Little Mermaid submersible digger scours Surrey river bed
Osborne and sheet piling sub-contractor Suttle Projects collaborated to provide a world-first scour protection solution while working on a project to dig out and replace the river bed at the River Colne near Staines in Surrey. The companies developed the Little Mermaid, a remote-controlled fully- submersible digger.
The solution was needed because the deep, fast flowing water and low headroom of the bridge meant traditional sheet piling techniques were not viable. The mermaid submersible digger was used to scoop out the existing river bed. Divers then set out grout bags that 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 far too low for a standard mini excavator to be used on a pontoon with an operative sitting in the cabin to work it. Also, the water was too deep to over pump the channel or divert flow to create a temporary dry work environment. The mermaid was able to carry out the scour protection work while on a floating pontoon.
Its lower height of 0.8m tall versus the standard height of around 2.7m meant it was able to work effectively under the bridge, and was safely 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. It works on a hydraulic pressure of around 150 bar, with a flow rate of 25-50l/min. It 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.”
Suttle Projects director 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."
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