Sampling and Testing Cretaceous Claystone near Darwin, NT

As part of a proposed $10 billion development of Timor Sea gas reserves, Woodside Energy Ltd in joint venture partnership with Shell Development Australia evaluated an onshore LNG processing plant to be situated at Glyde Point, on the Gunn Peninsula about 45 km north-east of Darwin, in the Northern Territory.

Gutteridge Haskins & Davey Pty Ltd was commissioned to perform a preliminary geotechnical investigation as part of the project feasibility study. The work included an assessment of the suitability of the site for the proposed development, to facilitate the determination of foundation design parameters for 120,000 cubic metre capacity LNG storage tanks and the prediction of settlements of shallow footings.

The site was found to be underlain by a monotonous sequence of weathered, lateritised kaolinitic claystone and shale, or clay shale, becoming montmorillonitic with depth. Difficulties experienced during the investigation included a slower than anticipated rate of drilling in “soft” rock, disturbance during drilling, remoulding of cored samples, and difficulty in recovering “undisturbed” samples.

The evaluation of the field and laboratory data included reconciliation of field and laboratory test results and charting variation in basic soil parameters with depth. A large discordance was found between various methods of strength and deformation characteristic evaluation. Due to lateritisation of an overconsolidated profile, the usual strength and depth relationships were not observed, and these anomalies had to be taken into account in developing a geotechnical site model.

The paper summarises the main conclusions drawn from the work, including identification of limitations of the investigation procedures and a recommendation of alternative testing methods for future detailed geotechnical investigations on the site.