geotextile provides separation between the gravel and the soft ground.  Geogrid provides strength

If you're placing a stone layer (for aggregate interlock witn the geogrid) then the grid would be better.  If you are considering a finer-grained soil, you may be better suited to use the woven geotextile.

You can get either with varying tensile strength, though the grid may be available with greater strength.  Also, the woven geotextile will have biaxial strength.  Grids are often uniaxial or biaxial.

f-d

¡papá gordo ain't no madre flaca!

Woven geotextiles usually have an order of magnitude or so less strength than geogrids (per unit width) but have small aperatures, and are thereofre more suitable for separation applications.  If you are going over peat, you may need to use both, a geotextile to separate the road base from the peat, and a geogrid layer within the road base to provide some additional strength/stiffness.   

Have a look at GMA Handbook for Geosynthetics.  It may be helpful.

• http://www.gmanow.com/pdf/GMAHandbook_v002.pdf

Biaxial geogrids work wonders for soft soil / peat situations.  I have used the Tensar BX1100 and BX1200 with some success.  If memory serves, there is a design circular on the Tensar website that may be of some use.

The lack of knowledge about geosynthetics is astonishing.  The strongest common geosynthetics are geotextiles (GT).  Geogrids (GG) fall off the chart pretty quickly.  The FHWA and most knowledgeable people use the term "Geosynthetic Reinforcement".  For slopes and walls, it doesn't matter what you use, the design strength is the design strength.

For your particular application, I would suggest a woven geotextile.  The separation benefits and cost outweigh any other considerations.  For an application over peat, a GG does not offer any advantage over a woven GT.

The only time that I know of where a GG is better than a GT is when you put it in the middle of a thin (8" to 10" or less) granular layer over a firm foundation.

FYI, "biaxial" is a manufacturing term meaning that the grid is oriented in 2 directions.  It does not mean that the strength in each direction is the same.

Be careful when looking at numbers.  The GT people tend to talk in lbs/in and the GG people like to use lbs/ft.  That may be where the one respondent made the "order of magnitude" error.  That statement is totally wrong.

The strongest GT that I have used is 1,000 kN/m (68,500 lbs.ft).  That is about 5 times stronger than UX1800 HS (210 kN/m or 14,390 lbs/ft)

OldGeoGuy,

Thanks for setting the record straight.  I too was ASTONISHED at the lack of knowledge of geosynthetics.  Of course most of this comes from the slick marketing of some of the geosynthetic companies.  One in particular that only makes geogrids will tell you that only geogrids have strength and can provide reinformcement, geotextile can't.  This is just salesmen being salesmen.

It is important to note that not all GT's are created the same, there are nonwoven GT's, woven slit tape GT's and woven high strength GT's.  These all have very different properties.  Nonwovens have high flow rates and filtration properties; they are used for drainage and don't have very high tensile strengths, they also elongate significantly in tension.  Woven slit tape are separator fabrics and also don't have a lot of strength, used for separation only.  Woven monofiliment and multifiliment fabrics have very high tensile strength, good soil interaction and good permeability.  These are used for reinforcement, both in base reinforcement applications and in walls and slopes.

The strongest uniaxial geogrid on the market that I know about has an ultimate tensile strength of around 25,000 lbs/ft.  This is a polyester geogrid.  As you noted, Tensar's UX1800 has an ultimate tensile strength of 14,390 lb/ft, this is a HDPE geogrid.  The strongest geotextile that I am aware of has an ultimate tensile strength of 1,600 kN/m (approx. 110,000 lb/ft).  It is a mater of material.  Geotextile and geogrid are made from the same basic polymers, and you have a lot more material in a geotextile than you do in a geogrid, since there are no holes in a GT.  This is also true in the biaxial market, you have GT's with higher tensile strengths than biaxial geogrids.  

The other thing to look at when comparing a GT to a GG is interaction with the surrounding soil.  Generally speaking these values are similar for both products.  With geogrids you will hear people talk about interlock with the surrounding aggregate.  This is a good story, but if you have a material with a fines, like a crushed aggregate base, or soil, then there isn't really any interlock.  The geotextiles interact with the surrounding soil through friction, and sice GT's don't have open areas like a geogrid, they have very good interaction.  These values come from actual pull-out testing with various soil types.

Roxndrt, If you think geogids work great on soft peat and soils, you should try high strength multifilament geotextiles.  They are made from the same polymer as the BX geogrids you mentioned, but have higher tensile strengths, similar interaction characteristics and provide separation, filtration, drainage and reinforcement.  

There are 4 basic functions a geosynthetic can provide, separation, filtration, drainage and reinforcement.  A geotexile can provide all of these functions, a geogrid can provide only 1 - reinforcement.

Go to Geosynthetics Materials Assoc. (GMA), www.gmanow.com for more information.
  

in subgrade stabilizations i've gone with geogrid everytime. i never really gave much weight to the separation item.  seems to me that if the stabilized stone doesn't pump, where's the contamination??

i always throw out a little stone before laying the grid (1"-2") to ensure aggregate interlock and remove sliding failure from the picture. (sliding failure scenario: imagine stabilized clayey subgrade + regular irrigation at adjacent planting beds that keeps the stone saturated + stop signs + truck traffic)

i agree that the GG fabricators are blowing the GT fabricators out of the water as far as marketing goes right now. or at least that is my perception in the Carolina region market of USA. Where is Mirafi and why haven't they fed me lunch yet?

  

 

If you are working on a soft subgrade, you really are "throwing out" the 1"-2" of stone.  The subgrade will mix with the stone above over time.  Remember, for every action there is an equal and opposite reaction.  The reaction to a live load is the pumping of fines into the stone above.  Over time, stone on stone contact becomes stone on clay.  That is where separation comes into play.

As for "interlock", it is measured by a pullout test.  The resulting coefficient of interaction will be equal or better for a GT except in cases of a very low confining pressure (4" -5").  Beyond that, the friction offered by the much greater surface area of the GT overcomes the "interlock" of the GG.  That's math, not opinion.

If Darthsoilsguy2 gets a free lunch out of this, I would ask him to send them my way, I'm always hungry.

JML and Oldguy are you implying that geotextile can be substituted for geogrid under any condition?  What about the elongation under these loads?  Are they comparable to the geogrid also?   

Hi Riggly,

Yes, you can.  Except for the situation that I noted above, a geotextile will work as well or better than a geogrid.  That's why the proper terminology is "Geosynthetic" Reinforcement.

The numbers are the numbers.  If you are designing an embankment over a soft subgrade, the standard design procedure is based on a required strength at a specified strain, many times 5%.

Except with extremely low confining loads, the coefficient of interaction (pullout, interlock, friction) will be equal or better with a GT.  Even then, over a weak subgrade, the separation offered by a GT will overcome the "interlock" of a geogrid over time.

In walls and slopes, the long-term design strength of the geosynthetic reinforcement is determined at a specified strain, usually 10%.  Again, the numbers are the numbers.

When you get past the marketing, it's all in the numbers, including the final one of cost.  When you design with geosynthetic reinforcement, then you don't care whether a fabric or a grid is used.  You're designing for a result, not a product.

Riggly,

Sorry for the slow reply, I do not check this board very often.

Yes, a geotextile can be used, and often should be used instead of a geogrid.  By definition, when you are looking a subgrade stabilization, the subgrade soil is soft, with a CBR<3.  In these instances you want to use all of the functions a geosynthetic can provide; separation, filtration, drainage and reinforcement.  There are woven geotextiles (not slit tape) that can provide these functions as one product.  High strength (this is a relative term) woven PP fabrics have similar or better elogation than geogrids also made of PP.  Nowoven fabrics elongate significantly and are not typically used for subgrade stabilzation applications.

Most geosynthetic references will recommend using a separator fabric in conjunction with a geogrid in CBR<3 soils.  Instead of using 2 products you can get the same result, or better results with the right geotextile. It is also more economical.