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Technical Excellence: A structured approach to erosion

Immortalised in the Dire Straits song ‘Tunnel of Love’, Whitley Bay’s Spanish City amusement park is now defunct, significantly reducing this North Tyneside seaside resort’s attractions for tourists. 

A regeneration project was launched three years ago, but since then another key landmark popular with tourists has come under threat. 

St. Mary’s Lighthouse, now a museum, visitor centre and café, is located on the tiny Bait Island to the north. A tidal causeway links it to Trinity Road, the only access, which sits on top of low boulder clay cliffs partially protected by a conventional seawall. Unfortunately, continued erosion of the soft boulder clay left the southern end of the seawall exposed. Seawater was penetrating behind the wall causing further erosion, and the long term stability of Trinity Road was at risk. 

Responses

A conventional response would have involved sheet piling and/or rock armour. Both had drawbacks, not least the significant area of the relatively narrow beach that would be lost to rock armour. Something different was needed, something that looked better and had a much lower beach take. 

North Tyneside Council highways and infrastructure manager Mark Newlands says the coastline around Whitley Bay is constantly at the mercy of the elements. 

“We identified that the sea wall was at risk and remedial work was needed. The aim was to protect the interface between the existing sea wall and the softer cliffs to the south.” 

A collaboration between Lafarge Tarmac, the Environment Agency and North Tyneside Council eventually decided on a solution involving the first full scale use of the new, patented TBlock (see box). An interlocking wall some 26m long and 6m high would be constructed beneath the threatened cliff to protect the end of the existing sea wall. TBlocks were chosen for a number of reasons, not least that visually the TBlock wall would closely match the appearance of the seawall, and block by block: It only took nine days to install the 162 blocks that were required for the project to approximately mirror its profile.

Design

Design was carried out by Capita, North Tyneside’s technical services partner. Capita engineering project manager Peter Woods says that rock armour was originally the preferred solution. “But we found that by using TBlocks the beach take would be less than 2m compared to the 9m needed by rock armour.” 

Lafarge Tarmac produced the necessary TBlocks at its Gateshead readymix depot, using local marine aggregates to achieve a C40 mix. Each block weighed in at 2.4t, but four recessed lifting eyes in the top surface made handling safer and more efficient. 

“I’m greatly impressed by the speed and efficiency with which the project was completed, despite the difficult weather and tidal conditions at the outset” 

Mark Newlands, North Tyneside Council 

The ESH Group, North Tyneside’s framework construction partner, was responsible for the TBlock installation as a sub-contractor to Lafarge Tarmac. Several challenges faced the project team. Between the main road access and the site there is 1.5km of public beach that had to be kept open. And autumn had arrived, with exceptionally high tides and the likelihood of storms and tidal surges. 

Keeping the beach open was achieved by only transporting the blocks into position during quiet periods in the early morning and late evening. During the five week construction period the works were submerged for three days, an apparent setback which the team turned to their advantage by using the time to move substantial numbers of blocks onto the beach. 

Dry stacking the TBlocks speeded construction, although it was found that inserting a thin sheet of plastic foam between the horizontal faces enabled very precise vertical alignment to be achieved. Conventional lifting equipment was all that was needed. In all, it took only nine days to install the 162 blocks needed. 

More efficient

Afterwards it was calculated that 88% less excavation was needed compared to the rock armour alternative and the blocks were installed in only 20% of the time. 

Part-funded by the Environment Agency through its Flood Defence Grant in Aid scheme, the £210,000 project was finished on time and to budget. Newlands, who originally championed the TBlock alternative, says they represented an ideal solution. 

He added: “I’m greatly impressed by the speed and efficiency with which the project was completed, despite the difficult weather and tidal conditions at the outset.” 

Woods also admits to being impressed. “The scheme has secured the structural integrity of the sea wall while maintaining the beach at the north end of Whitley Bay intact. TBlocks have provided a solution that not only protects the seawall but can be extended without costly engineering enabling works.” 

There is quiet satisfaction in the Lafarge Tarmac camp as well. Whitley Bay is seen as a successful demonstration of the TBlock’s potential, given the unfavourable conditions in which they were installed. “Compared to alternative sea defence solutions, TBlocks are cost effective, which was crucial in getting this project off the ground,” says regional director with responsibility for construction solutions Richard Vine. 

Long term

And he also points out other, longer term advantages. “Unlike rock armour, for example, a TBlock design requires little long term maintenance. Indeed, one of its major benefits is that the blocks don’t clog up with sea waste or provide an ideal habitat for vermin.” 

This first showcase for the TBlock is less than 20km due south of Newbiggin-by-the-Sea, where the basic concept was first born. It would be nice to think that in due course TBlocks would be chosen to provide lasting protection to St Bartholomew’s church out on its headland – but no such project is yet planned.

This article was produced in association with Lafarge Tarmac.

An ingenious solution

Coastal erosion has always been a problem for the little Northumbrian town of Newbiggin-by- the-Sea. Over the years, millions of pounds have been spent on recharging the beach and building breakwaters. 

Local freelance inventor Alan Thompson, however, was particularly concerned about the threat to the historic 13th Century parish church of St. Bartholomew. Located on Church Point beyond the breakwaters, it was under constant threat from North Sea storms. 

Thompson’s ingenious solution was to utilise the WWII concrete tank traps that littered the beach. He persuaded the local authority to move some into position to protect the church – and in doing so made the key discovery that led to the development of Lafarge Tarmac’s innovative precast concrete TBlock. 

“He observed that the orientation of the tank traps determined how effective they were in absorbing wave energy,” explains Lafarge Tarmac commercial development manager Tom Bateson. 

“Those that had their corners facing the waves were much more effective than those that were straight on.” 

Over the next couple of years Lafarge Tarmac developed Thompson’s ideas, modelling the effectiveness of a ‘sawtooth’ vertical profile and developing a three dimensional geometry that provided a two plane lock to ensure excellent resistance against wave impact. 

Thus the TBlock was born. Interlocking to create a distinctive jagged profile, a TBlock wall utilises near field diffraction effects to significantly reduce wave energy, Bateson says. “And a TBlock installation is much more vertical than the conventional alternatives, significantly reducing the ‘beach take’ – the area of the beach lost to the defences.” 

If the client should so desire, he adds, TBlocks could be coloured or textured to match existing structures, or even given a drystone wall finish. The design is notably flexible, different basic shapes are available, and end uses go beyond coastal defence. 

Lafarge Tarmac believes the TBlock could make significant inroads into the river and flood defence sectors, and be a viable option for retaining walls and railway embankments. The ‘guaranteed interlock’ is the key characteristic here, Bateson says. 

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