The variable ground across the Potteries, from glacial till to weathered mudstone, means subgrade strength can shift dramatically between two boreholes on the same site in Stoke-on-Trent. We run laboratory CBR tests because a pavement designed on an assumed 5% CBR that encounters a real 2% subgrade will fail within the first winter. The combination of Stoke-on-Trent's damp climate and clay-rich drift deposits makes soaked CBR testing mandatory under BS 1377-4, and our lab processes specimens under controlled moisture conditions to replicate the worst-case scenario the subgrade will face after construction. Before finalising the pavement layers, many schemes in Stoke-on-Trent start with a grain-size analysis to confirm the fines content driving the saturation behaviour.
A 2% CBR subgrade needs roughly twice the granular layer thickness of a 5% CBR — the test pays for itself in the pavement section alone.
