The upgrade of a bridge that carries four railway lines over a busy roadway required innovative design and construction methodologies to minimise track possession times.
For three days, between Boxing Day (December 26th) 2014 and New Year’s Day 2015, the T1 Western Railway Line and Parramatta Road in Sydney’s west were both closed down to facilitate the upgrading of the underbridge1 over Parramatta Road (see Figure 1). The upgrading work was needed “…to keep the bridge safe and reduce the disruption and cost of ongoing maintenance” and involved replacing aged bridge girders, addressing insufficient clearance for vehicles on Parramatta Road, strengthening abutments and piers, and improving drainage and maintenance access by replacing the timber transom deck with a precast concrete deck slab.
The underbridge crosses Parramatta Road in Granville, on the T1 Western Railway Line between Granville Station and Harris Park Station. The bridge comprises two approximately equal spans and has a total length of 35.8 metres (117.5 feet). It carries four railway lines over Parramatta Road on a skew (oblique angle) of 47 degrees and consists of four separate bridge decks each supporting a single track. The two central bridge decks are each comprised of two riveted steel girders and were built in 1928 to replace a level crossing. In 1983 the two outer bridges were added as part of the quadruplication of the line and comprise twin steel welded I-girders.
The upgrading work involved:
- replacement of the two central bridge spans built in 1928;
- strengthening work to both abutments and the pier in the road median;
- replacement of the timber transoms with a concrete deck and provision of a direct fixed track on resilient rail plates to each of the four bridge spans;
- replacement of all bearings;
- raising the bridge spans to provide the 5.3 metres (17 feet) minimum clearance to vehicles on Parramatta Road and adjusting the track vertical alignment upwards by approximately 100 millimetres (3.9 inches);
- reconstructing the ballast walls to suit the new alignment;
- adjustment of overhead wiring to suit revised alignment;
- providing deck drainage; and
- provision of walkways and refuges in accordance with Sydney Trains standard ESC310.
Innovative Construction Methods and Design Features to Meet the 3-Day Construction Window
Due to the limited construction timeframe available and the high risks involved in delaying completion of the bridge upgrade during the track possession2, the construction delivery model employed was unusual in that Sydney Trains used their construction team (Major Works group) to manage the works by engaging a number of specialist construction subcontractors.
The reason for this model was the high risk of consequential damages if the construction period was delayed and the major rail corridor (both commuter and freight) as well as the major arterial road were not open in time. This risk would have disproportionately increased the price for a construct only delivery model. Instead Sydney Trains decided to take on the risk themselves using experienced project managers who have completed this type of accelerated bridge construction work before.
It was also why WSP | Parsons Brinckerhoff had to work very closely with the Sydney Trains team to carry out the bridge structural design, the overhead wiring design, and the construction traffic management plans we were engaged for.
During the detailed design development for the upgrade of the bridge, consideration was given to the constructability, speed of construction, and safety-in-design assessments in close consultation with Sydney Trains. The following major issues and design features were identified:
- The mass of the precast concrete deck panels was to be limited to 6.5 tonnes to allow the panels to be easily lifted in place by a small mobile crane capable of driving on the deck of the bridge.
- Bracing was provided between the new girders for the Up Main and Down Suburban line tracks3 to ensure stability during the lifting of the girders in pairs. These pairs of girders were designed to be fabricated in approximately three equally weighted assemblies. This minimised the number of lifts on site, speeding up construction while keeping each component small enough to be transported to site and lifted into place.
- The piers and abutments were strengthened by installing threaded rods vertically through the abutments and by core drilling and grouting the piers. These needed to be installed prior to the replacement of the bridge girders and as such were located so that they could be installed with the existing girders in place.
- The work was designed in consultation with Sydney Trains to be completed within an extended track possession.
- Stability of the abutments for the Up Main and Down Suburban line bridges requires that continued monitoring is put into place. Although stability limit states were marginal, it was noted that no movements have been detected over the many years of service to date.
- As the track was being raised by 100mm, the height of the ballast wall also needed to be raised. This was achieved by replacing the existing wooden ballast logs on top of the ballast walls with deeper precast concrete ballast beams.
- The rails were to be supported on resilient rail plates. High density polyethylene (HDPE) packers were specified beneath the track fixings rather than grout which would typically be used if time was not an issue. HDPE packers speed up construction time, as there is no setting of curing time, but require tighter tolerances as a result of the maximum allowable height of the packers.
- The increased level of the track required the overhead wiring to be adjusted. The design of the overhead wiring is not addressed in this article.
A number of options for the replacement of the deck were considered. The original option was to replace the precast, prestressed concrete deck plank units of all four bridges with a composite deck, while keeping the existing substructures.
Each track was to be supported with four 950mm (3 ft.) deep girders with a 200mm (7.8 inch) thick cast-in-place composite deck slab as shown in Figure 2.
Subsequently, this option was revised for a steel girder and precast concrete deck solution as shown below in Figure 3. This design solution was chosen due to the construction methodology needed and to reduce the track possession time required to complete the construction. The added weight of the concrete girders in the original design discussed above also placed additional stress on the already highly stressed footings of the original bridge.
A further factor in the decision to go with the steel girder solution was to achieve better productivity from the existing rail infrastructure. The Down Main and Up Suburban line bridges (the outside bridges) were only 31 years old and had many years of service life remaining. By reducing the width of the new precast concrete deck from 4.4 metres (14.4 feet) in the original option we were able to limit the loading on the existing steel girders under the critical load case of train derailment. This allowed the existing girders to have sufficient capacity to continue to be used.
The original construction methodology adopted was to construct the deck replacement with a two staged approach. This involved working under a track possession, removing the overhead wires and, with a heavy lift crane, removing the central two bridges and replacing them with the new structures. In a second track possession, the bearings for the outside bridges would be replaced and the girders raised. The transoms would then be replaced with the precast concrete deck units.
The overhead wiring configuration over the bridge revealed a number of wiring crossovers which had to be moved out of the way and required significant track possession time for this operation. To overcome this challenge, an innovative construction methodology was devised by Sydney Trains which utilised a hydraulic travelling and lifting system.
The system comprised two sets of two self-propelled heavy hydraulic lifters that travel along pre-assembled tracks. This method lifted the old girders out and the new girders in by supporting the structures from underneath on top of the tie beam, using hydraulic lifting (see Figure 4).
The main disadvantage of this method was that the edge deck needed to be temporarily removed and set aside while the central two decks were removed and replaced.
This project successfully utilised innovative design and construction methodologies to meet the 3-day construction window, minimising disruptions to the public as well as gaining greater productivity out of our aging rail infrastructure. Reusing the existing bridge girders with new bearings and deck meant significant cost reductions and minimal disruption to commuters. The innovative use of tracked hydraulic lifters for replacing the girders meant the track possession times were reduced and disruption minimised. The strong working relationship and close collaboration of the design team and the construction team has been a critical element in the success of these works.
1Underbridge: A bridge carrying the railway and allowing a roadway to pass underneath.
2Track possession is the term used for closing a stretch of railway for rail improvement projects. They are strictly time-limited to minimize passenger disruption.
3Up refers to the direction “toward Sydney” and down refers to the direction “away from Sydney”.