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Defender of the lights

Blackpool - Blackpool's sea defences are being rebuilt to protect 1,500 properties and the seafront tramway from ooding. John McKenna reports on how precast concrete is playing a vital role.

By 2009, Blackpool's donkeys will have more adventurous routes to take tourists than the usual trot up and down its sandy beach. The Lancashire resort is undergoing a four-year, $123M sea defence improvement scheme that will see its black wall defence replaced with a curvy beachfront and sea wall.

Five headlands will be created along a 3.3km stretch, sweeping out by as much as 50m towards the sea. This is architect EDAW's way of introducing a more natural feel to the area. A year into the project, work is underway along nearly half of the seafront.

Client for the scheme is Blackpool Council, whose on site resident engineer Lee Cunningham believes that the new design will help reconnect the town with the sea. 'The current sea wall acts as a barrier. But the new scheme uses sand-coloured concrete revetments that not only help absorb wave energy and cut scour, but act as steps that give the public access to the beach at any point, ' he says.

The current sea wall is about 100 years old and at the end of its serviceable life. With maintenance demands increasing and the wall being overtopped more frequently ? leading to the seafront tramway, road and 1,500 properties being flooded ? the council decided to take action. Replacing the Victorian sea wall is part of a ú1bn regeneration strategy for Blackpool (see box opposite).

As well as providing protection from coastal erosion, flood alleviation and improved public access, the new defences will create a further 5ha of open space. 'This scheme is not about land reclamation ? but that is a welcome by-product, ' says Cunningham.

He points out that the entire ú66M came from the Department for Environment, Food and Rural Affairs, which classes the project primarily as a coastal protection scheme with the secondary aim of acting as a sea defence.

The new defences have a 100-year design life and are built to withstand a one-in-200year storm. They are being built seaward of the existing ones and generally comprise two rows of sheet piles with a sloping revetment between them.

The revetment is formed from fill material, placed at a slope of 1:3, capped with a concrete blinding layer. Precast concrete step units, 5m by 3.5m, are then laid on top. These lock together to form a concrete apron.

The steps act as a barrier to the tides and a seating area for holiday makers. Behind this stepped apron is a at, cast insitu berm and precast wave wall.

Behind the wave wall and up to the tram tracks, the promenade is being replaced with coloured concrete paving (see diagram).

Consultant Halcrow and contractor Birse Civils (recently acquired by Balfour Beatty) opted for 30,000m 3 of precast concrete to be incorporated into the design on the revetments, wave wall and rear wall. A total of 100,000m 3 of concrete will be used on the project.

One of the biggest challenges for Birse project engineer Anthony Burgess is coordinating construction with the tides ? there is just a six-hour window every day for some operations.

Working with Halcrow project manager Gareth Robertshaw, Burgess's solution was to incorporate as much prefabrication in the form of precast concrete as possible so that on-site work would just involve assembling components like the pieces of a jigsaw puzzle.

The precast units come in four-step sections, 2,791 of which are needed to cover the 3.3km stretch of beach between the toe beam and the promenade. Each unit measures 5m by 3.5m, weighs 20t and contains 8m 3 of Strux-reinforced concrete (see box).

Dealing with these volumes with no sizeable storage facility on the seafront was identied as a problem early on by Birse, which enlisted a dedicated local precast facility as part of its successful bid for the job.

With its partner SLP Precast, Birse constructed a building to house a precastconcrete factory a few kilometres up the road at Thornton.

The site was ideally located, according to SLP Precast project manager Graham Hoole.

'It is next door to the Tarmac batching plant at Thornton, ' he says, adding that the Blackpool job was instrumental in keeping the batching plant in business.

The site was up and running by December 2005, four months after Birse had been awarded the main contract for the job. Being near the construction site, it also prevented haulage delays.

The Thornton precast yard also produces the wave wall sections, which are 2.5m by 1.5m and weigh 16t. These are steelreinforced, with a cage that is stainless steel for the sea-facing half to prevent corrosion.

Synthetic acro fibre reinforcement is being used for the revetment sections, which are also produced at the precast factory (see box).

The only challenge presented by using these bres came when the units were lifted into position ? the bre concrete was not designed to withstand the point loads from traditional four-point crane lifts.

The solution was a vacuum lift manufactured for the project by Dutch rm Moderniek. It has a suction pad for each step of the revetment and if there is a power failure, it has an emergency cutoff valve that keeps the vacuum closed so the revetment stays attached.

A problem posed by the revetments is that there are 81 different types of mould to accommodate the architect's concept of sweeping headlands.

This makes the jigsaw puzzle more complicated, according to Cunningham. Hoole also had to use three mould suppliers, as no single company could cope with such a large order.

Once positioned, the revetments are held in place by a combination of their own weight and the toe beam.

Should a wave dislodge one of these sections during construction, there is a safeguard: every 50m there is a small break in the revetments where piles are driven and insitu concrete is poured.

'If a revetment unit did wash out, ' says Robertshaw, 'you may lose a section, but you wouldn't lose the whole wall.'

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