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Nonwoven geotextile reinforcement

Nonwoven geotextile reinforcement

Nonwoven geotextile reinforcement

(OP)
I am working on mitigating housepads that were improperly constructed atop 15-20 feet of fill.  Part of my recommendation was to remove several feet of soil, install a woven geotextile, and properly place and compact suitable fill material.  Some of the "powers that be" at the site insist on removing 5' of material, install a NONwoven fabric, and place and compact fill material.  I don't have much knowledge of strength/reinforcement characteristics of a nonwoven fabric (i typically use woven for such problems), will the nonwoven provide adequate strength?  I should point out that there are many voids in the subsoil that the nonwoven fabric will have to bridge.

thank for the help

RE: Nonwoven geotextile reinforcement

Let me preface by saying that my knowledge of geotextiles is limited.  I typically specifiy non-woven fabrics for drainage applications because I have seen where woven fabric came unravelled.  I have observed this in the field typically where the fabric ends are exposed.  The non-woven fabrics I have seen can meet the same strength specs as the woven, but I believe that they are needle-punched to produce the holes to allow drainage.

RE: Nonwoven geotextile reinforcement

Hmmm,

My first thought has nothing to do with the geotextile.  Who the heck decided to build houses atop 10 to 15 feet of improperly compacted fill?  Or do I misunderstand what's going on?  (Your comment, I should point out that there are many voids in the subsoil that the nonwoven fabric will have to bridge concerns me greatly.)

As to the geotextile: I prefer the spun bonded, non-woven, needle punched stuff.  It is just as strong, but is immeasurably more effective as a filter.  And the fill should be providing full support anyway.  I haven't recommended the woven stuff in more than a decade -



Please see FAQ731-376  by VPL for tips on how to make the best use of Eng-Tips Fora.

RE: Nonwoven geotextile reinforcement

Hello civilmg:

Rather than using woven or non woven you may wish to consider a geogrid as this has been advocated for use in bridging sinkholes etc. Many manufacturers/suppliers have such products. Tensar is one well known product.

Now the above may not really be the answer as I am not entirely sure of the extent of your problem.

Good Luck
  

RE: Nonwoven geotextile reinforcement

Looks like I may be going against the grain here but I would not recommend a non-woven in use as a base reinforcement unless there was also a geogrid used in conjunction with it.  Non-wovens cannot reach the same strengths as high strength polyester woven geotextiles, they can actually exceed the strength of geogrids.  If the primary purpose is filtration or drainage then the non-woven makes sense as long as it also meets the strength properties, otherwise I would stick to wovens for base reinforcement.

One consideration is that many construction companies keep a stock of nonwoven for drainage applications and may want to use that up.

Focht3 is correct that continuous length, spun bonded, needle punched non wovens can reach the strengths of standard slit film polypropylene woven geotextiles but please ensure that it IS a continuous spun bond and not a staple fiber.  The staple fiber gives high Grab tensile strength as the one inch wide clamps tend to actually grab and test the fiber strength and not the fabric strength.

RE: Nonwoven geotextile reinforcement

The nonwovens are fine for temporary projects such as haul roads; their life is limited by high rates of creep (polypropylene creeps worse than polyethylene).  Nonwovens have a much higher strain at a given stress than do wovens (i.e., lower modulus), so I wouldn't suggest using them when you want them to work for a long time or when you have voids to span.

Geogrids are the best bet for situations like this.  Polyester has the best stress-strain properties of any geosynthetic, and most geogrids are classified as uniaxial (one predominant direction of strength), but the only truly uniaxial ones are the HDPE grids made by drawing a punched sheet of polyethylene (such as the Tensar UX series).  Note: the drawing orients the polyethylene chains into a predominately parallel configuration, thereby increasing the density and strength, hence HD polyethylene.

The woven polyester geogrids all have a significant cross-direction strength even if they are classified as uniaxial.  They have different stress-strain properties depending on their construction.  They will all work for such an application in making a pad that will settle as a unit rather than differentially.  They will not change the fact that overall settlement will take place.

The most impressive polyester geogrid I've ever seen is Stratagrid, because it is knitted.  When I built retaining walls, it is the only geogrid I used and I used to work for one of the competing polyester geogrid brands!

They have a biaxial version (same or similar strength in both machine and cross-machine directions).  I would say its a toss-up between a biaxial polyester geogrid like the Stratagrid product and the Tensar BX-1200 or stronger product (and equivalents).

I hope this is informative.

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC

RE: Nonwoven geotextile reinforcement

Hmmm,

I'm going to disagree with the following statement made by Bruce Nothdurft (DBNodurf):

Geogrids are the best bet for situations like this.


No, the best bet is a properly compacted fill, which might entail removal and replacement of the questionable fill.  As I said on October 3, 2003: ...the fill should be providing full support anyway...  The choice of geogrids, geotextiles, etc. as alternate approaches will depend on things the members of this forum can't see or touch.  

Geogrids won't provide any filtering - an important property considering that civilmg has indicated that the remaining fill will contain voids.  Of course, this still ignores the very real risk of damaging differential movements as those voids collapse or fill in over time...

And if the decision is made to leave the uncontrolled fill in place, then it should be done on a lot-by-lot basis with a pad-specific geotechnical study used to select the appropriate design and construction approach.  All those studies too expensive?  Then yank it all out and put the fill in properly!



Please see FAQ731-376  by VPL for tips on how to make the best use of Eng-Tips Fora.

RE: Nonwoven geotextile reinforcement

True, Focht3; I actually meant geogrids were the best bet of the different geosynthetics, but the issue with the fill itself is the bigger picture; I went off on the geosynthetics sidetrack.

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC

RE: Nonwoven geotextile reinforcement

(OP)
I appreciate the input.

For whatever reason (cost, i'm sure) no one was called to verify compaction of the fill during original construction.  Now, for the same reason, the "powers that be" do not want to remove and properly replace the uncontrolled fill (which was my original recommendation).  

I decided against geogrids simply because we needed separation between the sandy fill and the underlying silty clay.  

once again, thanks for the knowledge

civilmg

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