Tunnelling through the alluvial deposits of the Severn Vale is not a theoretical exercise. Last winter we reviewed borehole data for a storm overflow tunnel near the Gloucester Docks—the client had lost two weeks sorting through mixed samples of soft silty clay and loose sand lenses. The water table sat less than 2 metres below street level. You cannot design a closed-face TBM drive in those conditions based on desk-study assumptions alone. Our team runs a full ground investigation programme under BS 5930, extracting undisturbed samples from the Lias Clay interface and the overlying Quaternary sequence to define the stiffness, permeability, and stand-up time. When the face pressure is miscalculated, the consequences show up fast: settlement troughs cracking brickwork on Southgate Street, or worse, a blowout in the gravel layer beneath the canal. We combine standard penetration testing with detailed triaxial testing to build a soil model that the TBM operator can actually trust.
In Gloucester's alluvial clays, the difference between a controlled drive and a sinkhole is often 10 kPa of face pressure.
