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Tarmac turns the tide

The Somerset resort of Minehead is being protected from flooding with a precast concrete wall and stone brought in via a tourist railway.

TO WALK from one end of the seafront to the other at Minehead in Somerset is to step out of one British seaside stereotype and into another.

At the harbour end is a picturesque fishing village with squat cottages and elegant houses stretching up the hillside. Moving east, wriggly tin amusement arcades and souvenir shops lead to Butlins' Somerwest World, with its mini-funfair.

But all the buildings, old and new, face a common threat: they are flooded when the sea breaches the harbour wall.

Minehead has suffered from flooding for hundreds of years; most recently there have been floods in 1989, 1990, 1992 and in January and October of 1996. it was therefore no wonder that the (then) National Rivers Authority put it high on its list of priority sea defence schemes.

The project to improve the town's coastal defences got under way in 1992 with feasibility studies.

Work on site started in February this year, when Tarmac Construction arrived - then stopped again in July.

One of the conditions of the contract is that no work affecting the seafront must take place during the six-week school holidays in July and August.

By the shutdown, Tarmac had completed the first 200 m of defences at the harbour end. Since then it has installed another 400 m and has built 250 m at the other end of the job, working towards the middle. The total eventual length of the sea wall will be 1,800 m.

Two different designs of sea defence have been adopted for the two sections of the seafront (see diagram), although the topmost wall, curved to turn the waves, is the same all the way along.

The eastern section sits on tubular bearing piles and has a stepped concrete block revetment sloping down to the beach. The toe of the revetment is founded on sheet piles. All the piling was completed before the summer.

The steps of interlocking, precast blocks serve both sea defence and leisure purposes.

At the harbour end, the defences consist of the curved wall in combination with rock armour sloping down to the beach to absorb the waves' energy. Here, the precast curved wall pieces sit on the existing harbour wall.

'We have got to admit that the masonry wall, which dates back to God- knows-when, is in better condition than the later concrete wall,' says site agent Barry Fletcher.

Most of Tarmac's new wall is made from precast units supplied by Tarmac Precast Concrete from its Henlade works, near Taunton. There are three types of unit: the curved wave tops, which come in 2 m lengths and weigh in at 5 tonnes each; interlocking blocks for the stepped revetments, which weigh 7.5 tonnes, and wall beams weighing more than 7 tonnes, which span the tubular piles.

The precast units have no fixings in them, since the consultants did not want unsightly grout patches to mar their appearance. This stipulation obliged the two Tarmac companies, Construction and Precast, to develop new lifting methods.

For the wall units they came up with a huge scissor grab, a large-scale version of those used for kerbstones. For the large blocks, they opted for a suction pad.

'It works on the same principle as the things you use to unblock the toilet,' says Mr Fletcher.

But when Construction News visited the site, the vacuum pad was not working.

'It's French and a bit temperamental,' says senior engineer Richard Murray, by way of explanation.

There are 3,000 precast units in total. During the first stage of the contracts, three loads a week were arriving but that is rising.

'We have more than doubled that rate and we have got to double it again, although the quantity we can take is somewhat dependent on tides,' says Mr Fletcher.

He adds that this job is somewhat unusual. Work on marine contracts often has to fit in with the tides but on this contract, work only proceeds from Monday to Friday between 7 am and 6 pm.

Minehead lies on the Bristol Channel, where there is a huge tidal range. The level of high tide fluctuates by up to 5 m. Nevertheless, Tarmac has finished most of the rock placing behind the wall and two out of four rock groynes are complete. A third is under way.

'We have made quite good progress on the rock and we have got ourselves ahead of programme. The sea has been kind to us so far,' says Mr Fletcher. 'The biggest threat is high tides getting the wind behind them, leading to storm damage.'

That, of course, is an even bigger threat to the businesses and residents of Minehead - which is why Tarmac is there.