Standard Penetration Testing (SPT) in Gloucester: Reliable Subsurface Data for Geotechnical Design

For typical two-to-three-storey residential schemes on Gloucester's river terrace gravels, we generally recommend boreholes to 10–15 metres depth. This captures the full gravel thickness, the underlying Lias Clay or Mercia Mudstone, and any soft alluvial pockets. NHBC Standards require that the ground investigation extends to a depth where the stress increase from the foundation is less than 10% of the original overburden pressure, which in practice means at least 1.5 times the building footprint width. On sites near the Severn floodplain, we often go deeper to check for buried channel deposits that can introduce differential settlement risk.

A typical SPT investigation in Gloucester with three boreholes to 10–15 metres, including mobilisation, drilling, sampling, logging, and a factual report, runs between £440 and £660 per borehole, depending on access conditions, depth, and the number of test intervals. The total project cost depends on the scope, but for a standard residential plot you should budget in that range per exploratory position. We can provide a fixed-price proposal once we review the site location and proposed foundation layout.

When refusal is reached — defined as 50 blows for any 150 mm increment — we switch from SPT sampling to core drilling to recover an intact rock sample. This transition is critical because the refusal depth becomes the key design parameter for pile socket length or for confirming that shallow foundations are viable on rockhead. We log the refusal depth, the drilling method change, and the rock quality designation (RQD) from the core run, all in accordance with BS 5930:2015.

We apply the correlations recommended in BS 8004:2015 and the NHBC Standards, which relate SPT N-values to allowable bearing pressure for different soil types. For granular soils like the river terrace gravels, we use the relationship between N-value and friction angle proposed by Peck, Hanson, and Thornburn, then apply bearing capacity factors from Brinch Hansen. For cohesive soils like the Lias Clay, we convert N-values to undrained shear strength using the Stroud correlation (cu = f1 × N × pa), with the factor f1 calibrated to local experience. Every correlation is clearly stated in the interpretative report so the design team can review the assumptions.