It is possible to store stormwater in attenuation structures below buildings–but there are important considerations for the design engineer, says Stuart Crisp, UK manager of Advanced Drainage Systems.
In dense urban areas, such as sites where a new building covers the entire footprint of the development plot, it may make sense to install SuDS attenuation assets below the building itself. However, during optioneering, there are multiple factors that the designer should consider including the foundation design, construction sequencing, ground conditions, level of the water table and how the system will be inspected andmaintained.
This arrangement is most often deployed below multi-storey buildings, such as offices, residential or mixed use, where the lower level is dedicated to parking. This allows for easy access to the below-ground attenuation system so that regular inspections and maintenance interventions can take place, maximising the service life of the system.
When initially assessing whether below-building attenuation might be possible, the first requirement is that no loading from the building should be transferred to the attenuation asset. Attenuation assets should not be installed under buildings with a mat or raft foundation system as a continuous slab would transfer weight to the underlying soil and hence onto the attenuation device.
Geo-technical and structural engineers should also investigate the soil conditions to determine whether the ground around the attenuation asset is moisture sensitive and whether water would adversely affect the building’s foundation systems. It may be necessary for the attenuation device to be contained, for instance with a thermoplastic liner, to prevent water infiltrating into the underlying soils or contacting the surrounding foundation system.
Another important consideration is the position of the ground water table, throughout the year. Even if buoyant conditions can be managed, it is not recommended that below-building attenuation systems are installed below the level of the seasonal high groundwater table.
A designer should also consider temporary loading on the below-ground attenuation system during the construction phase. The phasing and proximity of heavy construction plant, mobile and tower cranes, form work and false work should be tightly controlled to ensure that loading on the buried attenuation system is not excessive.
As with all such installations, it is important to carry out regular inspections throughout the installation of the attenuation system. Ensuring that a system is installed in accordance with the design and with the manufacturer’s instructions is vital in assuring that the system will perform as intended. Depending on the footprint of the building and the sizing of the attenuation device, space around the perimeter of the installation may be limited, which could impact on the ability to inspect the installation.
In conclusion, installing attenuation assets below a building can be a good solution in limited situations. Given the potential cost and difficulty of replacing a system that has failed for whatever reason, it is more important than ever to assure the quality of the installation and to check that no damage has been caused through inadvertent overloading of the ground during construction.
