Some factors influencing the engineering properties of Hawkesbury Sandstone

Abstract

The Hawkesbury Sandstone is a major rock unit within the Permian-Triassic sediments which form the Sydney Basin, Australia. It covers an area of about 12,5000 square kilometers and has a thickness of about 300 meters. As a result of its widespread occurrence in the region, particularly in the metropolitan area, it is of considerable importance to both the civil engineer and the geologist. This thesis examines the factors causing variations in the engineering properties of Hawkesbury Sandstone. An investigation has also been included to determine the suitability of crushed sandstone as a concrete and road aggregate. The determination of the mechanical properties of fresh Hawkesbury Sandstone was carried out in the laboratory on small specimens of intact rock, free from obvious macroscopic discontinuities, sampled at different locations in accordance with the standard test procedures. A number of sandstone exposures, in the form of quarries and roadside cuttings, were examined to understand the natural processes of weathering occurring in the field. Samples of weathered rock, representing all stages of weathering, were also tested in the laboratory to determine the effects of weathering on the properties of this sandstone. Thin section studies were made on all fresh and weathered samples to study the mineralogy and the texture of the rock. The clay fraction of the rock was analysed using X-ray diffraction and differential thermal analysis techniques. Samples of fresh Hawkesbury Sandstone were also collected in the field to produce crushed aggregates having a nominal particle size of 19 mm. Two trial mixes of concrete were made in the laboratory using crushed sandstone as the coarse aggregate. After curing, the strength and the deformation measurements of concrete were made. The stress-strain relationship of Hawkesbury Sandstone subject to uniaxial compression exhibits strong non-linearity at low stress levels. The results obtained in the measurement of mechanical properties of sandstone showed wide variations. The analysis of these results based on statistical techniques, revealed that the material characteristics of the rock, such as density, porosity, moisture content, mineralogy and texture, significantly influence the strength properties and the modulus of elasticity of the rock. The physical processes of weathering appears to be more widespread than the chemical process and cover a number of changes. These changes include opening of discontinuities and formation of new discontinuities in the rock mass; opening of grain boundaries and grain fracturing in the rock material. Alternate wetting and drying was found to be the most significant single process of weathering which contributes largely towards the degradation of the rock. The strength and modulus of elasticity of the weathered rock were appreciably lower than those of fresh rock. The failure of certain properties of the sandstone aggregate to satisfy the limits set by the currently available specifications has proved it to be an unsatisfactory material in the upper courses of a road-pavement. The success of its use in the lower courses, mainly in the sub-base, depends on its grading, durability and stability to environmental and traffic variations. The performances of crushed sandstone as a concrete aggregate where much poorer than those of conventional types of aggregates.