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Construction equipment: The little mermaid

Suttle Projects has developed a bespoke excavator to help with a tricky scour protection project in Staines. Ben Cronin reports.

A difficult scour protection project on a bridge in Staines has inspired an innovative equipment solution from contractor Osborne and its subcontractor Suttle Projects.

When the contractor was commissioned to dig out and replace the river bed at the River Colne near the town in Surrey it soon realised that a traditional ­excavator would not be viable due to the deep, fast flowing water and low headroom of the bridge. So it worked with sheet piling subcontractor Suttle Projects to develop a bespoke excavator nicknamed ‘The Mermaid’.

As the name implies, the excavator has been designed to work underwater, and further down the line that is exactly what Suttle Projects and Osborne propose to do. But for this particular project, its main virtue was its diminutive size.

Little Mermaid

Action stations: It took 18 months and cost £65,000 for Suttle to convert a conventional mini excavator into The Mermaid

“The bridge in question has less than 2m headroom, with deep, fast flowing water,” says Suttle Projects director Joe Paine. “Traditional sheet piling techniques would not be ideal for this set-up, so we created our own remote control excavator with the cab and engine removed so that it can be floated under the low headroom bridge on a pontoon and operated by remote control.”

The remote control and the excavator’s lower height of 800mm versus the standard height of 2.7m allowed it to work effectively under the bridge and be safely activated by an operative working a few metres away.

This enabled the contractor to excavate the river bed so that divers could hand-place grout bags underwater before silt was laid over the top of the hardened grout bags.

“We created our own remote control excavator with the cab and engine removed so that it can be floated under the low headroom bridge on a pontoon and operated by remote control”

Joe Paine, Suttle Projects

Paine says the innovative new piece of plant was born out of the frustrations of a previous scour protection project in Pevensey, East Sussex where extreme rainfall and a similarly low bridge had necessitated the use of a type of Venturi water suction pipe. “I think they’re used in harbours for dredging and in the third world in particular, but in an environmentally sensitive fluvial stream in East Sussex the thing was hopeless,” he says.

“You couldn’t control it to produce an even amount of excavation as you moved around.”

It took 18 months and cost £65,000 for Suttle to convert a conventional mini excavator with support from Pirtek Poole, a specialist fluid transfer solutions company.

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, weighing 4t.

Although it was floated on a pontoon for the Staines project, the Mermaid is currently designed to work underwater up to 25m deep, and Paine adds that it could easily be set up for deeper water conditions.

“When you break it down it’s about eight hydraulic circuits that you’ve got to find a way of powering, so why not remove anything that creates an electrical current or sparks, anything that uses fuel and anything that relies upon getting exhaust fumes out and remove the need for someone to sit on it?” he says.

The removal of any electrical current is particularly important for working in intrinsically safe environments, and especially around submerged vegetable matter.

“There are quite a few culverts that we come across where as soon as you stir up any vegetable matter inside the culvert, you get loads of methane coming out of the water,” Paine adds.

“By creating this excavator I thought we would always have a solution to any kind of underwater or semi-submerged or difficult-to-access excavation.”

Given that Suttle was recently awarded a scour protection subcontract worth £1.4M by Osborne covering Network Rail’s Wessex route, The Mermaid could be seeing plenty more ­action in the future.

“There are three upcoming projects where The Mermaid will be used underwater, where the water is either too deep for the excavator to reach the river bed from a pontoon, or where the headroom is so low, it would have to be deployed underwater,” says Paine.

“Companies have tended to pick off the easiest scour ­protection schemes, and put the more difficult ones on the back burner,” he adds. “We’re negotiating to do several additional schemes this year and it looks like we’ve got a good chance of winning them with this excavator.”

Osborne site manager Paul Fagan agreed that The Mermaid would prove useful on future projects and suggested that it had influenced the company’s decision to award further ­business to Suttle.

“The use of this self-developed technology simplified a complex operation, significantly reducing the time divers had to spend in the water meaning the Staines project finished a week and a half earlier than scheduled.

“This collaborative approach and the willingness to try out these innovations has delivered value for our customer Network Rail and our ultimate customers, the end users,” Fagan says.

The new piece of equipment, which is also available for ­companies to hire, is said by Fagan to be relatively easy to ­operate. Suttle searched the market for a remote control unit that mimicked the two-lever controls configuration on modern excavators meaning it can be used with a good degree of confidence by operators.

Paine adds that it runs on bio-oil, although this is less of an issue because the engine has been removed.

“With a normal excavator you have to exercise a degree of caution about the fuel, battery chemicals and mineral based engine lubricants when using excavators to ford a live watercourse, or work within a dewatered area that might become inundated,” says Paine.

“Because The Mermaid has no capacity to deposit harmful mineral oil into any given watercourse it offers the opportunity to get into more problematic situations without risk of damaging itself or the watercourse eco system.”


Jo Paine thinks inexperienced contractors make some common mistakes on scour protection projects. To help, he provided NCE with some guidelines.

Try to dewater if at all possible, and use sheet piles if you can. If these are installed to toe into a competent cohesive strata there is little chance of water ingress from below, (which of course can be calculated).

Pile clutches will silt up once a cofferdam is pumped down, even if clutch sealant is not used, so persist in trying to get the cofferdam as dry as possible, with a large pump to get the water low and find any leaks quickly, before starting excavation work within it.

Once it is pumped down, maintain the pressure forcing silt into the pile clutches by pumping 24/7. Use of sand bag dams will seem cheap at the outset of the project but in water of any depth it will mean constantly needing to treat silt laden water that has come into the cofferdam and been contaminated by silt stirred from plant operation.

If it is necessary to work in a live channel, because the flow is too large to over-pump within the space provided, and the geometry of, say, a bridge structure prevents splitting the channel with piles, separate the works into two halves with a silt curtain. Allow the flow to pass down one side, creating still or near still water within the other “working” area.

Some structure will be necessary at the upstream end of the work area to hold the silt curtain out in the channel and create a flow shadow, if you like.

This might be sheet piles, bulk bags, or simply driven poles to support the curtain. If the curtain runs out into a taper against the upstream bank, this can work well.

Thus if a filter type curtain is used, and allowed to stabilise in terms of water levels being equal across the curtain, the water will flow at double speed in the live channel and remain fairly still in the work area.

It is surprising how effectively any water borne silt is retained in the working area rather than migrating into the live channel and contaminating outside the work area.

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