A recent multi-storey residential scheme on the east bank of the River Exe encountered highly variable ground within the first 8 metres — loose silty gravels overlying weathered breccia — and the structural engineer refused to sign off the pile design without site-specific penetration resistance data. That is the reality of building in Exeter, where the transition from the New Red Sandstone highlands to the floodplain deposits of the Exe Estuary can shift over a distance of less than 50 metres. We mobilised a track-mounted rig, ran the SPT at 1.5 m intervals in accordance with BS EN ISO 22476-3, and delivered a factual report within four working days that gave the design team the N-values they needed to size the foundations correctly. When ground conditions are this unpredictable, relying on desk-study geology alone is a gamble that no sensible developer should take. Our approach combines the Standard Penetration Test with targeted Cone Penetration Testing where continuous profiling is required, so the investigation strategy fits the geology rather than the other way around.
An SPT N-value alone is not a design parameter — it is only as useful as the logging discipline behind it, and in Exeter's mixed geology that discipline makes the difference.
Scope of work in Exeter
Demonstration video
Critical ground factors in Exeter
Exeter's population has passed the 130,000 mark and the city is absorbing significant housing growth along the Exe corridor, yet some of its most developable land sits on alluvial deposits with a known history of poor bearing capacity. The 2014 winter floods that inundated the St Thomas area were a reminder that groundwater rises fast here, and SPT results obtained during a dry summer can paint an optimistic picture of soil strength if seasonal saturation is not accounted for. A low blow count in a saturated silty sand at 3 m depth — typical of the Exe floodplain — signals a real risk of settlement or bearing failure under concentrated loads unless the foundation solution responds to the measured N-values. We have seen projects where skipping the SPT in favour of a cheaper window-sample survey led to a £70,000 piling variation because the ground was weaker than the desk study suggested. In a city where the geology changes block by block, the cost of the test is negligible next to the cost of getting the ground wrong.
Our services
Every SPT programme we run in Exeter is designed around the specific geological unit being investigated, not a one-size-fits-all grid. The two service levels below reflect what most local projects actually need.
Borehole SPT with factual reporting
Track-mounted or restricted-access rig, full-depth SPT at prescribed intervals, logging by an experienced geotechnical operative, groundwater monitoring during drilling, and a factual report with N-values, N60 corrections, and soil descriptions to BS 5930. Suitable for foundation design, retaining-wall verification, and slope-stability assessments across the Exeter region.
Combined SPT and laboratory testing package
SPT fieldwork paired with selected laboratory tests — moisture content, Atterberg limits, particle-size distribution, and undrained triaxial where required — so that the engineer receives both index and strength parameters in a single coherent report. This package aligns with Eurocode 7 design requirements and is particularly useful on the Mercia Mudstone sites north of the city centre.
Common questions
How much does an SPT borehole typically cost in the Exeter area?
For a single borehole to 10 m depth with SPTs at 1.5 m intervals, most Exeter projects fall in the £430 to £680 range. The final figure depends on access conditions, traffic management requirements, and whether we are drilling through the Volcanic Series — which wears tooling faster and can push labour time up.
What is the difference between the raw N-value and N60?
The raw N-value is the blow count recorded by the field hammer. N60 corrects that value to a reference energy ratio of 60 percent, accounting for the efficiency of the specific hammer used. In the UK, most automatic trip hammers have an energy ratio between 70 and 85 percent, so N60 is typically higher than the raw N. We report both so the designer can see the conversion.
How deep can you drill for SPT testing in Exeter's geology?
With our standard track-mounted rigs we routinely reach 25 to 30 metres in the Mercia Mudstone and sandstone formations. In the Exe Valley gravels, depths are sometimes limited by cobble content or high groundwater inflow, but we can switch to a sonic rig for deeper or more difficult profiles if the investigation requires it.
Can SPT results be used directly for pile design?
Yes, N-values are a key input for empirical pile-capacity methods widely used in UK practice, including the methods referenced in the ICE manual of geotechnical engineering. However, we always recommend that SPT data be combined with a site-specific ground model and, for critical structures, verified by a static load test. The test provides the data — the engineer provides the interpretation.