Melbourne’s new Metro Tunnel goes further under the city than ever before, and features a spectacular triple-vaulted concourse
It takes four long minutes to descend from the centre of Melbourne to the platform at State Library station – but it’s worth the trip. Eleven storeys below the city, a single monumental space has been carved out of the bedrock, triple-vaulted like the nave of a cathedral and 10m high at the central aisle.
State Library is one of five stations on the new Melbourne Metro Tunnel, the largest infrastructure project in Victoria for nearly half a century, and the deepest excavation in the city’s history. Designed by a global team including Rogers Stirk Harbour + Partners, Weston Williamson + Partners, and Hassell, it was inspired by the Moscow Metro, the 1930s “people’s palaces” intended to boost Soviet citizens’ morale with ornate chandeliers, heroic murals and rococo columns. The 2020s Melbourne version has sweeping red-metal arches and art deco inspired pendants hanging beneath exposed concrete soffits.
But the most striking thing about it is undoubtedly the scale. The platforms and concourse are joined together in a 30m-wide cavern, lined with exposed concrete, and the platforms stretch for 220m, the length of three football pitches.
The underlying rationale for such a spectacular space is the mundane but critical consideration of pedestrian circulation. The Metro Tunnel carries about 40,000 passengers an hour at peak times, with up to 1,800 on each of the 160m-long trains. Many of these will be disembarking at State Library for the central business district, or changing for connections at Melbourne Central station via a short pedestrian tunnel. Opening up the platforms and the concourse alleviates congestion and claustrophobia at a potential bottleneck.
But how to create such a huge space, 35m below one of the densest urban districts in Australia? A single space spanning 30m would naturally have a very high vault, generating huge amounts of excavation material to be carried to the surface. Flattening the arch would reduce the amount of spoil, but wouldn’t be nearly as structurally efficient.
Instead, structural engineer Arup has devised a unique “trinocular” solution, forming the cavern from three tunnels that overlap like the Olympic rings. This resulted in a smaller excavated volume, but with more concentrated useable space. The vault over the central concourse was slightly lower, but the separate arches over the outer tunnels left room for service channels to run above the trains.
The overlapping trinocular tunnels were initially lined with 350mm of fibre-reinforced shotcrete, secured with 4m rock bolts. When all three were finished, a permanent concrete lining was cast in situ. At the overlaps, the arches merge into a 2m-deep downstand beam supported on ovoid concrete columns and another 2m-deep upstand beam cast into the basement slab.
Above ground, the entrance echoes the neoclassical State Library of Victoria opposite, maintaining its composure on a congested corner next to an eight-lane tram corridor. A portal of 12 geometric precast columns topped by 1.2m-deep, 18m transfer beams frames the escalator shaft. These beams, which will eventually support an over-station development, weigh 70 tonnes and had to be lifted into place by a 400-tonne crane.
Low-carbon concretes were used throughout the project, including the shotcrete linings, with an average cement replacement level of 51%. State Library is also notable for the use of recycled crushed glass in a structural-grade concrete. The mix, which swaps the glass in for 25% of the industrial sand used in concrete production, was used to build temporary concrete “blinding” slabs as part of the construction of the primary structure.