A new distributor road on the eastern edge of Coventry cut through a shallow clay layer over Keuper Marl. Within weeks, the formation turned to slurry. Standing water softened the subgrade, and the temporary haul road became impassable. That job taught us a simple lesson: in Coventry, surface water management is not enough. You need a geotechnical road drainage strategy that deals with groundwater seepage from the underlying Mercia Mudstone Group. We design filter drains, fin drains, and blanket drainage layers that keep the formation dry during construction and throughout the pavement life. Before we specify any drainage system, we always run a groundwater infiltration test on site to measure the actual permeability of the local soils.
In Coventry's clay tills, a geotextile-wrapped fin drain outperforms a granular trench drain by a factor of three in long-term flow capacity.