Over recent years, the increasing frequency of flood events causing disruption and closure of the railway in the southwest of England should have caused even hardened climate-change deniers to begin to question their beliefs.
Investment in several projects designed to reduce the frequency and/or severity of such railway disruption is now being rolled out. Two of these, and ones that have recently been successfully completed, are on the Great Western main line near to Exeter and on the South Western main line near to Axminster. There are similarities between the two schemes that lend themselves to a combined report.
Both projects share similar features, though they have been devised independently from each other. In each case, relief from flooding damage has been provided by the removal of sections of embankment and the installation of precast reinforced-concrete box culvert units. The two rivers with regular flood events are the Exe and the Axe, both names derived from the ancient Brithonnic word “iska”, meaning abounding in fish.
River Exe project
The scheme to protect the railway from the effects of the River Exe is at Cowley Bridge junction, and is one of ten sites forming Great Western’s resilience plan, a significant suite of investments projected over the next few years. The site at Cowley Bridge is the first of these to be commissioned, another site at Chipping Sodbury, on the South Wales main line, being the next.
As the Great Western main line approaches Exeter from the London direction it runs across the flood plain of the River Exe for just over a mile. The branch line from Barnstaple also crosses the flood plain to join the main line at Cowley Bridge junction.
The positioning of this junction and the associated railway embankments were perhaps not seen as critical at the time of their construction, but they have created a literal bottleneck. When the river overtopped its banks and made full use of the flood plain, the floodwaters had nowhere to go at the junction, except straight through and over the railway. This has caused closure of the line on several occasions in recent years as a result of the washing away of the ballast and damage to signalling cables.
Sometimes these closures have lasted for several days whilst the infrastructure was restored, with London trains being diverted via the Yeovil single line or replaced by buses to pick up a train again at Tiverton or Taunton. This was clearly an unacceptable situation.
At this particular location, there have been severe flood events, which could not be mitigated, no less than 22 times in the past 10 years.
One particularly bad event in autumn 2012 prompted a study of better remedial options. This work commenced in 2013 and has become known as the “Resilience Study”, also encompassing proposals for another nine sites on Great Western.
A special case
In the sphere of railway flood events, Cowley Bridge junction is a special case due to its geographical location and the topography surrounding it.
When the floodwaters reach this location, they have built up considerable speed and cause ballast mobilisation, which leads to extensive infrastructure damage and large-scale wash out. In the past, after the floodwaters had subsided, an inspection would take place with the line remaining closed until it had been extensively repaired, requiring the use of heavy machinery, engineering trains and many tonnes of new ballast. Sometimes, repairs to signalling cables were also necessary.
Prior to the construction of the railway, the River Exe flowed directly to the site of the junction. It now takes a meandering course over the flood plain, with the railway crossing it on two major bridges. In times of flood, the excess floodwater heads straight down the abandoned river course and encounters the railway embankment. In 2000, after a flood event closed the railway, a new culvert of large concrete pipes was installed to cater for this flow. A decade later, this capacity was found to be inadequate and further cross-sectional opening was provided.
However, it was apparent from even more recent events, and from the Resilience Study, that a more radical solution was required. The study recommended selective lowering of the flood plain on its approach to the embankment, removal of the concrete pipework and the replacement of a significant extent of the embankment with twin concrete box culvert sections. The objective of the proposal was to provide sufficient flow capacity to prevent any erosion of the embankment or ballast and to equalise the water pressure on both sides of the embankment.
To confirm the design and dimensions of the box culvert sections, hydraulic modelling was carried out. This was also to ensure that scour patterns would not be altered. A topographical survey of the flood plain was made on a five-metre grid with the use of LiDAR data. This then enabled design of the profiles of the flood plain lowering in the approach to the embankment.
The main works were designed by AmcoGiffen, utilising Ove Arup as design consultant. Twin culverts, sitting side by side, cross beneath the railway at a skew angle to suit the natural flow of the watercourse. Each culvert consists of 12 box units, each unit being 4.8 metres wide, 2.1 meters high and 1.5 meters in depth. The run of 12 units are tensioned together longitudinally with Macalloy bars. The flow capacity of the new culvert units is 4,000 litres per second.
To provide further dissipation of the energy within the floodwaters and a robust channel out to the river, Salix pre-filled rock mattresses were specified as these are quicker to install and are an environmentally friendly alternative to gabions, concrete and blockstone. Salix also supplied Vmax P550 turf-reinforcement mats to be used on the slopes for erosion control.
Tony Gee and Partners was responsible for the design of all temporary works, including a bridge to carry signalling cables during the works to excavate the embankment and install the culverts.
The precast reinforced-concrete box sections were manufactured in Ireland by Shay Murtagh before being shipped to Wales and then transferred to the site by road. The main contractor for the works was AmcoGiffen. Access to the site required construction of a substantial access road from the adjacent public road and across the flood plain.
Ground stabilisation trials were undertaken ahead of the possession, as soon as access could be obtained to the adjacent field. Soft alluviums were present in the ground, so two trials were undertaken to decide on the most suitable foundations for the culvert units.
One trial was of a shallow excavation, whilst the other was deeper, removing the soft material. Fill, including large boulders measuring a minimum of 500mm, was pushed into the ground and surrounded with smaller, gabion stone. Backfill was then installed on top of this, which was compacted and rolled in layers. Both trials passed the California Bearing Ratio tests and a suitable combination of the two trial methods was recommended by the designer, Arup, for use underneath the new culvert units.
Each individual box culvert unit weighs 13.8 tonnes. The culverts were designed to be composed of the 12 small units, so that they could be handled by a large 80-tonne excavator. This avoided the use of any craneage, minimising weather-related risks, and also ensured a safer worksite with its restricted working space.
The main possession for the installation work was of 75 hours duration, from early on a Tuesday to early on the Friday morning in mid-June. This unusually timed possession was selected in agreement with the train operating companies so as to not disrupt Monday morning and Friday evening Cross Country passengers, who make particularly long journeys. London-bound services were diverted.
At the same time, the local delivery unit took the opportunity of this generous line closure to undertake some track renewal works and some works to a number of level crossings.
In total, 1,300 tonnes of excavation was undertaken, 450 tonnes of ground stabilisation materials imported, and 600 tonnes of backfill used to recreate the embankment after installation of the box culvert units.
The cost of the project, which included the two new flood relief culverts, 70 metres of new track, river embankments and spill ways, and also some repointing to another culvert nearby, was approximately £3.6 million.
Deborah Elliott, project manager for Network Rail Infrastructure Project Enhancements, told Rail Engineer: “AmcoGiffen has provided a quality installation of two new flood relief culverts, delivered within a 75-hour blockade of the Great Western main line despite challenges with ground conditions. These culverts have been designed to provide improved flood resilience at this strategic location to help us to keep trains operating more of the time and to enable the infrastructure to recover quicker during prolonged periods of flooding.”
River Axe project
Interestingly, the project near to Axminster shares many similarities with the work near Exeter – apart from the derivation of the river names.
The South Western main line from London Waterloo to Exeter follows the valley of the River Axe as it approaches Axminster. Problems with flooding affecting the railway in this area have called for the installation of concrete culverts at two separate locations, one is known as Axe and the other as Broom, these being the local names of the level crossings adjacent to each of the new structures.
Early development for the scheme was undertaken by Mott MacDonald, working jointly for Network Rail, the Environment Agency and Devon County Council with input from East Devon District Council, and was funded in part by a fund created to help East Devon recover from severe flooding that occurred in 2012. The relationship between the four statutory bodies was formalised through a partnership agreement, and was set up, in no small part, to ensure that the complex environmental controls that had to be complied with to deliver a project in a protected riverine site were met with minimal inter-body disagreement, and therefore minimal cost to the public purse.
The structures were designed by Arcadis, with continued involvement from Mott MacDonald for hydraulic modelling. At each site, the structure consists of seven box culvert openings, each opening consisting of a male and female unit, joined together along the centre line of the railway. Each unit at the Broom site weighs 38 tonnes while those at the Axe site weigh slightly more at 40 tonnes, essentially being a larger version of the same design to give a greater cross-sectional flow area.
In addition to these 28 precast culvert units, there are precast concrete “restraint” units, placed throughout the length of each structure on each side, to prevent any lateral movement that might occur over time.
Finally, there are precast wing wall units. All of this added up to a significant quantity of material to be conveyed to site.
Shay Murtagh, as at the River Exe project, was the manufacturer of the precast units, which were shipped from Ireland to Liverpool and then brought to site by road.
Stephen Carr, project manager for main contractor Osborne, told Rail Engineer that, in anticipation of the disruption to a local village by these intensive lorry deliveries, a lot of effort went into liaison with the inhabitants regarding mitigation. It was planned that the delivery of all the units would require nine days, with five low-loaders passing through the village each day.
Having involved themselves in the local community, Osborne installed a soft play area for the local primary school, repaired an Armco barrier and did some planting. These generous goodwill initiatives were motivated by the fact that, apart from coming through the village, the only other way to site was over a minor road bridge with a three-tonne weight restriction.
However, at the eleventh hour, the local council enabled this other route to be used by strengthening the bridge to 40 tonnes! However, the word strengthening is not quite accurate – what was actually done was to install a completely separate new truss bridge spanning above the old arch bridge and founded above the original abutments. Quite an unusual but useful first aid measure!
Work on-site started in mid-May, leading up to the main possession, which was to be a nine-day closure of the line in September. Fixed-jib cranes were brought onto each site two weeks prior to the main possession. Once rigged, their first task was offloading the concrete units as they were delivered. Trial placement and coupling up of a pair of units, male and female, was then carried out on a hardstanding area, clear of the railway, to check the ease and manoeuvrability of the units for this process before having to do it ‘for real’ in the main possession.
Removal of the embankments and placement of the box culvert units went well at both sites during the main possession, with only a slight stoppage as high winds above 35 mph prevented the cranes from working. In fact, despite this, the overall work finished with almost a day in hand.
The joint between the male and female units is sealed by a Denso strip while the waterproofing on top of the culvert units, applied before reinstatement of the track ballast, is Wolfin, with installation sub-contracted to Waterseal.
The foundation specified for the box culvert units was quite a thorough one, to take account of the flood plain soil conditions. It consists of four layers: Terram T1000 textile, followed by 575mm of 6F4 (50mm to dust) material, then two 150mm layers of DoT Type 1 material and finishing with 75mm of a small single-sized gravel. After placement of the restraint units, riprap rock armour was provided as further protection against scour.
During the works, temporary scaffolding bridges were provided at both sites to carry signalling and telecommunications cables. There was a fibre-optic cable joint which had to be particularly carefully managed to avoid disturbance.
Impressively, at both Axe and Broom sites, the contractor’s office, stores and messing facilities were powered solely by frames of solar panels, on hire from Speedy Hire.
The cost of this project was £9.5 million.
Prevention not cure
Returning once more to the similarities between both projects, they can each be visualised as valiant attempts to provide fast-filling bathtubs with larger outlets than previously. But, recognising the great forces of nature, the message is “prevention not cure”, and occasionally there will remain the risk of a closure.
For the River Exe site, the project team estimates that a severe event, maybe closing the railway, will now occur only once every 10 years instead of once every two. And, for the River Axe, Mott MacDonald, on behalf of the project team, has estimated that the new culverts will have extended the severe event frequency from 1 in 5 years to 1 in 20.
Nevertheless, in terms of confident operation of the railway and with minimal disruption, these are very significant gains achieved by these two similar projects.
Read more: Andrew Haines and the future of UK rail