Terzaghi developed his bearing capacity equation for strip footings by analyzing the forces acting on the wedge abc in Fig. 12.6. The equation for the ultimate bearing capacity qu is
ult = ultimate load per unit length of footing, c = unit cohesion, γ the effective unit weight soil, B = width of footing, D,= depth of foundation, Nc, Nq and Nγ are the bearing capacity factor
They are functions of the angle of friction, Ø.
The bearing capacity factors are expressed by the following equations
where Kpy = passive earth pressure coefficient
Table 12.1 gives the values of Nc, Nq and Nγ for various values of Ø and Fig. 12.7 gives the same in a graphical form.
Table 12.1 Bearing capacity factors of Terzaghi
Figure 12.7 Terzaghi's bearing capacity factors for general shear failure