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Retaining Wall Surcharge Load

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ASTM C857 Minimum Structural Design Loading for Underground Precast Concrete
Utility Structures recommends that you use a surcharge pressure of not less
than 0.5 percent of the wheel load, and says the surcharge can be neglected
when you are more than 8 feet below the surface. For an H20 wheel load of 16
kips, this works out to 80 psf. 
It's pretty common to use the equivalent lateral load from 2 feet of soil
surcharge for traffic loading (AASHTO guideline for abutment walls and H20
loads). So if you had a unit weight of say 120 pcf, and active or at-rest
coefficients of .33 or .50 respectively (phi = 30 degrees), the pressure for
the active case would be .33 * 2 ft*120 pcf = 79.20 psf and the at-rest case
would be 120 psf. I think the ASTM C857 value is based on active soil
Either way, I wouldn't carry the traffic load deeper than 8 feet.

This is a repost. Original said it bounced because I copied the original
message, which was too long. Probably due to image file in original post to
which I was responding.

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