Laboratory-controlled cone penetration test results for two silty tailings in a variety of saturated and unsaturated states, obtained using two calibration chambers, are presented then interpreted using a state parameter-based approach. For each, the cone penetration resistances, which increase due to the presence of suction when the tailings are unsaturated, can be normalised using the initial mean effective stress to establish a relationship with the initial state parameter. The relationship is applicable to saturated and unsaturated conditions, as long as the presence of suction hardening as well as the influence of suction on the mean effective stress are accounted for, and as long as the cone penetrations occur under drained conditions. The relationships enable state parameters to be back-calculated from normalised cone penetration resistances. The state parameters enable estimations of the tailings’ peak friction angles for drained loadings as well as their propensities to liquefy during undrained loadings. Application is demonstrated using cone penetration test soundings in the tailings storages from which the samples were taken, showing how in situ void ratios and state parameters, as well as future state parameters if the tailings were to become saturated, can be determined. Close agreements with direct measurements of void ratios are shown.