SPT (Standard Penetration Test) in Dublin for Foundation Design

For a standard borehole with SPT sampling at regular intervals, budgets typically fall between €440 and €680 per borehole depending on depth, access, and the number of tests. A site with two 10-metre boreholes and six SPT runs generally sits in the middle of that range once mobilisation and factual reporting are included.

Depth depends entirely on the foundation type and the bedrock profile. For a typical two-storey extension on glacial till, we usually stop 1.5 to 2 times the footing width below formation level, often 6 to 8 metres. For piled foundations in the city centre, we continue through the till until SPT refusal on limestone bedrock, which can be anywhere from 12 to 25 metres below ground level.

Yes, but it requires experience to interpret the blow counts correctly. Fresh limestone gives refusal (50 blows for 50 mm or less) consistently across three boreholes. Weathered rock often produces N-values of 40 to 60 with gradual increase, while boulder clay can spike to refusal on a single cobble then drop to 20 on the next drive. Our geologists log the cuttings and core fragments to confirm the boundary.

Trial pits give excellent visual information down to about 4 metres, but they cannot tell you the penetration resistance of strata at 8 or 12 metres depth where your piles will found. SPT provides that quantitative N-value at depth, which trial pits simply cannot reach in Dublin’s water-bearing ground. Most structural engineers request both: pits for near-surface and SPT boreholes for the bearing stratum.

For sites within the M50, we can typically have a track-mounted rig on site within 5 to 7 working days of instruction, once the underground service clearance drawings are received. City centre sites with parking restrictions and traffic management plans may take an additional 3 to 4 days for permits.