Coalwash is a granular waste by-product that results from the processing of coal. While its potential use as fill material for port reclamation has been recognised, the effect of particle breakage on its geomechanical performance has not been investigated. This paper presents an experimental study of the stress-strain behaviour and associated breakage of compacted coalwash. The characterization of basic index properties is first presented and stress-strain response is investigated using standard undrained triaxial tests over a wide range of effective confining stresses in line with the expected performance criteria of structural fill materials for port reclamation. Furthermore, to account for the effect of moisture variation on particle breakage, alternate wetting and drying cycles on coalwash aggregate was performed. Substantial particle degradation was observed after 25 cycles of wetting and drying. Results show that while the compacted coalwash conforms to typical structural fill criteria in terms of shear strength, the excessive deformation associated with change of particle size distribution due to particle breakage may render the material unsuitable in practice.