You can approximate a bearing value based on the N value from your SPT (BS 1377 test 19) if your soils are cohesionless.
If you are in clays you can approximate bearing values from the undrained shear strength tests. The insitu test is the vane shear test (BS 1377 test 18). However, there is evidence to suggest that the test results can be quite different to the real life case of a full scale project, so the results need to be corrected using factors.
Foundation Design and Construction by MJ Tomlinson is a useful reference.
Step one: Understand the geology of the site.
Step two: Assign strength characteristics.
Step three: Evaluate the compressibility of the soils.
Step three: Determine the building loads.
Step four: Evaluate the increased stress distribution from the new loads.
Step five: Determine to what extent the strength will be exceeded by the new loads.
Step six: Look at whether the soils will compress too much for the servicability of the structure.
Step seven: Talk to the structural engineer.
There is really no "test" for the bearing capacity without looking at the whole soil-structure interaction. Somebody that goes to a bearing surface and decrees a bearing pressure from a field test may just blow it. Let's say, you have a hard layer immediately below the footing subgrade (i.e., newly placed fill), but three feet lower there is a former swamp. You'd likely realize a foundation failure of some sort, even though the field test said every thing was good.
There is a reason that geotechnical engineers drill holes into the ground 15 to 50 ft (just a range) below the ground.
to prove bridge falsework pads i have poured a 4' x 6' x 12" thick reinforced concrete slab and loaded it with temporary concrete barrier (krail or jersey rail)
we needed 4000 psf so we loaded
it took (24sf x 4000psf)= 98000lbs/8000lb barrier = 12ea pieces of 20' long barrier. we stacked 2 wide by 6 tall and measured elevation changes every day for a week. did not settle more than an eighth inch.
justbuilt: Did you also have geotechnical engineering exploration data at (near) the location of the bridge pad? I like your test - just wondering whether you had background information to correlate the load test to. . . .
As a field rule of thumb without a geotech, I have heard of contractors using a #4 or #5 bar, applying a small amount of pressure to the rod (about 3 to 5 #), and seeing how far the rod penetrates. If the penetration is in the order of 1/2" to 1", then you have soil bearing somewhere in the 2000 psf range. Very primitive though.
Always assuming that the soil conditions are as good or better than the surface layer, that primitive test might work. But you're playing Russian Roulette by doing so.
