How bridges get built · Site investigation and testing · Cone penetration testing (CPT and CPTu)
Also called: piezocone, static cone
How it is done
- Push the 35.7 mm cone from a 20 t truck or a ballasted rig; on water, from a seabed frame or a jack-up.
- Record qc, fs and u2 every 10-20 mm; dissipation tests at chosen depths give consolidation parameters.
- Interpret soil type from the friction ratio and pore pressure, and derive undrained strength, relative density and pile shaft and base resistance.
- Pair with a few boreholes for samples and for calibration of the correlations.
Where it fits
- Soft clays, silts, loose sands, reclaimed fill, liquefaction assessment.
- Long viaducts over alluvium where dozens of profiles are needed cheaply.
Where it does not
- Gravels, cobbles and cemented layers: the cone refuses or breaks.
- Rock, obviously; and depths beyond 30-40 m in stiff ground without predrilling.
Plant, pace and money
PlantCPT truck or crawler with 100-200 kN push capacity; seabed CPT units for offshore.
Productivity3-6 profiles of 20-30 m per day on land.
CostAbout a third of a borehole per metre; the profile is continuous, which no borehole gives.
Risks and controls
What goes wrong
- Refusal on a thin gravel lens can be mistaken for bedrock.
- Pore pressure element de-saturating if handled carelessly.
Quality assurance
Cone calibration certificates, zero readings before and after every push, saturation procedure recorded.
Examples
Standard in the Netherlands, the Gulf reclamations and Korean west-coast estuaries; used with the LCPC and UWA methods for driven pile design.
Related methods
Further reading
ASTM D5778 · ISO 22476-1 · Lunne, Robertson and Powell, Cone Penetration Testing in Geotechnical Practice