Design and Analysis of Ribbed Raft Foundation Resting on Swell-Shrink Soils

Expansive soils shrink and swell with drying and wetting cycles. The shrink-swell capacity of expansive soils can result in differential movement beneath foundations which is a very common cause of foundation problems. Ribbed slab, often called waffle slab, is one of the efficient and inexpensive foundation solutions for use under light structures resting on shrink-swell soils. This paper presents a numerical analysis that is used to calculate displacement and internal forces in ribbed slab foundations resting on swell/shrink soil. The stiffness of the ribbed slab is modelled as an equivalent flat slab of constant thickness and the straining actions are calculated for the case of central heave. A parametric study of the effect of beam dimensions and rib spacing is performed to assess the optimum configuration. Dimensioning of the waffle slab system; slab thickness, depth and width of beams and spacing, is performed according to the Egyptian Code of Practice for the design of concrete structures. The results show that beam depths of 0.4 m and 0.45 m were safe and recommended under this condition of heave scenario. All beams with depths of 0.3 m cannot resist the resulting bending moments, While beams with 0.35 m depth are only safe when the beam width is not less than 0.2 m.