Re-development of the King Street Wharf area of Sydney continues with a project now well under way by Multiplex on the site of the original Wharves 9 & 10.

The project includes a multilevel basement, just metres from the waterfront on one side and an elevated expressway on the other.

Multiplex sub-contracted the design and construction of the basement walls to specialist contractor Bachy Pty Ltd (now trading as GFWA) – part of the international Bachy Soletance Group.

Bachy elected to install diaphragm walls and entrusted the geotechnical design to MPA Williams & Associates.

The site presented many significant challenges in both the design and construction phases of the work including:

• High water table with significant tidal fluctuation;
• Numerous obstructions arising from earlier site developments;
• Low strength surficial soil profile;
• Extreme variability of the underlying sandstone surface which dipped steeply to the north.

The latter of these represented the most significant challenge for the geotechnical designer. The excavation for the proposed basement extended to a maximum depth of over 14 m below ground surface, with the depth to the top of sound sandstone varying across the site from around 2 m to below the bulk excavation level.

The transition from relatively weak soil to strong sandstone generally occurred over a narrow depth, limiting the extent of toe support for the diaphragm which could be provided by the ground without expensive chiselling.