Foundation Investigation In Weak Slaking Rock, Darwin, Australia

Most of the Darwin CBD is underlain by a Mesozoic rock locally termed porcellanite. Much of this is a silcrete which can be up to 8 m thick and is typically high to very high strength. Many of Darwin’s buildings are supported on shallow footings founded on porcellanite and there is little available geotechnical information of relevance to foundation design in the weathered Proterozoic phyllite underlying the porcellanite.

This paper describes a foundation investigation and design for a 34 storey building with sunken lift core in Darwin which, when complete will be the tallest in that city. Preliminary footing design by others was based on the precedent of shallow footings supported in the porcellanite. A preliminary geotechnical investigation (by others) comprising boreholes to a depth of about 15 m, indicated that a significant thickness of porcellanite underlies only part of the site and that footings would need to be supported on older deeply weathered phyllite or a thin layer of porcellanite overlying phyllite. Based on low soil stiffness properties estimated mainly on the basis of low SPT ‘N’ values large diameter deep bored piles were proposed as a means of reducing differential settlement.

To further investigate the proposed footing solution options, a supplementary geotechnical investigation was undertaken to obtain estimates of the engineering properties of the phyllite. Diamond core drilling with pressuremeter testing was undertaken. However core recovery was poor and the results of the pressuremeter testing were affected by the slaking of the phyllite. The stiffness of the phyllite was subsequently estimated using dynamic load testing of precast concrete piles placed into pre-drilled sockets at select depths. Analyses using the estimated stiffness properties of the phyllite suggested that differential settlements could be kept within acceptable limits using a shallow footing solution, which was subsequently adopted. The footing system has since been constructed and construction is near completion. Settlement monitoring indicates settlements less than design predictions.