Geosynthetics - Use and Abuse

[VAD]

I recently attended a forum on Geosynthetics related to highway and environmental applications sponsored largely by by the geosynthetics community re manufacturers but endorsed by others. The speakers were internationally recognized "gurus". At the end of the day, I came away with the feeling that the objective was to promote the sales of geosynthetics rather than leaving one with a clear picture of why this product should be used and its limitations etc.

This is not the first occasion that I have objected to this type of brainwashing. It seems that while there is no doubt benefits in the use of a product there is a trend in parts of our business to take advantage of the lack of knowledge of others by crafty salesmanship. In my books I see some of our gurus now becoming what I term "Technical Snake-Oil Salesmen".

What is your experience on this matter?. Are you very clear on the use of this material?. How do you rationalize its use for a particular application?. Do you just go with the flow - throw a piece of geotextile down if the ground is soft. Is it immediate relief that is your concern and you are happy- it works?. What has been your experience of performance in the long term?. Do you invoke aspects of soil mechanics, construction practices, ground conditions in the short and long term in your decision making etc.or are you pressured by the Contractor to use this magic carpet and you have to because everyone knows it works. Have you ever asked yourself and reflected that many roads were successfully built and have performed well without this product?. Do you sometimes feel that you are losing your fundamental grasp of geotechnical engineering because of lack of proper integration of this product in the gfundamental geotechnical thought process?

Comments, please

 

[Mccoy]

Vad,
my brief comment from Italy is that the construction industry here often looks like being pretty traditionalist and wary of new contraptions, pretty much sticking to the old ways.
For example, I've never seen geotextiles used or reccomended for foundation purposes, their main use still being limited to drainage systems.
I've often wondered about the potential benefits of geotextiles in soft soil consolidation for foundations, never being able to put them to test, but your words are not encouraging in this sense.
I'll just stop wondering...

 

[SlideRuleEra]

In coastal South Carolina there is a lot of soft soil and a lack of "suitable" fill material to replace it. Cost/benefit ratio is easy to compute - the avoided trucking cost of the fill material (that is not needed) is usually enought to justify cost of the proper geotextile. Then there are benefits of faster construction & reduced environmental impact. When correctly selected and installed, the geotextiles seem to perform well over time.

http://www.SlideRuleEra.net

 

[jheidt2543]

VAD:

I think I understand the concerns you bring up and I would have the following comments:

1. "I came away with the feeling that the objective was to promote the sales of geosynthetics rather than leaving one with a clear picture of why this product should be used and its limitations etc.” The manufacturer or trade association or sales rep for each material we, as engineers, may specify promote their product. Each of their products are developed and engineered to satisfy a particular need. ACI, AISC, PCI, APA all were started to promote the safe use of their respective products. And, there have been some pretty heated advertising campaigns through the years between concrete vs. steel, masonry vs. precast, stucco vs. EIS and wood vs. steel studs to name a few. It is our job to sort through and decide on a job by job basis what product best suits the project at hand from a structural and cost perspective.

2. Every material has a place and was developed to meet a need, not on EVERY job, but on suitable jobs. Someone has to sort that out and that is part of every engineer’s job description.

3. It is not unethical for a sales rep to highlight a product’s attributes and soft peddle the short comings as long as he covers both. Selling products is what makes the business world go around. However, when we go to “trade shows” or “trade demonstrations” we should be asking tough questions. Find out if the product is applicable to your project and ask for site visits by the rep., product warranties and installation guarantees, where applicable.

4. Your "Technical Snake-Oil Salesmen" a little over the top and connotes something sleazy about the process and I really don’t think that is the case the majority of the time. But, we do have a responsibility to come to the presentation with enough basic knowledge to ask questions.

 

[dmoler]

Geogrids are used in Florida not commonly, but not rarely, for everything from major highways in the Everglades to parking lots and drives in subdivisions. Based on a number of ones I either provided recommendations and/or inspections and testing, the term I use to clients to dash their hopes is 'servicable'.

They are usually servicable in that they will need maintenance, they will not be perfect and flat, and the quality of the road will depend more on the amount of maintenance than on the construction in the beginning.

Here are several cases. A medium size parking lot that was supposed to be temporary was constructed for a school mainly for bus parking every day. The site consisted of about three feet of non-plastic silt or sandy silt with an organic silt layer (very bad) about 1.5 feet thick layer on the surface or somewhere near the top. A geogrid (I think it was a Tensar BX 1200) was laid down with a 1 foot limerock base on top (limerock CBR here is usually quite high, maybe 120 to 140). This is a sucess story for the geogrid, since I stopped off to take a look a few days ago. After 10 years, very few cracks but the standard 'swales' that usually form, but the swales were not bad at all, less than 3 inches over maybe 50 feet. And the parking lot seems now to be a permenant one with heavy bus traffic. I did not see any evidence of patching.

However, another parking lot was constructed about the same time that I drove by a few days ago to see, and it did not fare so well. The original ground had something like 4 feet of organic silt/sandy organic silt followed by sand. Same base and geogrid as above was used. Swales in excess of 2 feet vertical for about 30 feet horizontal had formed, with some as high as about 4 feet vertical(yes, 4 feet). The parking lot was heavily used with small trucks and cars, but no big trucks. Patching was not too evident. The parking lot was still being used, people just have to drive strategically and slowly.

Another major highway (I had no part in the initial construction) in the Everglades was constructed about 7 years ago (based on Highway Dept.) and about 2 years ago I did borings and the pavement/geo recommendations. For that highway, the embankment was built with about 8 feet of organic fibrous silt (muck is slang term here for that) and interbedded with geogrids spaced vertically about 2 feet apart. Heavy truck traffic (sugar cane trucks) virtually destroyed that road. Very excessive rutting, some waves about 1 foot high. Very bad, even with extensive maintenance as observed by large patches. It only lasted about 5 years total.

Geogrids make roads strangly resiliant to nomal deterioraton/cracking/pothole development. They really make the flexible pavement flexible. Waves and swales seem to form with mixed results for deterioration of the driving surface and cracking. Sometimes only hair cracks and sometimes larger cracks. The 4 foot waves from above actually had no cracks on them! Pavement evaluations for remilling/resurfacing are somewhat more difficult for that reason. Sometimes remilling/resurfacing is impossible due to the construction equipment having to deal with the 'waves'. Also, classifying swales and waves are somewhat difficult with classical pavement condition survey classifications.

So, I usually tell clients that using geogrids is like a box of chocolates, you never know what your gonna get. But expect maybe 10 years of 'servicable' life with anticipated maintenance costs to be probably equal to 10 to 20 percent of the cost of original pavement construction and possible remilling/resurfacing to be required after 6 years.

 

[MRM]

VAD.
Thank you for bringing up the question. I've been wanting to get more info about these products as well. They aren't used that much in Michigan, except for maybe some road applications as the others discussed above.

I too have been curious about geogrid or geotextile materials for use in creating a better foundation subgrade. Be it in soft clayey soils, or very loose sandy soils. For me to feel good about using it, I'd need to really understand how much "development length" to use when overlapping the material, and what the creep characteristics really are of the material. Both of these concepts should be relatively easy to hammer out through testing and research, and on paper, or one would think.

I haven't been able to locate any good "third party" research on this subject. To me, the concept looks like it could help improve the foundation subgrade, but it would be nice to see someone else do it first!

 

[SlideRuleEra]

MRM - Here is a starting point

http://ceprofs.tamu.edu/briaud/buchanan web/Lectures/Ninth Buchanan Lecture.pdf

Perhaps you can find other links here

http://www.geoengineer.org/books.html

http://www.SlideRuleEra.net