Performance of Slab‐on‐Grade Foundations on Expansive Soil

A slab‐on‐grade must be designed to resist two types of expansive soil movement: the short‐term cyclic heave/shrinkage around the perimeter of the foundation and the long‐term progressive swelling beneath the center of the slab. The purpose of the conventional slab‐on‐grade on expansive soils is to provide deepened perimeter footings that are below the active zone of seasonal swell/ shrinkage and to soak the soil subgrade in order to reduce the long‐term progressive swelling beneath the center of the slab. Two common problems with this foundation are that the perimeter footings are not deep enough to resist the short‐term cyclic heave/shrinkage and the presoaking beneath the slab is not done properly. The purpose of the design of the posttensioned slab‐on‐grade is to build a foundation that is strong and rigid enough to resist the expansive soil forces using the center‐lift and edge‐lift deformation configurations. Three potential problems are that the expansive soil movement is cyclic, the edge moisture variation distance is based solely on climate, and the center‐lift deformation may be underestimated.