The engineering significance of silcretes and ferricretes in Australia

Abstract Silcrete and ferricrete are silica rich and iron rich rock-like pedogenic materials formed by superficial physicochemical processes close to the surface of the earth. They are formed by the mobilization, transport in solution and deposition from solution of the appropriate oxide. They tend to be formed in areas with a warm climate and seasonally controlled rainfall. Silcrete occurs in many forms in Australia. It may form a cap-rock on a surface or a hard pan in soils. There are also lacustrine and riverine silcretes. The “gibber” mantle of the desert is derived from silcrete. Ferricrete is usually associated with a soil profile. This consists of soil overlying a ferruginous zone which in turn overlies a partly leached mottled zone and a completely leached pallid zone. Some forms of ferricrete tend to harden upon exposure and are self-bonding. The engineering significance of silcretes and ferricretes lies in their location in relatively unsettled areas where there is a lack of conventional constructional material and where engineering development is proceeding. The efficient use of the materials depends upon a knowledge of their properties; some problems have been encountered in their use due to lack of this knowledge and of tests appropriate to them. The engineering properties of silcretes and ferricretes depend upon their mineralogical composition and the physico-chemical properties of their constituents. Some forms of silcrete and ferricrete should be tested using standard tests but others, particularly self-bonding varieties, should be tested differently. When the engineering behaviour of these materials is properly understood, they will possess a big potential in the engineering development of Australia.