The collapse and reconstruction of the Malahide Viaduct – part 2
31 January 2017
(Image: Roughan O'Donovan)
In the first of this two-part series, Liam Meagher outlined the actions taken immediately after the Malahide Viaduct collapse in 2009, initial proposals for reconstruction and how the discovery of a previously written paper changed the initial proposals.
Diving conditions during the spring tides were particularly hazardous between Pier Nos. 3 and 5 because of the potential for instability of the fallen beams and the velocity of water flowing around them. It was necessary to restrict diving in that area to that brief period between tides when the incoming tide just reached the same level as the out-flowing waters.
On Tuesday morning, the engineer-divers reported on the depth of scour between Pier Nos. 3 and 5. The maximum scour was measured to be approximately 3.6m below the general top of weir level and occurred where Pier No. 4 had been.
Knowing then the pier construction details and the importance of the weir to the stability of the entire structure, it was clear that unless the weir could be secured against further erosion and scour effects, there was a significant risk that the scour hole at Pier No. 4 would spread and undermine Pier Nos. 3 and 5 and potentially undermine others as well.
At that time, there was only a narrow strip of weir remaining intact between the scour hole and the inner estuary.
There was concern that a full breach of the weir, with a scoured channel running from inner to outer estuary, could cause significant environmental damage because tidal flows would be dramatically changed; the ebb tide would flow from the inner estuary for a longer period; the head of water would be much higher; and consequently further erosion and scour could develop very quickly.
The primary concern at that stage switched from reconstructing the bridge to stabilising the weir and all efforts were directed to that end. The Hydrology Department of the University College Cork was engaged with a first priority to advise in regard to plugging the scour and reducing the volume and velocity of water discharging between Pier Nos. 3 and 5.
It was decided to construct a haul road on the eastern side of the viaduct, with access through Malahide Marina, in order to fill the scour hole with stone.
The weir and tides
The haul road was constructed on top of the eastern side of the weir with a top surface about 500mm higher than the general weir level. It was recognised that the weir would need to be substantially strengthened on both the eastern and western sides and a total of 25,000 tonnes of stone was laid to construct the haul road.
The contractor placed 1.2m diameter steel pipes east-west in the middle of each span, a metre below haul road level, to assist in accommodating tidal flows. Heavy boulders, 15- to 20-tonne weight, were placed along the eastern slope of the weir with smaller boulders and stone placed on top.
Special permission was granted by the local authorities and the Garda Síochána to permit the contracted quarries to operate continuously and for the boulders and stone to be delivered to site throughout daylight hours seven days a week.
By Sunday,, the flow through Pier Nos. 3 and 5 had been arrested. The haul road acted as a buffer, preventing the water from flowing through the scour hole, and the high tide flow over the haul road was seen to be equalised through each span. The high and low spring tides extend from +2.22m OD to -2.38m OD, a range of 4.6m. The weir level prior to collapse was approximately +0.5m OD.
Spring tides at high water cause a significant volume of water to enter the inner estuary where it is retained. On the ebb tide, the water in the outer estuary falls to a low water level almost 3.0m below top of weir. The high water retained in the inner estuary cannot fully discharge before the tide turns again and the incoming tide replenishes the inner estuary once more.
It was found that during spring tides, there was always a flow of water across the weir to a depth of at least 600mm. In order to allow works to continue unimpeded during spring tides, it was necessary for elevated stone platforms to be formed to allow construction equipment operate in dry conditions.
The high water during neap tides sometimes only exceeds the weir height by approximately 400mm. This causes a lower volume of water to enter the inner estuary and the water retained can fully discharge to top of weir level during the ebb tide before the tide turns and the incoming tide replenishes the inner estuary once more.
Some of the works had to be restricted to neap tide periods: pouring concrete to the pile cap on Pier No. 4, installing the micro-piles and some other works.
The cause of the collapse
On Sunday 23 August, a chance encounter with a leader of the Malahide Sea Scouts led to copies of their photographic records of flow conditions through the weir being made available to the engineers. The Sea Scouts have for many years used the weir as a training ground on their kayak courses. The ebb flow over the weir created conditions suitable for training those who then progressed on to white water rapids elsewhere.
The Scout leaders had often taken photographs of participants and had thereby captured the weir flow conditions in the background. It is now clear that general erosion had been taking place for some time over the entire length of the weir.
In the past, stone had been placed on the weir following storms but no replenishment had taken place since the mid-1990s. It appears that the top level of the weir had been slowly eroding, with a loss of stone over time. From the Sea Scout records, it was established that the scour which caused the collapse of Pier No. 4 was not evident in March 2009. By July, however, it is clear that spans Nos. 4 and 5 had suffered significant scouring, with deep channels having been formed in the middle of each span.
As the erosion developed, these channels became deeper and wider and consequently more water was drawn into the channels, exacerbating the problem. In the days leading up to the collapse, more stones were scoured away, deepening the channel until finally the pier foundation was undermined and the pier collapsed.
The sketch by the engineer-divers shows the development of two areas of scour to the east of spans 9 and 10. The eastern crest of the weir was locally eroded away progressively until in plan the weir crest was ‘horse-shoe’ shaped. On the ebb flow, the water passing over the weir crest gathers momentum and the flow becomes turbulent. This can cause erosion along the crest and the side slopes of the weir.
In the case of the ‘horse-shoe’ shaped crest the turbulence occurs within the weir, around the ‘horse-shoe’ crest, and causes erosion which draws more water into that area exacerbating the problem.
In spans Nos. 4 and 5 similar erosion had occurred but in that case the weir crest was eroded away until the ‘horse-shoe’ shaped crest progressively grew and eroded the weir directly beneath the bridge and then beyond the western edge of the bridge until it had almost broken a channel through the entire width of the weir.
At that stage, the increase in water volume and velocity caused progressively more destructive forces to undermine the weir and ultimately undermine the pier foundation causing the pier to collapse.
In 1967/68, the wrought-iron spans were renewed. At that time, the construction details of the bridge were known and the importance of the weir was well understood. When re-constructing the wrought-iron spans, it was recognised that the previous practice of removing deck timbers and unloading stone from ballast wagons to replenish the weir would no longer be possible and would be difficult by alternative means. A decision was taken to grout the top level of stone in the weir to a depth of 1.5m over the entire length of the weir.
The grouting was successfully carried out and the weir was stable for many years. Over time, however, the grout broke down and was itself eroded. Unfortunately having ‘fixed’ the problem, the knowledge of the weir’s importance faded, and with the passage of time there was a loss of corporate memory and the structural importance of the weir was not appreciated. This was found to be the underlying cause of the incident.
Having constructed the haul road and equalised the flow through each span, it was realised that despite the initial reluctance to ‘get our feet wet’, a new opportunity presented itself. It was realised that cranes could be brought along the haul road to remove the collapsed bridge beams, and other works, including the re-construction of Pier No. 4, could now be carried out relatively easily.
It was decided to abandon the design work then underway and develop a new design, to re-instate Pier No. 4 and construct two new spans in a style similar to the original but using precast pre-stressed beams and an in-situ deck.
Tracked crawler cranes were used to tandem lift-out the tracks, which were then cut into 15m lengths and placed on span 3, immediately behind Pier No. 3. Then the collapsed beams were removed and transported to the railway gantry-yard in Mullingar. The signalling cables on the up side of the line were-routed to the down side and all were supported by catenary wire spanning from Pier No. 3 to Pier No. 5.
The cables remained in use for the duration of the works. Having cleared the site, the scoured hole was then filled with 150mm stone, that size being chosen in order to facilitate the installation of driven steel piles.
A second haul road was constructed from Bissets Strand Road, alongside the western side of the southern causeway, as far as the southern abutment. This facilitated the importation of stone to be laid on the western side of the weir for strengthening purposes.
The importation of stone from the northern side of the bridge was discontinued once the decision had been taken that Pier No. 4 could be re-instated, working from the haul road on the eastern side of the bridge.
In Mullingar, six of the eight larger bridge beams were load tested to confirm their load carrying capacity. Having completed the load tests, the beams were cut into sections in order to examine the condition of the post-tensioning tendons and to confirm that the grouting had completely filled the tendon ducts. The tests confirmed that the beams can safely take the loads to which they are subjected to in service. The level of corrosion on the tendons was quite small and no loss of grout was observed.
No site investigations had been carried out before completion of the haul road, but because there was little time available, it was decided to drive 273mm diameter tubular piles for the foundation of Pier No. 4 and thereby determine the depth to rock.
Fortunately the main contractor was at that time constructing the Clonmore Bridge on the M52 Link Road near Mullingar where such piles were being used and a sufficient number was immediately made available for this project. 22 No. piles were driven at Pier No. 4 and rock was found at approximately 25m below top of weir level.
A decision was taken that while the works were under way, micro-piles would be installed below all the other piers and the abutments. 15 No. raking micro-piles, using 50mm diameter Dywidag hollow bars, were installed at every pier, and an additional 8 No. vertical piles, using 75mm diameter Dywidag hollow bars, were installed at Pier Nos. 3 and 5.
The micro-piles were designed as friction piles and were not taken to rock level but were terminated approximately 15m below top of weir level. These micro-piles do not carry the entire applied load but were installed to give supplemental support to the piers.
When the micro-piling started and the vertical piles were being installed at Pier No. 5, it was noted that some settlement was taking place. At that time, the piling contractor was drilling a steel casing through the stone rip-rap beneath the pier, the rig sitting on the bridge deck and the drill passing through vertical holes previously cored down through the masonry pier.
When the figures showed that there had been settlement of 20mm at the eastern side of the pier and 7mm on the western side, a cross-level of 13mm, the micro-piling was stopped. It was concluded that the method of installing the steel casing by percussive drilling, using high air pressure at the toe of the casing to drive the drill head, had led to the disturbance of the sand/silt layer beneath the weir rip-rap which caused voids to develop, with the consequent effect of pier settlement.
At that stage the installation of the 76mm diameter Dywidag hollow bar was to be carried out in three stages: core a 200mm diameter vertical hole through the masonry pier; drill and install a 168mm diameter steel casing from the base of the masonry pier through the stone rip-rap to the sub-strata sand/silt layer; and drill the 76mm diameter Dywidag hollow bar to the required depth and grout.
Following consideration of the problem it was decided to stop the casing installation 1m before the bottom of the rip-rap layer. At that stage the casing was filled with grout which was allowed to permeate into the rip-rap and the underlying sand/silt. The grout was then allowed to set overnight before the installation of the 76mm diameter Dywidag hollow bar which was then drilled through the grout and in to the sand/silt layer. Subsequent monitoring showed the process to have been successful.
Pier No. 4 was completed by the 9 October and the new spans were completed two weeks later. The track was re-instated by the 10 November and all micro-piling was completed by Wednesday 11 November.
At that stage, load testing of the bridge was carried out over two days using an 071-class locomotive and laden Tara mines wagons with axle weights of 18.5 tonnes. It was found that the loading had no effect on the bridge piers and no settlement occurred.
Completion of the works
Possession was handed back to the Operating Dept at 18:00 on Friday 13 November, twelve weeks after the bridge collapse. Train testing of track circuits was carried out over the weekend and trains were positioned for passenger services which commenced on Monday morning, 16 November.
Following the re-opening of the line, further works were carried out to strengthen the north east causeway and finish off the weir. A total of 112,000 tonnes of stone, including the 25,000 tonnes used to form the haul road, was used to widen and strengthen the weir.
The top of the weir was profiled, with V-shaped channels formed between the piers extending from the eastern to western crest. The effect of these channels is to draw the main flow of turbulent water away from the piers and thereby further mitigate the risk of scour at the piers.
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).
The efforts of all involved in investigating the initial collapse, developing and approving the selected scheme, and completing the project in a very short timeframe, require acknowledgement.
The following contractors were engaged directly by IÉ:
- Main contractor: Jones Civil Engineering;
- Piling contractor: PJ Edwards & Co Ltd;
- Concrete coring: Core Drilling Services Ltd;
- Masonry stitching: Ulster Damp Proofing Group;
- Precast beams: Banagher Concrete;
- Steel Handrails: McGuirk Steelworks Ltd.
Engineering Consultancy services engaged directly by IÉ were:
- Engineer-divers: CEI Collins Engineers;
- Topographical & hydrographical surveys: Murphy Surveys;
- Site investigation surveys: Irish Drilling Ltd;
- Site investigation surveys: Apex GeoServices Ltd;
- Aerial photography: Peter Barrow Photography;
Already mentioned in the main article but noted again here for completeness are:
- Civil, Structural, Geotechnical & Hydrology Consultant: Roughan O’Donovan Consulting Engineers (for independent peer review);
- Hydrology consultant: Dr Eamon McKeogh, University College Cork;
- Environmental consultant: Michael O’Sullivan, Creagh House Environmental Ltd;
- Geotechnical consultant: Dr Eric Farrell, AGL Consulting Geotechnical Engineers.
Mary Molloy BE CEng MIEI, Principal Inspector (Approvals), Railway Safety Commission, has also advised the following further details pertinent to this article.
Under the Railway Safety Act 2005 the re-construction of the Malahide Viaduct was subject to approval by the Railway Safety Commission (RSC). The work was independently checked by Atkins on behalf of the RSC. The Atkins team was led by Doug Watt BSc (Eng), PrEng, AIStructE, FSAICE and was supported by: Dr Yiping Chen – Estuarine Hydrodynamics; David Meikle and Steven Lyall – Estuarine Erosion; George Smith – Environment; Pankaj Garg – Bridge Design; Nic Depezay – Geotechnical Design.http://www.engineersjournal.ie/2017/01/31/malahide-viaduct-collapse-reconstruction/http://www.engineersjournal.ie/wp-content/uploads/2017/01/Malahide-Viaduct.jpghttp://www.engineersjournal.ie/wp-content/uploads/2017/01/Malahide-Viaduct-300x300.jpgCivilMissedbridges,Iarnrod Eireann,Roads and Transportation Division