At about 18:30 on 21 August 2009, two spans on the 12-span viaduct at Malahide collapsed. The collapse of the structure was due to scouring of a pier by tidal action. The line was immediately closed to rail services and placed under an engineer’s possession. Following reconstruction works, possession was handed back at 18:00 on 13 November, some 12 weeks after the incident and the line was re-opened to early morning traffic on 16 November.
This paper will describe: the actions taken immediately after the collapse; the initial proposals for bridge reconstruction; how the uncovering of a previously written paper changed the initial proposals; why the bridge failed; the development of the final design; and execution of the works.
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CLICK TO ENLARGE Fig 1: Extract from the 1:50,000 Ordnance Survey map showing the Malahide Estuary and the Broadmeadow with the causeways north and south of the viaduct (pic: Iarnród Éireann)[/caption]
Malahide Station is located nine miles north of Connolly Station, the terminal station on the Dublin-Belfast line, and Malahide Viaduct is located half a mile north of the station. The line is double track, the track comprising 54kg/m continuously welded rails on concrete sleepers, and the maximum line speed is 90mph (145km/h). The line is electrified from Connolly Station to Malahide Station and the over-head line equipment (OHLE) extends beyond the station but stops short of the southern abutment of the viaduct.
Trains are controlled by colour light signals, and continuous automatic warning system (CAWS) and train detection is by track circuits. The signalling is controlled through solid state interlocking. Operationally the signalling is monitored and controlled from a control panel at Central Traffic Control (CTC) at Connolly Station.
The viaduct and the approach causeways to the north and south of the bridge cross the Broadmeadow Estuary at Malahide. The construction of the railway created an inner estuary, measuring approximately 2-square miles in area, which retains water permanently throughout the tidal cycle. The bridge has 12-spans, 8-spans of 15.85m and 2-spans at each end of the viaduct of 12.25m, giving a total length of 175.8m.
Broadmeadow Estuary is protected by extensive environmental legislation. It is a candidate site for designation as a Special Area of Conservation (SAC); it is also a proposed site for designation as a National Heritage Area (NHA); it is designated a Special Protection Area (SPA) site; and it is a designated site under the Ramsar Wetlands Convention.
Any works in that area consequently must adhere to the environmental conservation requirements. A particular requirement for the viaduct works was that the mud-flat exposure areas at the western end of Broadmeadow estuary not be changed. This is to ensure that bird populations, in particular winter migrating birds, are not diminished through loss of feeding grounds. The mud-flat areas have a very gentle gradient towards deeper water consequently small variations in water level have a significant impact on mud-flat exposure areas.
Immediate actions
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CLICK TO ENLARGE Fig 2: Facing north, the collapsed section of Malahide Viaduct, 22 August 2009, showing the severe water flow at the bottom of the illustration. The ruined structure to the north on the up side was the office of the resident engineer Marcus Hearty during the 1860 reconstruction (pic: Iarnrod Eireann)[/caption]
Aerial photographs taken by the Irish Coast Guard Service early on Saturday morning 22 August show the extent of the damage. The collapsed pier, Pier No. 4 counting from the northern end, was completely washed away and the 12 precast beams spanning on to the pier, six on each span Nos. 4 and 5, are seen to be collapsed into the sea.
During the course of the subsequent works, no trace was ever found of the remnants of Pier No. 4. It is believed that it broke up on collapse and, such was the force of the water, the masonry blocks were scattered a considerable distance from the bridge.
On the evening of the incident, a decision was taken that a main contractor and other contractors could be engaged immediately under emergency procurement procedures and the main contractor attended site that evening. Other contractors commenced work over the next few days: for the provision of topographical surveys, hydrographical surveys and bridge monitoring; the provision of engineer-divers to inspect the bridge piers and scour damage; and the provision of mobile cranes and piling equipment.
Within a few days of the incident, an independent geotechnical consulting engineer, Dr Eric Farrell, was engaged to advise on all geotechnical matters. Dr Eamon McKeogh, Hydrology Dept, University College Cork (UCC), was engaged to advise in regard to the hydrology of Malahide. His colleague from UCC, Michael O’Sullivan, was engaged to advise in regard to the environmental issues. Roughan O’Donovan Consulting Engineers was engaged to carry out a peer review of all design proposals prepared by Iarnród Éireann directly and the geotechnical and hydraulic designs carried out by the other independent consultants.
It was known by engineers who attended the site on the evening of the collapse that the deck comprised of precast post-tensioned beams had been renewed sometime in the late 1960s and that the beams were simply supported and were not attached to each other. There was ballasted track over the viaduct and each track was supported by two beams. There were also two outer beams incorporating ballast retaining upstands and steel handrails.
Former US Secretary of Defence Donald Rumsfeld is quoted as stating,
‘There are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns; the ones we don’t know we don’t know." In the first few days after the incident, Iarnród Éireann engineers were operating in a position of ‘unknown unknowns’; the remaining piers and bridge spans appeared to be in a robust and stable condition and it was assumed at that time that all piers and abutments were supported on solid foundations bearing on good ground or rock. In the months following this incident, it became clear that other railway administrations do not always have construction details of their bridges, in particular bridge foundation depth and foundation construction details.
Initial thoughts were that the reconstruction activities should be carried out working from the existing bridge, with two independent work sites; one on span No. 3, to the north of the collapsed spans; and the other on span No. 6, to the south of the collapsed spans. This was to avoid marine works which it was thought would have necessitated specialist equipment with possible delays in procuring a suitable contractor. It was also thought that marine based works would take considerably longer to complete.
Initial design concept
The initial design concept was that 4 No. pile clusters would be installed alongside each of spans Nos. 3 and 6 in order to support crane out-riggers for cranes positioned on these spans. The cranes were initially to remove the suspended track and the collapsed beams. At that stage piling rigs were to be used to install bored-pile foundations to the front of Pier Nos. 3 and 5. The intention was that the collapsed Pier No. 4 would not be replaced but that precast pre-stressed beams with a concrete cast in-situ deck would span from Pier No. 3 to Pier No. 5.
Given the importance of the Dublin-Belfast line, a significant element of the national transport infrastructure, there was an expectation at the highest levels that the line would be re-opened as soon as possible. Having discussed the initial design concept with the main contractor, the Department of Transport was advised on the day after the incident that the line would be re-opened within three months.
In the days following the incident monitoring equipment was installed to detect possible movement and/or settlement at any point on the entire viaduct. Survey monitoring targets were attached to each end of the outer beams on each remaining span. The x, y and z position of each target was measured daily to determine the three dimensional movement at the top of every pier and abutment. Tilt monitors were installed at Pier Nos. 2, 3, 5 and 6 to measure north-south and east-west rotations and vibration monitors were installed at Pier Nos. 3 and 5 and read-outs were available continuously.
Engineer-divers commenced an underwater survey on Sunday 23 August. Their first priority was to survey the remaining piers, Pier Nos. 1 to 3 and 5 to 11, and the abutments for scour damage. The second priority was to determine the depth of scour erosion between Pier Nos. 3 and 5.
On Monday morning, the engineer-divers were able to confirm that there was no immediate risk to the remaining piers and abutments. Some scouring had occurred to the east of spans 9 and 10 but the scour did not pose an immediate risk to Pier Nos. 8, 9 and 10. They also reported that almost 66% of the entire ebb flow, then in spring tide, was passing between Pier Nos. 3 and 5 with a velocity of approximately 4.5 to 5.0 m/sec.
A problem familiar to all railway infrastructure staff is that of access and in particular access to incident sites. In this case access to the southern abutment was easily achieved by arrangement with the owners of the adjacent Malahide Marina and boat-yard, located on the Up side of the line beside the southern abutment.
To the north of the viaduct, public road access was approximately 2km from the bridge and necessitated access through private lands. It was possible to make use of an access road previously constructed across these lands to facilitate previous railway works. By 24 August, ballast was being drawn in to the site to create a ballast bed over the track in anticipation of bringing in heavy-lift cranes and piling rigs.
Historic paper
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CLICK TO ENLARGE Fig 3: Drawing of the original wooden viaduct and the 1860-replacement structure. Inset: Drawing of the initial 2009-proposal for a single span replacement of the two collapsed spans[/caption]
On Monday evening a copy of a paper published in
Journal No. 143 of the Irish Railway Record Society, October 2000, written by Oliver Doyle, Manager Resources & Central Traffic Control at that time, was turned up. It described the history of the viaduct and contained information which was of considerable significance to the works then planned to commence on site.
When constructing the railway line from Dublin to Belfast, a very direct route out of Dublin city had been taken, hugging the coast as far as Laytown Station. To achieve this route, it was necessary to construct causeways across a number of estuaries, including Fairview, Malahide and Rogerstown.
The map extract (see Fig 1, above) shows the causeway at Malahide. The inner estuary to the west of the railway measures approximately one mile north-south and two miles east-west. Prior to the construction of the railway, the tidal waters flowed in and out of the estuary unimpeded. With the construction of the causeway and viaduct, all of the flow was concentrated into a narrow channel.
The original viaduct was constructed in 1843. It had 11 spans of 52 feet (15.85m) with a total length of 572 feet (174.4m). The abutments were of masonry construction, supported on timber piles. The piers and bridge deck were of timber construction also supported on timber piles.
The obstruction to the natural tidal flow caused a very powerful current through the bridge spans and within a very short time after the railway coming into operation, it became clear that erosion of the soil into which the timber piles were driven was causing settlement.
To address the problem, approximately 90,000 tons of stone was placed beneath the bridge, encapsulating the timber piers. The stone formed a rip-rap weir, extending between the north and south abutments. The purpose of this weir was twofold, to protect the seabed against further erosion and to reduce the volume and velocity of water flowing through the bridge spans. The weir was 130 feet (40m) wide at its base and 30 feet (9.2m) high with steeply sloping sides.
The settlement of the deck was dealt with by packing up the rails, by up to 3 feet (0.92m) in places. By 1859 however it became clear that the timber was suffering decay and a decision was taken to renew the bridge. The construction requirements were quite onerous in that it was necessary not to disrupt rail traffic during the course of the works.
Having considered a number of options, it was decided that the new structure would have wrought iron spans supported on masonry piers. The piers were to be supported directly on top of the stone weir. Given the concerns in regard to the condition and stability of the original timber piles, it was decided not to disturb the original piers and to retain the original abutments.
It was also decided to minimise obstruction of the waterway and to construct the bridge in three stages: to construct three piers at the northern end, renew the first three spans on both the up and down lines, take down the original piers for those spans and clear and level the waterway; then construct four more piers and spans in a similar manner; and finally complete the last four piers and spans.
Construction of the piers
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CLICK TO ENLARGE Fig 4: Drawing of an original pier of 1860 showing the method of construction. Lower illustration shows the cross-section of the weir created to protect the earlier wooden structure from scour[/caption]
One of the options that had been considered was to use cast-iron girders with cross-spanning brick arches (jack-arch construction) and having a track laid on ballast. It was known that the cast-iron would have been more durable in the marine environment; however the board of directors were mindful of the risk that cast-iron could fail suddenly with catastrophic consequences and it was directed that a wrought iron structure be used instead.
It is worth noting how the piers were constructed. The piers were initially built-up dry at Dublin Terminus, now known as Connolly Station. Each stone was marked up, the piers disassembled and the stone delivered to site. There the piers were again built-up dry alongside the causeway, but this time the piers were built upside down. The weir had to be prepared in order to receive the new piers. At low tide, the water in the outer estuary can fall to a level approximately 3.0m below top of weir level. It was possible to restrict the flow of water from the inner estuary by damming one span at a time.
Sections of track were laid on top of the weir, spanning from pier to pier, having convex plan-curvature towards the western face, filled with sods and pitched with stone on both faces. While the dam did not prevent all of the water from passing through it allowed the masons to construct the pier foundations under a shallow depth of water.
To prepare the pier formation level, loose stones were first removed to a depth of 15 inches (0.45m) below the general level. Around the hole thus formed round stones were laid on edge without cement. Then stone fragments, gravel and iron turnings were placed in the hole and rammed with a 5 cwt (260kg) block of iron. When a firm level base was achieved, a framework of rails, having the profile of the base of the pier, was laid down and adjusted to the correct level using blocks of stone, the intermediate rails being packed with flat stones.
The interior space was then filled with flat stones and grouted-in. At this stage, the first layer of ashlar was laid and the remaining stonework for the pier carried on from there. It only took 5 months from the laying of the first stone on the first pier to the completion of the last pier, during which time seven deck spans had also been completed.
The information contained in the historic paper was of significant importance to the engineers in drawing attention to facts not known to them at that time; that the bridge piers and abutments were founded directly on top of the weir.
Author: Liam Meagher, programme manager (structural and architectural design), Iarnród Éireann
This article was originally published in the
Irish Railway Record Society Journal Volume 24, No. 176 (October 2011).
On 31 January, the second half of Liam Meagher's article will outline why the bridge failed, the development of the final design and the successful execution of the restoration work.