Single Size Backfill

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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?


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 someone else 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.

I suspect it depends on how the aggregate ‘interlocks’… same sized angular aggregate would be different than compacting ‘marbles’. Likely somewhat correct.


It’s not likely that the aggregate is a “single size”. That probably refers to a singular designation such as #57 stone…which is not single sized but a range of sized particles. I have never seen commercially available aggregate or drainage stone provided in a single particle size.

Washed sand with no filter fabric could eventually clog up if there are clay and silt particles in the neighbourhood.