Changes to the Design and Construction Guidance for adoptable sewers gives the green light for arch-shaped attenuation chambers.

The Design and Construction Guidance (DCG) for sewers offered for adoption has been updated to include information about arch-shaped below-ground water attenuation chambers. This will make it easier for water companies to adopt such drainage infrastructure – given that certain criteria are met.

“Previously, there was no reference to arch-shaped attenuation structures in the DCG which has meant that it has been more difficult for water companies to adopt them. That all changes with the addition of this extra section,” explains Stuart Crisp, UK manager at Advanced Drainage Systems (ADS) whose StormTech system complies with the new DCG requirements.

The DCG first came into force on April 1^st 2020, replacing the long-standing Sewers for Adoption guidance. Developers who design and install sewerage systems in line with the DCG can expect to have their systems adopted by their local water companies – although it should be noted that water companies must be involved at the earliest stages of design and specification to ensure that local requirements and nuances are met.

One of the notable things about the DCG is that it includes information on sustainable drainage systems (SuDS) and that the definition of a sewer has been broadened to include certain SuDS components. This means that water companies in England can adopt SuDS components which are mentioned in the DCG under Section 104 of the Water Industry Act 1991 in the same way that they can adopt pipes, manholes and other infrastructure.

Version 2.2 of the DCG, dated 29 June 2022 and first available through the Water UK website in November last year, means that arch-shaped attenuation structures have now been added to the adoptable SuDS family.

The relevant information can be found in section C7.8 of the document which covers tanks, in clause C7.8.4d which says:

“d) where half pipe or arch structures are proposed, the design must (in addition to the above) demonstrate how the system can be cleaned/jetted and done so without damage or erosion of base materials or membrane. Further design evidence should outline how, in areas of a high-water table, groundwater is kept out of the system and, when positioned under highways, that the loading criteria is acceptable to both undertaker and adopting Highway Authority (if applicable).”

Any changes to the DCG must first be assessed and accepted by the Independent Sewerage Adoption Panel, which is made up of representatives from water companies and developers and then a recommendation made to Ofwat for their approval.

The inclusion of arch-shaped structures has been initially well-received by water companies, reports Stuart Crisp – and paves the way for developers to offer StormTech for adoption, without having to do additional work to prove its suitability. “With the ramping up of legal requirements around pollution control and nutrient neutrality, the timing of this change is good,” he says. “StormTech’s inbuilt water quality treatment stages can enhance removal rates, reduce the need for other treatment and cuts costs.”

For more information on Advanced Drainage Systems, visit www.adspipe.co.uk.

StormTech arch-shaped SuDS attenuation chambers from Advanced Drainage Systems have been used on over 50,000 projects worldwide. New Eurocode modelling demonstrates they pass muster for projects in the UK and Europe too.

With a global track record that stretches back decades, an underground SuDS attenuation system that exploits the structural properties of the arch is now being designed and installed on construction projects in the UK.

The heart of the StormTech system is its corrugated thermoplastic chambers which have an elliptical arch-shaped cross section. This elliptical profile shapes the embedment around the chambers into stone arches and structural columns, transferring loads away from the chamber into the stiffer material surrounding the chambers so that they can be installed at both shallow and deep cover. Designed for flexibility of layout, ease of installation and transportation, the StormTech system can also incorporate an integral means of removing surface runoff pollutants at no extra cost – which is easy to maintain and can remove the need for costly pre-treatment systems.

Produced by US drainage giant Advanced Drainage Systems (ADS), which is also the largest recycler of plastic in North America, StormTech chambers are designed to US codes and Standards, AASHTO and ASTM International. To ease their acceptance in the UK and other European countries, ADS commissioned a study to model their performance under the Eurocode design methodology.

ADS’s UK manager Stuart Crisp explains: “The US design philosophy is different to the Eurocode one,” he says. “This study translates the US approach and demonstrates with complete certainty that the StormTech system performs under the Eurocode design models, when installed using our standard construction details.”

Testing scenarios

To investigate the performance of the StormTech system, the seven different sizes of chambers were put through their paces using a finite element analysis (FEA) model, which looked at limit state modes of failure as set out EN 1991-2 – Eurocode 1 – Actions on Structures – Part 2. Some engineers may be familiar with CIRIA C737, which covers the design of thermoplastic crates for underground water attenuation, which also suggests Eurocode modelling as a means of demonstrating structural adequacy.

As per ISO/DIS 4982 which covers arch-shaped chambers, the FEA model was used to test the various chambers in the most demanding loading scenarios. At shallow depths, it is live traffic loads at the surface that are most likely to cause failure. For maximum cover, it is the long-term loading of the backfill material which must be considered.

Load models for four different stress and fatigue cases were applied, according to EN 1991-2 with cover in accordance with the ADS StormTech Construction Guide.

The modelling considers the shape of the arches and material properties. The sections are injection moulded from a thermoplastic, which means that the long-term performance of the material under loading must be taken into consideration.

Performance proven

The FEA modelling proved that all the StormTech chambers are structurally adequate for each of the load cases detailed above. For minimum cover situations, there is significant additional structural capacity; for maximum cover, more of the chambers’ capacity was used but they were still comfortably within their capacity.

Crisp hopes that these calculations will help engineers and contractors to make the case for using StormTech. “Contractors are already using the system because they see the benefits in cost-effectiveness, particularly when expensive pre-treatment systems can be eliminated and when excavation depth can be reduced for installations under roads with HGV traffic,” says Crisp. “This study means that when designers and installers want proof of structural performance to Eurocodes, evidence is to hand.”

Click here to download our Eurocode Modelling Study

The Francisco Street Reservoir now serves as a source of useful non-potable water at a park for the local community, thanks to a stormwater collection system underneath.

Francisco Street reservoir was the first large reservoir in San Francisco, California when originally built in 1859. Decommissioned in the 1950s, the site was to be redeveloped but the Francisco Park Conservancy fought to keep it as a natural resource that would include harvesting rainwater. Underneath the 1.8 hectares Francisco Park is a stormwater capture and reuse system which was installed in 2021 to perpetually provide water for the park’s irrigation and toilets.

How the stormwater system works

The stormwater is stored in a 1.9M litre cistern at the top of the hill before being transferred to the service building, where it flows through a series of filtration and disinfecting processes. This ensures that the water meets public health regulations, while saving 5.7M litres of potable water every year.

The engineers decided to use a system of arched chambers because they would provide the largest storage volume per square metre.

“The reservoir is on a slope that is just under 20%. It’s a very challenging site from a variety of perspectives,” explains Sherwood Design Engineers (San Francisco) principal Cody Anderson who is responsible for the stormwater system.

“Normally you’d have your catch basin at the bottom of the site. Here, the historic reservoir is midway up the slope, so half the runoff is collected via gravity flow and the rest collected at the bottom of the site and pumped. All captured runoff flows through the screening filtration and into the chambers for storage and later use.”

Equipment used

Three hundred and seventy two ADS StormTech chambers, were installed using a 35m by 45m area of the existing reservoir and then covered with soil. This gave a total storage capacity of 2,000m³ of water in a 1,682m² footprint. ADS StormTech chambers provided the best ratio of storage volume to footprint area.

The StormTech chambers are independently tested, BBA-approved and fully compliant with ASTM F2787, F2418 and F2922 stormwater storage systems standards. StormTech is typically used for below ground SuDS attenuation projects and more than 2.5M chambers have been used successfully around the world in over 40,000 projects. Three StormTech Isolator Rows are included in the Francisco Street reservoir system. These patented water quality treatment devices are integral to the StormTech system. They capture the “first flush” and trap sediment and other pollutants coming from stormwater runoff.

StormTech chambers come in a wide range of sizes, making them easy to install for all conditions. They are highly adaptable and can be configured around obstacles as well as affording multiple inlet and outlet positions. Standard pipe manifolds, manhole and access chamber inlet/outlet structures and flow controls can be used.

Reclaiming an area of San Francisco

“There are competing products on the market,” says Anderson. “We needed to store as much water as possible in the given area. We work on projects around the globe with an emphasis on sustainable development and we’re known for having the vision and the technical capacity.

“The Francisco Park is one of those projects of a lifetime. It’s reclaiming an area in the city of San Francisco that is now a beautiful park for the people.”

For more information on Advanced Drainage Systems, visit www.adspipe.co.uk.

As British weather officially gets wetter, ADS StormTech – the world’s preferred proprietary stormwater attenuation system – arrives in the UK.

With Britain hosting the UN’s Climate Change Conference this November, the recent State of the Climate report was greeted with interest by politicians and construction professionals alike. Published by The Royal Meteorological Society, the report showed that Britain had become 6% wetter, on average, over the last 30 years (1991-2020). Additionally, six of the 10 wettest years since 1862 have occurred since 1998.

This wetter weather will challenge the UK’s drainage infrastructure and the management of water quantity and quality. Fortunately for engineers working on rainfall management and stormwater attenuation systems, StormTech – the world’s preferred subsurface water management system – is now available in the UK.

StormTech is manufactured by ADS, America’s largest manufacturer of corrugated thermoplastic drainage pipes and a specialist in water management systems. StormTech has a long and successful track record with over 40,000 installations – including St Petersburg and Guernsey airports – ranging from the DC United stadium to Lidl stores in the USA.

StormTech provides a durable solution for commercial, residential, industrial and infrastructure installations. As part of an integrated SuDS system, StormTech helps provide valuable surface space for parking, playgrounds or other amenities, and has many benefits over alternatives such as geo- cellular crates, large diameter pipes and vaulted systems.

Flexibility is designed-in to StormTech. The chambers are injection moulded to ensure consistent quality and performance, and even the colour and semi-elliptical shape are deliberate engineering choices. The chambers shape the embedment stone into structural arches forming load bearing columns. This makes StormTech more adaptable than other systems, as it’s the stone that transmits the majority of the load.

StormTech chambers lock together, providing uninterrupted access for easy inspection and maintenance, and the wide range of sizes makes it easy to optimise systems for all conditions, including shallow invert depths and reduced footprints. They’re also highly adaptable and can be configured around obstacles and afford multiple inlet and outlet positions. Standard pipe manifolds, manholes, access chambers and flow controls can be used.

StormTech incorporates the unique, patented Isolator Row, a built-in water quality treatment device, removing silt, sediment and other surface water pollutants. Isolator Row has been independently tested by universities and respected industry bodies and proven to remove over 80% of Total Suspended Solids (TSS) and other water-borne pollutants including metals, hydrocarbons, phosphorus, and nitrogen. Isolator Row provides two treatment stages: settling (gravity separation) and filtration. Unlike other sub-surface systems, Isolator Row provides a “free” water quality treatment device as an integral part of the stormwater attenuation system.

StormTech’s Design Tool 2.0 provides quick and effective system design optimisation that integrates with CAD and other design, engineering and specification documents. Sample designs and layouts are available, as are a team of specialist design engineers. For installers, the SiteASSIST app includes comprehensive instructions and videos, making working with StormTech fast, easy and efficient.

StormTech affords low embodied carbon per m 3 of attenuated water compared with alternative systems and is fully recyclable. It is independently tested, BBA approved and compliant with ASTM F2787, F2418 and F2922.

Originally published in NCE.com November 2021

For more information on Advanced Drainage Systems, visit www.adspipe.co.uk.