BENEFITS TO THE CLIENT

Tensar provided a proven, quick and safe way to stabilise the highway embankment.

The integrity of the critical bottom geogrid layer was ensured by the simple way that the overlaps could be formed and the layer installed.

THE PROBLEM

A new dual carriageway had to be constructed over a large pool contaminated with heavy metals and mining lagoon waste with a water depth of 4m and a 1m thick bed of silt. The highway embankment needed to be stabilised over this very soft material and any geosynthetic used had to be able to be installed correctly and easily without the use of divers, whose use was ruled out due to the contaminants.

THE SOLUTION

Two layers of Tensar geogrids were installed in the embankment to form a stable platform. The lower layer was prefabricated on site, using simple cable ties to form a continuous mattress, which was floated out, over the pond. Coarse rock fill was then placed in a controlled manner. Vertical band drains were installed once the fill was above the water level followed by a second geogrid layer and general embankment fill to surcharge the soft deposits.

PROJECT DESCRIPTION

Part of the A39 Carnon Gate Project entailed building a new dual carriageway over very difficult ground to provide much needed relief to the existing narrow A39 through Devoran, Cornwall.  Woodbury Pool exists as a legacy of the mining industry and is contaminated with heavy metals and lagoon settlement waste.  One of the complexities of this job was the contaminated silts at the base of the pool which, due to Health and Safety implications meant divers would not be able to check  geogrid laid underwater.

The first layer of Tensar biaxial geogrid was installed by floating a ‘mattress’ of geogrids secured by cable ties across the 12,000m2 area; it was positioned by winch. These geogrids are manufactured from polypropylene which has a specific gravity of <1.0 and therefore float. Granular 6A fill was then placed in a controlled manner by 360o excavator and the geogrids were “sunk” into place under the weight of the rock fill with the aim of displacing a proportion of silt on either side.  Once the embankment was out of the water, a further 1 metre of quarried crusher run was compacted to allow safe installation of the band drains, followed by a second layer of Tensar Geogrid and then general embankment fill placed to a maximum height of 5.2metres during the surcharge period.

Actual settlement during the initial filling and surcharge period were well within the anticipated range.  Cormac Site Agent Derek Dunn said “In excess of 40,000 tonnes of fill was placed directly on the Tensar Geogrids straight from the delivery lorries, which were supplying quarried aggregate to various grades.  By using Tensar Geogrids we saved a large proportion of this aggregate that otherwise would have been lost in the contaminated silts.  The Geogrids were easily secured and literally floated into position”.