Contents
Background
Design/ Check: WSP
Contract: NEC 3 Option B
Project Management/ Supervision: DCC Engineering Design Group
Works Duration: 17 months
Year completed: 2022
Works value: £1m
This site is located on the A377, just south of Umberleigh. A major route for HGV’s and the travelling public in getting between Exeter and Barnstaple. A 45m length of cracking was visible in the edge of the carriageway. DCC undertook a survey of the bank which was supporting the highway, and this showed that the toe of the bank was subject to serious scour from the River Taw below. The bank was also constructed oversteep, so wasn’t providing the required support to the carriageway. Two previous areas along this section of the A377 had slipped into the river over the past 15 years, so catching this one early aided in allowing us time to plan a scheme, rather than react to an emergency repair.
What were the options?
Ground investigations were undertaken to determine the conditions, failure mechanism and to also aid with optioneering. Due to the sensitive location of the site and the need to keep the road open, a scheme was developed which didn’t require a road closure for long periods of time.
A lengthy consultation and land negotiations was also required to enable the works to take place. Due to the River Taw being within the works vicinity, the EA were consulted and various surveys (including freshwater pearl mussels) were undertaken to be able to grant the works permit. The landowner of the bank also has the fishing rights to this section of the Taw, so they were also consulted on the works proposals to ensure the solution was acceptable by all parties.
The final design involved an ODEX piled retaining wall with reinforced concrete capping beam. Trief kerbs with drainage outfalls incorporated were installed and the carriageway and fencing reinstated. Large boulders (approx. 2.5t) were placed into the river and voids backfilled to slow down the scour processes and protect the bank from further erosion. The bank was re-planted with trees and shrubs to allow the vegetation to take hold again and provide a natural barrier between the road and river.
Who was involved?
WSP Senior Engineer and DCC Technician to deliver the scheme.