Use of dredged materials in a materials offloading facility embankment, Barrow Island, Western Australia

David Foulsham, Srijib Chakrabarti, Bob Semple and Bob Whiteley

This paper describes aspects of the geotechnical design and construction of a Materials Offloading Facility (MOF) embankment up to 20m high, that consists of material sourced from dredging and plant site development works for the Chevron-operated Gorgon Project – the largest single resource project in Australia’s history. The MOF is located on the eastern side of Barrow Island (BWI), a Class A Nature Reserve. Several significant challenges were faced in sourcing the range of materials required for the MOF. These challenges related to the local geological conditions on BWI and in the near shore environment proposed for the dredging works. Source rock from BWI was used in construction of the initial section of causeway from Town Point to the Pioneer MOF (PMOF) platform, however, this paper only refers to the PMOF and beyond to the MOF Head where the LNG Jetty Abutment structure is located. The materials for these works were sourced from the near shore environment and generally comprised carbonate rich limestone and calcarenite. These dredged materials were considered primarily as rock fill and methods for sourcing and characterisation of the rock fill, selection of parameters, geotechnical analyses and relevant construction details are also discussed in this paper. Design parameters, based on particle size distribution, unconfined compressive strength, particle shape and roughness characteristics were selected and slope stability analyses were undertaken using the limit equilibrium method. Load deformation analyses applied the finite elements coded in the latest readily available geotechnical software. Potential creep and seismic effects were taken into consideration. High strength geotextile reinforcements were used to achieve the required long term embankment stability. Various compaction methods including the relatively new Cofra Dynamic Compaction (CDC) method were adopted at different levels and locations of the embankment and innovative Seismic Surface Wave geophysical methods were combined with Plate Load Testing for verification of the heterogeneous fill materials to ensure the design assumptions would be realised.

The Gorgon Project is operated by an Australian subsidiary of Chevron (47.3 percent interest), in joint venture with the Australian subsidiaries of ExxonMobil (25 percent), Shell (25 percent), Osaka Gas (1.25 percent), Tokyo Gas (1 percent) and Chubu Electric Power (0.417 percent).