What fill material do you specify behind your retaining walls?

[Greenalleycat]

A topic of discussion in the office presently as I have just been getting some RFIs from a contractor on retaining wall backfill.
We have been specifying 'free-draining compacted backfill' and leaving it vaguely there, which opens it up to interpretation
I have now been presented with three mixes for consideration

CA40-20
CA20-14
CA20-8

Basically, rounded riverstones crushed to have at least 2 angular faces (CA) and 100% passing 40mm 0% passing 20mm (for 40-20, etc)
In my head, all of these will be very free-draining due to lack of fines - even the smallest stones at 8mm are large so big voids
The angularity is important as rounded stones do not compact well, but they're all equivalent for that
So it just comes down to the max-min grading, and I feel that 40-20 is quite large and may not compact that well, so a 20-14 mix is a reasonable economy?
But this is all based on some subjective thoughts with less factual reference than I like to have when making decisions on 150 tons of backfill

So, anyone got their own thoughts or experiences to share?

 

[milkshakelake]

Here's my state's guide on this. I don't know the answer, but you can find it in here. It lists different types of fills, when they're suitable, and the mixes.

https://www.dot.ny.gov/portal/pls/portal/mexis_app.pa_ei_eb_admin_app.show_pdf?id=12742

 

[BridgeSmith]

A lot depends on how conservative the design was, specifically in regard to the assumed internal friction angle (phi) of the retained backfill. If a high angle (>30 degrees) was used in the design, I'd want documentation that the material they're proposing to use meets or exceeds what was assumed in design.

 

[slickdeals]

Why don't you ask the Geotech to specify it? They provide the values for the earth pressure and IMO they should be the ones specifying the backfill also.

 

[Greenalleycat]

@slickdeals, it's a bit of a grey area here
The majority of the retaining walls in my city are on the hills and standard guidance is provided from the central government as to the design process and parameters, subject to geotech confirmation that the expected materials (loess materials) are found
So the geotechs have little involvement (and frankly I have very little respect for them anyway in the majority of situations)
The backfill usually ends up falling back onto us as structural engineers, which is fine, but our spec is normally 'performance based' aka a bit vague due to the wide variety of suppliers and aggregate mixes around