Equivalent fluid pressure

[Gregory32]

I will soon be constructing a precast foundation system. The precaster's caveat is "Backfill must not exceed 60 pounds per cubic foot equivalent fluid pressure.."

I had intended on backfilling some areas with 3/4" clean stone.

Can someone help me along in how I figure out if this is acceptable??

 

[DRC1]

Equivelent fluid pressure is a fancy name for a triangular pressure distribution. Pressure is measured in pounds per squae foot, so I assume the limiting pressure is 60 pounds per square foot. 3/4 inch stone has a unit wieght of approximately 115 pounds per cubic foot. The "at rest" or Ko pressure can be conservatively taken as 0.5. This is the portion of vertical load that is transfered to lateral load. Thus the lateral pressure of the stone is 115 x 0.5 or 57 psf. You can check the unit weight of the stone from a local quarry.
This does not consider the effects of a surcharge or high water table. This would cause the stress to exceed the required values.

 

[strucsteel]

If you have an IBC 2003 handy, you can look at Table 1610.1 (similar tables can be found in just about any building code, either in the structural design or possibly in the soils and foundations section). Basically, gravels and sands can be assumed to be about 60 pcf for an at-rest condition, although the number quoted by the IBC will have to be adjusted for surcharges and/or a high water table, as DRC1 noted. Unless you have something like that, you should be fine with the 3/4" stone.

 

[RCEJD]

You must also look at what the other soils that may occur within the active wedge of the retaining wall so as not violate Coulomb's and Rankin's principles.

You do not mention whether there is an additional loading from a sloping backfill.

Simply consult your geotechnical engineer .. this is a simple question to answer, and he should be abel to provide you with a more educated answer.

Good luck