A2 Limavady Bypass - Earthworks
Overview of the A2 Limavady Bypass scheme earthworks
The Ice Age
The north coast of Ireland was over-ridden by major ice sheets on at least two occasions during the Quaternary Ice Age. In Ireland this is referred to as the Munsterian and Midlandian stages; most of the surface or near surface glacial deposits were laid down during the Midlandian stage. The ice sheet appears to have remained static for a considerable period and it is probable that an extensive glacial lake existed some 15,000 years ago in Lough Foyle. Vast quantities of glacial silts, sands and gravel were laid down and are over 5 metres thick in places.
Limavady town sits on a raised terrace of glacial sands and gravels some 15 metres above the River Roe, Curly River and the Castle River floodplains. The floodplains consist of estuarine alluvium underlain by soft grey silts.
The bypass
The bypass is built on embankment across the floodplains using sands and gravels obtained where the bypass cuts through the raised terrace at Killane and through the Drenagh Estate.
Site investigation
54 'shell and auger' boreholes were sunk; these recorded the type of and depth of the underlying strata and provided samples for laboratory analysis. 44 'cone penetrometer' tests were carried out; amongst other things this provided an indication of how ground water travels horizontally in the soft grey silts in the floodplains.
Embankment construction
Vertical drains were installed in the soft grey silts in the floodplains to dissapate groundwater trapped beneath the road embankments, greatly reducing settlement periods. The settlement was closely monitored using an extensive array of instrumentation installed beneath the embankment.
Construction of the fabric reinforced embankment was a lengthy operation. Thin layers of site won material were deposited with the 'rest period' between each layer determined by results obtained from the array of instrumentation.
Updated 6 August 2015
