Here’s one we made earlier

The first pre-cast concrete combined sewer overflow in a box, designed by MWH engineers Rob McTaggart and Ian Lovering, has dramatically cut delivery times

Like all good ideas, the initiative to create a standardised design for combined sewer overflows (CSO) seems blindingly obvious in retrospect.

During AMP3, MWH engineers Rob McTaggart and Ian Lovering were working in a water company catchment area with 16 CSOs designated unsatisfactory and looking for ways to improve the efficiency of the design process. Their eureka moment came when they realised that each of this group of CSOs fell into one of four generic groups based on a number of key parameters.

The largest group, which included more than a third of the total, fitted into the category of a new structure with a mechanical screen and inflow of less than 1,000l/s. This led them to the realisation that the whole delivery process could be hugely streamlined by developing core designs covering the four generic groups.

The next step was to develop a pre-cast concrete system around one of the core designs. The core design was then split into two variants covering the main category of “new chamber with a mechanically driven screen with an inflow of less than 1,000l/s”. Over time, the core designs were adapted to include other screen types and other water companies’ operational needs but the principal design has not changed. In fact, MWH now anticipates that one of the pre-fabricated chambers variants will be suitable for application in one in six of the UK’s 2,500 CSOs that need to be addressed during AMP4.

Each CSO has an optimum bespoke design to meet its particular hydraulic, spacial, operational and structural needs. However, the beauty of this approach is that you can look in more detail at specific aspects of the design and improve on the overall efficiency of delivery. A parallel can be made with bicycle design – the original designs focused on making the bike weigh less, but we now know that it is the overall aerodynamics which make the difference. The wall thickness of the CSO in a Box is 200mm, while still meeting a design loading of 45 units of HB loading. A conventional in-situ wall thickness would be 300mm or more.

Installing a traditional CSO can take 10-15 days, while a CSO in a Box can be installed in one day, offering significant programme and resource savings. The roof slab has variants which are specifically tailored for different types of screens and different water companies’ access needs.

“Each variant has been worked back to a set of hydraulic parameters which become the design limit or envelope. MWH has developed design manuals, which cover the core design and the variants design envelopes, allowing the designer to make an informed decision as to which variant best matches their particular needs. The important thing is to know what you can change and what is outside the design envelope,” explains Rob McTaggart.

MWH, in conjunction with Hanson, one of the UK’s leading manufacturers of pre-cast concrete, produced an off-the shelf, pre-cast concrete CSO chamber that can dramatically cut delivery times while still meeting the specific needs of a project. The concept was jointly developed by MWH engineers, Ian Lovering and Rob McTaggart, who have extensive experience of structural and sewer network design and management.

Ian Lovering says: “We have specified and installed standardised chambers and tanks throughout water company AMP programmes. So it was a logical progression to complement our solutions with a pre-cast structure that retains the design flexibility and all the benefits of conventional WaPUG-compliant designs that users are familiar with. CSO in a Box can offer programme time and direct cost savings, particularly when the construction of a CSO is on the project critical path.”

The pre-fabricated CSO chambers provide a quick and flexible CSO solution from a familiar and assured design. “We found that by backward engineering the different variations of the core designs, including the main screen types, we developed a range or band which complies with a standard set of design criteria.

This approach provides flexibility in meeting design criteria yet still retains the benefits of standardisation. The result is that we can design CSOs more easily and they are much easier to build, install and operate,” he says.

By standardising the chamber, they are able to match a particular screen model selection from a screen manufacturer with the operational and design criteria of a water company. This was a particular benefit at that time as water industry clients were having real problems getting individual screens. One bespoke screen was more expensive than buying five banded screens. They were able to show clients that adopting a slightly larger standardised design created all sorts of efficiencies and improvements in repeatability, speed and bulk-order cost reductions.

Industry screen suppliers HydrRok saw the potential and were instrumental in introducing the benefits to Northumbrian Water (NW) when they were invited to present to them. Initially, Lovering and MacTaggart say most interest came from small- and medium-sized contractors rather than large contractors due to the programme approach to resourcing sites. However, attitudes are changing.

Since Northumbrian Water installed the first precast concrete CSO in a box earlier this year on the busy Geneva road in Darlington, in less than five hours, Hanson and MWH have received seven more requests for detailed quotes. Looking forward to AMP4 and AMP5, Lovering and MacTaggart say that the pressure will be on contractors to work more safely while still meeting challenging programmes and causing the least disruption to consumers.

The CSO in a Box design could prove an invaluable tool in the delivery of a large CSO programme as it will enable workers to get out of the ground more quickly and complete projects in much less time. The speed with which it can be installed and the minimum disruption to consumers was a key attraction for Northumbrian Water’s contractors, Lumsden and Carroll (L&C).

MacTaggart explains how they got to hear of it. “MWH, as a company, went through an internal process called the ‘fast-track solution development initiative’ at the back end of AMP3 before starting AMP4 to see how we could do things more quickly and cleverly.

“Our work on CSO in a Box was recognised as part of this programme, and its benefits were promoted through our knowledge intranet. Sarah Davies, a consultant who works with NW saw how it could help some of their CSO programme needs and in particular would help NW framework contractor L&C meet a tight AMP4 Year 1 regulatory deadline for construction of a CSO in a busy road.”

The Type 5 CSO in a Box was installed in less than five hours from the first lift through to the completion of base, main and weir walls. Following delivery of the static screen, the installation was completed with the addition of bespoke roof slabs. This meant that overall construction, road closure time and inconvenience to the water company’s customers was numbered in days – rather than weeks.

Northumbrian Water’s Paul Laybourne says: “Northumbrian Water is always looking for ways to improve our processes and to minimise disruption to our customers. The CSO in a box product provides flexibility to the project programme and significantly reduces timescales.

“This type of construction method has many advantages, including the improvement to the health and safety of the construction workers. MWH and Hanson have succeeded in tailoring their solution to meet the requirements of Northumbrian Water’s operations and the fast track nature of the AMP 4 schedule,” Laybourne adds.

“Saving time equals reduced disruption to the public and also cuts construction costs,” he says. “With increased emphasis on water companies’ efficiencies from Ofwat, community relations and regulatory targets, we anticipate that this type of standardised solution will play a significant part in the delivery of the AMP4 programme.”

Steve Walker of Hanson believes there is great potential for the CSO in a Box. “In addition to offering a pre-cast concrete CSO as a stock item, we think it has the scope to be modified to suit one-off applications, of which Geneva Road is a prime example. It provides a quick and flexible solution to most typical CSO improvement needs for all UK-based water companies.”

The beauty of the design is that it comes in standardised banded parts and does not require a huge amount of kit to be put in the ground. Going forward in AMP4, all contractors will be required to work more quickly and safely. This design could help them work more efficiently because anything that needs to go in is inside the pre-cast box or chamber.

This enables contractors to get workers out of the ground much more quickly. The design concept may also have a future in areas other than CSOs. One example is wastewater storage because it can be installed so quickly in highways. Others uses are likely to centre on storage or as inlet works for WwTWs.

Overall, the consensus is that the potential for CSO in a Box in the water industry is very good. Its success will be determined by whether the industry becomes more pro pre-fabrication. And it depends on whether the industry recognises its merits and its ability to speed up the process of replacing an unsatisfactory CSO or supplying a new one.

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