Old Treasury Building Tower

Background

The historic Cathedral and Treasury precinct, located in the Perth CBD, has undergone a major redevelopment, including:

Restoration of the existing 135 year old, heritage listed Treasury Building
Construction of a 35 storey office tower

Mirvac subcontracted geotechnical specialists GFWA to construct the foundations, tunnels and temporary works for the Old Treasury Building (OTB) basement ower development. Mirvac Design had previously commissioned two geotechnical investigations and permanent retaining wall design. The precinct is one of the most iconic sites in Perth and the $280m tower is enclosed by both the old Town Hall and the existing Treasury Buildings.

Scope

CMW carried out the detailed geotechnical design under GFWA’s contract. Throughout construction, CMW provided further geotechnical input to verify, optimise and formalise the already completed designs.

Two boreholes were drilled by CMW to depths of 52m in order to confirm the depth to rock and rock strength parameters for the design of deep foundations. Design components included:

Underpinning works using permeation grout techniques to create a grout block;
Retaining walls with grouted anchors, using continuous flight auger techniques;
Piled foundations encompassing barrette piles and pressure grouted tension piles;
Development and documentation of construction methodologies for temporary needle beam jacking techniques in order to construct the tunnel from the OTB tower to

Findings/Challenges

Protecting the heritage buildings during construction presented a significant geotechnical challenge.

A high degree of uncertainty existed regarding the predicted behaviour of the underpinning needle beams under jacking. CMW was able to provide a simple, time efficient temporary support solution that prevented delay to Mirvac’s schedule.

In addition, weekly site visits by CMW provided opportunities to suggest alternative construction methods which could better meet the demands of construction schedule and reduce costs.