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Shuffling the deck

A tight time schedule, a confined site and live railway tracks are just some of the challenges facing the engineers of a new showpiece development. Mary Munro reports from Melbourne

The prosperity of Melbourne during the last century was built on the railways. Unfortunately the Victorian engineers took little thought for future development when locating the rail tracks. As a result a prime site in the heart of the city between the business centre and the River Yarra has for many years been occupied by vast marshalling yards.

Although the city authorities had earmarked the site for a major scheme some time ago, it was not until the railways were reorganised, reducing the number of tracks from 54 to 12, that redevelopment became a practical possibility. It also coincided with the city's proposal for a landmark project, Federation Square, to commemorate the meeting in May 1901 of the first Parliament of the Commonwealth of Australia.

The plan was to deck over the rail lines to create a 40,000m 2space on which a civic centre would be constructed. The project was the subject of an international competition, which was won by the London-based architectural group LAB and Bates Smart of Melbourne.

The buildings, none more than four storeys high, are grouped on the eastern half of the deck, leaving a wide plaza. They comprise a museum, a covered atrium, a cinema and media complex including a TV centre, two commercial buildings and two other slender blocks known as shard structures. The open plaza provides a link between the city centre and the river, and will also act as a meeting point for celebratory parades, and other events in the city calendar.

Well before the final design selection had been made however, preparatory work on the site had to be started if the tight schedule set by the commemoration date was to be met. The first stage kicked off three years ago when a joint venture of mechanical engineer Kinhill and consultant Connell Wagner began the rationalisation of the rail tracks and design of the crash barriers and support walls for the deck.

The geology in the area is complex.

It is overlaid with thick layers of soft alluvial silt which is aggressive to concrete and corrosive to steel, and the water table is high. Within the silt are some basalt inclusions at a depth of 15m to 16m, but in other areas the soft material goes down 30m or more to the underlying mudstone. Over 800 bored piles, taken down to bedrock, have been installed between the rail lines to support the walls.

'At this stage we had no idea of what was to be erected on the deck, or where it would be located, which made design something of a problem, ' explains Brian Dean of Connell Wagner. 'In addition to meeting the high parallel and horizontal loadings demanded by Australian rail regulations, the crash walls also had to be capable of accommodating column loadings of up to 1,200t at locations which had yet to be determined.' Furthermore, the schedule was tight and construction had to be carried out under high voltage cables, and with trains moving close by.

To overcome the problems of space, access and programme, designer and contractor Leighton evolved an innovative sandwich design for the walls. This consisted of two 150mm thick precast panels, 4m high and 3m wide, with a 450mm gap between filled with reinforcement. The panels acted as permanent formwork, with the area between pumped full of concrete. Using this method it was possible to erect the 1.35km of 4m high concrete wall at high speed during the limited possession times. The record was 300m of wall plus 150 piles to 15m depth constructed over a 12 day period.

With the development design still under discussion, the walls were not taken up to final height. Their completion, the design of the deck slab and the structural design of the buildings have been in the hands of consultant Hyder, acting as engineer to the winners of the architectural competition, with Multiplex as management contractor.

The deck consists of a 300mm thick concrete slab, about 150m by 250m, sloping in two directions, west to east and down towards the river. It is supported for the most part on steel beams spanning between the crash walls. The crash walls run parallel to the rail lines, so they are not on a regular grid nor continuous, and were not necessarily in the most convenient place to support the buildings. And with the final design still subject to changes, loadings had to be calculated largely on a judgmental basis.

There were also more detailed concerns. One was the need to limit vibration from the trains. This is particularly important for the building which houses the cinemedia centre, with its videotheque, two cinemas and the Melbourne operations of national TV company SBS. Low natural frequency spring damper bearings have been placed below the deck to isolate the structure, reducing the vibration from between 20Hz to 40Hz down to 3.5Hz.

For other buildings, elastomeric bearing pads have been installed.

The possibility of electrolysis from the high voltage lines below the slab had to be taken into the design considerations, and there were also complications due to the architectural design of the buildings themselves. Paul Strickland of Hyder admits it has been one of the most complex design jobs he has ever tackled. 'In addition to the problems of vibration attenuation and electrolysis, loadings varied considerably in different sections of the deck, depending on whether there was a building there or not, and this had to be taken into account in calculating the dimensions of the steel beams. These also had to fit in the restricted profile between the rail track clearances and the suitable top surface level.'

The need to work over live tracks, made no easier by a change of ownership after the railway was privatised, meant that a great deal of planning was needed to make the best use of the available time.

'We tried to push as much as possible into the daytime, but this wasn't possible for all the work, ' said Multiplex project manager Tony Hodder. The wall extensions were completed first, but the steel beams, up to 24m long and 1,200mm deep, had to be placed overnight. 'Possession was between 12.30am and 4.30am, but by the time we had everything set up we had only two and a half hours to get the beams and bearings in position. Accuracy was essential as there was no time for adjustments, ' he adds.

The deck has been constructed in seven sections, the last of which was completed in October 1999. Construction of the buildings is currently under way. These are complex in shape, with sharp changes in angle, and 'setting out has been a bit of a nightmare', Hodder admits. The facades of the main buildings combine panels of stone, glass and steel arranged in an irregular pattern, and placing and fixing the cladding will be no easy task.

Close co-operation between the client, Melbourne's Office of Major Projects, the rail operators, contractors and consultants has helped to keep the project on programme. Tony Hodder is confident that all will be completed by May 2001 when Melbourne celebrates the founding of Australia as a unified country.

Graham.Whitehead@Hyder-con.co.uk

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