Two Walls Are Better Than One

Quick Facts

Builder:
CPB Contractors

Architect:
Woods Bagot

Owner:
Transport for New South Wales

Structural Engineer:
Taylor Thomson Whitting

Type:
Commercial

Double Wall precast concrete panels:
Austral Precast (manufacture, transport and installation)

Location:
corner Clarence Street and York Lane, Sydney

Function:
Government infrastructure

Austral Precast Double Wall concrete panels are at the bustling heart of Wynyard Walk,
a world-class pedestrian tunnel linking Wynyard Station to the Barangaroo precinct.

Up to 20,000 commuters and visitors per hour to Sydney’s CBD flood through the Wynyard Walk tunnel from Wynyard Station. The Clarence Street Entrance building is the hub of Wynyard Walk, a pedestrian thoroughfare that will transform access between the city centre and the Barangaroo precinct.

The vertical elements in the building’s towering, multi-level atrium are constructed with revolutionary Double Wall concrete panels from Austral Precast, Australia’s foremost precaster.

The Wynyard Walk project combined excavation, demolition, tunnelling and underpinning of heritage buildings, as well as construction. All this took place in the middle of the city, without disrupting station operations!

Wynyard Walk’s Clarence Street Entrance building on the corner of Clarence Street and York Lane is accessed from the railway concourse level via a tunnel under York Lane. From there commuters and visitors can exit through the Kent Street tunnel or take the 180-metre long Wynyard Walk tunnel which passes diagonally under Clarence Street and then tracks under Margaret Street.

The pedestrian tunnel exits via stairs, lifts or escalators at Napoleon Plaza, a new pedestrian precinct near Sussex Street. The final leg of the journey is a pedestrian bridge with lifts and escalators that pass over Sussex Street, connecting directly with the renowned Barangaroo cultural, residential, retail and commercial development.

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The walk from Wynyard Station to the Barangaroo waterfront takes approximately six minutes, unimpeded by street crossings and steep inclines.

The Clarence Street Entrance required excavation to a depth of 18 metres to accommodate the five-level building, with three levels below ground. There are voids on every level with no level being typical. The large spaces between floors – for example, the street level to plant level exceeds nine metres – made construction with conventional structural concrete panels a challenge. It would be difficult to form one face of a wall, install reinforcement, form up the other side and then pour.

As the name suggests, Double Wall panels comprise two reinforced precast shells linked across a cavity by a grid of embedded steel ties. (For this application, trusses were specified, more about which later.) Each precast shell can be 70mm or 125mm thick, with a cavity from 40mm to 150mm, enabling a total panel thickness between 180mm and 400mm.

The system’s flexibility allows for steel reinforcement, edge and opening forms, and built-in windows and doors to be engineered off-site.

Manufacturing a Double Wall panel is a multi-stage process. Reinforcement for the first shell is laid up and the ties or trusses to link the two shells are positioned. The concrete for the first shell is poured and left to cure before removal. Reinforcement for the second shell is laid up and the concrete poured. Before the second shell cures, the first shell is lifted into position and lowered onto the second, allowing the tips of ties or trusses to be embedded to the specified depth into the setting concrete. The assembly is left until the second shell is cured, ready for removal and transportation to site. The inner face of each shell is lightly textured to maximise the bond between the shells when the cavity is concrete-filled on site.

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As well as being ideal for walling for high-rise, residential, commercial and industrial buildings, Austral Precast Double Wall applications include:

  • Liquid-retaining structures
  • Retaining walls and service pits
  • Propped cantilever walls, such as basement walls, with or without significant vertical loads
  • Simply-supported two-way bending structures, such as complex basement walls, again with or without significant vertical loads.

Austral Precast’s experienced team of engineers can advise on design and construction requirements, and the Double Wall Technical Guide, an invaluable resource for engineers, is available to download from their website. This comprehensive manual sets out design procedures for a wide range of applications including shear calculations, joint detailing, propping requirements, water-proofing, and the calculation of fire, thermal and acoustic performance.

For the Clarence Street Entrance, Austral Precast manufactured, delivered and installed 107 panels in 250mm and 400mm thicknesses. A typical panel is 2.5 metres by 7.5 metres.

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Speed and simplicity are hallmarks of the Double Wall system, even in demanding applications such as this. Pour the slab, drop the panels in, butt them together, drop in reo cages at all the corners, and pour the cavities.

Placing prefabricated cages at the corners was initially a challenge on the Clarence Street Entrance project but a system soon fell into place. There were some clashes with starters at the first level, so at the next level the cages were installed first and the panels dropped in on the cages. To save on site work, Austral Precast blocked penetrations with Styrofoam before the panels arrived on site.

The relatively light weight of each panel was a further bonus for this project. The heaviest panel was 10.6 tonnes, a considerable reduction on a similar-sized solid panel of 18.3 tonnes. Solid panels would have been even more of a challenge at street level where the tallest panel is 10.6 metres. Larger panels mean fewer needed, and faster installation with fewer joins.

The Double Wall advantage also came to the fore along a rear wall where there was a gap of just 50mm to the neighbouring building, making the use of a rear formwork shutter impossible. With Double Wall system there is no formwork, simply drop all the panels in, join them, and pour the cavities. The result is an in situ structural concrete wall without the hassle of conventional formwork.

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One significant change for this project from the usual practice was the specification of trusses to tie across the panel cavities. Normally standard ties would be fine, but the Clarence Street Entrance has a 100-year design life which required the additional embedment that the trusses allow.

Another deviation was the specification of 65 MPa concrete with a 180mm slump for filling the panel cavities. Usually, 40 to 50 MPa is adequate. This was a requirement because the walls are structural and designed to carry a further 10 storeys at a future date. The high slump required running a hose down the cavities but so far testing has not exposed any voids.

The Clarence Street Entrance disproved the conventional wisdom that double wall precast construction requires additional trades and more site work when compared with conventional solid panel construction.

However, the ultimate benefit is speed of construction. This was probably the most difficult application imaginable for Double Wall panels. But despite that, the construction team believe they saved three to four weeks off the program. Using Double Wall on a simpler building it would have been even more efficient.

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