The Fylde Peninsula, where Blackpool rises to just 10 metres above sea level along its famous promenade, conceals a subsurface story written in glacial and interglacial cycles. Beneath the piers and tower lies a complex sequence of Devensian till, glaciofluvial sands, and soft post-glacial estuarine silts that extend deep below Ordnance Datum. For any tunnelling project in Blackpool — whether for a new utility crossing beneath the railway approaches or a stormwater storage scheme under the dense Victorian street grid — the geotechnical analysis for soft soil tunnels must confront these water-charged, low-strength horizons head-on. Our team has logged boreholes from St Anne's to Fleetwood that reveal just how rapidly the till consistency can shift within a few hundred metres, a variability that demands a phased investigation approach. The CPT testing we deploy provides a near-continuous profile of tip resistance and pore pressure, essential for identifying the thin sand lenses that can destabilise a tunnel face within Blackpool's glacial sequence, while in-situ permeability testing quantifies the inflow risk that shapes the dewatering strategy before any TBM launch.
In Blackpool's boulder clay, the stand-up time for an unsupported tunnel face is measured in hours, not days — the undrained strength dictates every metre of advance.
Our approach and scope
Local geotechnical context
The drill rig on a Blackpool site typically works from a compact crawler platform, sinking dynamic sampler boreholes through made ground that often contains Victorian-era rubble, old sea wall foundations, and uncompacted sand fills placed during the resort's rapid expansion. The first risk is always the unexpected obstruction — a timber pile from a former pier railway, a brick culvert diverting a buried watercourse — that can deflect a borehole and give a false impression of depth to competent bearing strata. Beyond obstructions, the real challenge specific to soft ground tunnelling in Blackpool is the transition zone between the glacial till and the underlying mudstone: this zone of completely weathered bedrock behaves as a soil when excavated but swells and slakes on exposure to air and water, changing its volume and eroding the passive resistance at the tail of the shield. Ground gas is another factor that appears frequently in investigations along the Fylde coast, with methane and carbon dioxide generated in the organic-rich estuarine clays requiring continuous monitoring during any confined-space excavation. The borehole log for a Blackpool tunnel project must therefore capture not just SPT N-values and recovery ratios, but also gas readings, standpipe piezometer readings over at least one tidal cycle, and detailed lithological descriptions at the scale of the laminated silt-clay couplets that control permeability anisotropy.
Applicable standards
BS 5930:2015+A1:2020 — Code of practice for ground investigations, BS EN 1997-1:2004 (Eurocode 7) — Geotechnical design: General rules, BS EN 1997-2:2007 — Ground investigation and testing, CIRIA C760 — Guidance on embedded retaining wall design, PAS 8810:2016 — Tunnel design — Design of concrete segmental tunnel linings
Complementary services
TBM Feasibility Ground Model for the Fylde Coast
A comprehensive ground investigation and interpretative report specifically structured for mechanised tunnelling in Blackpool's glacial sequence. The package includes rotary cored boreholes with in-situ pressuremeter testing, a full suite of laboratory classification and strength tests on undisturbed samples, and development of a 3D geological model that maps the till-mudstone interface along the proposed alignment. The deliverable is a geotechnical baseline report compliant with BS EN 1997-2 and suitable for TBM specification and lining design.
Tunnel Face Stability and Settlement Risk Assessment
A focused analysis package for shallow tunnels beneath Blackpool's urban core, where the dense heritage building stock and tourist infrastructure demand tight settlement control. The work combines CPTu profiling to define the sand lens distribution with advanced laboratory testing to constrain the critical state parameters of the tills. Outputs include a face pressure envelope for EPB or slurry TBMs and a ground movement prediction using both empirical Gaussian trough methods and 2D finite element modelling calibrated against local case histories.
Typical parameters
Frequently asked questions
What are the typical ground conditions for tunnelling in Blackpool?
The typical sequence beneath Blackpool is 2–5 metres of made ground and dune sand overlying Devensian glacial till — a stiff to very stiff silty clay with cobbles and boulders — which in turn overlies weathered Mercia Mudstone. The till contains discontinuous lenses of water-bearing sand and gravel that can cause localised instability at the tunnel face. Groundwater is high, often within 2 metres of the surface, and tidally influenced near the seafront.
How much does a soft ground tunnel investigation cost in Blackpool?
A site investigation programme designed for a soft ground tunnel alignment in Blackpool typically falls between £3,470 for a targeted supplementary investigation with a few CPT soundings and laboratory tests, and £15,200 for a full feasibility-stage campaign involving multiple deep boreholes, pressuremeter testing, advanced triaxial testing, and a comprehensive geotechnical interpretative report. The final cost depends on the tunnel length, depth, and the density of the urban access constraints.
What laboratory tests are essential for Blackpool's glacial till?
The essential suite includes moisture content, Atterberg limits, and particle size distribution for classification, consolidated undrained triaxial tests with pore pressure measurement for strength and stiffness, and oedometer tests for consolidation parameters. Given the laminated nature of the tills in the Fylde area, we also recommend a series of multistage triaxial tests to capture the post-peak strain-softening behaviour that controls face stability and ground movements.
Can a tunnel be built through the water-bearing sand lenses under Blackpool?
Yes, and it has been done successfully on several utility tunnel projects along the Fylde coast. The key is characterising the sand lens geometry and hydraulic connectivity during the investigation phase using a combination of CPTu profiling and multilevel piezometer installations. With a reliable ground model, the TBM face pressure can be specified to balance the pore pressure in the sand while preventing blowout in the surrounding clay. In some cases, a programme of pre-treatment grouting from the surface is used to reduce the permeability of the sand bodies before the TBM reaches them.