Melbourne Oz Innovative Sound Tube

MELBOURNE OZ

Innovative Sound Tube

The Melbourne Australia CityLink toll project is most notable for its long tunnels and its toll automation features, but it also has an innovative sound tube over an elevated section. Located on the western section which links the airport-oriented Tullamarine Fwy with the central business district and the motorways heading southeast and west, the tube is part architectural, part functional.

For traffic traveling into central Melbourne from the airport the tube will be the first element of a signature entryway called the Gateway, a kind of modern sculpture-in-motion that begins with a smooth curved enclosure over the road and then outside the tube presents the contrast of large (30m long) slightly inclined square section posts and a huge 70m beam that looms at a low angle over the roadway. An elaborate light display will make the thing glow at night. Opinions will differ strongly over the merits of the whole urban sculptural event, and I reserve judgment until I see and travel it.

But the tube at least is also functional, in that it allows the road’s noise to be attenuated at highrise apartment buildings nearby in an elegant way.

Three apartment blocks about 150m (500') from the road soar to 21 stories and the tube allows high angle noise waves to be captured that would have required very high vertical walling. Roger Rugless the engineer in charge told us that the $5m for the 300m (985') long tube represents some cost premium over walls, but that the walls would have been both unsightly and difficult to construct with sufficient strength to withstand storm winds.

Curved and tapering steelplate frames that form oval shapes enclosing the twin roadways are the main architectural element of the tube. The frames are attached to the edges of the concrete box girders of the 2x3-lane elevated roadway beneath the cantilevered roadway deck. They rise up from under the roadways well clear of the parapets. Ten thin struts span each gap between frames longitudinally down the roadway stabilizing the structure which is a kind of hightech pergola. The sound attenuating cladding forms a C-shaped enclosure on one side and roofs over one outside lane of traffic completely. On the other side cladding rises only a little above parapet level.

63d(B)AL10(18hour) is the government required sound limit at the face of the highrise residential buildings, compared to 70d(B)A currently experienced. Clearly more cladding could be readily added to other parts of the tube if further highrise buildings needed noise protection.

Rugless says the structure was designed by a local architect: “We believe the skeletal design may be original but don’t know if the use of a tube for acoustic purposes has been done before. It seems an obvious solution for attenuating noise for highrise buildings close to an elevated motorway. Standard type acoustic walls would have done the job. However they would have been quite high, unsightly, and a problem structurally being on the edge of the bridge cantilever.”

The tube is 300m (985') long, 42.4m (139') wide, and at its high point about 8m (26') above the roadway deck, providing minimum clearance in the outside lanes of 5.5m (18'). It is all done in a regular galvanized steel finish.

CityLink is an inner area investor-built toll road currently under construction in Australia’s second city and is expected to take over 100k tolls daily when it opens around the end of the year. Shares in the concessionaire trade on Australian stock exchanges and no taxpayer money is involved in its construction. Construction is by Transfield-Obayashi, a joint venture of the Australian Transfield and the Japanese Obayashi construction companies. It will be the first toll road in the state of Victoria. Sydney, the country’s largest city has a bunch of toll roads but none as spectacular in engineering terms, or as strategic to area mobility, as Melbourne’s CityLink. (Contact on the tube - Roger Rugless rrugless @transurban.com.au)