Contractor Osborne has used an underwater excavator to help with a scour protection project on the River Colne in Surrey.
The river bed needed to be dug out and replaced near Staines, but due to the deep, fast-flowing water and low headroom of the bridge, traditional sheet piling techniques were not viable.
Osborne worked with sheet piling sub contractor Suttle Projects to overcome the problem by using a bespoke underwater remote control excavator nicknamed ‘The Mermaid’.
The firm believes it is the only one of its type in the world.
The Mermaid, which is fully submersible, scooped out the existing river bed. Divers then set out grout bags – which had to be hand-placed underwater and layered to the right levels. Finally silt could be laid over the top of the hardened grout bags.
In the case of the Staines bridge, the headroom was too low for a standard mini-excavator to be used on a pontoon with an operative sitting in the cabin to work it. In addition, the water was too deep to pump out the channel or divert flow to create a temporary dry work environment.
Therefore, the Mermaid was deployed, situated on a floating pontoon to carry out the scour protection work. It is 800mm tall compared to the standard height of around 2.7m, which meant it was able to work effectively under the bridge. It was activated a few metres away by an operative managing the remote control unit.
The Mermaid took 18 months to build in-house by converting a conventional mini excavator, with the project costing £65,000 overall. This was done by removing the cabin and engine and adding remote controls. The remote control unit was connected via hydraulic umbilical leads, supplying the machine with hydraulic power.
The machine works on a hydraulic pressure of around 150 bar, with a flow rate of 25-50 litres per minute, weighing 4t. The Mermaid is currently set up to work underwater up to 25m deep, but can easily be set up for deeper water conditions if required.
Paul Fagan, Osborne site manager, said: “The use of this self-developed technology simplified a complex operation, significantly reducing the time divers had to spend in the water. We are delighted with the outcome of this project and always look to new technology and better ways to deliver results.”
Director of Suttle Projects, Joe Paine, added “On a previous scour protection project with extremely heavy rainfall and a very low railway bridge, we used divers to place concrete, and also had to excavate with a type of venturi water suction pipe, suspended from our pontoon. Although this did provide a solution, it was time consuming and costly.
“We soon realised that a conventional small excavator that was able to work freely under water would provide the ideal, innovative solution. We could find nothing of this type on the market globally, so with support from Ross Welsh at Pirtek Poole, who specialise in fluid transfer solutions, we decided to build our own machine, the mermaid, which we can also hire out to other companies, too.
“By collaborating with Osborne to use this equipment in the project, we were able to provide an efficient and cost-effective solution for what was historically a complicated and expensive task.”