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Highways: Island access

Floodwater stranded residents of one Somerset village for almost three months at the start of 2014 but recent highway work means that problem should be a thing of the past. 

Few can have failed to have seen the media coverage of flooding across the Somerset Levels in the early months of 2014, or have missed the ensuing debate on the merit of dredging. While the flood waters have long since receded and progress is being made on carrying out £100M of repairs from flood damage, the risk of another similar incident remained high – until now.

One of the defining images of the flooding was of children from Muchelney being taken to school via boat, but some recent work has delivered a route in and out by road that is accessible whatever the weather. This work was fast tracked from design to completion through a collaborative approach by client Somerset County Council (SCC), contractor Skanska and designer Atkins with technical input from geogrid specialist Tensar.

The work focused on raising a 500m section of Law Lane by up to 1.27m to the west of the village to create a safe road route in the direction of Drayton.

“Law Lane flood frequently and the residents of Muchelney are used to being cut off for short periods at a time,” says Skanska business director Simon White. “In 2013 it was a few weeks but last year the road was submerged for months rather than weeks.”

SCC carried out a feasibility study that looked at all the routes in and out of Muchelney but the section of Law Lane to the west offered the shortest section needing improvement, making it more cost effective than the other options. The study was undertaken between April and July and design work followed straight away in August and September with construction starting soon after.

“The work was delivered in a very collaborative way with a masterplanning approach in order to complete the work by Christmas 2014 so that if January this year was as wet as it was in 2014, the residents wouldn’t be stranded again,” explains White. “We opened the road on time and we are now just finishing the new drainage channels.”

Atkins technical director James Apted says that time was a challenge on this scheme but the ground conditions also complicated the work. “We always knew that the ground was soft but the ground investigation undertaken by Structural Soils at the end of August gave some surprising results,” he says. “The alluvium varied from 5 to 10m across the site and although the alluvial crust showed strength values of 20 to 30kPa, in some areas where there was up to 8m of peat the strength was halved to 10 to 20kPa. The weaker area coincided exactly where the new embankment was at its highest.”

The solution to these challenges came through use of geogrids from Tensar. “The conditions were challenging and we really benefitted from Tensar’s design experience at an early stage with this project,” says White.

“The design is bespoke to the challenges at this site”

Tom Jefferis, Tensar 

In total the scheme used 25,000m2 of Tensar Triax geogrid and 15,000m2 of Uniaxial material, which when put in context of a 500m long, 3m wide road that is being raised by 1.27m the scale of engineering design in the new embankment is clear.

“We carried out a two-stage design to address initial access and then the embankment construction,” says Tensar product and technical manager of structural systems Tom Jefferis.

The Triax TX190L geogrid was used with a 6F5 stone to line drainage ditches, level the site and stabilise the ground before work moved onto using a mix of Triax and Uniaxial geogrid to create the soil reinforced structure of the embankment using Type 1 material.

“Providing near isotropic 360° radial load spread properties, Triax geogrids provided reliable, initial stabilisation over the site’s extremely soft ground,” says Jefferis. “Further layers of Triax have been used in the embankment to mitigate differential settlement of the road structure, while uniaxial geogrids provide lateral restraint to prevent the sides of the embankment from slipping.”

Tensar’s design used seven layers of geogrid that were overlain by the Type 1 aggregate in layers as thin as 150mm to take the embankment up to the required 1.2m height.

“The design is bespoke to the challenges at this site and it is unusual for us to mix these products in such a way,” says Jefferis.

Atkins carried out modelling of the settlement by assessing the load from the embankment on the long section to derive the compressive value. This was analysed with the elastic values from the ground investigation to calculate that settlement over the 60-year design life would be around 200mm at the eastern end of the work, while the figure would rise to 500mm at the western end.

The design will achieve an embankment above expected flood levels with slow settlement that can be managed with a maintenance regime.

“Ongoing maintenance is normal for roads in this area,” adds Apted. “When we were working on another route nearby the ground investigation showed that the made ground below the road extended below the level of the alluvium through centuries of the route being built up to compensate for subsidence.”

Other options for Law Lane could have been constructing a wide berm, using lightweight fill or creating a piled raft but the geogrid approach was considered to be the best option to minimise the land-take while managing the differential settlement. “It was a faster, more cost-effective and robust design than the other solutions,” concludes Apted.

20-YEAR PLAN

Flood Action Plans (FAPs) were set up in the wake of last year’s flooding to bring together decision makers from the local authorities and environmental groups and other stakeholders and provide funding for flood resilience initiatives over the next 20 years.

The Somerset Levels and Moors FAP will be delivered under the recently established Somerset Rivers Authority (SRA), which is an umbrella organisation led by SCC and includes Department for Environment, Food and Rural Affairs; the Environment Agency; Farming and Wildlife Advisory Group; Natural England; Sedgemoor District Council; Somerset Internal Drainage Boards; South Somerset District Council; Taunton Deane Borough Council; and Mendip District Council.

Defra is providing £1.9M funding for the SRA’s first year, with SCC providing £600,000 and a further £200,000 provided by the district councils.

The new SRA will help identify what is needed, and fund and deliver projects similar to the Muchelney scheme. SCC has said that the organisation will provide a higher standard of flood protection in Somerset than is currently provided, as well as ensure the large number of separate flood risk management authorities in the county work together through a combined programme. 

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