Construction methods · Site investigation and testing

Boreholes with SPT and sampling

The backbone of bridge ground investigation: a borehole at every pier and abutment, logged metre by metre, with the standard penetration test giving an N-value every 1.5 m and samples for the laboratory. Rock is cored and its quality logged (RQD).

How bridges get built · Site investigation and testing · Boreholes with SPT and sampling

Also called: rotary drilling, cable percussion, standard penetration test

rockrigSPT every 1.5 m: N-valuesamples to the labcore 3-5 m into rock fill N 7soft clay N 3medium sand N 18dense gravel N 42rock RQD 70borehole logBorehole with standard penetration tests
Concept diagram (TheBridgeEng)

How it is done

  1. Position holes at each foundation, plus one or two along the approaches; on water, from a jack-up or a moored barge.
  2. Advance the hole by rotary wash boring or cable percussion; in rock switch to double or triple-tube coring.
  3. Run the SPT at 1.0-1.5 m intervals: a 63.5 kg hammer dropped 760 mm, blows counted for 300 mm of penetration after seating.
  4. Take disturbed samples from the split spoon and undisturbed thin-wall samples in clays; log water strikes and standing levels.
  5. Install standpipes or piezometers where groundwater matters; backfill the rest with grout.

Where it fits

  • All bridge foundations; the minimum is one hole per support to a depth below the pile tips or 1.5 times the footing width.
  • Sands and gravels, where the N-value carries the design.

Where it does not

  • Relying on SPT alone in soft clays and silts: use CPT and vane tests too.
  • Stopping the hole at the first rock: prove it is bedrock, not a boulder, by coring 3-5 m.

Plant, pace and money

PlantTruck- or crawler-mounted rotary rig, or a cable percussion tripod; barge or jack-up on water; SPT hammer with energy calibration.
Productivity10-25 m per shift on land in soils; 3-8 m per shift in rock coring; marine holes cost three to six times land holes.
CostRoughly USD 150-300 per metre on land, USD 600-1,500 per metre over water including the platform.

Risks and controls

What goes wrong

  • Boulders mistaken for rock head.
  • Uncorrected hammer energy: N-values from different rigs are not comparable without calibration.
  • Artesian water in confined sands blowing the hole.

Quality assurance

Energy-calibrated hammers, continuous logging by an engineering geologist, photographs of every core box, standing water levels on every log.

Examples

Marine investigations for sea bridges typically run 40-70 m holes from jack-ups; the game's borehole cost per hole is scaled from this.

Related methods

Desk study and walkover surveyCone penetration testing (CPT and CPTu)Rock coring and rock mass loggingGeophysical surveys

Further reading

ASTM D1586 (SPT) · ISO 22476-3 · Eurocode 7-2

← Desk study and walkover surveyCone penetration testing (CPT and CPTu) →