Geogrids - Sub-base improvement

[lemoncurd1702]

Does anyone have experience of using geogrids (mesh type not cells) as an aid to sub-base improvement?
Any thoughts on this appreciated. I am considering using these in order to reduce excavation depth and to improve overall construction.
I have read some literature on manufacturers websites but none seem to provide information on the use of geogrids in domestic applications.

[Tony McC]

I've seen them used on civils jobs but not on residential projects, and I suspect that is due to depth and cost. Most web-cells are 150mm depth whereas many residential sub-bases will be only 100m, and then there is no potential to save money as the minimum depth of granular material remains at 100mm, but with the added cost of a web-cell textile.

If there was, say, a 50mm web-cell product, I'm not sure it would be capable of providing the same load-bearing potential as 100mm of unbound sub-base, as the web provides only lateral restraint (side-to-side) rather than vertical. On deeper sub-bases, the lateral restraint is beneficial over larger areas because of its effect on the tendency of thicker sub-bases to spread or slump, but when you've only 100-150mm and potential restraint via kerbs or edgings at relatively short separation distances, all combined with what are relatively minimal loads, the case for using web-cell systems is hard to make.

[lemoncurd1702]

Cheers Tony.
As I said in my post I'm looking at the mesh type see link( there are other manufacturers such as Tensar) and not the cell type that is used for decorative gravel.
Not sure of the depth of these but they look to be a just few mm.

These seem to act as a reinforcement much the same as rebar in concrete.

The soils in my area of operation are predominantly clay and occasionally I come across some jobs where I could excavate a mile before coming to a suitable sub-grade.
The economics of this are a nightmare when one works on a fixed price quotation.

[lutonlagerlout]

We have used Geo grid mesh twice where there was no sub grade I.e. made up ground 900 deep and undetermined on the other
We still dug out 250 and put 2 80mm layers of stone on the geo grid
Afaik they are both ok
It cost £200 for a 4M by25M roll IIRC
Pain to ideas it likes to roll back up
Also used multitrack fabric under the geogrid
Cheers LLL

[lemoncurd1702]

Thanks LLL.
Were you installing block paving and also have you revisited these installations to see how they have fared over time.

Hope you don't mind the questions.

Cheers
Mark

[Tony McC]

Sorry - when you said "mesh type not cells" I thought you were trying to differentiate between web-cells and rigid plastic cell pavers.

For reinforcement meshes (geo-grids), the argument is pretty similar. They work best when you have a multi-layered sub-base (so usually >250mm) where they are placed between the compaction layers, but I have seen, and actually used them, between sub-grade and sub-base when we know the sub-grade is unreliable. But again, for a residential driveway type project, you can't really cut down the sub-base depth to below 100mm whether you have a geo-grid or not, so it's hard to see how it can save money on a typical project.

[lemoncurd1702]

Thanks Tony.
To clarify though do you think on unreliable sub-grade that a layer of terram then geogrid and 100mm of sub-base or is it a case of, may as well excavate further and more sub-base.

[lutonlagerlout]

Both are still fine 3 years on and 15 months on
I re sanded both after 6 months
Maybe cos I was paranoid
Hope this helps
LLL

[lemoncurd1702]

Thanks LLL.
All helps, the more info the better.

Cheers
Mark