Geogrids vs. Geotextiles - Setting the Facts Straight 7

[theanimal1999]

thread260-221555

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.

 

[TDAA]

Darth:
1) Water movement can migrate fines into the aggregate materials. As the fines mix into the aggregate, you can start to see the material change properties, and therefore, pumping/motion may begin. This will lead to premature failing vs. having the material be separated.

2) Your lower grids should develop tension (therefor distributing the loads). This will happen due to the soft nature of the material you are working on, as you place the first layer of aggregate. A second layer would help distribute the load the same way as the bottom layer. The difference is that you are working over something that is already stabilized to some degree. While the grid will keep the aggregate from locking in amongst itself, the confining action of the grid apertures should do more good than bad. I would suspect that the one you tore out had issues concerning the size of aggregate, the need for separation, or some other construction related issue (ie, insufficient lift thickness to stabilize the are to be able to place the next lift.)

 

[theanimal1999]

To add to the excellent answer by TDAA:

Separation is only needed less than 5% of the time generally. Use these equations to check if separation is required. (Developed by Army Corp for Damn Construction.)

D15(filter)/D85(soil) must be less than 5
D50(filter)/D50(soil) must be less than 25

Also, Multi-layer systems are beneficial when you have thick layers of soft soil and you are trying to limit differential settlement or you have high loads such as load transfer platforms or buildings. Otherwise, it is better to use a single layer higher strength grid than a multilayer system. You should not have had issues with the two layers, it just isn't cost effective or necessary.

 

[darthsoilsguy2]

thanks for the replies tdaa and animal.

seems reasonable... i think i would normally go for some additional stone depth over the fabric. i think the aggregate base course stone we have around here is pretty well graded and won't let many problems wash in.

to satisfy some curiosity on item 2... to be honest, i can't say why we had trouble in that area. it was a small portion of the overall project and it wasn't too much rework. the project was a USA Track & Field certified running track built on an existing no fill CH site a while ago. After paving, there would never be traffic. We needed to stabilize for paving operations. i'd like to think the owner appreciated knowing that i wasn't being over-conservative with recommendations since the whole job was a time and materials change order with undercuts being called out on a case-by-case basis. The grader was very pleased with the end product (they pay a stiff penalty for out of tolerance track surface) and the stone looked like cured concrete. We did have two areas that we had to redo though... gotta go wife wants to watch a movie

 

[VAD]

Well well. Glad to see that there are more people that share some common views about the use and misuse of geotextiles. What we do today is to get a few Univesrity Profs to be at local seminars wher young graduates flock and indoctinate them. I have battled this since around 1986 nad have made a lot of people upset by mentioning the same things as the animal1999. Everyone gets upset.

However, I have seen that woven geotextiles beneficial in reducing lateral spreading of embankments on muskegs. settlements are not significantly reuced. My take on geotextile under culverst etc is require to keep the boots of the cionstruction crew fairly clean. However, this is abillion dollar industry today. I tried years ago to place some approach to use and ordering of material re quality assurance and have seen material shipped out by secretaries from well known suppliers. Cloth is Cloth who cares.

Improper installation is a important factor and I agree that lots are installed incorrectly.

Most roads still in service were built without the use of geotextiles. However, toady our first crutch is to recommend geotexttile in all cases when we are constructing roadways, just in case we need to have a clean surface to walk on. Suppliers and manufacturers like this.

I will have to continue later to provide comments on geogrids and where geotextiles may be of some benefit

 

[JML4TCM]

It is good to have this type of thread to get some of these myths and marketing “stories” out into the open. Very interesting comments, and my friend 'theanimal1999' has only reinforced the misconceptions about geosynthetics.

In theanimal1999’s post to begin this thread, he indicates in paragraph 5, “for combination applications (i.e. separation and reinforcement) …you would want to select a high strength woven fabric made of polypropylene”. This is exactly what I stated in my earlier post. Sorry if there was any confusion. So I am glad we agree on this major point. When selecting a geosynthetic for subgrade stabilization you are usually talking about soft, wet soil conditions. In these cases a woven geotextile, not a slit tape woven, but a high strength, high permeability woven will provide you with the most benefits, separation, reinforcement, filtration and drainage. Geogrids are over used in soft subgrades and the benefit is diminished as the geogrid gets pushed into the soft soil where it can no longer provide much benefit.

To correct some inaccurate statements: First, Mirafi 140N is a nonwoven geotextile, not a woven as 'theanimal1999' indicated. Yes, Mirafi 140N does look like felt and is used for drainage and filtration applications. It is a filter fabric, NOT a reinforcement fabric. This is the trouble, many people lump ALL geotextiles as being the same or similar when in fact they are VERY different and PERFORM VERY DIFFERENTLY.

There are several types of geotextiles. First there are 2 main types of manufacturing processes, woven and nonwoven.
Woven is exactly what is says, continuous yarns woven to create a fabric. But you can’t lump all woven fabric together. The type of weave and type of yarn used can create VERY DIFFERENT fabrics. The Mirafi 500X that ‘theanimal1999’ mentions is a slit tape geotextile. Slit tapes are created by taking sheets of polypropylene (PP) and cutting it into flat tape like yarns. Talk about your 1990’s computer, this is it. This material was created by the carpet backing industry and is old technology. Slit tape fabrics have low tensile strengths, low permeability, are very smooth with poor soil interaction and should ONLY be used as a separator fabric in dry conditions or as silt fence. This type of geotextile is the most misused.

As the industry has evolved, several types of woven fabrics have been created with different types of yarns and weaves, higher tensile strengths, higher permeability, rougher surfaces to create a better performing material. The problems is, uneducated people refer back to old research projects that compare a slit tape or nonwoven geotextile compared to a geogrid. Of course the geogrid will perform better. It is comparing a separator geotextile with low modulus to a higher strength geogrid. There is no comparison, but people either on purpose for their own agenda or because they don’t know better, constantly compare these studies and then lump ALL geotextiles as not reinforcement material. But, as ‘theanimal1999’ indicated, and I agree, high strength, woven polypropylene (PP) geotextiles (not slit tape), do perform better than geogrids in soft subgrade reinforcement because they can provide separation, reinforcement, filtration and drainage. And these types of woven PP’s cost the same or less than biaxial geogrids.

The other type of geotextiles are nonwovens. These are manufactured by spunbonding or needle punching fibers to create a felt like geotextile. These materials have very high water flow rates but are low in tensile strength and elongate more than 50%, so should never be used as a reinforcement material. Nonwovens should only be used as filtration and drainage fabrics. Unfortunately, several researchers, in particular the Army Corps, have used nonwovens in subgrade stabilization research and compared performance to a geogrid. This has resulted in statements that ‘theanimal1991’ has referred to, that the Army Corps says not to give any value to fabric in subgrade applications. This should be clarified to indicate that the fabrics they are talking about are nonwovens and slit tape geotextiles. Not high strength, high modulus PP woven geotextiles.

I don’t recall ever stating that the FHWA is an authority on geosynthetics. They have updated several of their design and construction guides related to geosynthetics which is a good thing. I agree they are still a little behind the state of practice of the private market, but you will always have that with a government agency. It is funny that ‘theanimal1999’ thinks the FHWA is not an authority on anything but seems to blindly follow the Army Corps of Eng. The Army Corps, like the FHWA, is still behind on state of practice of the private market. However, while the FHWA has worked with several well known independent industry experts to update their manuals, I am not aware of any new publications (not papers but publications) regarding geosynthetics from the Army Corp. In fact, most of the Army Corps publications use research done in the late 80's and early 90's, there, once again is your 1990's computer. The FHWA has done more contemporary work with geosynthetics, but I will agree with you that they are still behind some current state of practice. One of the most referenced Army Corps reports is the Webster report, dated 1992, once again, there is your 1990’s computer. To think a research paper from almost 20 years ago is state of practice, or uses current materials, is misguided at best, not to mention the problem with the type of geogrids that were compared in this research. It was good in that it showed significant improvement in performance by using a geosynthetic. Many of the subsequent Army Corps publications were developed using this report as the basis.

It is interesting in your picture you show a geogrid with a nonwoven geotextile underneath. This is the benefit of separation. Without that fabric, that geogrid would disappear under the soft subgrade soil. This is why a high strength woven (not a slit tape) should be used instead of the more expensive 2 layer system (geogrid with nonwoven).

I am not sure what ‘theanimal1999’ means that wovens provide no stiffening enhancement and if you read the extensive research provide a maximum thickness reduction of 25% in subgrade applications. Your 6th paragraph seems to contradict your 5th paragraph. I have no idea where you got this 25% reduction from, but it is simply not accurate. High strength woven geotextiles can give traffic benefit ratio’s higher than comparative geogrids. Particularly in soft subgrade conditions. You have highlighted perfectly the misunderstandings that I have been trying to explain.

Stiffness in geosynthetics is not applicable. This is why ASTM has not adopted a test method for geosynthetic “stiffness”. You used the example of a plastic bag compared to plywood on soft soil. Like ‘dgillette’ correctly pointed out this would be a good comparison if there were any geosynthetics that were “as stiff as a board”. The problem is that ALL geosynthetics have properties more like the plastic bag (at least a lot closer to the bag than the board, I can’t bend plywood with my fingers). The “stiffness” of ANY geogrid is so small that there is no benefit contributed to “stiffness” in a roadway. A material that can’t hold the weight of a small cup of water in cantilever is going to give benefit to supporting truck traffic? What a joke. It is good marketing, just not good engineering.

As “Creepisnotcrap’ said, Giroud-Han is not state of practice. Giroud-Han did a paper review for one specific geogrid and came up with some design methodology. This “design methodology” HAS NEVER BEEN CALIBRATED WITH OTHER GEOGRIDS, you can ask J.P. Giroud and Jie Han yourself. The only property that is included in the Giroud-Han method is aperture stability; tensile strength at low strain, tensile modulus, aperture size, coefficient of interaction is not included in the Giroud-Han calculation. Not even stiffness or junction strength is included in the Giroud-Han formula, only aperture stability. What is aperture stability? Where you clamp around the junction of the geogrid and twist. Hmmm? What does that tell you? Aperture stability has not been accepted as a valid test method by ASTM.

Finally, since this post is getting too long, ‘theanimal1999’ said the reason other geogrids are junk is that they are woven. The most recent (July 2009 – FHWA/MT-09-003/8193) full-scale research performed on a wide variety of geogrids was done by Montana State University with the Montana DOT (and some other DOT’s). In that research project, the greogrid that performed the best was a woven PVC coated polyester geogrid. And that research included Tensar and Naue.

 

[CreepisnotCrap]

Short clarification based on AASHTO M288

SEPARATION: CBR >= 3.0
STABILIZATION: 1.0 < CBR > 3.0

M288 does not address any issues with reinforcement (CBR < 1.0)

Note that AASHTO M288 is being in use and continue improvement for the last 20++ years.

 

[Bluefoot]

theanimal1999,
"If you need some references I would be glad to provide them."
Can you please send me some of the Army Corps reference regarding the design guide?
Thanks.

 

[theanimal1999]

JMLTCM has yet to reference any study in which "high stregth" geotextiles have been found to provide any benefit in subgrade stabilization. It is always funny to me when people claim things work or are incorrect, but have no actual research to back up the claims. Please post any actual research that has shown any benefit of fabrics that installs them as they would be in real life.

Also, the Montana study quoted was complete junk. If JMLTCM actually read the study he would have realized absolutely nothing performed well. All the geogrids failed miserably. This was due to the sudy simply placing the stone without compaction and trafficking it. No one would do this with any material, even fabrics, as the resulting rut depths, just like in the study, would be unacceptable.

I am guessing tha JMLTCM works for or is in bed with Mirafi as the claims made are almost verbatum what the Mirafi guys have said during presentations I have attended. However, I have asked and have yet to recieve any proof of improvement in trafficking or subgrade improvement with the high strength fabrics from the Mirafi representatives.

There are however many worldwide studies that show the benefits of various geogrids. I have attached one of the most recent studies out of Europe which pits all grids against one anoth as well as a high strength fabric. It was done in conjunction with Tensar it appears, but TRL is a large well respected lab in Europe. They are similar to the turner-fairbanks lab here in the US.

I have also included the ACOE design reference some have requested.

 

[theanimal1999]

This is the ACOE study. It did not attach to the last post.

 

[theanimal1999]

ACOE Engineering Technical Letter.

 

[JML4TCM]

It is funny that you call the Montana study junk but then post a paper from Tensar. The geotextile material used in the Tensar paper is not a high strength geotextile. Geotextile ultimate strength of the 1 geotextile used was 40 kN/m, that is low to moderate strength material. Also, looking at the properties of that material it looks like it is a slit tape woven fabric. Maybe that is the problem.

TheAnimal1999 does not understand the different types of geotextiles avaialbe and that exactly highlights my point. Slit tape woven fabrics are typically compared to geogrids, usually by geogrid manufactureres, to show better performance of a geogrid over an inferior geotextile. A slit tape geotextile is a separator fabric, it should not be used in soft subgrade reinforcement applications. There are several properties you have to look at when selecting geosynthetics, strength is 1, but also modulus, permeability, water flow, yarn type, etc. Slit tape materials have low strength, low soil interaction due to their smooth surface, and have low permeability so you get the "waterbed" effect on soft saturated soils.

The USACOE document you provided, "Use of Geogrids in Pavements" expired in 2007. That is not such a big deal but it does show how old the document is and with the evolution of products in the geosynthetic industry it should be used carefully. That document states that a nonwoven separator fabric should be used with the geogrid for subgrades with CBR's < 4. So that is all soft subgrades, CBR's > 4 are generally firm to stiff. This 2 layer system, a geogrid and a nonwoven geotextile, was used because that was what was avaialable when this document was developed. Now that you can get separation and reinforcement with one high strength geotextile product (but not a slit tape geotextile), there is no need to use the more expensive 2 layer system.

There is a trend to say that the nonwoven separator is not needed with the geogrids because the geogrid can provide "separation". But that just insults our intelligence. A material with a 1 inch opening can not separate fine grained soil from aggregate. This is highlighted in the Army Corp. paper by Tingle and Jersey from 2005 titled "Cyclic Plate Load Testing of Geosynthetic Reinforced Unbound Aggregate". This study compared geotextiles and geogrid and concluded: "The separation function appears to be the dominant geosynthetic function for fine-grained soft subgrades. Since the reinforced pavement sections demonstrated high TBR's (TBR is traffic benefit ratio) at low levels of pavement deformaiton, the geotextile's improvement is attributed more to separation than to reinforcement via the tensioned membrane effect. The use of a geogrid only provided some reinforcement benefit, but the geogrid's reinforcement potential was minimized due to it's inability to effectively separate the base and subgrade as evidenced by staining of the bottom of the based and observable aggregate punching."

It takes time to re-learn things we think we already know. But geogrids should not be used in soft subgrade reinforcement applications without a separator fabric, so why use 2 materials when you can use 1. It is very simple, 1 layer costs less and is quicker to install than 2.