Doctor of Philosophy
Department of Civil and Mining Engineering
Hii, John Kwong, Development and use of coal washery refuse for strata control in coal mines, Doctor of Philosophy thesis, Department of Civil and Mining Engineering, University of Wollongong, 1990. http://ro.uow.edu.au/theses/1237
This work involves the development and analysis of a new pump packing material, which utilizes coal washery refuse (CWR) as a raw material, for strata control in underground coal mining operations. A review on the state of the of pump packing systems is presented, together with an analysis of the cost effectiveness of the new pump packing material as compared to other commercial pump packing materials.
A method is proposed for use in the design of cement and coal washery refuse (CCWR) material mixes to give a required early strength and degree of workability when incorporating CWR. The usefulness of the method is shown by comparison of the results from a large number of tests on CCWR specimens with previously published results. It is shown that the relationship between the strength and curing time of a mix can be accurately described by a simple equation. The effects of the compositions on the strength within the experimental regions are discussed. As expected an increase in the strength of CCWR materials can be obtained either by decreasing the water content or increasing the quantity of OPC used in the mixtures.
This study also investigated the influence of moisture content on the mechanical properties of CCWR material. The unconfined compressive strength, Young's modulus, indirect tensile strength, triaxial strength and drying shrinkage were the primary properties investigated. Results indicate that the moisture content has a significant effect on both the strength and elasticity of CCWR material at all curing times. It is evident from the tests that the drying shrinkage of wet mixes is larger than that of dry mixes. In general, moisture content affects the shrinkage of CCWR material as it reduces the volume of the restraining CWR.
An empirical criterion is proposed for predicting the strength and yield characteristics of CCWR materials. A numerical procedure has been developed to determine the parameters contained in the proposed yield function. It has been demonstrated that the proposed logarithmic yield function can accurately predict the strength and yield characteristics of CCWR materials.
Preliminary tests have also been conducted to determine the influence of moisture content on the flow properties of CWR. The test results indicate that the presence of very fine grains in CWR enhances its flow ability remarkably.
Further work investigated the effect of specimen geometry on the strength and elasticity of CCWR material. CCWR models were tested, simulating underground monolithic pack support with different widths in a coal seam of uniform height. The study also investigated the load deformation characteristics of CCWR models under the tests conditions. It is shown that geometry has a significant effect on both the mechanical properties and load deformation characteristics of CCWR models.
A yield pillar technique analysed by the finite element method appears suitable for simulation of excavation of roadways under different virgin stress conditions. Comparison of stability of yield and conventional pillars in several examples substantiates the feasibility of the technique and indicates that yield pillars are practical when the factor k, the ratio of horiwntal to vertical stress exceeds 1.6.