Compressibility and Creep Behaviour of Hydraulically Placed PFA and Mine Tailings Fills.

Many industrial processes result in the production of silt sized waste materials which may be disposed of by being pumped as a slurry to settling lagoons where the solids settle to form fills which often have a high voids ratio. These are described as hydraulic fills. These processes are commonly found in the coal based power generation and mining and mineral processing industries, amongst others. The geotechnical behaviour of such deposits is of interest with respect to the integrity and safety of the impoundment systems and possible future use and redevelopment of the sites. Such deposits are very difficult to sample and in many cases it is almost impossible to obtain notionally undisturbed samples, and so it is necessary to prepare samples in the laboratory using techniques which simulate field conditions. The paper describes the results of a series of one dimensional compression tests, up to very high stress levels, on pulverised fuel ash (PFA) and fluorspar mine tailings. Sample preparation methods attempting to recreate the in-situ structure and density are described. The results presented consider the initial 1-dimensional loading of the initially loose specimens, their unloading response and their creep behaviour. An attempt is made to define the normal consolidation line for these materials and the importance of the initial voids ratio is discussed. A general conceptual model of one dimensional compression behaviour is presented. The secondary compression, or creep, response is discussed, and creep behaviour may be important in considering their behaviour. The creep behaviour displays a linear relation in e-log(t) space. Contours of equal time of creep (isotaches) are suggested which are parallel to the normal consolidation line.