EVALUATION OF TIME RATE OF CONSOLIDATION AND UNDRAINED SHEAR STRENGTH OF HYDRAULICALLY PLACED FINE COAL REFUSE

Fine coal refuse (FCR) refers to the fines generated during the processing of raw coal. FCR is usually mixed with flocculant and water and hydraulically placed behind impoundments. It is generally assumed that the FCR in these impoundments will consolidate over time due to the overburden weight of the materials above, losing some of the fluid-like properties that it possessed when initially placed. However, some in situ observations have shown that there exists under-consolidated material within slurry impoundments even after many decades of deposition. These under-consolidated materials can be prone to destabilization and flow, which can result in fatalities and environmental disasters. The purpose of this study is to investigate the consolidation behavior and effect of flocculant on the material properties and flowability of the FCR.Traditional consolidation tests were conducted on in situ FCR samples obtained from a range of depths behind an impoundment. The consolidation response of the in situ samples was compared with companion samples of fresh liquid slurry pre-consolidated to stresses corresponding to the depths of the recovered in situ samples. A finite difference model used to calculate time rate of consolidation of the FCR using variable coefficient of consolidation which was obtained by consolidating FCR slurry under different pressures. The results were compared to traditional Terzaghi method with constant coefficient of consolidation. Laboratory vane shear tests were conducted to study the influence of flocculant on undrained shear strength of the FCR and modified flow table tests performed on consolidated FCR slurry samples prepared with different background fluids.The results suggest that the variable coefficient of consolidation method may best predict the time rate of consolidation for the FCR slurry compared to traditional methods which use the in situ coefficient of consolidation. Although the particle size analysis revealed that the effect of flocculant degrades over the time, vane shear results suggest that the flocculant increases the undrained shear strength and can improve the FCR resistance to flow at early stages of consolidation. The results of this study give a better understanding of the consolidation behavior and undrained shear resistance of the hydraulically placed FCR behind impoundments.