Five underground attenuation tanks hold back surface water at the expanded junction 6 on the M42 near Birmingham Airport. The challenge was to prevent groundwater from lifting the tanks and to withstand lateral soil pressure.
ACO Water Management supplied its StormBrixx HD system for the £1.7m attenuation scheme. This heavy-duty geocellular solution offers vertical and lateral strength to retain stormwater runoff beneath the new carriageway and 2.4km link road.
Resisting uplift pressure, six metres deep
Groundwater uplift occurs when water pressure beneath a buried structure exceeds the weight above, causing the structure to rise and damage road foundations. One tank sits six metres underground, covered by four metres of soil, increasing the risk.
Philip Jones, ACO Technical Specialist, led the design to counter uplift. He calculated groundwater pressure and specified bespoke tanks made from StormBrixx HD crates, panels and connectors.
Strategically placed filter drains at certain invert levels reduce hydrostatic pressure. This removed the need for costly thrust wall protection. Engineered lateral backfill, matched to the tank’s strength, ensures stability under soil loads.
Eleven thousand cubic metres held
The scheme involved over 115,000 StormBrixx HD units. Across five tanks, total volume is 11,542 cubic metres. Design and testing followed CIRIA C680 guidance, confirming the system will remain secure for decades.
Optimising access cuts costs and carbon
ACO redesigned access to the tanks reducing five manifolds to a single 900mm pipe. This cuts materials and installation time.
StormBrixx HD units nest within each other, allowing more units per lorry load. Delivery required 48 artic lorries, about a quarter of the volume needed for standard tanks. Reduced deliveries saved time unloading on site and lowered the project’s carbon footprint.
Flood control aids local economy
StormBrixx installation is complete. The M42 upgrade between junction 6 and new junction 5a is due to finish in the first half of 2026.
Philip Jones, ACO Technical Specialist, said: “This scheme helps ease traffic congestion around Birmingham Airport and supports economic growth in Solihull and south Birmingham. Our experience in highways water management meant we could design tanks to resist uplift pressure and hold rainwater reliably for years.”
ACO’s StormBrixx HD system addresses a significant engineering challenge for surface water management on a key Midlands motorway upgrade.
