Tunnelling beneath the Georgian crescents of Bath presents a stark contrast to the open farmland just south of Combe Down. Where the Royal Crescent rests on stable Great Oolite limestone, the valley floor along the Avon corridor conceals pockets of soft alluvium and weathered Lower Lias Clay that make mechanised excavation unpredictable. A standard site investigation will flag the presence of these materials, but it tells you nothing about undrained shear strength below 40 kPa or the sensitivity of the clay fabric once remoulded. The soft ground tunnel analysis we provide quantifies exactly these parameters, feeding directly into face pressure calculations and settlement trough predictions. Bath’s UNESCO World Heritage status means surface movement tolerances are measured in millimetres, not centimetres.
In Bath’s Avon Valley, the difference between a successful EPB drive and a face collapse often comes down to knowing the remoulded strength of the alluvium, not just its peak.
Service characteristics in Bath
Critical ground factors in Bath
Bath’s development since Roman times has left a legacy of undocumented cellars, culverted streams, and backfilled stone quarries beneath the city centre. The 18th-century expansion under John Wood the Elder filled many natural drainage channels with rubble and ash, creating perched water tables that modern tunnel drives intersect without warning. When a TBM passes from competent limestone into a backfilled solution feature, the face pressure required to maintain stability can drop from 3 bar to near-atmospheric in the space of a ring build. Without a ground model that maps these transitions using resistivity tomography ahead of the drive, the risk of inadvertent returns or crown collapse increases sharply. Even small settlements can activate damage in the Bath stone ashlar facades above, triggering Section 61 claims under the Planning (Listed Buildings and Conservation Areas) Act 1990.
Our services
Our Bath tunnel analysis package covers the full sequence from desk study to construction support. Every test programme is designed around the specific geology encountered along the alignment.
Laboratory Strength and Stiffness Testing
CU triaxial with local strain measurement, incremental oedometer, and ring shear for residual strength of Lias Clay. All testing at our UKAS-accredited laboratory to BS EN ISO 17892.
In Situ Soft Ground Characterisation
Seismic CPTu profiling along the tunnel alignment, field vane tests in alluvial pockets, and piezometer installation for groundwater monitoring during the drive.
Quick answers
What is the typical cost of a geotechnical tunnel analysis in Bath?
For a soft ground tunnel investigation in Bath, budgets generally range from £3,010 for a targeted CPT and laboratory programme on a short alignment to around £14,980 for a comprehensive package including seismic piezocone profiling, triaxial and oedometer testing, and numerical modelling. The final figure depends on the length of the drive, the number of investigation points, and the complexity of the geological transitions along the route.
How do you determine face support pressure for an EPB machine in soft ground?
Face pressure is calculated from the effective stress at tunnel axis level, adding a component to balance pore water pressure and a surcharge to control surface settlement. The critical parameter is the undrained shear strength of the soft material, which we measure via CPT and triaxial testing. In Bath’s mixed-face conditions, we model several scenarios to bracket the range between active and passive failure limits.
Can you assess the risk of settlement damage to listed buildings above the tunnel?
Yes. We use the volume loss method combined with Gaussian trough prediction to estimate surface settlements. The results are compared against the limiting tensile strain criteria in CIRIA C760 and Burland’s damage classification. For Bath’s Grade I listed structures, we typically recommend a maximum volume loss below 0.5% and real-time excavation monitoring with automated total stations and in-tunnel levelling during the drive.