The Elizabeth Street Retaining Walls

From Engineering Heritage New South Wales

    In February 1922 work began in earnest for an underground electric railway in the centre of Sydney. Designed by John Bradfield it is a high-level system generally just below the surface. This allowed for shorter passenger accessways and easier ventilation. It also meant that significant parts of the route were above ground where natural levels were lower. These sections were across Belmore Park between Central Station and the tunnel entrances at Goulburn Street, and eventually across Circular Quay. The high-level scheme also made it just possible for the railway to reach the high deck of the proposed Sydney Harbour Bridge.

    On the above ground section just north of Central Station the route crossed three streets – Eddy Avenue, Hay Street and Campbell Street. The first named is crossed by a very sophisticated reinforced concrete continuous beam bridge over three spans, only the second reinforced concrete railway underbridge built in NSW, and the first to carry passengers, while the latter two streets were crossed with concrete arches. The bridges’ story will be told in another article, but between them, facing Elizabeth Street, high, elegant, sandstone-faced retaining walls were built. It is perhaps so well done that it is not noticed as being the huge engineering structure that it is - it just blends in as part of the glorious golden Sydney Sandstone face of the town.

The wall functionally completed, although the parapet is yet to be added in October 1923.

    The wall is a gravity concrete structure, or perhaps more accurately a sandstone and concrete composite as the sandstone blocks are massive and thoroughly bonded to the backing concrete.

    The wall is founded on rock at about 27 feet (8m) depth. Mass piers, about 7ft (2.1m) by 16ft (4.8m) are spaced 18ft 3in (5.5m) apart and the space between them is spanned with massive buried unreinforced concrete arches. The excavations, about 10,000 cubic yards (8,000 cubic metres), were all done by hand, with men loading large buckets which were lifted out by relatively small, usually electric, rail-mounted cranes and the spoil disposed of to form the embankment behind the wall. Upon this embankment concrete mixers were set up and their product fed straight into the piers and arches by chutes, or by buckets and cranes. The mixers were hand loaded from stockpiled sand, aggregate and bagged cement.

Work had been in progress for only two months, but many piers have been excavated and concrete is being poured. One crane is electric, the other is steam-powered. The concrete batching plant on the stage is also steam powered. April 1922
By July 1922 several courses of the wall were in place. Note the large cross section of the sandstone blocks.

    The sandstone for the wall came from the Government Quarry at Maroubra. Although the face of the wall shows perfectly fitted masonry, the blocks are deliberately of varying thickness to create a chequerboard effect to bond with the concrete of the gravity wall poured against it. This wall was placed in 3ft (900mm) layers stepping in from 10ft (3m) to 3ft 9in (1.1m). Keying between steps is achieved by the placement of large sandstone rocks in the wet concrete.

    There were two complications to this regular design. The Bondi Sewer crosses the site diagonally about 12ft (3.6m) below the surface and the pier spacing had to be adjusted to avoid it. An intrusive igneous dyke, wider than the standard pier spacing, also cuts through the sandstone and it had weathered to a great depth. Piers were positioned either side and a slightly longer arch utilised.

Placing concrete behind the sandstone face in January 1923. Note the uneven thickness of the blocks to ensure a monolithic structure, and the ‘keystones’ set into the wet concrete at the construction joint.

    The opposite side of the embankment, facing Belmore Park, was at first going to be provided with a brick retaining wall. It is not so high or visible as the earth ramp for the tramway access to the station colonnade obscures it. However, it was found that the igneous dyke had widened considerably in the width of the railway alignment such that spanning it was problematic. The solution was a batter with hand-packed stone and the planting of Ivy, Virginia Creeper, Climbing Fig and Bignonia Tweediana which soon turned the embankment into a mass of green and yellow. Was JJC Bradfield a botanist too?

    The photos in this article are from the Kathleen M. Butler albums in the Mitchell Library, though they exist in several other public collections.

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