The allowable bearing capacity (qall) is the maximum bearing stress that can be applied to the foundation such that it is safe against instability due to shear failure. The allowable bearing capacity is normally calculated from the ultimate bearing capacity using a factor of safety (Fs) where:
When excavating
for a foundation, the stress at founding level is relieved by the removal of the
weight of soil. The net bearing pressure (qn)
is the increase in stress on the soil and is equal to the
ultimate bearing capacity minus the stress release at the soil level:
where Df is the founding depth and g is the unit weight of soil removed removed. Applying a factor of safety for the net ultimate bearing capacity instead of applying it on the ultimate bearing capacity is preferred by certain engineers, and is equal to:
