Loss of support as a result of voids underneath concrete slabs can initiate and propagate slab cracking in jointed plain concrete pavements. This paper investigates the hypothesis that foundation voiding occurs because of localized plastic deformation of the depressurized foundation as a result of slab curling and repeated application of traffic loads. Plane strain and three-dimensional (3D) nonlinear finite-element models of the subgrade foundation are presented. The analysis shows that voids underneath concrete slabs usually occur along the edge of slabs adjacent to the shoulder. The sizes of voids are primarily determined by foundation properties (cohesion stress and internal friction angle), slab properties, axle loads, and temperature gradients. Voids along the edge of slabs adjacent to the shoulder lead to high transverse tensile stresses on the top surface, which may result in top-down longitudinal cracking close to the outer wheel path. These same voids increase the longitudinal tensile stresses on the bottom surface at the edge of the slabs, which can increase the potential for bottom-up transverse cracking at the middle of the slabs.