Degree Name

Doctor of Philosophy


School of Civil, Mining and Environmental Engineering


In Australia, a large amount of industrial byproducts are produced annually, mainly coal wash (CW) and steel furnace slag (SFS) from coal mining and steel making, respectively. The effective reuse and recycling of these materials rather than their disposal are the preferred and sustainable methods from the waste management perspective and these are economically beneficial. In recent years, the need for more land to accommodate new infrastructures such as expansion of existing ports has been increased significantly. The use of industrial waste as structural fill material is considered relevant for these types of projects. However, due to the lack of information on the geotechnical behaviour of waste materials, specifically mixtures of coal wash and steel furnace slag, they have been used in limited quantities. Individual CW and SFS can pose serious problems due to the swelling potential in SFS and particle degradation in CW that can cause differential settlements. However, the adverse effect of individual materials might be minimised by blending them together. Therefore, an in-depth study on the geotechnical behaviour of CW-SFS mixtures is essential.

This doctoral thesis is part of an industrial project for the expansion of Port Kembla Outer Harbour near Wollongong, Australia. For this project, mixtures of CW-SFS (five mixtures in total) were considered for the fill material. The suitability of different CW-SFS mixtures in terms of geotechnical characteristics to be used as structural fill is investigated through extensive laboratory tests. A comprehensive range of geotechnical parameters of CW-SFS blends was determined in order to establish the relationship between them and the CW percentage. These parameters include specific gravity, compaction characteristics and degradation due to the compaction energy, permeability of the mixtures compacted to the maximum dry density, California Bearing Ratio (CBR) both for as compacted and soaked condition, the Unconfined Compressive Strength (UCS), the swelling (free swelling and swelling pressure) characteristics and in-depth study on the stress-strain behaviour under monotonic triaxial conditions.

In order to investigate the effect of CW (or SFS) content in the mechanical behaviour of CW-SFS blends, series of drained triaxial tests on 100mm diameter and 200mm height specimens were carried out. Confining pressure was varied between 30kPa to 220kPa to mimic port loading conditions. Based on these tests, an empirical equation was proposed to predict the peak deviatoric stress for different CW-SFS blends and confining pressure. Furthermore, the ultimate adoption of CW-SFS mixtures as structural fill was supported by establishing a mathematical model for the stressstrain behaviour based on generalised plasticity and critical state concept. It was shown that within a unique framework, the model was capable of capturing the mechanical behaviour of different CW-SFS mixtures.

Based on the laboratory results, an optimisation method for the CW-SFS mixtures was suggested. Following this method, the most suitable blends can be identified by considering the required properties for site condition. The performance of the CWSFS mixtures on a large scale was assessed through field investigation, with the results confirming the suitability of the optimum mixture as structural fill.

The constitutive model proposed for the CW-SFS blends was implemented into the finite element software (ABAQUS) by developing the user-defined program (known as the UMAT subroutine). The numerical model was initially calibrated using the drained triaxial results from the laboratory and then verified and assessed by comparing them to the field trial investigation. The numerical model is capable of being used under different loading conditions for a range of CW-SFS mixtures, enabling practicing engineers and designers to evaluate the performance of a given blend for site conditions.