How would you relate % compaction with bearing capacity for foundation design?

[Tstruct]

I have to design a culvert resting on backfill. As an structure engineer, I only know relating soil subgrade modulus to foundation design. But incase of backfill there is no subgrade modulus present (to my knowledge at least), then how would I proceed with design? Thanks

 

[TORCHMAN]

Typically I call a geotech company to provide the bearing capacity of the soil subgrade. Even when you backfill and compact, creating a base, your load still goes down to your subgrade which is the weak part of the design.

If you want to calculate your own bearing capacity, you can use SPT tests/assume an angle of friction for compacted gran. muni budhu soil - mechanics and foundations is a good source