Integrated approach to coal mine washery waste disposal and land reclamation (geotechnical engineering aspects)

Abstract

The main aim of the thesis is to investigate the technical feasibility of a new concept in coal mine washery waste disposal, namely INTEGRATED COAL MINE WASHERY WASTE DISPOSAL, and to establish a basis for the proper understanding of the physical phenomena involved in this method. The objective was achieved through various stages such as information gathering, small scale field trials, laboratory modelling and testing, and numerical modelling.Small scale field trials of tailings disposal within a coarse reject embankment conducted at New Hope Colliery, helped to identify the key areas for further laboratory modelling and investigation. The laboratory modelling and testing generated a large amount of information on the engineering behaviour of coal tailings, coarse reject and mixtures of tailings and coarse reject. Subsequent numerical studies illuminated the nature of the seepage of tailings through a coarse reject embankment, and the critical factors controlling it.The consolidation of conventional tailings deposits require excessively long period of time, delaying or prohibiting altogether land rehabilitation work of the tailings storage areas. On the other hand, loosely dumped coarse reject are sources of both air and water pollution, aided by their potential for spontaneous combustion and leaching by rain water infiltration, respectively. Integrated coal washery waste disposal has the potential to ease this situation by eliminating the need for separate disposal areas for tailings and coarse reject. The combined coal washery waste formed by this method is simply a mixture of tailings and coarse reject with many improved engineering properties.Limitations of the conventional laboratory permeability testing equipment with regard to coal mine washery rejects, led to the development of a large diameter constant head permeability testing equipment with additional facilities to consolidate samples to required stress levels. The permeability test carried out in this apparatus clearly demonstrated that, the chief advantage of mixing coarse reject with tailings is the increased permeability of the resultant mixture. An increase in permeability of three to four orders of magnitude (from 10-7 m.s-1 for tailings to 10-3 m.s-1 for mixtures) were recorded from these tests.Consolidation tests carried out in large diameter Rowe cell demonstrated an improved consolidation behaviour for the tailings/coarse reject mixtures. This improvement is mainly attributable to the increased permeability of the mixtures in comparison to tailings only. As the proportion of tailings in a mixture increased (beyond 60% by weight) the mixture started to behave more like tailings. However, the mixture clearly demonstrated improved behaviour over a wide range of tailings to coarse reject mixing ratios (e.g. 20:80 to 40:60).Laboratory seepage modelling tests conducted in a specially designed seepage tank provided sufficient information regarding the filter cake formation and its effect in limiting the seepage through a coarse reject embankment. The transient nature of the seepage, the decreasing solids concentration of the effluent with the passage of time, and the depression of the wetting up line were studied in detail during these model tests.Laboratory tailings/coarse reject mixing trials were conducted in purpose built mixing tank with pore water pressure monitoring facilities. The mixing trials indicated that, to achieve good mixing of tailings and coarse reject, the coarse reject should be introduced into tailings within a certain time period after the deposition of tailings. Early introduction of coarse reject results in segregation, by coarse reject sinking to the bottom of the tailings. Late introduction of the coarse reject again results in segregation, where coarse reject is supported by the tailings, which gain strength by consolidation. Tailings can also be introduced into loose coarse reject, but effective mixing will occur to only shallow depth (of the order of 0.3 m).Consolidated drained direct shear tests carried out on tailings, coarse reject and mixtures thereof clearly indicated that these are essentially frictional materials, and the angle of friction is not very sensitive to the mixing ratios. The angle of friction remained high (above 30°) in all the tests.An existing finite element transient seepage analysis program TSEEP was calibrated using results from the laboratory seepage model tests and soil suction tests. The calibrated program was used to predict the field tailings seepage rate through a coarse reject embankment, simulating the small scale field trials conducted at the initial stages of the thesis investigation. Close agreement was found between the numerical predictions and the field measurements. The effect of the preferred seepage paths on the tailings seepage rate was successfully accounted for in the numerical modelling.The effectiveness of integrated coal washery waste disposal in a full scale mining operation was checked by field investigation carried out on a co-disposal site at Jeebropilly Colliery. The field investigation identified a few problems and the possible means of correcting these shortcomings are discussed in the thesis.