The Stress Computation Method determines how the 3-dimensional stress distribution due to loading is calculated.
The Stress Computation Method is specified under the General tab in the Project Settings dialog. Four options are available:
Multiple Layer
Boussinesq
2:1
Westergaard
Multiple Layer Method
The Multiple Layer stress computation method takes into account the stiffness (elastic modulus) of each soil layer, when computing the stress distribution due to loading. If the soil layers have significantly different stiffness values, then the resultant stress distribution can be very different from the Boussinesq solution, which assumes homogeneous elastic properties.
If the soil layers all have the same elastic modulus, then the Multiple Layer method should give the same results as the Boussinesq solution.
For further information and references, see the Settle3D Theory Manual.
Boussinesq Method
The Boussinesq method uses the theory of elasticity to calculate the vertical stress under a point load in a homogeneous, semi-infinite half space:
Where 
is the loading stress at any point and the meaning
of the other symbols are as shown in the following figure.
To compute the stresses under a distributed load, the Boussinesq solution is integrated over the area of the footing.
2:1 Method
The 2:1 method assumes that the zone of influence for an applied rectangular load has a slope of 2:1 as shown in the following figure.
The vertical loading stress at some depth is then calculated by:
For a non-rectangular footing, the stress is calculated by computing the area of the load at the surface. With increasing depth, the area over which the load is applied increases at a 2:1 ratio and the magnitude of the loading stress decreases correspondingly.
The 2:1 method is included mainly for purposes of comparison with existing calculations, however, its use is generally not recommended for 3-dimensional analysis, due to the simplistic assumption on which the method is based.
Westergaard Method
The Westergaard stress solution implicitly assumes a layered soil structure, and includes Poisson's ratio in the derivation. This tends to distribute loads over a greater area, compared to the Boussinesq solution.
For further information see the Settle3D Theory Manual.