Client name: Menard Bachy

Duration: 2010 - 2012

Location: Sydney, New South Wales, Australia

Coffey overcame a number of in-ground challenges to deliver the 770m long perimeter diaphragm wall for the basement at Barangaroo South, within a tight timeframe and meeting stringent performance criteria.     

Diaphragm wall delivery faced with numerous in-ground challenges

Barangaroo South is a major mixed use development located on Sydney’s CBD waterfront.  The development comprises three high-rise commercial towers, sharing a common two-level basement to be retained around its perimeter by a 770m long cut-off diaphragm wall.

Old harbour walls and buried wharf structures, steeply dipping rock levels and buried cliff lines presented many challenges for the design and construction of the wall. The design also had to account for the close proximity of existing building basements and complex bulk excavation staging. Provisions for the construction of future Sydney Metro tunnels which crossed the wall alignment at two locations were also made. At one crossing location, the diaphragm wall was required to carry large building loads, as the tunnel geometry restricted the use of piled foundations. 

Stringent wall deflection and settlement criteria needed to be met, as well as the need to consider seismic events and sea level rise due to climate change. An existing basement along the southern site boundary just 9m from the diaphragm wall precluded the use of conventional inclined ground anchors, while the design was further complicated by the limited rock cover over the tunnel crown.

Installation of cantilever wall with vertical anchors 

As designer for the D&C contractor Menard Bachy, we designed a diaphragm wall socketed 0.5m into the Sydney sandstone and temporarily supported by a single row of ground anchors, to be propped by the basement floor slabs in the long term. The diaphragm wall was designed using the finite element program PLAXIS to assess structural actions, deflections and associated ground movements, as well as loads in the ground anchors and floor slabs. 

The extremely variable bedrock level across the site ranged from only a few metres below ground surface to more than 30m. The design required a large number of wall cross sections, with analysis of each section including 13 stages to simulate temporary and long term conditions. 

After exploring several options, a cantilever wall with vertical anchors was chosen as the design. This solution comprised 1.2m thick wall panels embedded at least 3m into sandstone. Wall displacements were controlled by installing vertical prestressed anchors on the soil side, connected to the wall by a cast-in-situ concrete outrigger beam. 

Several wall panels were extended to greater depths to form a “trouser leg” arrangement to transfer building loads away from future tunnels. They will be constructed through several diaphragm wall panels, therefore ‘soft eyes’ using fibre-glass reinforcement were incorporated into some panels to facilitate future tunnel excavation.

The impact of tunnel excavation-induced ground movements was also considered using state-of-the-art numerical analysis to simulate the tunnel excavation process. The reinforcement design was then modified to accommodate increased structural actions.

Innovative designs met tight timeframe and stringent quality criteria

The diaphragm wall was delivered within very tight timeframes and overcame significant in-ground challenges. Innovative design solutions including vertical anchors, removable anchors, “trouser-leg” panels and detailed numerical analysis all contributed to the outcome. As a result, we achieved a design that accommodated the complex surrounds of the project, future proofed the wall in relation to future metro tunnels and supported the significant scope of the project.