Footing design for residential type structures in arid climates

Peter W. Mitchell

The parameters required for the design of footings on expansive (or reactive) soil by AS2870-1996 for arid regions of Australia are derived theoretically from established relationships based on experiences in the more temperate climates. Two critical parameters required for a footing design by AS 2870-1996 are the surface soil suction change (∆us) and the depth of the design soil suction change (Hs), and current recommendations for arid climates have a range ∆us = 1.2pF to 1.8pF, and Hs=3.7 m to 6.0 m. Using the results of solutions of the diffusion equation, with values for the diffusion coefficient for a soil profile in an arid climate that are extrapolated from the established relationships between the Thornthwaite Moisture Index, the annual cycle of wet/dry months and Hs in the more temperate climates, it was found that for an arid climate, ∆us=1.8pF and Hs=2.5 m. This finding was supported by a case history of a building in the Jackson oil-field, south west Queensland that had been distorted by the effects of an expansive soil profile. Three worked examples, using ∆us=1.8pF and Hs=2.5 m for the design of a footing for a residential type building on an expansive soil in an arid area, are given.