Network Rail began work on the second section of the new Dawlish sea wall in Devon in November. Nia Kajastie gives an update on progress made by contractor Bam Nuttall.

The new £80M sea wall, running from the Coastguard breakwater to Boat Cove, will protect a vital stretch of railway between Dawlish and Teignmouth.

In 2014, heavy storms along England’s southern coast destroyed a section of the old sea wall and left just a swing bridge of rails behind.

This resulted in an eight week closure of the line. Since then, Network Rail has been looking at options for improving the railway’s resilience, to protect it from rising sea levels and further extreme weather events.

In early 2019, the Department for Transport committed funding for the new sea wall.

In May of the same year, Network Rail awarded the £30M contract for construction of the first phase to Bam Nuttall, and work on it along Marine Parade began in the summer. The 360m section, running from Colonnade underpass, west of Dawlish station, to Boat Cove was completed in July 2020.

Teignbridge District Council approved Network Rail’s plans for the second section soon after that.

Bam Nuttall was awarded the £27.8M contract for the remaining structure in July last year. It entailed the construction of a new 400m section of sea wall between the Grade II listed station building and Coastguard breakwater.

This section is expected to take around two years to complete.

More specifically, the stretch from Dawlish station to the Coastguard breakwater east of the station is expected to be completed in late 2021. Work on the section between the station and the Colonnade breakwater, which  will link up the new wall at Marine Parade, will start shortly after that.

The Wave Walker jack up barge in position on the Dawlish seafront

Bam Nuttall is employing an eight legged, self-contained jack-up barge, called the Wave Walker, as a piling platform. According to the contractor, it is the only kit of its kind in Europe. This is also the first UK rail project for this type of barge.

The Wave Walker, developed by Fugro and Van Oord, can “walk” through bi-directional movement to position itself, without the need to be floated into place.

The piece of kit was chosen over a traditional jack-up barge, which would have meant excavating rock at the sea wall. Bam Nuttall also scrapped the idea of building a causeway to pile from.

With the Wave Walker, the contractor can access the sea face of the railway embankment along Marine Parade. The barge is able to operate across the high tidal ranges, which impact the south Devon coastline.

Bam Nuttall senior agent Yan Sayles explains: “The Wave Walker was chosen because it could undertake the movement along the beach during any tidal state, and without the requirement for a marine vessel.

“This was because the beach in this section is quite mobile, so it would require us to move potentially large volumes of sand to allow enough draft to float both a traditional jack-up barge and the vessel required to move it. The weather at the time of year of the piling would also restrict when vessels could get close enough to move a traditional jack-up barge.

“The beauty of the Wave Walker is its ability to reposition by sliding up to 4m in either direction and then being able to reset its legs to continue moving.”

Bam Nuttall’s scope of work involves the installation of 286 circular hollow section (CHS) piles into a sandstone breccia. The piles will range from 12m to 16m long.

“Following on from the installation of these, a precast facing panel will be installed followed by a low carbon, insitu concrete to build up a new wall in front of the existing wall,” says Sayles.

He explains that the contractor considered using 1,220mm outside diameter (OD) CHS piles with 30mm wall thickness and 762mm OD CHS piles with 25mm wall thickness.

“We undertook a trial exercise in the station car park in April using a Delmag RH34 [drilling rig] and both sized piles. The 762mm pile was successfully installed to the specified depth, but the 1,220mm pile was not able to be driven to depth,” he notes.

“We also trialled the use of no teeth and teeth cut into the piles, which directed us to needing teeth cut into the bottom of the CHS.

“The use of the 762mm CHS pile is also more compatible with our delivery method through Dawlish and under the railway due to the smaller size and lower weight.”

Bam Nuttall chose the Delmag RH34 for the work, as it was successful on the trial pile and is an acceptable weight at 110t.

The ground in the area consists of a layer of beach deposits, overlaying a thin layer of weathered Alphington and Heavitree Formation.

“We have had an offset axis kit and mast extension kit added to the rig to allow it to reach out further and also increase the up and over height to allow the rig to connect onto the piles when they are pitched into the gate prior to piling,” Sayles explains.

The drilling rig is sat on a bespoke temporary works cantilever, which forms a 3m projection over the barge side. The cantilever is required because the legs of the Wave Walker are located on the outside of the hull to allow the barge to walk and would otherwise clash with the installed piles.

“We are using a bespoke pile dolly designed by our regional engineering team,” Sayles adds.

“This is because the pile has to be driven below the deck height of the Wave Walker, so the extension piece is used for the last 2m of pile drive.

“We are using double start rock augers on the rig as the material is able to be retrieved from inside the pile and brought up to backfill the previous pile. We are using a hopper designed by our regional engineering team.”

The piles are being pitched through independent piling gates – a seven pile gate for straight sections and a three pile gate for changes in direction. The gate is moved along using the large Wave Walker crane.

The ground in the area consists of a layer of beach deposits, overlaying a thin layer of weathered Alphington and Heavitree Formation.

Then the rest of the pile is into  intact Alphington and Heavitree Formation.

“The final 1m of the pile toe requires a socket, so no augering is allowed in this final 1m, which makes the last drive using only the torque of the drilling rig key,” says Sayles.

“To give the rig the best chance of achieving this, we are preserving the teeth of the pile by hard facing and adding hardened metal wearing plates onto the teeth profile.

“We are using a 680mm diameter auger inside the 712mm internal diameter pile, which reduces the shoulder that the pile has to shave off as it is rotated into the ground.”

Towards the end of December, Bam Nuttall’s team at Dawlish  passed the halfway point in the piling process. Work on site resumed again on 4 January after a two week Christmas break.

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