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Dec 20, 2023

Bubble curtain speeds up Tyneside piling works by dampening noise

Working in estuaries brings the challenge of protecting marine life but

Working in estuaries brings the challenge of protecting marine life but contractors working on the River Tyne have found a solution.

In the coming years, East Holborn in South Shields on Tyneside will be developed into a new residential and commercial neighbourhood. But before people can move in, the needs of another type of creature must be considered.

The River Tyne is the UK's most important river for Atlantic salmon and the only one that the Environment Agency categorises as "not at risk" of having its salmon population fall below sustainable levels.

That poses a problem for contractors working on the riverside during the migration season in the spring and summer. Research by Cardiff University has shown that when exposed to increased amounts of noise for longer periods, fish can experience stress, loss of hearing and various changes to their behaviours. As a result, marine protection body the Marine Management Organisation (MMO) has set limits on operations that cause such loud, intensive noise.

From March to November, this work is not allowed to take place for three hours after low tide. In May, the busiest migratory month, no impact piling is allowed to take place until four hours after high tide.

This is a significant challenge for the riverside piling work needed to deliver the new residential and commercial neighbourhood at East Holborn.

Contractor John F Hunt has a £20M contract to reclaim and remediate four disused dry docks at East Holborn for South Tyneside Council. It is preparing the docks to be handed over to housing developers. The work involves impact driving more than 100 piles into the river bed in proximity to marine life. To ensure work is carried out without time restrictions, the contractor has been using a bubble curtain to limit the noise underwater during the work.

The air emerges through the perforations as a sound absorbing curtain of bubbles

A bubble curtain is created by a weighted and perforated polyvinyl chloride (PVC) tube lying on the river bed and attached to a compressor that pumps air through it. The air emerges through the perforations as a sound absorbing curtain of bubbles with diameters of less than 1mm.

John F Hunt marine and civils site manager David McLaren is particularly attuned to salmon protection practices, having worked as a civil engineer at the Port of Tyne for 16 years before moving to the contractor side in 2015. He is used to working around the time restrictions and warns that if the tides fall at the wrong hours in relation to the working day, you could lose as much as seven hours.

The bubbles create a "wall" to attenuate the sound from the piling

"When it comes to mitigation, there's not really much you can do to get around it. Up until now, the only thing you could ever do was carefully plan your works," McLaren says. "There's a lot of works you can carry out before impact driving, such as vibrating piles to a certain point rather than hammering them. But there are certain days you can't avoid it."

McLaren and his team decided to look into the possibility of using a bubble curtain to reduce noise impact to below the 160dB limit set by the Environment Agency for this project.

This technology has been used in other construction applications, including controlling the movement of silt on water-side construction sites and emergency aeration of rivers and lakes.

"It is a tried and tested innovation, but it has not been previously used to discharge this particular MMO-licensed condition," McLaren explains.

The contractor started figuring out the specifications for its bubble curtain in January 2022, with piling due to start on site six months later.

Assessments were made of the rate of current, depth and salinity in the East Holborn section of the Tyne. This information was shared with silt control and water quality specialist Frog Environmental, which collaborated with system supplier CanadianPond.ca Products to manufacture the tube.

John F Hunt's bubble curtain has to shield work on three 26m wide dock gates, so the curtain is formed by a 91m length of proprietary branded Bubble Tubing. This comprises dual 25mm diameter PVC tubes connected and running in parallel, one for the airflow and one with stainless steel running through it to weigh it down. The Bubble Tubing is connected to the compressor on the shore by a further 15m of PVC tubing. It cost between £35,000 and £40,000.

Details of the bubble curtain were submitted to the MMO and Environment Agency in April 2022, with work due to start in July. But the project team had to wait more than two months to see whether its use as a noise mitigation technique would be approved.

At the end of June, a week before piling was due to start, environmental regulation specialist Subacoustech was subcontracted to measure the noise propagation across the estuary and the background noise of ships moving in and out.

The MMO was unable to approve use of the bubble curtain in time for the start of piling, so work had to take place under the usual time and tidal restrictions for the first week. The MMO and the Environment Agency gave the green light for its use by the second week though. This meant that piling could continue at all hours of the work day, provided that the curtain's effectiveness was monitored continuously.

Subacoustech was again called in to monitor and set up hydrophones at intervals around the perimeter of the curtain to measure the sound that made it through the bubbles.

"The results were quite dramatic – in fact, startling," McLaren says. The sound of impact piling dropped from 163dB to 140dB – well below the danger level for salmon – when the curtain was running.

Subacoustech carried out several tests on different days to monitor the effect of the curtain under different tide conditions and with different piling activities.

The bubble curtain was used until the end of the 2022 migration season. McLaren believes that using the technology enabled John F Hunt to shave a week off the work in 2022 saving tens of thousands of pounds. It will be used again during this year's salmon migration season.

The rewards will be bigger when the technology is used on longer term projects, McLaren adds. Following John F Hunt's success, contractors at the Port of Tilbury have started using the innovation for sound attenuation.

"We have a workable solution that we’ve proved has a dramatic impact on noise propagation," McLaren says.

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