The purpose of a shallow foundation is to apply the structural loads to the foundation by spreading the load from walls or columns over a larger area. Theoretical solutions for the bearing capacity of shallow foundations are generally based on the theory of plasticity where the soil is assumed to behave as a rigid plastic material that fails according to the Mohr–Coulomb failure criterion. A combined footing may be necessary when a column load is close to a property boundary, and the foundation cannot be made large enough to support the load without encroaching onto the neighbouring property. The ultimate bearing capacity of a foundation depends upon whether it is loaded rapidly so that the soil does not drain or whether it is loaded slowly so that all pore pressures have dissipated. Solutions have been found for the case where the soil strength varies uniformly with depth or where the soil is layered.