During berthing operations at the jetty, propeller action produce highly turbulent water jets which increase the flow velocity at the seabed and cause scour. The loose material of the riverbed and the riverbank behind the submerged piled wall at the Shell Jetty area was found to be considerably eroded from the water jets produced by ships’ main and bow thruster propellers. The slope behind the submerged wall provides support to the existing Thames Flood Defences at the top of the riverbank, so it was imperative to ensure its protection.
Kaymac Marine & Civil Engineering Ltd were commissioned by Laing O'Rourke to install a grout mattress protection system to the existing slope on both the East & West side of the jetty. The works involved the installation of 35 Nr custom made Proserve fabric mattresses, with an average size of 20m long x 5m wide x 0.15m thick to the slope.
Kaymac first carried out a topographic survey on the structure to determine the exact location of the foot of each of the 42 steel tubular supporting piles, working in at least 6m of water. Using these co-ordinates, each mattress was individually manufactured to fit by Proserve Ltd. Due to the tidal nature of the works, filter Point FP225 mattress was used in the wave zone and Constant Thickness CT150 mattress below. Each mattress panel was designed to be zipped the neighbouring mattress and also to butt against the existing pile wall at the base of the slope.
The existing bed level was littered with debris which needed to be cleared prior to the mattress installation. This was carried out by a combination of land based (at low water periods) and marine plant, with the work being heavily diver orientated. The location and orientation of the mattresses were set out on the slope by Kaymac and guide wires fixed to the submerged pile wall. Once in position, working from Kaymac's marine plant, divers unrolled the fabric mattress, zipped it around the individual piles and positioned it along the guide wires- with only approximately 1.5hours of dive time being available due to the flow and tidal conditions.
Working from the shore, a concrete pump was set up with the tremmie hose end fitted to the inserts of the mattresses by the diver. Under the control of the diver, micro-concrete was then pumped into the mattress in stages to ensure that the fabric did not burst or damage. Stringent tests were carried out for each concrete load to ensure that blockages did not occur, as the mattresses were not designed to have any cold joints.
All 35 mattresses were completed in this way, with the top of the mattresses adjoining to the existing top asphalt revetment by trenching works.