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In April this year, Water UK published its latest sewers adoption Design & Construction Guide (DCG), introducing for the first-time new requirements for the adoption of Sustainable Drainage Systems (SuDS), including attenuation tanks. In this blog, we briefly explore the main implications in terms of open areas and surface-level car park fires and how different types of underground attenuation tanks can be affected by fire.

Every year in the UK, over 100,000 cars (around 300 a day) go up in flames. Thankfully, loss of life, livelihood and homes are usually rare with such incidents. However, the damage caused by such fires is not restricted to vehicles, vegetation or other assets visible above ground. There is potential for devastating loss if such fires find their way into the underground drainage and stormwater attenuation systems built under roads, lawns, squares and surface-level car parks. A vehicle fire can generate heat reaching well over 815°C with flames reaching up to 3 metres or more. In a surface-level car park a fire can spread reaching several parked vehicles, either through flames or spilled burning fuels such as diesel. Any tank structures under such car parks will be affected, either through the burning liquid fuel reaching the tanks through drains and vents, or through the extreme heat from ground level.

When such fires occur, underground drainage pipes and attenuation tanks can suffer significant damage, especially if such systems are made of flammable materials such as Polypropylene (PP), Polyvinyl Chloride (such as uPVC) or High-Density Polyethylene (HDPE). If such pipes and tanks burn, they can add further fuel to the fire. If they melt under heat, they may lead to the partial or total collapse of the space above ground. A temperature rise to 65-105°C could be sufficient for PP to soften and start experiencing deformation and loss of structural integrity. Concrete pipes and attenuation tanks behave differently. In such case as concrete can withstand high temperatures and will remain intact throughout such fire incidents. Using precast attenuation tanks wouldn’t only prevent further spread of fire but would also reduce the cost of any clean-up or rebuild (if needed).

In the next few years, it is expected that hundreds (or thousands) of attenuation tanks and pipeline systems will be installed under car parks, squares and lawns across England. The possibility of car park or vegetation fires reaching or affecting these underground attenuation tanks cannot be ruled out. As there are no adoption requirements or Non- Statutory standards on resistance to fire, it is up to the contractor/ developer/ asset operator to decide whether attenuation tanks’ fire resistance should be considered. Given the poor fire performance of PP and uPVC, is it really wise to specify attenuation tanks made of such materials near roads, or under lawns and car parks?

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