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Ground Engineering: Rising to the occasion

With more than 245,000m3 of fill needed to redevelop a site in the coastal Devon town of Seaton, an innovative approach was needed to avoid clogging up the narrow rural roads.

Some people view supermarket companies as corporate giants that are killing the high street, but in one Devon town local shop owners have welcomed the arrival of Tesco. When the store opened ahead of schedule and in time for Christmas, local people turned out in force to get a firsthand look.

What makes this new development significant to the residents of Seaton is not just that it has redeveloped a derelict site, but also the speed at which construction took place. Innovative use of marine material and surcharging helped consultant Pinnacle to fast-track the work to take it from earthworks to opening in eight months.

“The aim at the feasibility stage is to look at potential issues such as services, ground conditions and levels”

This may sound like a lot of work for new supermarket, but Tesco corporate affairs manager Emma Heesom believes it was worth the effort. “Tesco looks for gaps in the market,” she says. “At Seaton, people had to travel some distance to the nearest supermarket and the site also offered us a town centre location on a site in need of redevelopment.”

Pinnacle undertook a feasibility study of the site for Tesco in 2006 to carry out an engineering appraisal and assess the potential development costs. In 2008 Tesco started to exhibit its plans to the local community. It gained planning consent for the store in 2009 as well as outline planning permission for a residential and hotel development on the site, which Heesom says will happen at a later stage.

“Tesco wants to be a good neighbour, so bringing material in by road would have gone against this aim”

“The aim at the feasibility stage is to look at potential issues such as services, ground conditions and levels,” says Pinnacle project associate Ed Latham. At Seaton, the issues were obvious - the site is low lying, placing it at risk from tidal flooding. It therefore had to be raised by 2m to 2.5m across the whole development area. While this may sound simple in theory, bringing in the necessary 245,000m3 of fill by road was not a viable option.

Material gain

“Even if it was possible to source that amount of material in the region, bringing in that volume of material along the narrow roads into the town was a real problem,” explains Latham. “The local authority imposed a limit of 40, 20t trucks per day, which meant it would have taken two years to bring the material in.”

Heesom adds: “Tesco wants to be a good neighbour, so bringing material in by road would have gone against this aim.”

The solution was to bring marine dredged material into the site by sea, although the site is not located right on the beach, so some lateral thinking was required. Dredging contractor Westminster Dredging Company used its ship Oranje to bring material from the Bristol Channel to about 1.6km offshore, where it was transferred into a barge capable of carrying 15,000m3 before being pumped via a pipeline in a seawater suspension.

Noise issue

“The pipeline had to be raised up to carry material over two roads and onto the site,” says Latham. “Initially we were going to use sand and gravel from close to the Isle of Wight but the gravel rattled too much in the pipe as it was pumped onshore and we were concerned about the noise, so Westminster switched to using sand reserves from the Bristol Channel. We were bringing in one barge a day, seven days a week so the impact of the noise could have become an issue.”

Once on the site, each bargeload of the material was placed into a settlement pond to allow the seawater to be drained out and returned before the daily filling operation could get underway. With the use of sea water suspension came concerns about leaching of chlorides into the groundwater, so the settlement ponds were lined and a Huesker HAT 60.006SP geotextile membrane was also placed under the whole development area to stop any traces of chloride being carried off site.

Underlying geology

Once drained, earthworks contractor Raymond Brown used a fleet of excavators, dump trucks and compactors to place and compact the marine sand in 300mm layers. The design called for the store site to be raised by 2.5m so that flood risk remained above a 1 in 100 year flood event. The rest of the site was raised by 2m and the material needed for this was placed in just four weeks.

But placing the fill was not the end of the story. The underlying geology was formed from peat and alluvium so further work was needed to consolidate the newly placed materials.

“We calculated that without any additional work, the ground would settle by 250mm over 20 years which was just not workable,” says Pinnacle project associate Richard Carter.

While a piled foundation could have helped overcome the problem, the length of piles needed would punch through the geotextile and create a pathway for chloride leaching. Pinnacle’s solution was to surcharge the site and found the building on a flexible raft.

Finite analysis

“Across the store footprint, 4m of surcharge material was placed while 2m was placed over the car park and access road areas,” says Carter. “Ground engineering specialist Geotechnics used settlement cells, hydrostatic profile gauges and settlement plates to check the progress of the consolidation. We expected it to reach the required level that would result in 50mm of settlement over 20 years within four weeks but it took five weeks to reach acceptable levels.”

Carter’s team used finite element analysis to design the 70m square, 250mm thick reinforced concrete raft foundation for the store.

“The design allows for 25mm of differential settlement between column and 1:500 distortions to prevent any movement from causing cracks in the floor tiles,” he says. “A traditional pad foundation solution would have almost reached the base of the new fill and increased the localised bearing stresses on the underlying geology.”

Raymond Brown cast the slab in four sections using C32/40 concrete, while the car park has been paved using a conventional asphalt construction. “There are flexible transition plates between the thresholds so that differential settlement between the building and the surrounding areas can be accommodated,” says Carter.

“To be able to open two weeks ahead of schedule was a great achievement given the scale of the work”

Work on the slab was completed in June, clearing the way for main contractor ISG to start work on building the store. Bowland and Kirkland erected the timber hybrid frame at the end of July and the rapid construction work made the site a prime tourist attraction over the summer months.

“We erected a viewing platform during the consultation stages of the project,” says Heesom. “It remained on site during the construction phase and people seemed to like being able to check on the progress.”

Smooth sailing

Work on site went well with no real issues - something that is borne out by the early opening of the store. “We always planned to have the store open before Christmas, but to be able to open two weeks ahead of schedule was a great achievement given the scale of the work undertaken in Seaton,” says Heesom.

Tesco is also a hit with local people with the free car parking attracting visitors to the store and other shops in the town. It has also created 190 new jobs.

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