Design of stiffened slabs-on-grade on shrink-swell soils
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Stiffened slabs-on-grade are one of the most efficient and inexpensive foundation solutions for light structures on shrink-swell soils. After removing the top soil, the stiffening reinforced concrete beams, say 1 m deep and 0.3 m wide, are formed in the natural soil and placed every 3 m in both directions. The slab is typically 0.1 m thick. Such stiffened slabs are often called waffle slabs because of the geometric analogy with a waffle. They are commonly used in the USA for the foundation of houses or any light weight structure on shrink-swell soils and cost about 100$/m 2 (2010). This paper presents a simple design procedure and associated charts for calculating the depth of the beams required to limit the differential movement of the foundation due to bending to an acceptable amount. This bending of the foundation is due to the shrinking and swelling of the soil during the dry and wet seasons under the edges of the structure. The procedure consists of using the change in suction or the change in water content selected for design purposes in the region, the anticipate depth of influence of these seasonal changes, the soil properties, and the beam depth, to calculate the distortion of the slab under load. If the distortion is too large for the type of structure considered, the beam depth is increased until the acceptable value is reached. 2011 Taylor & Francis Group, London.
