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Renewables: Overview

Seaside special

The arrival of a big crawler crane on a sea defence project in Kent has brought construction efficiencies and a new tourist attraction, as Margo Cole reports

The Kent seaside village of Dymchurch is a popular destination for visitors in the summer. Its “blue flag” sandy beach stretches for miles in both directions and, during the holiday season, is home to everything from donkeyrides to kite surfing, while the village shops, cafés and amusement park are filled with day trippers and holidaymakers.

Things, not surprisingly, quieten down considerably once the school holidays are over, leaving the village and its beach to local dog walkers and those looking for some peace and quiet. This year, however, Dymchurch has
another - somewhat unexpected - visitor attraction, in the form of a construction site.

The project

Project: High Knocke to Dymchurch Sea Defences
Client: Environment Agency
Contractor: Birse Coastal
Designer: Jacobs
Precast concrete manufacturer: Macrete
Piling subcontractor: Commercial Marine and Piling
Contract period: February 2009-Autumn 2011

 

Birse Coastal has a £30M contract to build 2km of new sea defences along this stretch of coast over two years.
The contractor is currently building revetments on the village beach, and it is not uncommon to see 25 to 30 people
gathered on the promenade watching construction.

One reason for the interest is the kit Birse has assembled on the site to place precast concrete revetment units, which includes a 120t crawler crane and a vacuum lifting machine. This choice of plant is allowing the contractor to build more efficiently during the few hours a day between high tides when it can get access to the work site.

The original design for the defences was a series of small stepped revetment units placed on a slope to reach the existing promenade level. These units take the energy out of the waves, while a new “recurve” wall at the top of the revetment sends the waves back. A new wall at the back of the existing promenade, 700mm higher than the wall already there, will provide the ultimate line of defence against high water and flooding.

 

“We said we could increase outputs if we used larger revetment units, but that would mean using a bigger crane”

Paul Lilley, Birse Coastal

 

This original design would have necessitated all the revetment units being placed from the beach, with plant being
moved off at the end of every shift, before the tide came in. But once Birse had been appointed - on an early contractor involvement (ECI) basis - the contractor and designer Jacobs got together to see if the design could be improved.

“We are both on frameworks with the Environment Agency, and when the ECI was set up, we re-looked at the whole constructability of the scheme,” recalls Birse project manager Paul Lilley. “We said we could increase outputs if we used larger revetment units, but that would mean using a bigger crane.”

This, in turn, raised the issue of where such a big crane would sit. Putting it on the beach would have required a substantial platform - creating a huge amount of temporary works - and the crane would have to be moved off the beach before every tide. There would also have been the danger of contaminating the beach if any oils escaped from the machine, as well as the disruption to the public if the entire beach had to be closed off while the machine was in use.

Instead, Jacobs and Birse looked at the revetment design, and came up with an alternative, which involves putting
in fewer revetment units on a steeper slope, and then building a promenade at the top of that slope, beneath the existing promenade, with the new wave wall in between the two (see diagram). This “lower promenade” acts as a temporary platform for the crane during construction, but then becomes a permanent architectural feature once the work is finished.

High Knocke to Dymchurch readoubt sea defences

“The introduction of the crane has moved the whole frontage forward,” explains Lilley. “It wasn’t originally intended to build a lower promenade - it is a product of the method of construction.”

The new design now consists of steel sheet piles forming a 7m deep “toe pole” at the base of the revetment, to prevent scour, behind which the 1:3 slope is built up with 6A stone and a smaller stone drainage layer, and topped with a 75mm concrete blinding layer. In places the new revetments are being built over the existing stone revetment, which dates back to the 1930s, and some of this stone is being re-used in the slope formation.

The precast revetment units sit on packers on the blinding layer. Each revetment unit has five steps, and the wall is between three and five units high, depending on the level difference between the beach and the promenade. Once the units are in position, they are grouted in place.

“The job is very lineal. Once the crane’s in place you can’t get past it so you can’t commence certain activities”

Paul Lilley, Birse Coastal

 

Antrim-based precast manufacturer Macrete is making the 1,700 revetment units, as well as 900 curved units for the wave wall that will sit at the back of the lower promenade. The 20t revetment units are shipped - with 150 units per ship - from Northern Ireland to Rye Harbour, which is just 29km from the site by road.

Their size was dictated by two factors: their weight - 20t being the maximum that can be carried on the highway by one lorry - and size. The units are 5m long, but when tilted to go on a lorry they take up just 2.7m, so they can be driven to site without a wide load licence. The units are placed using a vacuum lifting machine attached to the 120t crawler crane. This machine, made by Dutch firm Moderniek to handle loads of up to 25t, has five pads - one for
each step of the precast unit - delivering a vacuum to each step equally. Using the vacuum lifter means there are no lifting eyes or sockets to be filled, giving a smooth, clean finish to the revetments.

To support the crawler crane ahead of revetment construction, Birse builds a platform using rock gabions with a reinforced earth mattress. This eventually gets incorporated within the revetment construction, and will eventually be topped with a concrete slab to form the lower promenade. “It starts off as temporary works but becomespermanent,” explains Lilley. Progress is necessarily slow on a contract like this, when operatives can only work for a maximum of seven hours a day, due to the tides, but Birse is progressing at between seven and 10 units a day.

Slope and blinding construction are done ahead of revetment placing, but, says Lilley, the blinding cannot be left too long at the mercy of the tides, so a maximum of about 35m is prepared in advance of the crane.

Lilley describes the job as “very lineal”. “Once the crane’s in place you can’t get past it, so you can’t commence certain activities until the crane’s out of the way.”

Vacuum lifting device

The precast units are delivered by a truck running on the line of the new lower promenade, which then leaves by the same route. However, keen to keep reversing to a minimum to avoid accidents, Birse has commissioned a specially built truck that incorporates a cab that can turn through 180°. Once a unit has been offloaded, the driver
swivels the cab and then drives off by pushing the trailer rather than pulling it.

This combination of unusual plant is certainly keeping Dymchurch residents interested. During the summer, Birse
agreed to stay off a 700m stretch of “amenity” beach that runs in front of the village, and worked at the north end of the job, away from the public gaze. The restriction was lifted at the beginning of September, and the contractor is now working right in the centre of the village.

A similar restriction will apply between Easter and September next year, so the contractor will spend this winter doing the precast step units, lower promenade and wave wall on the amenity beach, work on the rest of the beach in the summer, then come back to finish the upper promenade and sea wall next winter. For now, though, the people of Dymchurch have a new attraction in their midst.

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