The interplay of site reactivity, design practice and construction procurement in structural failures on expansive clay sites

The expansion of housing into the Western suburbs of Melbourne has involved, effectively, mass-production of conventional, masonry veneer dwellings on highly reactive sites. The design favoured by constructors for reasons of cost and speed is the “waffle pod” system, typically 385mm deep on highly reactive sites, with the performance of such footings generally predicated on adequate control of soil moisture in the foundations close to the footprint of the building, both during construction and in the permanent condition.

Damage to masonry facades and excessive floor slab deformations, evidently due to serviceability failure of footing systems, have led to recent publicity and litigation. Whilst the underlying geotechnical characteristics may be causative of soil heave, the unique characteristics of waffle pod footing systems and the associated landscaping and drainage provisions stipulated by the designers, along with the method of procurement of the house itself point to the contribution of systemic and contractual factors in footing failures.

A further aspect is the interplay of differing deformation criteria between AS2870 (“the Standard”) and the structural design standards such as AS1684, whereby, again, dissociated procurement and design of building components (footings and roof trusses in this instance) can lead to incompatibilities in structural behaviour and consequent damage to cladding and finishes. This paper examines the interplay of geotechnical, structural, construction detailing and contractual aspects that can lead down the path to poor footing performance, dispute, and litigation. The question is raised as to whether, given these relatively gross uncertainties relating to the input parameters for codified standard designs and design methods, the degree of refinement and accuracy implicit in the Standard is warranted, and whether more robust standardised designs are justified.