Stablilsed access provided by a Tensar Mechanically Stabilised Layer which enabled
construction operations to proceed quickly
BENEFITS TO CLIENT
The use of Tensar geogrids allowed the rapid installation of the new raised level causeway to enable the residents of Muchelney to benefit from their newly constructed road.
THE PROBLEM
Initially caused by a series of strong storms in late December 2013, the ordinarily picturesque the village of Muchelney and its access roads became submerged as flooding continued to worsen, making it the first village in the area to be completely cut by rising waters for months. There was a need to elevate the road levels to prevent loss of access in future.
THE SOLUTION
As part of wider flood defence measures put in place, the project consultant, Atkins, created a new design for the road. By incorporating Tensar geogrids, the road stabilised to allow construction operations to take place and achieve the required level increase above expected flood water levels, effectively creating a causeway and safe access route for the village.
PROJECT DESCRIPTION
In early 2014, the roads into Muchelney, Somerset were submerged for months cutting the village off from the outside world.
Somerset County Council (SCC) carried out a feasibility study that looked at all the routes in and out of Muchelney, and the section of Law Lane to the west offered the shortest section needing improvement , making it more cost-effective than the other options. Construction work started with the aim to complete the project in December 2014.
Time was a challenge on this project but the ground conditions added further complications. Ground investigations showed that alluvium depths 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 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 expereince at an early stage with this project," said Simon White, Skanska Business Director.
Tensar carried out a two-stage design to address initial access and then the embankment construction. A Tensar Mechanically Stabilised Layer (MSL) incorporating a Tensar TriAx® stabilisation geogrid was formed with a 6F5 stone to line drainage ditches, level the site and stabilise the ground for construction access before work moved onto using a mix of TriAx and Tensar uniaxial geogrid to create the soil reinforced structure of the embankment using Type 1 material as fill.
Tensar MSL’s have been used in the embankment to mitigate differential settlement of the road structure, while uniaxial geogrid provides lateral restraint to prevent the sides of the embankment from slipping. Tensar’s design used 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 specific to the challenges at this site.
Alternative construction options were considered but the Tensar system approach was considered to be the best option to minimize the land-take while managing the differential settlement. “It was a faster, more cost-effective and robust design than the other solutions” concludes James Apted, Atkins Technical Director.
Tensar stabilised causeway nears completion
Client: Somerset County Council
Designer: Atkins
Contractor: Skanska
