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Single Size Backfill

Single Size Backfill

(OP)
Can you help me please,

I keep being told that 'single size stone backfill is self-compacting'.  Do they mean 'single size backfill requires no compaction', is there a subtle difference between the two phrases?

What I specifically need to know is that if the surround material is "clean, single size angular granular material from within the range 20mm to 70mm", what settlement is there likely to be, I need to traffic over it.

If the installation (perforated pipe, wrapped in geotextile) is within the single size material, within a prepared trench, does the backfill 'compact' in a way that minimizes settlement?

Thank you.

 

RE: Single Size Backfill

(OP)
Having now dived around the forum a little I should also add that the excavation is wrapped with a clay liner, can this containment reduce settlement?

RE: Single Size Backfill

No backfill is "self compacting".  However, a clean (no fines) rock like you describe generally requires very little compaction and is not likely to settle very much if it is not compacted at all.

If I understand the situation this stone is being placed around a perforated pipe in a trench and then this trench will have traffic loads on it.  Depending on the type of pavement, if there is any, and the type of traffic; the trench could settle and require some maintenance quicker if there is no compactive effort.

It is always better to compact, but in this case the risk appears to be fairly low if you do not compact.  

Mike Lambert

RE: Single Size Backfill

this has been discussed many times, suggest searching the forums, key words are "french drain" and "self compacting".

the fine grained material can migrate into the voids of the rock and that will both clog the drain rock and cause additional settlement. It should not be considered "containment"

RE: Single Size Backfill

The generally accepted answers to french drain questions around here, if I recall, fall in two categories:

1)  Use a washed uniformly graded aggregate and wrap the whole thing in permeable geotextile filter fabric, or

2)  Use clean, washed sand, and no filter fabric.

Professionally I've always seen/used option 1, but a guy on engtips made a very compelling argument for option 2 last time around, and I've been considering specifying that for my next job requiring a french drain, to see how it works.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Single Size Backfill

Unless the particles are all perfect spheres, they are not "self-compacting."  How much a "single size" material will compact or settle in a given application is a function of particle shape and texture and it could be a lot or nearly nothing.

An estimate can be made using AASHTO T19 (ASTM C29) by comparing the volume of voids in both loose and rodded samples of the same material.  Consolidation under compactive effort can be estimated to be at least as much (and probably more) than occurs by rodding.

RE: Single Size Backfill

(OP)
Thank you all.

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