The line was back up and running within weeks of being swept away by huge waves, but 12 months on engineers are still busily repairing the damage from last year’s storms.
Project: Dawlish Lower Walkway
Client: Network Rail
Contract value: £10m
Main contractor: Amco Rail
Precast concrete supplier: Hanson Precast
Specialist subcontractor: Can
Life in the sleepy Devonshire seaside town of Dawlish might finally be getting back to normal for its inhabitants.
The hullabaloo of last year - the endless stream of concerned-looking MPs as they gawped over Brunel’s breached railway line, the swelled ranks of the press - all gone, consigned to the memory.
Even the army of orange-clad construction workers, assembled at a moment’s notice from all corners of the country, has dwindled.
The spotlight, so blindingly focused on Dawlish after two storms in February 2014 ripped away Network Rail’s artery to Cornwall, may have shifted since its subsequent triumphant reinstatement, but there is still work to be done.
The second phase of work will see some 340 m of the seawall repaired, raised and replaced under a £10m scheme, with main contractor Amco Rail carrying out the work.
It is a contract that was always going to happen, according to Amco Rail site agent and design engineer Chris Warburton.
He was one of the first on the scene at Dawlish, getting on site just hours after the first of last year’s storms caused all the damage.
“The ‘Dawlish Lower Walkway’ phase was always part of the plan,” he says.
“It was just a question of priorities. Get the railway up and running first - then come back later and sort out the lower walkway and seawall.”
Having hit the first of the objectives at Dawlish – to get the railway up and running again by the beginning of April, the project team set about working on the design of the second phase.
With the curved base of the original wall remaining in situ, the new cross-section was to provide a wave impact return wall and reinstate pedestrian access along its length, enabling walkers to promenade between Dawlish and its smaller sister village of Dawlish Warren a few miles to the east.
Tidal constraints inform design
It was clear that there were tidal constraints that would impact any potential work.
At Dawlish, the existing base of the wall is constantly washed by the upper reaches of all high tides. When the highest of spring tides come along it can easily lap halfway up its height.
Combine high spring tides with strong onshore winds and the result, as witnessed last year, can be devastating.
“We have been working on the design since we managed to hit our first target to get the railway open by April last year”
Chris Warburton, Amco Rail
That high tidal range is one of the reasons the team plumped for a design that features 2 m-wide precast concrete seawall sections fixed to the base of the existing seawall.
The new walkway is brought up to level using reinforced concrete fill poured between the back of the precast units and the front of the track wall.
The precast units are secured to the existing base by a 40 mm-diameter, 5 m-long dowel bar grouted vertically through the precast unit toward the front of its foot.
There is a further 1.5 m-long dowel bar grouted at the rear of its section.
The precast concrete units also feature eight holes cast longitudinally through each section that will receive 25 mm-diameter steel reinforcement dowel bars.
These are drawn through each of the 10 precast units that form a 20 m-long bay.
At both ends of the bays a precast unit, shaped differently from the other 15-tonne units, forms a diaphragm between the face of the precast unit and the face of the track wall.
Wrapped around this diaphragm are the steel reinforcement bar mats for the cast in-situ concrete fill, which help provide the bulk of the lower walkway wall’s strength.
“We have been working on the design since we managed to hit our first target to get the railway open by April last year,” Mr Warburton says.
“Our precast producer Hanson Precast had worked with us during the first stage and they have helped us out again during the second phase.”
The Amco site team used a polypropylene fibre reinforced concrete to bring the wall base up to level and provide a ‘landing strip’ on which to place the precast concrete sections.
This strip is cast across the width of the existing wall’s masonry base section at thicknesses of between 150 mm and 275 mm and was brought in across the beach using a dumper and a 38-tonne excavator.
This part of the work was carried out during the tidal window, but the introduction of a jack-up barge (see box) has enabled the precast placement work to continue through each stage of tide.
Jack-up prevents wet feet
To help ensure work could continue whatever the state of tide, the team has used a jack-up platform moored just in front of the site.
It is onto this platform that the precast units are unloaded, having been shipped around from the quayside at nearby Teignmouth.
Other materials including rebar mats and dowel bars are similarly loaded onto the platform and lifted into place by the crane located on its deck.
“Without the jack-up we would be limited when we could work. I don’t think we could have managed without it,” Chris Warburton says.
Despite having the platform in position the team found themselves beholden to the weather as, once more, high seas, winds and difficult tides made their mark on this part of south Devon.
“It meant that precast units were being stockpiled at the Teignmouth site so we had to stall production,” Mr Bolton says. “When you are working on projects like this you expect to be weather-affected at some point,” he added.
A non-shrink grout is placed behind the precast units once they have been located. This grout helps plug some of the surface gaps between precast base and landing strip.
The team then installs the longitudinal dowel bars and rebar cage before pouring the C50 self-compacting concrete to infill behind each 20-m bay length.
Excessive concrete pressure
Concrete is despatched from a static pump set at the top of the cliffs above the railway. The pump line snakes its way down to track level through the gardens of disused properties in a move to reduce excess pressure build up.
Thrust blocks have also been installed at the bottom of the pipe run where it dives beneath the railway in a further move to control the pump line, helping prevent it from rearing out of control when in use. In all, some 3,250 cu m of concrete will be placed.
“When you are working on projects like this you expect to be weather-affected at some point”
Martin Bolton, Hanson Precast
“We have been using self-compacting concrete because it helps avoid working in confined spaces,” Mr Warburton says.
“It is so congested behind the precast units that we realised it would be easier and we could still get the flow through the rebar.
“The pump line weaves through some disused gardens and down the cliff side before going underneath the railway, it’s around 750 m in all.”
The TV crews may have disappeared but the Amco project team has every intention of staying on track until the end of the line.
Precast solution helps beat tide
Amco is not the only member of the first phase project team that has moved onto the second at Dawlish.
Precast concrete producer Hanson Building Products is back on board manufacturing the 15-tonne precast seawall units that the team is installing.
These latest units are heavier than those used in the initial phase of work with a thicker profile and are the result of design collaboration with the project team. Using analysis of wave and storm data, the team designed the precast units which are 700 mm thick, tapering to 450 mm at the wave deflection head.
“They are a bespoke product, designed and manufactured specifically for this project,” says Hanson national sales manager Martin Bolton. “We started talking to the Amco team in July last year and started developing the moulds. The units used in the first phase weighed 5 tonnes. These are 15 tonnes.”
Each unit is packed full of rebar. The 32-mm and 25-mm-diameter steel is placed at 150 mm-centres each way, with concrete coverage to the bars a minimum of 65 mm thanks to the aggressive environment. In each unit there is around 2 tonnes of steel.
“We cast the units at our yard in Summercotes,” explains Hanson project manager Mark Shepherd. “There are four steel moulds and the units are cast on their side using 50 N sulphate resistant concrete.”
From the yard in Derbyshire the units are brought down by road to the nearby port of Teignmouth where the project has some wharf space. The units are then shipped from the wharf to the jack-up platform in the tidal zone in front of the site.