Degree Name

Doctor of Philosophy


School of Civil, Mining and Environmental Engineering


Coal washery reject is a byproduct of the coal washing process and is produced at over a hundred million tonnes per year in Australia. While its potential use as structural fills has been recognised in past research studies, the effects of particle breakage and the level of relative compaction on its geomechanical performance have not been investigated. This waste material contains fine-grained coal tailings and coarse-grained coal rejects (coalwash), and since it is readily available close to coal mining operations, its utilisation as structural fills have both economic and environmental advantages. Previous research studies have mainly focused on characterising the geotechnical properties of coalwash, which means less emphasis has been given to investigating the characteristics of particle breakage and compaction, and its influence on the stress-strain behaviour.

This research work investigates the behaviour of saturated and compacted coalwash at various levels of relative compaction and proposes a new constitutive elasto-plastic model. An extensive program of laboratory tests on compacted coalwash under a broad range of stress paths in both compression and extension were carried out. These tests involved probing the stress paths to investigate the form of the yield surfaces of specimens at different levels of compaction, and generally included shearing to failure to investigate the critical state conditions. The particle size distribution of specimens before and after compaction, and after shear probing, were also studied, after which the experimental data were used to develop the new model and calibrate its performance.

The experimental results showed that large particle breakage occurs during compaction and at shearing stages so its influence on the shear strength and change in volume was investigated. Particle breakage is linked to the distinct critical state line for drained and undrained shearing in volume space. The experimental results also showed that anisotropy had no apparent influence on the critical state of saturated samples. Critical states can be represented in the q-p' plane by a single straight line passing through the origin, while the yield surface can be represented by a distorted pear shape passing through the origin. Moreover, the level of compaction energy has a strong influence on the size and shape of the yield surface. A novel approach is proposed based on the shadow projection method to generalise the yield surface in a three-dimensional stress space.

A new constitutive model incorporating anisotropy and particle breakage was formulated based on the experimental observations; this model includes a capped yield surface that is a function of compaction effort, stress ratio, and anisotropy. A new critical state surface model is proposed that defines the change of critical state with an increase in particle breakage. To estimate the plastic strain induced by breakage, an empirical relationship between the amount of work done and particle breakage due to shearing was introduced. To integrate the model into a numerical solution scheme, the governing elasto-plastic rate equations were incorporated into the finite element software ABAQUS using a subroutine coded in FORTRAN. The constitutive model was calibrated and verified using the results of drained and undrained triaxial tests of CW. Finally, this model was also validated under plane strain conditions and then compared with the laboratory results of a model footing. Practical implications of the results are also discussed.



Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.