The most important part of designing mat foundations is calculating the values for the nodal spring constants (Modulus of Subgrade Reaction MSGR, force/cubic foot typ.)This value can be obtained from a Geotecnical engineer, By the results of plate load tests, by typical published values or by performing load-displacement calculations, given the tributary area and stress for each node. The last method is iterrative and tedious. Pressure/Displacement = MSGR