Underbridge 313 was a two span steel bridge located on the main Dublin-Cork railway line. The bridge is situated in a Special Area of Conservation and spans the River Awbeg, which is a major tributary of the River Blackwater. It is a picturesque part of the country, and the Awbeg contains a population of otters, as well as salmon and white-clawed crayfish.
In December 2010, the condition of UB 313 was assessed by specialist engineers within Irish Rail as part of their Asset Management System. Due to extensive deterioration to the central pier and the bridge abutment bearing plates, a design solution of steel strapping was applied to the central pier on the Down side. It was also deemed necessary to impose a temporary speed restriction of 10mph on the Down side and 25mph on the Up side across the span of the bridge in order to limit vibration and reduce further movement of the pier.
Irish Rail determined that the best permanent solution was to replace the life expired two span underbridge with a concrete box culvert structure and funding for the project was agreed and approved under the 2011 Railway Safety Programme.
Planning to improve
A 59-hour period on 28-31 October 2011 (the Irish October Bank Holiday weekend) had been determined at tender stage as the first window of opportunity. The existing bridge was to be replaced with two new culverts, and the work included diversion of the River Awbeg, removal of the existing track, dismantling of the existing bridge, excavation for culvert installation, placing of precast culvert units, construction of retaining walls and ballast retainers and track replacement.
The river diversion works used an old arch culvert under the railway line approximately 100 metres from the site, and were undertaken prior to the weekend possession by Irish Rail personnel. The precast culvert units were to be provided free issue by Irish Rail and two precast suppliers were used in order that they could be manufactured in time.
Following a tender process, on 3 October 2011 Coffey Construction was appointed as main contractor for the project. Coffey Construction is a civil engineering, environmental engineering and building contractor with operations in the UK and Ireland. Although better known for heavy civils works, it also specialises in slope stabilisation, trenchless techniques and excavator mounted piling. The company has been a principal contractor for Irish Rail for over 25 years.
Nine days before the possession started, it became apparent that the existing ground conditions required excavation of soft material two metres deeper than was originally thought necessary. This required a sheet piled cofferdam to be constructed around the area to be excavated and the excavation of 6,000m3 of material, a ten-fold increase. Coffey used three of its piling hammers to install the sheet piles in order to meet the deadline date of line closure, working on piling and excavation works 24 hours a day, as most of this excavation had to be undertaken prior to the possession starting. A supported section of original ground was left in place around the bridge piers and abutments. This work and other preparatory works were undertaken with look-out protection in place, which allowed work to be undertaken close to the railway during the day. As a result of these measures, the extra excavations were completed in time and the possession could proceed as planned.
Each of the precast units weighed 30 tons, and these were unloaded in advance of the possession into specific locations on site ready for lifting into place in accordance with the pre-approved crane plan. The existing bridge deck was estimated to weigh 100 tons. To safely lift, remove and replace the bridge components, two heavy task cranes, with lifting capacities of 500 and 750 tonnes were set up, one on either side of the railway line.
Bridge Replacement works
At 22.30 on Friday the 28 October, Coffey Construction was granted a TIII Possession of the railway line. The track was cut by Irish Rail, all cables were disconnected, and Coffey removed six sections of railway track each 18 metres in length. The ballast was removed down to bridge deck level using excavators, the holding down bolts of the old bridge were cut and the bridge deck was divided into two halves for lifting purposes by cutting the steel plate which connected the bridge beams.
Once the bridge was removed, the area around the bridge was dug out – up to 3.0 metres below river bed level – and backfilled to form a solid base for the new culverts. Three 45-ton excavators were used within the constrained space to dig out the remaining 2,000m3, with five A25 dump trucks drawing away the unsuitable material to a site stockpile. Over 2,000m3 of class 6N stone was built up in layers to the finished level for the base of the culvert. Coffey used old rails to act as screeds for the newly installed culverts to ensure that all culverts were placed on a level bed. An environmental consultant monitored all site works to ensure that Coffey Construction adhered to its agreed Environmental Management Plan.
As the working period was limited to 59 hours, Coffey planned the works with gangs working 12 hour shifts with 12 hour rest periods between shifts for all site staff. Coffey had carried out many similar projects to fixed possession deadlines. Each of the multi-discipline gangs was familiar with all aspects of the works, which meant that work could proceed at full efficiency whether the works were behind or ahead of programme at changes of shift. Coffey’s use of directly-employed labour for its civil engineering works allows it to respond quickly to arising situations and also facilitates robust management of safety and quality.
A total of 84 U-shaped culvert units were installed to form two box sections. Each unit was 7.5m wide by 2.85m high and 2.0m long. A gang of eight personnel were allocated to work with each crane, with each crane working away from the centre. A time of 17 minutes per unit was the maximum programme time available to sling and place each unit for the deadline to be met. A 50mm blinding layer was placed over the culverts to level up any irregularities between the units, and eleven precast ballast retainer units weighing 29 tons each were placed over the culverts. At the four corners of the bridge, six precast retaining units per corner were placed to form wing walls to hold back the railway embankment and the river banks.
Coffey placed the railway ballast in 2 layers of 200mm which were compacted using 4-ton double drum rollers. The site was handed back to Irish Rail for them to lay the new sections of track nearly a full day ahead of programme at 11:00am on Sunday 30 October 2011, completing the works in a 40 hour possession period.
Trains are now once again running over the picturesque River Awbeg at line speeds of 90mph, so passengers don’t have as much time to enjoy the view!