CSO renovation cleans up Digbeth
Plastic piping manufacturer, Polypipe Civils has designed a challenging combined sewer outfall system in Birmingham. On behalf of Polypipe, Tom Lay asks water management specialist Luke Dekonski to explain the process
Last year, Polypipe Civils designed and helped create one of its most challenging combined sewer outfall (CSO) projects to date. As part of a large capital works programme intended to bring improvements to the Cheapside area of Digbeth in Birmingham, three existing rectangular concrete combined sewer outfall (CSO) chambers were successfully removed and replaced with a single chamber that could handle their combined capacities.
The task was undertaken by the main contractors and achieved by using Polypipe Civils’ Polyethylene Ridgistorm-XL large diameter pipes and a series of bespoke elbow-joints.
Operated by the local water company, the original CSO chambers had been used to handle the excess flows experienced in the area during storm events and divert them into the nearby River Rea to reduce the risk of urban flooding. However, after a prolonged period in operation, the existing chambers required an upgrade to avoid the integrity of the sewerage system being compromised at some point in the future. The flap valve, which was used to prevent flooding, was in particular need of attention.
It was decided that replacing all three CSO chambers with a single combined system was a more cost-effective option in terms of both the time disruption and costs. After an initial approach from the main contractors, Polypipe Civils was able to get involved at the early design stages. Due to the confined space on site and the complexity of the structure surrounding the project, it was decided that a series of four bespoke bends would have to be designed and manufactured if the new CSO was to be connected to the outfall chamber successfully.
The main contractors gave Polypipe’s technical team four coordinates to work from – two where the outlets were based and two where they were to connect. Using only these four co-ordinates, the Polypipe technical team were able to calculate the precise angles needed for consolidation of the CSO chambers.
“Having to completely re-design the existing CSO system was complex enough in itself,” says water management solutions specialist, Luke Dekonski. “However, the site constraints meant that the new system also had to be value engineered to satisfy the contractor’s exact design specification.
“The existing system was made of concrete. The project required greater flexibility than this would allow to create a more efficient system. The main contractors contacted our technical team and we confirmed the feasibility of refurbishing the system with our Ridgistorm-XL plastic pipes, plus a series of bespoke elbow bend components. We worked closely with the main contractor at design stage, purpose-designed a bespoke system which matched their exact specifications, offering a very practical and effective solution to the problem.”
The technical team analysed the site conditions and installation parameters which applied at the location. To ensure that the specification process for the pipes was as accurate as possible, the team also had to identify the most appropriate pipe stiffness classification, based on soil characteristics, trench construction methods, cover depth and embedment.
“The native soil conditions on site ranged from stiff to very hard at invert,” explains Dekonski. “Due to continuous re-development which has taken place over the years, the top 1.5-2m of soil was soft and cohesive.
“Using specialist technology, and calculating many different parameters which included these soil conditions, we were able to identify a lower stiffness classification of SN1, which ultimately resulted in a cost-effective solution that was neither over, nor under specified.
“It was following this process that we were able to fabricate and supply the two Ridgistorm-XL pipes, in 1,200mm and 1,800mm diameters, which would be required for the project.”
One of the most important aspects to the project was the assurance that the pipes, once fitted, would not leak at the joints. To enable 100% seal tightness, as well as a guarantee of exceptional joint and pipe performance, Polypipe used its electro-fusion jointing method to seal the components of the system to each other. This eliminated any need for internal hot works within the pipe itself and also ensured reliable, long-term performance.
This process involved Polypipe’s fabrications team connecting an electro-fusion wire to the socket end of each component during manufacturing. Each one was given a unique barcode which could be used to identify the settings and installation parameters.
As the two main Ridgistorm-XL pipes and their connecting elbow joints were brought together on site, internal and external support rings were fitted which would apply pressure to the seals during the jointing process. The electro-fusion wire was then connected to an electrical supply, fusing the pipes together, ensuring no wastewater would leak from the new consolidated CSO chamber.
“The fact that no two projects of this nature are ever the same underlines the value of the bespoke approach that we take to each one,” adds Dekonski. “Whether it’s at the early design stage, the product manufacturing stage or during the installation process, we use the specific requirements of the project in question to guide each step that we take.”
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