Portsmouth's coastal geology presents a specific challenge for foundation design: thin Quaternary gravels overlying the Lambeth Group sands and clays, with the London Clay formation dipping beneath the harbour. We frequently see projects where conventional boreholes miss thin, critical soft layers that a CPT profile captures in detail. The cone penetration test provides a continuous resistance log, which is essential when you are dealing with the variable Wessex Formation deposits found across the city. For sites near the harbour walls or on reclaimed land in Portsea, combining CPT data with a grain-size analysis helps identify any loose silty layers that could be prone to settlement, and a targeted SPT drilling programme can then be used to confirm soil type at specific intervals where the cone friction ratio indicates a material transition.
A single CPT sounding in Portsmouth can replace multiple boreholes when the objective is to map the lateral continuity of the London Clay or identify dissolution features in the chalk.
Process overview
Local context
We reviewed a project on the northern edge of Portsea Island where a proposed block of flats was to be founded on pad footings. Three CPT probes across the site revealed a 1.2-metre-thick layer of peat within the superficial deposits at approximately 4 metres depth, a feature that had been entirely missed by the trial pits dug during the phase one investigation. The peat was draped over an irregular chalk surface, creating a variable compressible layer that would have induced unacceptable differential settlement. By integrating the CPT profiles with the geological mapping of the Portsmouth area, we were able to recommend a Improvement scheme using stone columns to transfer the loads through the soft horizon. Skipping the CPT would have led to a foundation design that looked adequate on paper but failed in service — a costly outcome given the density of development in this part of the city.
Reference standards
BS EN ISO 22476-1:2012 Geotechnical investigation and testing — Field testing — Electrical cone and piezocone penetration test, BS EN 1997-2:2007 (Eurocode 7) Geotechnical design — Ground investigation and testing, BS 5930:2015 Code of practice for ground investigations, UK Specification for Ground Investigation (ICE, 2nd Edition)
Additional services
Standard CPTU Profiling
Measurement of tip resistance, sleeve friction, and dynamic pore pressure at 2 cm intervals. Ideal for mapping the thickness of the London Clay and identifying sand lenses within the Lambeth Group.
Seismic CPT (SCPT)
Downhole shear wave velocity measurements using a triaxial geophone array. Provides data for site classification to BS EN 1998-1 and liquefaction assessment in the saturated estuarine deposits near Langstone Harbour.
Dissipation Testing and Permeability Estimation
Held-penetration pore pressure decay tests to evaluate the coefficient of consolidation (cv) in the Bracklesham Beds and any overconsolidated clays. Essential for settlement rate predictions.
Typical parameters
Quick answers
What is the typical cost of a CPT sounding in Portsmouth?
For a standard CPTU profile to 20 metres depth on Portsea Island, the cost ranges from £110 to £190 per linear metre, depending on access conditions and whether seismic modules or dissipation tests are included. Mobilisation is charged separately, though we can often share mobilisation costs if multiple tests are scheduled in the same week.
How does the CPT handle the gravel layers common in Portsmouth?
The Quaternary river terrace gravels above the London Clay can be problematic for CPT refusal. Our rig operates at up to 20 tonnes of thrust, which is generally sufficient through thin gravels. If we encounter thick, dense gravels, we pre-bore through the obstruction using a hollow-stem auger and then continue the CPT from the base of the borehole, maintaining a continuous profile.
Can CPT data be used directly for pile design in the London Clay?
Yes, the cone tip resistance correlates well with undrained shear strength in the London Clay, and we apply the LCPC and ICP methods to estimate pile shaft friction and end bearing capacity. The continuous profile allows us to identify the exact depth where the clay becomes sufficiently stiff for a good founding stratum, which is particularly useful for CFA and driven pile design in the city.
How many CPT soundings do I need for a typical residential development in Portsmouth?
For a small to medium residential plot, three to four probes spaced across the footprint is the standard approach, following BS 5930 guidance. The spacing is adjusted based on the variability seen in the first two probes; if the chalk surface is irregular or we detect infilled dissolution features, we tighten the grid to ensure the geological model is solid.