Bearing Capacity Formulations
Bearing Capacity Formulations
(OP)
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.
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.





RE: Bearing Capacity Formulations
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.
RE: Bearing Capacity Formulations
RE: Bearing Capacity Formulations
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.