Allowable bearing capacity | Safe bearing capacity

Allowable bearing capacity

It may be defined as

The net load intensity at which no failure occurs is called allowable bearing capacity.

Calculating the gross allowable load bearing capacity of shallow foundation requires the application of the factor of safety (FS) to the gross ultimate bearing capacity.

q allowable = q ultimate / FS

Net ultimate bearing capacity

It may be defined as

It is defined as the ultimate pressure per unit area of the foundation that can be supported by the soil in excess of the pressure caused by the surrounding soil at the foundation level.

If the difference between the unit weight of the soil and the concrete is negligible, then

q net(u) = q(u) – q

From the above equation it can be seen that net ultimate BC is less than the ultimate bearing capacity.

q net(u) = net ultimate bearing capacity

q = unit weight of soil * Depth of foundation (Df)

So

q allowable (net) = (q(ultimate)-q) / FS

Safe bearing capacity

It may be defined as

The maximum Pressure that a soil bears without shear failure is known as safe bearing capacity.

Conclusion

Maximum safe bearing capacity is always greater than maximum allowable bearing capacity.

Allowable bearing capacity is usually taken as 150kN/m^2 .

Generally these two terms allowable and Safe BC are confused. But by studying about definitions it becomes clear that these are a bit different due to load intensity.

Difference between ultimate and net ultimate BC

Whereas the difference between ultimate and net ultimate BC is that, in ultimate bearing capacity the ultimate pressure per unit area along with the pressure of the surrounding soil is considered. Whereas in net ultimate BC, pressure of the surrounding soil is not considered. Only the load intensity is considered in net ultimate BC.