Bearing Capacity Formulations 2

[GeoGrouting]

According to VAVFAC Ultimate Bearing Capacity is yDNq + 0.4YBNy. For allowable bearing capacity the ultimate number is divided by 3. Hence as the width of foundation (B) increases the allowable bearing pressure increases.

However according to the Terzaghi and Peck charts, for a given SPT, as the width of foundation increases, the allowable bearing pressure DECREASES.

Can somebody explain this discrepancy, please.

 

[BigH]

This has been discussed many times in the various forums on this site. (
1) bearing capacity is based on shear without regard for settlement. The formulation you give places a safety factor of 3 on the ultimate bearing capacity to determine the allowable bearing capacity.
2) Terzaghi's charts (and other similar ones, say, in Bowles and elsewhere) limit the settlement and should be referred to as the allowable bearing pressure (not capacity). The Terzaghi chart uses an allowable 25 mm settlement. Others might use 40 mm (India, I believe). If your project permissible settlement is less than 25 mm, one would have to adjust accordingly.

If one looks at some of the recent state DOT manuals, one will see charts of allowable bearing capacity and then the allowable bearing pressure (at various permissible settlements) superimposed.

 

[GeoGrouting]

I have noticed that DAS has formulated many allowable bearing pressures. Do you agree with those formulations. A university colleague told me that he usually disagrees and I have lost confidence in those formulation. Although I think the formulations come from a logical view points. Thank you.

 

[BigH]

I don't have Das. My suggestion: Why don't you sit down and try to work up a few charts yourself. This is what we had to do years ago before spreadsheets. Pick a "sand" deposit of infinite depth, say use N=20 and determine (1) the allowable bearing capacity for footings of size, 1,3, 5, 10 and 20 ft using a FS of 3 (or 2.5 if you wish). Then determine using correlations of "N" values with modulus values (e.g., Schmertmann) or other means to compute estimated settlements for these various footing sizes; or better, in reverse, pick several allowable settlements, say 15 mm, 25 mm and 40 mm and determine for the foundation width the bearing pressure that would cause the settlement. Plot these up and get your own chart. You will have a curve based on shear and another based on settlement. You will find that shear governs up, perhaps to 1 m or so, then settlement will govern. By doing the exercise yourself - and it won't take long - you will have a better appreciation of the problem you have in front of you .

Similarly, you can do the same for footings on clay. Assume an undrained shear strength of, say, 60 kPa. Assume slight overconsolidation, say 1.3sigma' and do the similar anlaysis of what bearing pressures you will get for a footing of a certain size to settle the various chosen allowable values. With clays, though, remember you have both immediate settlements (which might be important) and consolidation (long term) settlements.