Adoptable chamber assemblies and the benefits of plastic

The introduction of Sewers for Adoption 7 (SfA7) has meant that engineers can now specify plastic inspection chambers instead of those made from concrete. Offering a substantially lighter, structurally sound, watertight chamber and one that benefits from exceptional loading capabilities, Paul Grills of Brett Martin takes a fresh look at modern plastic materials which are fast becoming the go-to alternative to traditional concrete chambers.

With the intention of standardising the performance and installation of all ‘adoptable' drainage installations, the SfA7 guidance provides installers with general specifications for drainage.  But when is an inspection chamber adoptable and when is it not? Adoptable inspection chambers serve two or more properties and are the first inspection chamber back from an adoptable lateral drain.  Non-adoptable chambers are installed within the property boundary and serve individual properties.

Building a case for plastic

One of the major step changes with SfA7 is that tried and tested engineered plastic inspection chambers are now being accepted for use in adoptable areas as an alternative to traditional materials such as concrete and offer significant cost and time benefits.

Through the introduction of newly-defined design specifications and by bringing in installation standards for all adoptable drainage networks, SfA7 provides the specifier with Typical Access Chamber Details for flexible material (plastic) versions. This means that traditional rigid concrete chambers are no longer the only authorised option for adoptable drainage installations down to 3 metres.

Plastic inspection chambers are now required to be designed, tested and manufactured to meet the requirements of BS EN 13598 under SfA7 guidance. Part 1 of the standard covers installations down to a maximum of 1.2m invert depth, and Part 2 covers deeper installations, including critical areas. Traditionally there has been some reluctance towards plastic inspection chambers from water authorities, even for Part 1 compliant installations. However with any lateral drain or sewer serving two or more properties and connections to the public sewer now able to be adopted by a water company, a traditional 1.2m concrete inspection chamber would not be practical in a lot of these instances. This change, enshrined in SfA7, has begun to result in a change of attitude from the water companies.

Independently tested

One of the reasons why plastic systems are becoming more accepted is due to the rigorous testing program set out in BS EN 13598. These independent tests include dimensional assessments, load testing, pressure testing and elevated temperature cycling to name just a few. Furthermore, a product certified by a third party (such as BSI) will be subject to this robust test program on an ongoing basis, ensuring that companies are consistently producing a product which meets this high standard. This impartial testing and certification should not only give water companies confidence in the system which they have installed, but also builders, surveyors and homeowners as well.

Modern plastic systems are seeing increased interest due to their important set of benefits compared with concrete, including performance characteristics such as structural integrity, watertightconstruction and strong loading capabilities. And the substantially lighter weight offered by polypropylene chambers means there is no need for the craning required for the traditional concrete solutions, and that health and safety concerns on site are drastically reduced.

Plastic sewer systems are also known for a very smooth internal surface in comparison to a concrete system. This reduced friction coefficient helps the flow of foul water, reduces the risk of blockages and means that the jetting pressure required for a plastic system is significantly reduced against a concrete pipe. 

Ease of installation

Along with being durable and robust, these modern plastic systems can be integrally socketed for improved pipe alignment thereby easing installation and performance, a vital consideration for a long design life.  An additional benefit of the integral socket is the system can be manufactured specifically to be compatible with other standardised plastic sewer products (such as BS EN 1401 pipe and fittings). This further improves on installation time and removes the need for additional adaptor pieces which go between concrete and plastic systems.

With the official Sewers for Adoption 7 guidance now supporting the specification of plastic for adoptable solutions, local authorities can be sure they gain all the benefits of fit-for-purpose alternatives, and be fully in compliance with all standards.

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