Engineers Australia

Weathering Profiles of Bunya Phyllite in Southwest Brisbane – a Geotechnical Approach

E. A. G. Marques and D. J. Williams


This paper presents the results of a study of the morphology of weathering profiles of Bunya Phyllite, located in Western Brisbane, Australia. Several cut slopes located in the suburbs of St. Lucia, Taringa and Indooroopilly were analysed in order to provide accurate knowledge of their morphological characteristics. From these, six outcrops were selected for a more detailed description of their physical, geological, structural and geotechnical parameters, focusing on weathered rock materials, including several characteristics of the rock matrix (fabric, mineralogy, degree of weathering, etc.) and rock mass (rock:soil ratio, discontinuity characteristics, JRC, JCS etc.). Samples from the upper portion of the profiles, into the transition zone between rock and soil, were collected in order to determine physical indices, preparation and description of thin sections and point load tests (the last two are not included in the present paper). The results show that phyllite weathering profiles are relatively thin and the contacts between different material layers are sharp. A strong structural conditioning of weathering can be noted in all outcrops. Soil-like material is present only in portions of the rock mass close to the ground surface or along structural discontinuities (foliation and fractures). A remarkable and specific characteristic of the studied weathering profiles is the presence of stress relief joints parallel to the foliation, which plays an important role in the differentiation of the rock mass layers and weathering.