The challenge was to reroute the twin tracks of one passenger railway underneath both another passenger rail line and a freight line, on a site constricted and criss-crossed by a flood-prone creek, a major sewer, a nearby railway station, a tram overpass and a well-used local road.
As if that weren’t enough, the passenger lines were earmarked for electrification to replace diesel rolling stock, requiring interaction with the operatives putting in new track and signals; and the freight route had to remain operational throughout construction. Our input helped joint venture contractor Thiess-York deliver the entire underpass project, from design to construction, in only a year.
Passenger trains have right of way at two grade crossings: the Goodwood and Torrens junctions, which means freight trains have to park up if there’s a passenger train on its way. What’s more, the freight train has to be kept short enough to avoid obstructing a well-used road crossing. Removing the conflict will allow longer trains to move freely through the urban area and prevent the heavy trains powering uphill from a standing start, reducing delays at road crossings, cutting noise pollution for local residents and encouraging a shift in freight haulage from road to rail.
The frequency of passenger trains is set to significantly increase through the leafy suburb in the next few years, and capacity of freight routes is expected to expand too. Resolving the scheduling conflict is a priority of both the South Australian Government’s Department of Planning, Transport & Infrastructure, which owns and operates the passenger railways and is principal client for this project, and the Australian Rail & Track Corporation, which owns the freight lines. Complicating the project was the requirement to create an aqueduct for the flood-prone Brownhill Creek.
Simplifed solution halves time and cuts cost
As structures designer, we reshuffled route alignments to thread the twin Seaford passenger lines through an underpass, which would keep the Belair passenger line and freight line unobstructed and at grade.
The underpass includes two separate deck structures, one that carries the at-grade railways and one that serves as a level crossing for the Victoria Street road. The aqueduct is located between the two. A rail diversion was built to keep the freight line running for the three months while the underpass deck was built. The freight line was then routed across the deck before excavation of the underpass began. This ‘top-down’ construction method allowed the underpass works to continue unhindered by, and largely without hindrance to, the freight train schedule.
The 450m long underpass needed to be built within a six month window, when the passenger tracks would be closed for electrification and installation of new signals and sleepers. Six months is half the time this sort of project would usually take, so we simplified design and construction details to allow for a speedy build.
At one end of the underpass, the soil was cut into stable slopes and capped with non-structural concrete protection, forming revetment walls up to 6m high. This approach was last seen in the area some 80 years ago on a road underpass, which is still performing strongly. The method was made possible by the stiff cohesive clays, and allowed for straightforward construction that was easy to execute.
We employed the observational method, a continuous foundation and geotechnical engineering process that plans for the most probable conditions, rather than the worst case scenario, and monitors every stage of construction. Under the observational method, which we pioneered and popularised, designs allow for all contingencies. Under the best case scenarios, major efficiencies can be achieved. This approach unlocked the revetment wall application for the first time in 80 years, saving substantial time and cost.
Structural innovation saves weeks
The constricted corridor ruled out a similar approach at the other end of the underpass. There, retaining walls are formed from cantilevered piles with shotcrete arches in between. Capping beams tie piles together. Usually, the walls of this type of structure would be propped by the concrete track slab, but that would have added significant time to the build, so our track slab structure is independent, making it lighter and faster to build.
More construction time was saved in the design of the railway deck. We designed a deck 52m long, built from 750mm deep precast concrete planks. Usually, these would be topped with in situ poured concrete which we eliminated. Removing the top slab has no impact on structural integrity but cuts weeks from construction.