GEOSYNTHETICS played an important role in a challenging slope stabilisation project on the A38 in Glyn Valley, near Bodmin in Cornwall.
The £1.9M project, completed this summer, involved long-term stabilisation and realignment of 400m of the road immediately below and alongside the London to Penzance railway and only 10m above the River Fowey. The river is an important water supply and fishing area.
This section of the road has a history of instability and the problem was worsened by the wet winter of 2000/01, explains Mott MacDonald graduate geotechnical engineer Keith Halstead. Mott carried out the design for the Highways Agency. Construction was by contractor Dean & Dyball.
The slope comprises mixed fill dating from when the A38 was built, overlying the weathered sandstones and shales of the Lower Staddon Grit, with alluvial deposits covering the River Fowey floodplain.
Because the river is so close the slope had to be steep, which meant building a 7.5m high reinforced slope below the road, a minimum of 2.5m from the riverbank.
Reinforcement work included HDPE geogrids connected to 480 8m long soil nails installed in the original slope behind.
Where there was enough space at the toe, unreinforced embankments were built with transition zones blending into the reinforced sections. Gabions along the toe of the slope provide protection against a 100-year flood event.
'Design focused on environmental and economic sustainability, including hessian bags filled with a topsoil and seed mixture behind the face to encourage rapid vegetation growth and use of locally sourced China Clay for the class 6I/J fill material in the reinforced slope, ' says Halstead.
All trees along the riverbank were retained, and the floodplain restored after work was completed following Environment Agency consultation.