Compacted clay fill is a widely accepted construction material with measurable properties and predictable performance. This paper explores weakening of an adequately compacted clay fill that resulted in slope failures. Raby Bay Canal Estate is situated on the southeast Queensland coastline as a staged development of cut-to-fill earthworks in high plasticity clay soils. Final canal banks consist of various zones of compacted clay material overlying residual clays. Revetments consist of low concrete retaining walls with rockfill mattresses extending down to lowest tide level. During construction some failures occurred during excavation of slopes in fissured residual clays. More widespread post-construction slope failures have occurred in several stages of the estate, with repairs including piled support for the concrete walls. Initially there was reluctance to accept that these failures developed in compacted clay that had passed testing in accordance with specifications. Investigations were undertaken to identify weakening and potential future failures, with field and laboratory tests including simple wetting-up procedures. The processes leading to failure are not completely understood but involve clay mineralogy, initial fabric, and drying and wetting effects on plasticity. The constructed slope batter angle is almost identical to the softened peak friction angle, while the residual friction angle is significantly lower. The as-compacted clay fill fabric must be virtually free of defects, which is not easy to achieve at edges and between layers using typical construction specifications.


Many residential canal estates have been developed along the southeast Queensland coast, a region of rapid population growth and urbanisation. During and since this time there have been several areas of canal bank failure, some during construction but many developing after several years. The local government authority maintains the canalside of the concrete headwalls of the revetments, and was concerned about potential future failures.

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