To the editor:
The article by Dov Leshchinskey in the April/May 2008 issue of Geosynthetics raises a number of interesting points on the impact of water, which all designers should consider in their wall designs. The principles of wall design and what should be considered are well-known; however, where the process can fail is in the detail provided to the contractor whose interpretation of the drawings and specification can differ to that originally considered by the designer. Specifying “free draining” backfill can imply a wide range of materials that not only have a direct impact on water pressure development but also on damage to the reinforcing material. Backfill grading limits would go a long way to achieve the target permeability as well as providing a match to the installation damage considered for the reinforcement.
My concern comes down to applying the lessons learned in this article to the structures highlighted in the article on GRS bridges of Defiance County, in the same April/May 2008 issue (pp. 14–21), where a woven geotextile is used for reinforcement. What the author has not appeared to cover is how water impacts on the design of these structures, which are located in a fluctuating water zone. I would have thought the use of a woven geotextile would create a barrier to water movement through the backfill during drawdown conditions, thus reducing the shear strength of the backfill, which can be of a lower quality than that commonly specified for MSE structures.
One should consider the geotextile having to perform in the same manner as geotextiles specified in erosion control structures where pore size and permittivity are important design criteria. One may question why pore size, however if there is clogging or blinding off of the geotextile then the interface frictional properties could change, especially if lubricated during drawdown conditions thus reducing the pullout resistance. The use of a geogrid for these walls would ensure the backfill performs as designed and has a minimal impact on the overall cost. Even assuming the geotextile is 50% cheaper than a geogrid, the percentage increase in the installed wall cost is <10%, a small price to pay for higher level of confidence and a saving on attorney fees.
Gordon Stevens, Technical Manager, Maccaferri New Zealand.
The author appreciates the comments by Gordon Stevens regarding the importance of the impact of water on wall design. Mr. Stevens expresses concern and speculates that the use of the woven geotextile in the construction of the GRS abutments in Defiance County, Ohio, would not perform as well as MSE geogrid walls in a fluctuating water zone. Nonetheless, the good performance of the 11 bridges built in Defiance County using woven geotextile in the GRS structures under the water conditions of that area suggests that this concern is unfounded.
The terrain in Defiance County is flat, with rivers, many streams, and sequence floods. For this reason, the GRS abutments were designed to support bridge loads and high water with the potential for downdraw as the result of receding floodwaters. The GRS abutments were built with fine, open-graded, crushed aggregate (9mm) and closely spaced geotextile to produce a composite material that can endure a vertical stress of 25kips/ft2 and also allow the rapid release of water from the reinforced soil fill. All 11 bridges have been inundated with floodwater several times without any effect on performance.
While it is correct to note that the permeability of a geotextile needs to be considered for adequate long-term drainage, particularly when used in combination with fine-grained, poor quality soils, this does not mean that GRS structures built with a geotextile are not adequate or are not as robust in design and performance as MSE structures built with geogrid. The specification of a particular woven geotextile can be for reinforcement, separation, or erosion control. For the abutments in Defiance County, a woven geotextile was selected as reinforcement because of several factors, including cost ($1.5/yd2), ease of placement, and compatibility with frictionally connecting the block to the GRS mass.
Accordingly, the cost difference between a geotextile and a geogrid, although important, is not the only factor to consider. With the proven performance of the woven geotextile and the other advantages of GRS technology, users can confidently choose geotextile without fear of inadequate protection from the impacts of water on wall design or any increased risk of legal fees.