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Melbourne viaducts replace level crossings

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An ambitious programme to remove level crossings from an Australian suburban railway has proved a fertile ground for innovation.

It seems like an almost impossible engineering challenge: removing nine level crossings with minimum disruption to the running of a busy suburban railway or to local residents. But, with some lateral thinking, the Victoria state government in Australia and its construction partners have achieved it, by adopting a solution based on technology more commonly used for constructing long viaducts in open terrain.

The current Victoria government was elected in 2015 with a pledge to remove 50 dangerous and congested level crossings across Melbourne by 2022. The city and its outlying areas are criss-crossed by railway lines that carry suburban passenger and freight trains at ground level, often crossing roads via level crossings, which causes serious traffic disruption.

Programme starts

Immediately after the election, the government established the Level Crossing Removal Authority (LXRA) to deliver the improvements, committing a budget of $2.4bn (£1.3bn) to fund it. The programme is already well underway: 15 level crossings have been removed so far and replaced with safer and less disruptive alternatives. Eight stations have also been upgraded. By the end of 2018, a total of 28 crossings will have been removed.

Many of the removals have been achieved by building large open trenches and dropping the level of the trackbed to run below the roads that cross them.

Melbourne

Melbourne

But on the Caulfield to Dandenong section this was not an option. This 20km length of suburban rail on the south eastern side of Melbourne contains nine level crossings, of which four are extremely close together and in a very narrow corridor.

LXRA says an open cut solution to take in all nine of the crossings would have resulted in large and long trenches through local areas that would have divided communities. It would also have required around 230 days of rail line closures during two years of construction.

And trench construction would have been hampered by some very significant utilities, including a water main, a 66kV power cable and a major gas transmission main, which runs directly across the rail corridor. Relocating this main would have been incredibly disruptive to gas supply to a large part of Melbourne and would have taken many years to complete – adding significant cost to the project’s budget.

Long sections of open cut trenches would also effectively have cut off or redirected natural overland flood paths, impacting the viability of existing vegetation and risking flooding of the trench and rail line.

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Beam launching technology is being used for the first time in a live urban rail environment to build one section of viaduct in a constrained space

In addition, there is a high water table in this section of the rail corridor, adding to the engineering challenges. An extended open cut trench solution would have caused temporary – and possibly long-term – changes to the ground water conditions; and contaminated groundwater would have had to be pumped out and treated during construction.

To overcome all these constraints, LXRA decided to remove the level crossings by elevating the railway onto three viaducts. As well as taking around one third of the time to build than the trench option, it also has the benefit of opening up around 22.5ha of space beneath the railway that can be used for urban parks and community spaces.

For the most part, the elevated sections can be built by what LXRA project director Brett Summers calls “business as usual”: installing piled foundations, building pile caps and precast concrete piers, then lifting in precast concrete Super T beams using tandem crane lifts. But between Caulfield and Hughesdale, where there are four crossings in extremely close proximity, the rail corridor is so narrow that there is absolutely no space for a crane to sit.

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Viaduct section are moved along the structure after assembly

It is also Melbourne’s busiest stretch of railway, with so many trains running during the morning peak that the boom gates on the level crossings stay down for a total of up to 87 minutes. So the need for the crossings to be removed is as great here as anywhere else on the network, but the challenges are even greater.

“Once we had figured we would go up in the air, we had an issue to solve: how do we build an elevated viaduct on Melbourne’s busiest railway line and narrowest corridor?” says Summers.

“How were we going to build this and not disrupt passengers when we didn’t have room to bring in lifting cranes?”

Summers says LXRA and its alliance partners “looked around the world at what had been used previously”. And what they found was the beam launching technology that is regularly used in Asia for building huge viaducts through open landscapes, often with a straddle carrier being used to add each new span to the structure.

But this would be the first time this equipment would be employed to construct a structure above a live railway in an urban environment. “We looked at it and thought ‘why couldn’t we adopt a similar method – and do it through a very constrained corridor?’” says Summers. “So, we got some experts in, and finally figured out that we could get it to work; we could launch these beams and keep the trains running.”

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View from the cab of one of the transporters

A $1.6bn (£880M) contract to remove the nine level crossings between Caulfield and Dandenong was awarded in April 2016 to an alliance of Lendlease, CPB Contractors, WSP, Aurecon, LXRA and Metro Trains Melbourne (MTM). The idea to use a straddle carrier and beam launching system – the first ever Australian use of a straddle carrier and gantry crane configuration in a live rail corridor – was devised during the bid phase, when LXRA was talking to two potential alliance partners.

“We knew the limitations of the site, how narrow it was, the ground conditions etcetera, and we knew we needed an innovative solution,” says Summers. “We’d gone to the market without any preconceived solution. It’s a challenging environment, so we said: ‘you guys tell us what you think the best solution is’.

“As the client, we worked with two separate alliances to come up with two separate and independent solutions. Then the state government chose what we thought was the best solution.”

He continues: “We had two key objectives – minimising the impact to the travelling public, and minimising the impact to the local community – and we worked with our construction partners to achieve that.”

Slender structure

The solution was a structure that is slender in appearance, with long spans between slim supporting piers to reduce the visible bulk, built using a construction method that avoided excessive lorry movements and enabled the rail line to keep running almost unaffected.

 It was achieved by first establishing an assembly area close to Murrumbeena Station at the north west end of the site, equipped with two large gantry cranes. Precast concrete beam segments measuring 5.5m in width and 2.6m in depth were manufactured close by in the Melbourne suburb of Pakenham and then delivered to this assembly area, before being assembled at ground level into beams.

Each span is made up of between eight and 14 individual precast segments, depending on its location on the route. At the stations, the beams span 27m between piers, while on the rest of the route they are 40m long. The beam segments were stressed in pairs using stressing jacks, and then grouted.

Once stressed, each beam, weighing between 280t and 420t, was lifted onto the elevated rail deck by the two gantry cranes. Then the straddle carrier, which itself weighs in at 265t, picked up the beam and transported it into position at the far end of the structure. This machine was also used to install the spans.   the help of a 94m support beam. In some places, installation was carried out just 5.5m away from the fences of adjacent residential properties, and the straddle carrier was fitted with cable guidance technology to prevent it deviating off line.

174 spans

In all, 174 spans have been assembled and installed using this method since February 2017, to create the 4km long twin track viaduct between Caulfield and Hughesdale.  The alliance estimates that the straddle carrier has travelled more than 260km back and forth along the rail line during this time.

The beams are supported by precast concrete piers that were also delivered in sections and stressed on site. “The height varies, because the viaduct has to remain at the same grade, but the ground level fluctuates,” explains Summers. “But, typically, the new viaduct is between 8m and 12m in the air.”

The piers have a 2.4m by 1.4m cross section, and the main track beams sit on 2.5m by 3m by  2.5m deep crosshead beams.

The foundation design varies along the route due to changing ground conditions, but most of the elevated structures sit on piles, which are around 20m deep, and socketed into the underlying siltstone.

Nearing an end

The last beams on the Caulfield to Hughesdale viaduct were fixed into place earlier this year, and the straddle carrier was lifted off the deck and dismantled in April, along with the two 30m high gantry cranes that have been part of the Murrumbeena skyline for many months.

“At the moment we are storing the straddle carrier, because there is a broader programme of these level crossing removals across Melbourne, and there are other locations that are very similar,” says Summers.

“The machine could be modified to deliver a very similar outcome in another location.”

Work is now continuing on the remaining elevated structures that will enable the authority to remove the remaining five level crossings on this section of track. As soon as each viaduct is complete, the track is installed using a concrete slabtrack system, which will considerably reduce noise on the line.

The alliance’s contract also includes building five new stations at Carnegie, Murrumbeena, Hughesdale, Clayton and Noble Park.

Ground level stations demolished

“The existing stations at ground level have  been demolished, and are being replaced by new elevated stations, along with escalators, stairs and lifts,” explains Summers.

During the project, the alliance has been trying to minimise its environmental footprint. It has estimated that the option to elevate the railway resulted in 70,000 fewer lorry movements on local roads compared to an open cut trench solution. And it has also managed to use biofuels to power the gantry cranes.

Specialist fuel supplier Green Power Solutions was brought in to look at ways of using cleaner burning fuels to power the gantry cranes, and came up with a biodiesel mix used in combination with a 550KVA Scania generator that resulted in a 70t reduction in carbon emissions. The biodiesel consisted largely of recycled vegetable oil, sourced from a food manufacturing plant in the north of Melbourne.

The alliance’s construction director Simon Barnes believes the knowledge gained on the Caulfield to Hughesdale viaduct will be invaluable to the Australian construction industry.

“This project leaves an important legacy not only to the community but to the industry,” he says. “We have taken a construction technique usually associated with long viaducts built through open terrain, and successfully – and safely – applied that in an Australian residential environment.

“Adapting and evolving that technique to a local suburban environment has led us to form relationships with engineering experts around the world,” he adds.

These experts include Italian specialist equipment manufacturer DEAL, which fabricated the straddle carrier and shipped it to Australia in pieces for assembly on site.

Barnes says the new skills, processes and protocols associated with this “national first” required a lot of behind the scenes activity, as well as an extensive training programme: “We’ve trained around 300 workers to operate the carrier, gantry cranes and support beams, as well as [to] cast and tension the spans that make up the rail deck. “It’s not insignificant from an industry perspective as we will retain this knowledge in Australia along with a whole range of highly transferrable skills.” 

 

 

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