based on accepted design procedures but also provide default material property values for instances when actual design
procedures are not used. Table 1 lists the M288-96 default geotextile class and design class for the subsurface drainage, permanent erosion
control, separation and stabilization applications. The design class is a lesser class that may be used when field experience, laboratory tests on exhumed samples, and/or certain site configurations are taken
into account.
Material requirements
M288-96 is not focused on design or construction specifications but is, instead, based on geotextile survivability from installation stresses. When the specification
is applied, selection of the geotextile is based on knowledge of the anticipated installation stresses to which the material will be exposed. M288-96 covers six geotextile applications: subsurface drainage,
separation, stabilization, permanent erosion control, temporary silt fences and paving fabrics.
In M288-92, geotextile survivability was divided into classes A and B. Class A was used where installation
stresses--determined by aggregate shape, trench depth, and the size and height of an armor stone drop--were more severe than in Class B installations. There were no definitive measurements set for
differentiating between severe and less-severe installation stresses.
In M288-96, the general strength requirements for the subsurface drainage, separation, stabilization and permanent erosion-control applications
are broken into three classes of geotextiles. Class 1 represents the most robust and Class 3 the least. Within each survivability class, the strength requirements are established base on elongation at
break in the grab-strength test. The highest strength requirement is for materials that break at less than 50 percent elongation (typically woven) and the least for those that break at greater than 50 percent
elongation (typically unwoven). The requirements for the silt-fence applications are based on supported or unsupported fences. Paving fabrics are limited to fabrics with elongation at break of greater
than 50 percent.
Table 2 compares M288-92 survivability Classes A and B to the default survivability classes listed in M288-96.
Table 3 compares the general strength properties of M288-92 and M288-96 for the subsurface drainage, separation, stabilization, and permanent erosion-control applications. The values are shown in SI units; M288-96 made the conversion to SI units while respecting the approximate nature of these numbers.
Table 4 (p.41) presents a summary of all six applications, including default requirements for the apparent opening size, permittivity, and ultraviolet degradation. This table also lists current commercial geotextiles that satisfy M288-96 default specifications for each application. This information was obtained by polling the respective manufacturers. Additional manufacturers were contacted but did not respond to the inquiry.
No manufacturer currently provides its geotextile properties in a manner that allows a quick M288-96 selection by the designer. A quick review of the geotextile "identifiers" used by the
manufacturers shows that the convenience afforded by M288-96 has yet to be adopted by the producers. All manufacturers do produce products that meet and exceed the M288-96 criteria; the use of these specifications
does not limit the pool of potential suppliers.
Construction installation guidelines
After being omitted from M288-90 and M288-92, construction and installation guides
are now included as appendices to the current AASHTO specification book. The notes include directions on site preparation, material overlap, armor stone placement, post spacing for silt fences, embedding,
damage repair, preparation, material overlap, armor stone placement, post spacing for silt fences, embedding, damage repair, overlapping, direction of placement in water and on slopes, and installation suggestions
for paving fabrics. Proper installation is critical for the long-term performance of any geosynthetic. It is important that the notes and material specifications are published together.
Summary
The development of construction material specifications is an ever evolving process, because knowledge of performance and the types of available materials are always changing.
As a result, the manner in which such materials are specified must evolve with time. AASHTO M288-96 specifications reflect our current knowledge of geotextiles, based on more than two decades of use and
available materials. It will most certainly continue to evolve as our knowledge and materials improve. The cooperative process between users and manufacturers in developing M288 guidelines should continue to
serve as an examaple in this evolution.
The importance of standardization is not unique to geosynthetics. An article in the Sept. 15, 1997 Engineering News Record on the use of plastic composites in highway
bridge structures laments that, despite cost and service advantages, "the use of the material is still risky business and will be shunned by many owners and engineers." It quotes an industry
member. "What we need is more information on the basic mechanical and physical properties of the materials for standards and design specification development," and concludes, "Until then, it's
buyer beware, especially for construction." All building products have gone through the type of standards evolution that geosynthetics are currently experiencing. We feel that M288-96 can contribute to
the process and lead to fewer unique predications by the users, thus requiring fewer geotextile varieties from the manufacturers.
L. David Suites, P.E. is with the New York State Department of Transportation. He is a member of the AASHTO Committee that developed M288-96.
A regular and prolific contributor to GFR,
Gregory N. Richardson, Ph.D., P.E., is principal for G.N. Richardson & Associates, Raleigh, N.C.
References
AASHTO. 1990. Specification M288-90 on Geotextiles. Standard Specifications for Transportation Materials and Methods of Sampling and Testing 15th ed. Washington, D.C.: AASHTO.
AASHTO. 1993. Specification M288-92 on Geotextiles. Standard Specifications for Transportation Materials and Methods of Sampling and Testing 16th ed. Washington, D.C.: AASHTO.
AASHTO. 1997. Specification M288-96 on Geotextiles. Standard Specifications for Transportation Materials and Methods of Sampling and Testing 18th ed. Washington, D.C.: AASHTO.
Phair, M. and W.J. Angelo. 1997. Composites Astound & Confound, Engineering News Record. (239, 11, September 15), New York: Mcgraw-Hill pp. 34-38.
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