<p>The stability analyses for tailings dams in engineering practice rely on the critical state lines (CSL) for in situ tailings determined from laboratory tests, normally assuming a unique CSL. However, it has been found that there are some soils with a transitional behaviour, for which there are no unique Normal Compression Lines (NCLs) or CSLs as they would be in the critical state framework. These tend to be soils with a mixed grading and/or a mixed mineralogy. Given their gradings and mixed mineralogies, transitional behaviour could be expected in some tailings, as presented in previous studies, but a complete characterisation of this mode of behaviour is lacking in such materials. To investigate it, a series of oedometer and triaxial tests were conducted on samples of mineral sand tailings with a wide range of initial density. The transitional mode is identified by the non-unique NCLs and CSLs, and the characteristics of transitional behaviour are investigated. Notably, the spacing between the NCLs and CSLs in the specific volume logarithm of effective stress plane decreases as the initial density decreases. Nevertheless, regardless of the initial density, even in the densest samples, the tailings predominantly have compressive and strain-hardening behaviour, and the initial density has no significant effect on dilatancy behaviour or the stiffness. These results reveal an intrinsic characteristic of transitional behaviour that initial density has a clear but only minor impact on mechanical response, in contrast to conventional soils where initial density governs key aspects such as dilatancy and stiffness.</p>