The calculation tool given above for calculating bearing capacity of a shallow foundation follows the procedure given in "Foundation Analysis and Design, 5th Edition (Joseph E. Bowles) - Chapter 4. The details of the Terzaghi (1943) method for calculating bearing capacity of shallow foundations is given below.
Ultimate Bearing Capacity
The bearing capacity is calculated according to the following equations for Terzaghi (1943) method.
$$q_{ult} = c N_{c} s_{c} + \overline{q}N_{q} + 0.5 \gamma_{e} B N_{\gamma} s_{\gamma} $$
Bearing Capacity Factors
The bearing capacity factors Nc, Nq and Nγ; are calculated as follows:
$$N_{q} = {{a^2} \over {acos^2(45+\phi/2)}},\qquad a=e^{(0.75 \pi - \phi/2)tan\phi}$$
$$N_{c} = (N_{q}-1)cot\phi$$
$$N_{\gamma} = {{tan\phi} \over 2} \left({{K_{py}} \over cos^2\phi}-1\right)$$
Shape Factors
Shape factors sc and sγ are selected according to shape of the foundation. For strip foundations sc = 1 and sγ = 1, for round foundations sc = 1.3 and sγ = 0.6 and for square foundations sc = 1.3 and sγ = 0.8.
Effect of Water table, γe Term
The average effective unit weight of the soil (γe) considering the depth to water table below base of footing (dw) and the depth of wedge zone (H) is calculated as follows;
$$ \gamma_e = (2H-d_w){d_w \over H^2}\gamma_{wet} + {\gamma' \over H^2}(H-d_w)^2 $$
Where,
$$ H = 0.5 B tan(45+\phi/2)$$
Allowable Bearing Capacity
$$ q_{all} = {q_{ult} \over FS} $$