Client name: FIFE Capital on behalf of TAP Sydney

Duration: 2011 - 2014

Location: Sydney, New South Wales, Australia


Developing tailored models to provide better advice - ensuring project success and protecting a vital rail tunnel. 

Situation

This 38-storey tower in the heart of Sydney required excavation in a geological fault zone adjacent to old rail tunnels. To make the development economically feasible, a third basement level, which had been previously rejected by the rail authority due to potential adverse impacts on the rail tunnels, was needed.

The complex geological setting of the proposed site required specialised rock modelling techniques.

Due to the historical relevance and current public transport importance of the two rail lines operating in the tunnels, the rail authority imposed strict conditions where the excavation should not induce new cracks in the tunnel liners nor increase the width of existing cracks.

Solution

Based on the site investigation carried out by Coffey, our team developed a detailed geotechnical 3D model which focused on the important mechanism of rock behaviour which are typically simplified or ignored in similar types of assessments in Sydney.

Our analyses involved not only a realistic representation of the proposed development but a detailed interpretation of the in-situ stress conditions and its relationship with rock quality and a cutting edge rock constitutive model developed to capture the specific behaviour of the sedimentary rocks in the Sydney Basin.

This assessment indicated that a third basement level was feasible, with an observational approach proposed to confirm and validate the model assumptions during excavation. Such observational method involves a thorough geotechnical monitoring program which was also developed by our team.

Several meetings were held by our team and the rail authority to identify and understand their concerns. This collaborative approach allowed our team to develop modelling assumptions and techniques that would focus on the concerns of the rail authority.
Our analyses demonstrated to the rail authority the low damage risk to the tunnel which resulted in the project approval for the proposed three basements.

Result

Coffey developed a tailored numerical model to target the unique geological conditions at this site.  In doing this, we were able to balance the commercial needs of our client while ensuring the 80 year old rail tunnels underneath were protected.