The geological study of the formation and structure of in situ residual soils is well advanced; the study of the engineering properties of these soils much less so. In particular, no methods exist whereby mechanical properties can be related to geological descriptions, or to simple index tests, as is done for sedimentary soils. The methods for sedimentary soils are inappropriate for residual soils, yet no alternatives have been developed.
Most residual soils are weakly bonded, and they may have widely varying void ratios. A laboratory study of how these two features influence engineering behaviour is being conducted, based mainly on the testing of an artificial material in which both can be varied under control. Initial results from this study show that engineering behaviour depends on the void ratio of the soil related to that which the soil can have in the un-bonded ‘de-structured’ state. This indicates that it is useful to index the in situ void ratio of residual soils in relation to void ratio in the de-structured state, in a manner similar to the indexing of sands by relative density or of sedimentary clays by liquidity index. Simple methods whereby this may be done are suggested and discussed.