Strength, liquefaction and cone penetration test results in unsaturated silty tailings

The behaviour of two silty tailings are explored, especially their propensity for static liquefaction, and how they may be characterised using the cone penetration test (CPT), when unsaturated. The focus is on loose states and high degrees of saturation. Firstly the tailings are characterised, using triaxial tests involving constant suction and constant mass (closed system) conditions to establish stress-strain behaviours, and filter paper and pressure plate tests to establish water retention properties. Consideration of the closed system condition is novel. It is important because an instability commonly develops quickly, and deformation may be rapid, meaning the air and water inside the tailings may not be able to exit. A bounding surface plasticity model is adapted to suit the closed system condition. The triaxial test results are used to calibrate the model, with good agreements achieved. Model simulations are then used to identify which factors influence instability. Additional simulations to mimic rising water tables under constant total stress states in the field are also shown. Results are added to charts which relate peak and post-liquefaction strengths, as well as collapse lines, to measures of initial state, for unsaturated conditions, for use in practice. A number of CPTs are performed on the tailings in a calibration chamber. Interpretations are made, incorporating other CPT results, correlating the measured cone resistances to the tailings’ states for saturated and unsaturated conditions. A cavity expansion analysis is then performed, using the bounding surface model and the similarity technique, novel in its consideration of the closed system condition. Cavity expansion results are compared to the CPT results, a linear proportionality between cavity wall pressure and cone resistance is identified, and charts are produced enabling CPTs to be interpreted in unsaturated tailings whatever the initial state. Two types of behaviour are observed, depending on the volume of air present relative to the overall tailings volume, i.e. v_a/v. Specifically, a closed system condition is relevant to when v_a/v 0.15 the penetration rate is unimportant, as a pseudo drained condition prevails around the cone tip