Agenda item

Paul Frankland, Engineering Design Manager and David Johnston, Design Team Leader.

4.1    Mr David Johnston, Design Team Leader (Engineering) advised that the A72 was the main east-west link between Tweeddale and the Central Borders and it carried approximately 6,400 vehicles every day.   The steep slopes adjacent to Dirtpot Corner had had a history of instability and previous landslips had led to lengthy road closures and significant disruption on this main east-west link.   To reduce the likelihood of landslips on the slope above the road the Council had undertaken slope stabilisation works, the most recent of which added further structural netting and was completed in January 2013.    The existing road geometry at Dirtpot Corner was sub-standard.  The carriageway width and road alignment was such that two larger vehicles could not pass each other if they met in the vicinity of the bend.   The narrow carriageway width was exacerbated by the lack of any roadside verges as masonry walls sat immediately adjacent to the carriageway.  A number of personal injury accidents had occurred at the corner, with 2 personal injury accidents occurring during 2015.   However, the site was not currently classed as an Accident Cluster Site (i.e. 3 or more personal injury accidents over 3 year rolling period).   The topography at Dirtpot Corner was particularly challenging and the road was located 7m above the River Tweed and sat below very steeply rising ground which had a gradient in excess of 1 in 1.   A riverside masonry wall supported the road above the river and in some places retained up to 1.5m.   Sections of the retaining wall were leaning and the wall was in a poor state of repair.  The wall was damaged during the recent traffic accidents and the wall did not meet modern standards as a road restraint system.    Previous assessment work had discounted the very large and expensive schemes involving bridges over the River Tweed.  Officers and Members were keen to develop a more efficient solution with less environmental impact.   Ground investigation works were undertaken in October and November 2015 to inform the design and several structural solutions had been reviewed and developed and initial costs obtained.    Following a review of options widening on the hill side was discounted in preference to widening on the river side. 

4.2    Mr Johnston explained the preferred solution involved an “innovative” hybrid structural solution to mitigate the environmental impact and to keep costs to a minimum.   The proposed solution involved stone filled baskets connected to drilled anchors into the rock slope at the lower level.  The stone filled baskets would give a sympathetic/rustic finish to the lower sections of the wall and would ensure that in the event of flooding the wall would not be adversely affected.    At higher levels it was proposed to utilise a reinforced earth solution.  This solution involved a facing system connected to a “geodgrid” – a type of polyester/plastic mesh which extended back into the area to be backfilled and which was connected to the front face of the slope.  Where room permitted the design would utilise a green reinforced earth solution above flood levels.  It was anticipated that the improvement works would be concluded during November/December 2018.   The Chairman thanked Mr Johnston for his presentation.

DECISION

NOTED.