DrainsAid recently completed a challenging underwater sewer repair and rehabilitation project on a 200m section of a cast iron 375mm diameter combined sewer which is attached to the wall of the River Ouse in the historic city of York. DrainsAid was chosen to provide a solution by long-standing partner Yorkshire Water.
DrainsAid, one of the leading sewer renovators in the UK, was called to investigate the combined sewer which runs through the city from the Guild Hall Offices to the rear of Coney Street. A section of the sewer is fixed to the river wall and is often submerged. This section of sewer contained a large block of concrete which had settled on the silt, forming a 10m long potential blockage which had begun to interrupt the flow and was a pollution risk. Due to the location of the sewer and its proximity to the popular York City Centre, DrainsAid was keen to provide a quick solution to avoid any pollution risks.
From the outset, DrainsAid was faced with the unpredictable British weather, and weather conditions many miles upstream in the Yorkshire Dales. If heavy rain was to fall in the Dales, the river level would rise and the sewer would be submerged, including the access points at the Guild Hall, thus making it difficult for DrainsAid to carry out their work.
“We knew that working within the River Ouse would be a challenge, but I was certain our experienced team would find the best solution possible,” commented Ben McCluskey, Project Manager at DrainsAid.
DrainsAid go underwater – Concrete Removal
DrainsAid was tasked to find a quick and effective solution to the partially blocked pipe, and used their expertise to formulate the best possible solution. As the pipe was fixed to the river wall and at times, likely to be submerged, it required a different approach to conventional methods. To ensure the work could be carried out continuously and safely, DrainsAid utilised a team of divers and a boat fitted with a crane to access the sewer, and were able to cut open the 15mm thick cast iron pipe. The underwater team used specialist cutting equipment, a hydraulic Power Grit saw, to cut through the pipe in 3m sections, to remove the concrete obstruction. The work took five days to complete and 12m of concrete was removed. Each day the pipe was pieced back together, allowing flows to return overnight. The flow was diverted to another system whilst the work took place.
“Using the divers and boats was fundamental as it meant that if the river levels did rise, we would be able to continue with the works. We needed a permanent and accessible way of working alongside the sewer; the solution to this was a boat with a loading crane, which was capable of lifting the pipe out of the river to remove the concrete,” Ben continued.
Working on the River Ouse – Lining of the Sewer
To improve the structural integrity of the host pipe, and to stop the ingress of river water into the sewer, DrainsAid proposed that once the concrete was removed the pipe should be relined. If the concrete removal wasn’t challenging enough, the only stable working area was a concrete pad at the upstream manhole, which was not large enough to position the equipment required for the lining operation. Another solution would have to be found in order to access the upstream manhole and complete the lining works.
DrainsAid utilised a pontoon and tug capable of carrying 40 tonnes to use as a workstation and to house the equipment for the lining installation. Through careful planning, DrainsAid calculated that the concrete pad was just large enough to support a bespoke scaffolding system and associated supports for inverting the liner. A boat carrying a crane was positioned alongside the pontoon, and the team was able to lift into position and invert the liner from the scaffolding, successfully installing a full length CIPP Hot Cure Liner and rehabilitating the 200m x 375mm sewer. Whilst the work took place the flows were diverted to a nearby sewer system.
The lining work needed to be carried out when the pipe was exposed therefore the team had to monitor the weather continuously and wait over two weeks for the perfect weather conditions.
“The process of using boats and barges to load our usual road mounted equipment really made this project a unique one!” explained Ben.
“The location of the sewer was a real challenge, but once the different aspects of the project came together and I saw our boiler unit with the liner set off from its loading bay on a boat alongside the picturesque restaurants of York, it was a real sight to be seen.”
Due to the innovation and efficiency of DrainsAid’s work, Yorkshire Water now has a rehabilitated critical sewer free from infiltration issues or blockages, and a reduced risk of polluting the River Ouse.
“Even though we were battling the weather – and the river - the project went extremely well and was completed quickly. As always, the DrainsAid team worked seamlessly in spite of the underwater location and the proximity of the river. The success of the project relied on meticulous planning with different elements all working together. This has been the most exciting project I have managed in my career so far, and I am really proud of the DrainsAid team and what we have achieved.” said Ben.