Standard Penetration Test (SPT) in Swindon: BS 5930 Compliant Ground Investigation

For a standard residential extension in Swindon requiring two or three SPT boreholes to a depth of 6-8 metres, the cost ranges from £390 to £690, depending on access constraints, the requirement for traffic management, and whether the site is underlain by the Kimmeridge Clay (which drills quickly) or the Cornbrash limestone (which requires slower rotary coring through the hard bands). This price includes the mobilization of a tracked rig, the SPT execution, soil logging, and a factual report with the N-value logs.

On the weathered Kimmeridge Clay in north and west Swindon, we typically encounter SPT N-values between 8 and 18 within the upper 3 metres, which correlate to an undrained shear strength of approximately 40 to 90 kPa. This is generally sufficient for a conventional strip footing at 1 metre depth, provided the bearing pressure does not exceed 75 kPa. However, the clay’s high plasticity index often triggers a settlement analysis, and we use the SPT data to estimate the constrained modulus via Stroud’s correlation for stiff clays.

For a piled foundation in Swindon, the SPT borehole must extend at least 5 metres into competent ground below the pile toe level, or to the point where N-values consistently exceed 30 blows per 300 mm, whichever is deeper. In the town centre, where the Forest Marble and Cornbrash provide a solid bearing stratum at depths of 3 to 8 metres, this often results in a total borehole depth of 10 to 15 metres. The BS EN 1997-2 guidelines require that the investigation depth demonstrates that no weaker layers exist beneath the founding stratum.

Groundwater in Swindon fluctuates seasonally, perched on the Kimmeridge Clay or within the limestone aquifers, and can be encountered anywhere from 1.5 to 6 metres depth. When the SPT is conducted below the water table in granular soils, we apply an overburden correction to the raw N-value using Liao and Whitman’s method, and a further correction for the reduced hammer efficiency due to rod buoyancy. The resulting N60 value is what we use for liquefaction assessment and bearing capacity calculations, as mandated by BS EN 1997-2.

While the SPT itself does not directly measure sulfate content, the disturbed sample retrieved from the split-spoon during each drive is immediately bagged and dispatched to our UKAS-accredited laboratory for chemical testing, including water-soluble sulfate, pH, and total potential sulfate. In Swindon, the Kimmeridge Clay and Oxford Clay formations are classified as Class 3 or Class 4 aggressive ground in many locations per BRE Special Digest 1, and the SPT sampling provides the physical evidence chain from depth to laboratory that the design sulfate class is correctly assigned.