Over the last few years, Rail Engineer has written a number of articles about the Dawlish sea wall. There have been many pictures showing this vulnerable piece of infrastructure with lengths of wall being breached by storms and high seas and with rail tracks suspended in mid-air, no longer protected by the ageing sea wall.
The sea wall protects the high Devon cliffs, the town of Dawlish itself and its local inhabitants. It also provides a vital rail trade route into west Devon and Cornwall, helping to connect many west-country communities with the rest of the UK.
Emergency action taken to limit the effects of storm damage in 2014 involved the importation of old shipping containers filled with rock. The containers were designed to absorb temporarily the forces of the tides and provided an engineering lifeline to those involved in saving the Dawlish sea wall from total collapse.
These concerns must be seen against a backcloth of rising sea levels and more extreme weather patterns predicted for the future. These are messages that we cannot escape and that is why Network Rail has decided that the warning signs are clear and that a detailed plan needs to be developed now, to ensure that this railway route is fit for purpose for the next 100 years.
Key areas of concern
Detailed studies, designs and joint working between world-leading marine, coastal and railway engineering experts instigated by Network Rail are well underway. Three key areas of concern are emerging, although there may be more to follow.
The first relates to the coastal resilience of this section of railway, specifically, in the short term, around Dawlish – Marine Parade and Dawlish Station. The second is the condition of the tunnel portals and geotechnical risk between Kennaway and Parsons tunnel, this covers five tunnels in all on the coastal route. The third concern is the instability of the cliffs behind the railway formation in the Holcombe area between Parsons tunnel and Teignmouth.
Detailed proposals for the resilience of the sea wall in Dawlish – Marine Parade have been developed by engineering consultants from Arup, the outcome of which was to build a new sea wall, 2.5 metres higher than the existing wall.
The new wall
The profile of the proposed wall includes a curved upper edge, designed to direct the waves back towards the sea. There will also be a wider, safer promenade, with seating, which will enable visitors and locals to continue to enjoy the clear views of the coast. The plan is that a new coastal defence will be constructed in two phases, at Marine Parade and around Dawlish station. However, it was agreed that work would stop during the peak summer season from July to September this year, to minimise disruption to the local community and tourism in the area.
The new wall will cost around £80 million. The first phase extends from the Colonnade underpass, west of Dawlish station, to Boat Cove, a length of approximately 360 metres, and will take nine months to complete. The first phase of the work, valued at £25 million, was awarded to BAM Nuttall, with work starting on 1 June and completion planned for spring 2020.
The second phase, which is currently being planned, will extend the defences from Dawlish station to Coastguards Breakwater, around 420 metres, and cost an estimated £5 million. Dawlish station is a listed building and Network Rail is determined to dramatically improve the access to, and mobility within, the station as part of the work, the planning of which is proving to be quite a challenge for Arup. Network Rail expect to share more information on phase two in the summer 2020.
BAM Nuttall is now well established on site, concentrating on preparing for work and mobilisation. Access to the sea wall is a significant challenge. Pipework has been installed around and under the tracks, so that concrete could be pumped from mixers into the foundations at the foot of the new sea wall.
As substantial volumes of concrete would need to be delivered, appropriate routes through the sensitive, local countryside, had to be established for the delivery vehicles, with Newton Abbot emerging as a more favourable route than Exeter.
Noise, vibration, track and structural monitoring equipment needed to be installed to ensure that, once the excavation of sand and rock to construct the new foundations started, the overall structural integrity of the sea wall remained intact ready to withstand the next incoming tide.
The workforce is required to work 10-hour shifts, planned to coincide with the timing of the tides. Close monitoring of the tides was necessary as the specification stated that no work could take place on the foundations if the forecasted significant wave height was greater than 0.9 metres within the next 24 hours. This is due to the temporary stability of the sea wall during the works while the new concrete foundations are gaining strength. A 36-hour advance warning of wave height was therefore introduced. A small number of shifts have already had to be cancelled and winter has yet to come!
The work that has been carried out so far has focussed on the new sea wall foundations and well over 300 metres of foundation work has been completed. After clearing the existing beach material, the sand in front of the existing sea wall is excavated to a depth of five metres, down through the sand to competent rock. Excavation then continues, another two metres down into the rock.
Timber shuttering is then installed and held in temporary position by concrete Jersey Barriers, designed to add additional stability.
A specially designed concrete mix is then pumped into the excavation, displacing both water and slurry. Once the concrete has cured, the top 150mm, which includes the slurry, is removed using a milling machine, leaving a strong and firm foundation. Each foundation is constructed in intermittent five-metre bays to ensure that the stability of the existing wall is not compromised. With favourable conditions, up to two bays can be cast in one shift.
Dangerous ‘swallow’ holes
Engineers who have been involved in maintaining sea wall structures will be interested to know that, during investigations, a number of voids have been identified with ground-penetrating radar (GPR) within the existing wall. When coupled with tidal energy, small holes in the face of the wall can result in the wash-out of large volumes of material from behind the sea wall, with minimal visual signs. Usually, they are hidden by sand and evidence of their existence is only revealed when a significant amount of track formation has suddenly disappeared.
Voids that could accommodate a double decker bus are not unknown and, of course, it means that, when the backing to a sea wall is removed, the stability of the wall itself becomes very fragile when the next tide comes in. Suffice it to say, Network Rail has ensured that every such hole has been well grouted as part of the process.
The design then requires the installation of vertical dowels that will anchor precast concrete U-channel units to the in-situ pumped concrete foundations and form a base for the new precast concrete wall. As has been mentioned, the wall will be 2.5 metres higher than the existing one and designed with a distinctive recurve to deflect the waves back into the sea. Also, the front of the precast units will be scarified to create a rough stone finish.
Improvements for residents
The new walkway promenade will be raised by 1.4 metres and there will be a 1.1-metre parapet wall that will form part of the overhang that will deflect the waves. Improved lighting and seating will help to enhance the image of this new sea wall.
As stated, plans for stage two are well advanced and significant efforts are being taken by Network Rail to keep the local community well informed and up to date. Monthly newsletters are published for the residents and alongside the station is a community information hub that is open on Wednesdays from 11:00 to 14:00. Network Rail staff are on hand to answer any questions or to discuss any concerns. In addition, Network Rail is hosting an evening session from 17:00 until 19:00, giving commuters an opportunity to talk to the team.
The second area of concern, tunnel portals and the geotechnical risk between Kennaway and Parson’s tunnel, centres round the limited construction access to the site as well the engineering solution. Access to these tunnels is very difficult and there is a specific requirement to build a 210-metre structure at the Dawlish end of Parson’s tunnel, designed to capture the regular rock fall and protect trains. Detailed plans are expected to be ready before the end of the year.
Moving the railway?
Then there are the cliffs at Holcombe. These have been a worry for engineers for many years and close monitoring of movement has been in place since 2014, with drones regularly surveying the site and monitoring for live feedback on any cliff movements.
The solution proposed by design consultancy Arcadis is to move the tracks 30 to 40 metres out to sea, leaving room to stabilise the cliffs. This is a significant undertaking that will involve constructing a new length of railway, although it will be only 5-10 metres longer than the existing track due to it taking a straighter route across the bay. Apart from the obvious engineering challenges, there is a 400-year-old shipwreck in the way, along with a whole host of marine related issues.
Network Rail’s senior programme manager David Lovell and project manager Phil Morton, who kindly outlined all the detail for Rail Engineer, have an enthusiasm for the success of this programme that was clearly evident, as was their appreciation of the challenges ahead if all that is being proposed comes to fruition.
Given the opportunity and the funding, there will be many exciting challenges for engineers to overcome ensuring that there will be a vibrant railway service along the Dawlish coast for at least the next 100 years.