Transitional behaviour in sands with plastic and non-plastic fines

Two model soils, one made from sand with plastic fines and one made from sand with non-plastic fines, were tested in a triaxial apparatus to examine the basic mechanics of transitional soils, that is, soils whose initial density has a very strong influence on the behaviour. The sand with non-plastic fines defined parallel critical state lines and state boundary surfaces at the medium strain levels that can be reached in triaxial testing, the locations of which depended on the initial specific volume of the soil when it was created. Overconsolidating the soil, or shearing it a second time at an increased stress level, was not found to change this behaviour. There was also no significant effect of the initial specific volume on the soil stiffness, which was predominantly a function of the current stress level for normally consolidated states. A variety of different sample preparation methods was used, but the methods did not have any clear influence on the critical state volumes. Although the testing for the sand with non-plastic fines was more limited in extent, the patterns of behaviour seemed to be similar. The relationships between the initial specific volume and the critical state line intercept, Γ, gave gradients of 0.52 and 0.97 for the sands with plastic and non-plastic fines, respectively, showing significant differences in volume remaining after isotropic compression and monotonic compressive shearing.