To the editor:
Thank you for your response to my concern about a photo in Geosynthetics magazine, February-March 2007. While including a photo of the Bowman Road Bridge in the article, “Junction strength requirement for roadway design construction” was done [solely] for aesthetic purposes and not meant to mislead the reader, it has, in fact, done so. I offer the following rationale, as it may facilitate a better understanding of this issue.
The article focuses on the junction-strength requirement for geogrid reinforcement. [One] photo accompanying this article is of the Bowman Road Bridge in Ohio. The Bowman Road Bridge was built using geosynthetic reinforced soil (GRS) technology, rather than the standard mechanical stabilized earth (MSE) methodology. GRS abutments are built with polypropylene geotextile (not a geogrid) reinforcement and are designed without any requirement for junction strength or special reinforcement connection. Hence, the distinction between GRS and MSE construction is significant.
It is important to note that while I believe in the fundamental mechanics leading to reinforced soil design for some MSE wall systems, I do not believe that it is appropriate for all reinforced soil systems to be constructed under the rules that dictate MSE design. The placement of this particular photo in the article infers that the Bowman Road Bridge is just another MSE feature built with segmental retaining walls (SRW) blocks, when in fact it was built with geotextile reinforcement and without SRW block. Instead, the facing blocks were lightweight split-face concrete units frictionally connected to the GRS mass between the sheets of geotextile without the need for a vendor approved mechanical connection system.
Furthermore, the GRS abutments were designed without real consideration for reinforcement creep, lateral loads at the face of the walls, pullout, and the recommended MSE base to height ratio 0.7 with a 2-ft. wall embedment.
Please realize that it is a challenge to implement this very affordable, simple method of bridge support because it does not follow current MSE design methodology. There is a reluctance to change or to acknowledge alternative design methodologies in the industry.
There is a clear distinction between the MSE and generic GRS designs, and this distinction needs to be identified and understood to give the next generation of wall designers the option to build affordable walls and abutments to meet the demand for these features on highways.
Accordingly, the inclusion of the Bowman Road Bridge photo creates a factual inaccuracy in relation to the subject article and, not insignificantly, undermines the importance of the Bowman Road Bridge project as a cost-effective and structurally sound alternative to standard MSE bridge design.
The correct representation of new geosynthetic reinforcement applications and concepts is necessary to advance the state of the practice.
Letter submitted by: Michael Adams
Research Geotechnical Engineer
Federal Highway Administration