Bachy Soletanche in A46 Coopers Hill landslip stabilisation scheme
The Anchors, Grouting and Mini-piling team of specialist geotechnical contractors, Bachy Soletanche, has just completed a restricted access landslip stabilisation scheme on the A46 Coopers Hill for main contractor Dawnus and Gloucestershire Highways, a JV between Atkins and Gloucestershire County Council, in a rural location in the Gloucestershire countryside.
Starting on 15 November 2010, the multi-phased scheme corrected the slip effects that had appeared over recent years, most notably in a 40m long section. Just 30m along, the 120m long retaining wall was showing the effects with its kingpost and concrete plank structure gradually tilting over in the ground with the rotation particularly pronounced at one end. Cracks and tension damage were also apparent in other areas.
Bachy Soletanche used 70 508/450 mm diameter ‘shear key’ restricted access laterally loaded piles with 406mm diameter 10mm thick CHS tubes up to 18m long and drilled 5m into Lower Lias mudstone installed in 5 rows in the existing carriageway, with 3 inclinometer tubes for later remote monitoring.
108 22m long, 130mm diameter tension pile/self drilled bar anchors were installed through the existing 120m long retaining wall, drilled at 10-15 degrees, built c.1925 for later connection to the new retaining wall which was built by Dawnus. The new retaining wall was constructed from 54 300mm diameter 11m deep King Post piles with 203X203 S355JOH grade Universal Columns. 54 300mm diameter ‘shear key’ piles up to 15m long were installed into the slope stripped by Dawnus with a 5m rock socket with 273mm CHS tubes.
Richard Piggin A.G.M. Contracts Manager at Bachy Soletanche, said:
“The work on the ancient slip slope was adjacent to a 40m long tension crack. As the work was carried out close to live traffic, we controlled the situation with traffic lights to cause as little disruption to road users and the travelling public as possible. Restricted access rigs were chosen to avoid further obstruction and we used lighter access rigs which were safer on the potentially unstable slope.”
Jonathan Merrick, Project Manager for Gloucestershire Highways, said:
“The shrinkage was vertically downwards but, on the slopes, the heave back tended to be perpendicular to the slope angle which had the effect of gradually inching the ground downwards. Over the years this has allowed the wall to rotate, particularly at the end. To repair or demolish the old wall would have been too risky so it was “encased” by a new one just in front of it, this time with H-sections grouted in for reinforcement. Unlike the multidirectional forces possible on the slide piles, the H-piles had to essentially take vertical load for a concrete pilecap above and the new concrete wall posts and slats.”
Once the new wall was constructed by Dawnus, the 50mm steel pile reinforcement was extended in 3m lengths to pass through the new structure and restrain it after tensioning. The gap behind the new wall in front of the old structure was then pumped with a foamed concrete to fill the void and the “boulder” fill behind the old wall, to fill any interstices.