Crossing from the heavy Gault Clay beneath Lewsey into the marly chalk of the Barton Hills presents two entirely different tunnelling behaviours within a single drive. The clay swells, softens, and exerts sustained squeezing pressure on a lining, while the chalk can stand unsupported for a time before joint-controlled block failure kicks in. We run a full CPT test across the transition zone to map the exact interface, then pair that with triaxial testing on undisturbed cores to lock in effective strength parameters for each lithology. In a town where the railway corridor and the Luton Airport Parkway expansion keep pushing underground works into marginal ground, knowing the mixed-face boundary to within half a metre changes the entire TBM setup.
Mixed-face transitions in Luton shift from stable chalk to squeezing clay over a few metres — missing that boundary during ground investigation is the fastest way to a stalled TBM.
