A tough rock to crack at Rosyth Dock
A tender winning design recently sent Bachy Soletanche to the large scale Rosyth Dock expansion project to install a piled retaining wall as part of the entrance extension. Awarded by main contractor Bam Nuttall, the geotechnical firm’s innovative solution used dowels to extend piles into the extremely strong rock as a more efficient and cost effective alternative to piling several metres deeper for the same effect.
The overall project requires intensive engineering works to increase the capacity of its Number 1 Dock and widen the current entrance by 4m. This is due to a new contract which will see two Queen Elizabeth class aircraft carriers get finally assembled at Rosyth. Bachy Soletanche’s contribution to the expansion was to install 50m (linear) of secant pile wall to a 25m depth – with 3m dowel extensions – installed behind the existing entrance wall. The next step is to excavate between the temporary piled wall and existing dock wall and then construct the new dock entrance.
To construct the wall, 50 1.3m diameter piles were installed to a depth of 25m using the typical large diameter rotary bore technique. What differed from the usual process was the installation of the permanent cage which incorporated a tube in the centre of the pile wide enough to insert a 300mm diameter dowel. A string of drill rods was placed down the tube and a rock socket was formed with a ‘down the hole hammer’. A 6m long dowel was then grouted into the pile and the rock.
Bachy Soletanche Contract Manager, Steve Mallinson explained:
“Using the dowel ‘pinned’ the base of the pile. The alternative would have been to drill the 1.3m diameter pile several meters into the very strong Dolerite rock which was deemed a time consuming and costly exercise.”
The piles were also heavily reinforced to endure massive forces, as the eventual excavated wall is 24m deep with five levels of props. Subsequently, the full pile length cages weighed up to 14 tonnes.
Prior to excavation, Bachy Soletanche recently returned to site to install 12 ground anchors through the existing dock wall. The ground anchors, each with a load capacity of 3000kN and consisting of sixteen steel strands, were drilled and installed 20m through the mass concrete wall in order to cater for additional forces which will arise during excavation.
Bachy Soletanche’s 12 week contract ran from Easter 2009 and ended mid July.