On the compression behaviour of reconstituted soils

Transitional behaviour has been recognised in a diverse range of soils in the literature to date, from gap-graded soils to well-graded silts and sands. It is typified by non-convergent compression paths and critical state lines that are non-unique and which are dependent on the initial sample density. Many soil mechanics theories assume soils to have unique normal compression and/or critical state lines, which is not the case for a transitional soil. For such a soil it would therefore be difficult to identify the effects of structure on the mechanics since there is no unique behaviour of the soil when reconstituted. This paper describes series of oedometer tests that were performed to investigate in more detail when and why non-convergent compression behaviour might be expected, as a first step to identifying which soils are transitional. The effects of mixing soils of different grain sizes and mineralogies were explored, the tests revealing that convergent or non-convergent behaviour could be brought about either by relatively small changes to the proportions of the soil particles or by changes to their nature. It was also found that in some soils with non-convergent compression behaviour there was particle breakage while in others there was not. Since the factors that influence the mode of compression behaviour were found to be complex, it is concluded that each new soil encountered must be assessed individually for whether it is transitional, and that the accuracy of void ratio measurements is of particular importance in establishing this reliably.