Geogrids vs. Geotextiles - Setting the Facts Straight
Geogrids vs. Geotextiles - Setting the Facts Straight
I am starting this thread to set the facts strait on the use of geogrids and geotextiles in subgrade applications because of a recent thread Entitled Geogrid vs. Geotextile where a lot of bad advice was given by OldGeoGuy and JML4TCM, who both after bashing everyone's knowledge showed there lack thereof.
First and foremost the FHWA is not an authority on anything. Referencing them as such is the same as calling a 1990's Personal Computer state of the art. The FHWA is more than 20 years behind the current state of practice at any given time. That said, in 2008 the FHWA separated fabrics and geogrids in their geosynthetic design guide. They give advice for design and usage of geogrids and geotextiles.
For everyone's reference, the best source for design and usage of geogrids and geotextiles is provided by the Army Corp of Engineers. They continually run test and develop and refine their design methodologies. If you need some references I would be glad to provide them. Here are some design basics to consider.
In the world of geotextiles, there are a number of types of materials. By far the most common are woven and non-woven geotextiles. Woven geotextiles (e.g., Mirafi 140N) are only for separation, and provide absolutely no reinforcing value. (They look like felt.) They are selected based on survivability during construction as well as aperture size. Yes, geotextiles have apertures and you need to check that they will not clog when installed based on the gradation of both the subgrade and the fill material placed. Each manufacturer can provide you with the equations to check whether or not the material will clog.
Woven fabrics can be used for a number of different applications. They provide separation and can provide some reinforcement. Selection of a woven fabric is based again on the aperture size versus gradation of the surrounding materials as well as survivability. You will also want to check flow rates as wovens provide less water passage than non-woven's even if they do not clog. If separation is the only function, a lower strength fabric by comparison (e.g., Mirafi 500X) can be used. It should be noted that many of these lower strength fabrics are made of polyester which elongates as much as 20% and is subject to significant degradation in pH environments outside the range of approximately 5 to 10.
For combination applications (i.e., separation and reinforcement) or in high/low pH environments, you would want to select a high strength woven fabric made of polypropylene. These fabrics will provide reinforcement in some applications but very little in others. A good example of the use of a high strength woven is at the bottom of an embankment over soft soils if settlements are not a concern (e.g., surcharge embankment). They will provide modest subgrade improvement to start the construction and can provide enough tensile support to prevent slope failures, many times with just 1 layer of fabric. However, fabrics are a poor solution for subgrade improvement contrary to what has been posted before.
High strength woven's provide no stiffening enhancement and if you read the extensive research provide a maximum thickness reduction of 25% in subgrade applications for the most aggressive research. The preponderance of research gives little to no value to these fabrics in subgrade improvement applications. The Army Corps of Engineers blatantly state not to give any Fabric any value in Subgrade applications.
In Subgrade applications, fabrics work by providing reinforcement through what is termed as the "Hammock" effect. Basically, the ends of the fabric are held by friction and the tensile strength of the fabric supports the load. This would work well if traffic never moves from the same wheel paths and if you can get the fabric stretched prior to completing the work. The reality is that no one can ever get the movement of the fabrics completed before completion of construction. This is especially true in situations where the subgrade softens after construction, as fabric installed over a relatively stable subgrade will not have elongated at all. Generally speaking, high strength woven fabrics must elongate 4 to 5% before they engage their tensile strength. This is due to the crimped nature of the fabric fibers. These must elongate (stretch) to relieve this crimp as well as they must elongate in elastic deformation for the loads present. This is problematic as soils fail at 1 to 2% strain. A 4 to 5 % strain in the fabric will result in a failure.
In addition to elongation issues, fabrics suffer from being very flexible and do not spread loads. This is basic load transfer and common sense, I guess except for everyone but OldGeoGuy and JML4TCM. If you throw a plastic bag on a mud puddle and step on it your foot will sink in as it has no ability to transfer load. On the other hand if you used a piece of plywood the same size you would sink far less. Why? The stiffness of the plywood spread your load over a greater area, better utilizing the bearing capacity of the soils and switching the failure mechanism from shear failure to bearing capacity. Soils as we should all know are far better in bearing than they are in shear.
The long and short of all this is that fabrics are best for separation and have limited benefit as reinforcement. Fabrics as reinforcement must be selected wisely based on the application and must be sewn together for and reinforcement work as a discontinuity means failure. If you examples of the disasters created when people who think they know about Geosynthetics, like OldGeoGuy and JML4TCM, use fabrics when they should be using something else I will be glad to provide them. I can tell you the #1 failure in geosynthetic design is use of woven fabrics inappropriately. I would suggest you call you local DOT and ask, I can guarantee they can show you hundreds of places.
As a final note of fabrics, most fabrics specified by engineers are unwarranted, incorrectly designed, incorrectly selected, or improperly used. I would say 95% of the time separation is not an issue yet fabrics are specified at nausea for this purpose. There are equations provided by the Army Corps of engineers that allow you to check separation. You will be surprised how often nothing is needed because soils will maintain natural separation/filtration. If you take one thing away, fabrics are to be designed carefully for an intended purpose. Grabbing something off the shelf or just jamming something in because you use it before or it has a high tensile strength can lead to a hole slew of problems and if pursued can lead to sanctions against the engineer including loss of license.
To finish out this post I will touch on geogrids. Geogrids are used to provide mechanical enhancement to a layer of aggregate material. Simply put, the use of a geogrid can reduce a layer of something like well graded gravel by upwards of 75% while maintaining the same serviceability as the thicker layer without geogrid. This same effect was noted before for high strength woven fabrics, though to a lesser degree.
Quality geogrids work utilizing two main mechanisms, soil confinement and stiffness enhancement. Basically, the geogrids traps and locks the aggregate particles at the bottom and prevents their movement under load. The confinement allows better compaction to be achieved as well as an overall stronger structure. The best way to think of this is cue balls stacked in a pyramid inside a cue rack. If you push on the top ball the bottom balls move until the hit the rack, which through the strength of the balls and the rack resists the load. The same principle works for good geogrids.
Stiffness enhancement is as discussed before. In this case good geogrids are stiff and produce a "snow shoe effect" over soft soils. This in conjunction with stiffening the overlying soil results in better load distribution. Specifically, loads propagate at a 1:1 angle or greater depending on selected fill and geogrid. This stiffness enhancement increases with each layer of grid added. (Multi layer systems have been use as giant soil mat foundations for distribution of building loads over larger areas.)
(As a small note, geogrids do get a small amount benefit from the "hammock" effect discussed before, but not a significant amount.)
Unlike fabrics, geogrids have design methodologies for their use in subgrade applications. The state of the practice is the Giroud-Han design methodology as published in the ASCE geotechnical journal back in 2004. It allows for the design of both reinforced and unreinforced sections using geogrids.
As noted in previous threads, geogrids do not provide separate, thought they enhance it. That means you must check whether or not natural separation exists between your selected fill and the existing subgrade. If it does not, an appropriately selected geotextile (woven or non-woven) should be used under the grid for separation.
I will give OldGeoGuy and JML4TCM credit in that most geogrids on the market are junk and you would be best to use a high strength woven fabric. The only companies who have geogrids that have been shown to provide significant improvement in lab testing are Tensar, Naue, and Etsong. All other geogrids are junk and a waste of you clients money. (There are thousands of pages of research showing this to be true.) The reason is that most others are what are known as woven geogrids, which are nothing more than fabric with holes, which is to say they work the same way as a fabric without providing separation.
As a final note for this post, no two geogrids perform remotely the same even with similar material properties. The key to a geogrids performance is its interaction with the soil, which can not be measured at this time. As such, you must do design comparisons using a standard design methodologies. The differences will amaze you!!!
Thanks for reading and I hop this helped get rid of some of the junk that has been posted. As a note I will be happy to put you in touch with the president of any number of Geosynthetics associations to confirm what I have said in this post.