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]

I did not read the other thread, but what you have stated is pretty much in line with my experience.

The one point where my experience differs a bit is with _very_ soft soils. There, I have found that the placement of fill tends to give a fair stretch to the fabric, provided it has been placed well. This, along with the separation, and a suitable fill above can make a good working platform for the placement of additional fill. As noted, "the placement of additional fill". I did not mean to imply this was a good solution right below a pavement.

Yes, I know this is not really following the current, traditional thinking, and that there can be some other slope stability issues, etc., depending on the situation.

I agree fabric is often used where grid should be, but sometimes good separation with marginal load distribution, and a good interlocking initial layer(have used really cheap, 2-3 inch crushed, recycled concrete) works wonders.

 

[theanimal1999]

Fabric and enough fill in any situation will work, you just have to be wary about the long term issues. If it's not a critical application ubder dynamic loading (i.e., under a pavement) and fill is cheap fabric can be a cost effective solution. Grid is generally best when fill prices are very high because then the cost of grid is offset by the cost of stone. It is also best in dynamic situations because it operates at 1% strain or less when designed properly. I also love crushed concrete, it in and of itself can work wonders.

 

[ACtrafficengr]

For textiles, could you please touch on bursting strength vs tensile strength? I've been told that although woven textiles have greater tensile strength, tears are more likely to propogate once a sharp piece of aggregate hhs punched through them.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[cvg]

which USACE document has good references on geosynthetics?

 

[theanimal1999]

Burst Strength refers to the ability of a geotextile to resist aggregate punching through the material (i.e., ripping). While burst strength increases with tensile strength, due to the addition of more polymer, you will find that the resistance is more closely associated with the type of polymer (i.e., certain polymers are less easily damaged). As for ripping between wovens and non-wovens, the non-wovens rip more easily (i.e., under less load); however, they have a lot of stretch to them and may not propigate a tear easily. Also, due to their nature it is easy to have an overstress at one isolated location (e.g., The contractor drops the fill onto one location and causes a rip or may be dozer pushes to thin a lift over the non-woven and catches a stone.). On the otherhand wovens generally have a high resistant to tearing and generally only do so when their strength has been exceed. As such, tears are likely to propigate because either the design or installation methods have resulted in a overstress of the material. While this can occur at an isolated location it is more likely that there is a global (project wide)issue. The long and short is that if a woven rips something is wrong with the design or installation. The design may need to be adjusted (i.e., stonger fabric or different fill should be selected) or construction methods need to be modified (i.e., thicker lifts, different equipment, etc.).

 

[dgillette]

"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."

Are you implying that the BENDING stiffness of the grid itself is providing a significant benefit (like the plywood)? Or are you referring to the composite action of the fill with the grid acting in tension (because the grid has greater tensile stiffness than the textile does)? Please clarify that. Your plywood analogy appears to say the former, but that doesn't seem too likely for a material that I can bend with my fingers.

 

[theanimal1999]

Good geogrids, as previously noted, supply significant load spread on their own due to their innate stiffness. Literally, it is the reason you can roll a good geogrid out on a swamp and walk on it. I have attached a photo of a Job I worked on in Savannah building roads over paper plant sludge. The picture is of me standing on a grid with a 4 oz non-woven separator fabric. There is 4 ft of paper mill sludge at this spot. If you were to step off the grid you would sink waste deep (CBR of 0.4). So the grid itself is stiff enough to spread my load far enough so that my 160lbs did not sink. However, the grid allows for more thorough compaction of a layer making it stiffer.

If you were to run a DCP on a grid reinforced section versus one without you would see the DCP resistance hold constant throughout the grid reinforced layer and taper off significantly at the midpoint of the unreinforced, if not sooner. Because of the confining property of the grid the soil layer is compacted more fully and evenly making it stiffer than a similar layer without grid. This composite section of grid and soil spreads the load even further than the grid itself.

 

[theanimal1999]

CVG I will get you a list and a link to the Army Corps website for your reference.

 

[dgillette]

From the picture, it looks like the grid is in tension around your foot, which I believe is the hammock effect referred to above. I believe what you are seeing there is the tensile stiffness (greater than that of the textile below it), rather than the bending stiffness.

 

[theanimal1999]

The hammock effect only works when the edges are pinned between layers of soil. this is not the case here. I attached another picture to show what it looked like before I walked out. If tension had anything to do with it, you would be able to step on a fabric and not sink because of the hammock effect. Many fabrics have 10 times the tensile strength of this grid. In fact, if you were to place a fabric, if you could, and step on it you may end up dead as it would be like stepping on a pool cover. You would get wrapped in the fabric and drown, same as if you stepped on a pool cover. Since a fabric with 10 times the strength clearly wouldn't work you have no hammock effect. The result is in fact solely from the bending stiffness. Take a look at Tencate's (Mirafi) website. You will notice that HP570, a very high strength woven, has a higher tensile stiffness than most if not all geogrids. You would still sink and drown due to its low bending stiffness.

 

[TDAA]

I have seen guys walk on fabrics with similar deflections to what you show in that picture. This was on boiling sands that had been pushed out in front of the fill.

A guy was sent out by the contractor to move adjust some part of it. As soon as they guy stepped off (accidentally), he was in up to his waist, holding the fabric to keep his head up.

So, yes, while you do get the stiffness of the grid to help once the fill locks into it, what you are showing is mostly the hammock effect from the fabric, and the separation properties of the fabric to provide you with short term buoyancy.

 

[theanimal1999]

I would be interested to see pictures of that as I have never seen any fabric that most people would specify perform in such a manner. There are some extraordinarily high stregth fabrics that may have the same effect, but they are very expensive and not commonly used. I could see one of them doing the same thing as the grid because they are so stiff.

The bouyancy thing does not happen. The flow rate of water through that woven is incredible. once it got wet, it would sink on its own. The only reason it did not sink was we fastened it to the grid.

 

[TDAA]

I do not have pics of that as it was previous employment. It was a middle of the road fabric - woven. At the time and place, we would have spaced an Amoco something or other, or equiv.

The bouyancy thing does not happen. The flow rate of water through that woven is incredible. once it got wet, it would sink on its own.

So you are saying that they could have dewatered the material you were on by placing the fabric on it and letting it settle? Leaving a lower moisture sludge at the bottom and the water on top of the fabric? I do not see it.


The fact is, fabrics and grids behave differently. Not only from each other, but to different loadings, and differently over time. Under long term design, we expect that the guidelines you outlined are the way they behave, but under short-term construction situations, you can get them to do much more.

 

[theanimal1999]

The fabric would float in the middle. It would not have enough weight to dewater. Though it would not stay on the surface. It would end up as a blob in the middle. Niether at the surface nor the bottom.

 

[CreepisnotCrap]

Well...
I will try keep the product/manufacturing bashing to the minimum, I do not like the idea of pointing fingers to any of the geosynthetic manufacturers that are good or bad... I keep that as an opinion with no bear on the technical aspects of reality.

1.- Separation and stabilization is NOT reinforcement. You can not compare this two initial uses of fabrics as part of the reinforcement. For ones reinforcement is typically for CBR < 1 and would require special installation techniques other than just roll the fabric or grid on place.

2.- While I accept that stiffness helps a lot on the initial process of installation, correct installation process are rarely used when REAL reinforcement is necessary. Techniques such as pre/post tension trenches, side ballasting have being successfully used in extremely soft soils (again CBR < 1) so the stiffness is very good marketing exercise to exploit the ignorance of the engineering community in correct installation of geosynthetics.

3.- Soil confinement in grids will ONLY works with an extremely limited range of coarse aggregate that corresponds directly with the opening of the grid. You can read all the reports from the Vicksburg Waterways station (USACOE) on the track testing and find that aggregate use outside of the range of gradation of the geogrid in question will not support effectively the load intended. More over if you now move outside of the range of coarse aggregate you can find that sands and sand/soil will not benefit from the confinement at all BUT work in the same way than other geosynthetics... by pure friction.

Using the two analogies used here... try to perform the same experiment of the pool balls but now with racking the balls with a thin frame of about 1/3 or less the diameter of the pool balls...

Now... have you realize that currently 99.9% of the modern snow shoes are made from FABRICS... and the so call stiffness came from the FRAME where the fabric is stretched.

In this matter please check with the literature of the grid manufacturers... this can set the record straight as well.

4.- Geosynthetics used for reinforcement WILL require deformation in order to work this is regardless if they are grids, textiles, strips, etc...

5.- The statement "...The state of the practice is the Giroud-Han design methodology..." is inaccurate... since there are at least 2 more methodologies in use for calculating the benefits of a geosynthetic used in enhancement (NOTE THE SELECTED WORD) of road ways. The actual current state of the practice should be reference as: Recommended Practice for Geosynthetic Reinforcement of the Aggregate Base Course of Flexible Pavement Structures, AASHTO, PP 46-01, 2003 not Giroud-Han

For the record... you may not be aware but one of the many methodologies used for fabrics but probably the most complete is the work from Christopher & Holtz, 1991 based on the work of Steward (1977). By the way... this last reference is also the bases of most of the papers for use of grids in roads including Giroud-Han.

By the way... and now for closing...

Mullen Burst was a textile method used to simulate the elbow penetration in shirts. Mullen Burst have being removed from use with geotextiles for over 10 years now... in fact is not longer part of ASTM D35 - 01 (mechanical properties) AND Burst and Tensile strength does NOT correlates at all... it only correlates with the mass and the textile process used for weaving.


Regards...

 

[theanimal1999]

I would agree with a lot of what you said. However, PP46-01 is not a design methodology. It is general guidance. Based on PP46-01 you could claim nearly any benefit of a geosynthetic you wanted so long as you have 1 test to back it up. That why you get ridiculous claims from geosynthetic manufacturers of Traffic Benefit Ratio's (Reinforced ESALs/Unreinforced ESALs). Giroud-Han is for subgrades not pavements and is the state of the practice. Also, the 2008 FHWA geosynthetic guide supercedes PP46-01, which to my knowledge was not formally accepted and may now be defunct. PP46-01 was a provisional standard and I believe last year failed to be voted into a full standard, meaning it no longer exists.

As for installation, I CANNOT AGREE WITH YOU MORE!!! 99.9% of all geosynthetics are installed improperly. Some with more severe issues than others. I point to the fact that wovens used for subgrade/reinforcement applications are to be sewn together. When was the last time you saw that happen, if ever?

 

[Geostructsparks]

Much of this discussion reports on the somewhat over-rated differences between geogrid and geotextile. While there are some geotextiles that are only for separation or filtration (light ones or non-woven fabrics with considerable elongation), my experience has been that there is a geotextile comparable to almost any geogrid (and vice versa) for almost any strength application. That is because many companies manufacture geotextiles that can provide nearly-equivalent stiffness and strength, and in some cases, higher, for an equivalent or lower cost.

The analogy of stepping on geogrid is a red herring in terms of "real" applications. I have worked on oil-soaked sludge (100 - 300 psf strength) on a path made of a half-roll of a closely-spaced biaxial geogrid, and there is no question that that is a really good use of geogrid, and appears it will work far better than (any?) geotextile. I would not use any other. It is possible that geogrid MAY work better on a similar application, strength under tire loads. (The emphasis is that the additional stiffness of the grid is good, perhaps critical to small load support, e.g. foot and tire loads). (My apologies, the H&S guy at that refinery certainly thought it was a really useful application, and it was) For "real" applications, (reinforced earth for embankments or walls, reinforcing under footings, etc.) which may extend load over 5 to 20 feet length, I don't think the stiffness of some geogrids provides a benefit that is significantly greater than geotextile.

The greater longitudinal stiffness of the geogrid material helps overcome the fact that the geogrid is perhaps 1/10 of the aperture area, i.e. there are huge holes in it. This stiffness in or out of plane may help the so-called gravel interlocking. However, since the geotextile consists of many more fibers providing "continuous" coverage (and provides intimate contact with each gravel piece, not just at the ribs), they will have comparable stress transfer, strength, and stiffness when properly selected.

That said, there are applications that I use geogrids (specific attachment methods to blocks needed, less blow-over in wind, where specified by manufacturer, and applications where I use geotextiles (where need to be seamed, where need filtration in same layer as the reinforcement, where flexibility in placement is more useful), and some where I instinctually/habitually pick one over the other for no reason.

 

[theanimal1999]

Walls and slopes are completely different than subgrade applications. In walls and slope, long term design strength is just about all that matters. So anything with a high LTDS and low creep work well. However, in wall applications fabris is a bad choice for wall heights over 15 ft because the upper blocks actually have pressure on them. You will get a pinch point at the back of the block which wil cause fabric to rip. I can show you 100's of examples where some engineer or contractor thought that uniaxial grids and fabrics where interchangable.

 

[darthsoilsguy2]

Should have labeled this thread "The geogrid manifesto"


2 things that i never understood or never received a good answer to...

1) why do we care about separation anyway? the only way i can see caring about separation is if you try to stabilize a subgrade and fail... then you have to take up your old work to redo (maybe with a deeper undercut next time) and find your stone has mixed up with the soil. seems to me that without pumping, there should be no need to separate (stability = separation). i don't get how stuff that doesn't move can mix.

2) it seems to me that one layer of geogrid works better than multiple layers for subgrade stabilization for parking. i understand there are studies out there where the super-strong geogrids are put in layers to greatly increase the bearing area of foundations.... However, the loading condition is a much different ballgame. I see the geogrid as providing a catalyst stability to set the stone into. after that you build up well-compacted stone to give you a strong cross-section. I feel like a 2nd midlevel geogrid just disrupts the way the stone ties in together. Also, i wonder if a 2nd layer hurts the system by removing the ability to further tension that lower grid to maximize the strength of the material. I've unfortunately done a little experimenting with this on my own. took out a multilayer system that pumped and replaced with one grid and stone. We couldn't increase the depth due to underlying utilities... sure we could have done a better job on installation the 2nd time around... but it has made me think about it for years.

-dsg