Redevelopment Of Heritage Buildings In Sydney: Case history of complex underpinning and support works

ABSTRACT

The redevelopment of Sydney’s Central Business District has prompted a trend toward updating heritage buildings to include modern functionality while preserving their historical integrity. The Lands Department ‘Sandstone’ project transforms a heritage sandstone building into a luxury hotel, requiring a deep shaft excavation within the heritage building footprint, followed by a mined tunnel to connect to the adjacent heritage which is also updated.

The project geotechnical challenges included numerous rock joints due to proximity to the G.P.O. Fault Zone, resulting in rock wedges and other mechanisms that required underpinning of the heritage and new foundations around the new shaft. The presence of sensitive heritage structures with limited deformation tolerance, coupled with restricted site access, further complicated design and construction. Site investigations demonstrated the variable geotechnical conditions, prompting downhole camera and trial trench inspections to confirm the rock jointing locations with precision for underpinning design.

A key structural challenge was to support an existing five-story sandstone block wall directly above the shaft. A precast concrete beam with integrated steel members was designed as a load-transfer structure, redistributing wall loads onto new underpinned foundations. The underpinning of the new transfer beam and the other existing footings alongside the proposed shaft excavation were supported by rock bolts. To address building settlement risks, flat jacks were incorporated into the transfer structure foundations with staged excavation and design verification. Through innovative engineering solutions, and meticulous planning construction and verification, this project successfully solved the geotechnical complexities associated with the redevelopment of the heritage building, and provides a valuable precedent for future similar developments.