CPT PROBING AND CPTU PROBING

CPT probing (cone penetration test) and CPTU probing involve inserting the probe (penetrometer) of a conical shape into the soil with a set of rods, maintaining a constant rate of penetration (~2 cm/s). During the investigation, the following is recorded: cone penetration resistance (qc) and friction on the friction sleeve (fs). In Poland, these investigations are known as a cone static penetration test or, in short, "static probing."

Electric cones CPT and piezocones CPTU are precise measuring tools designed specially for geotechnical investigations of soil. These probes are equipped with strain gauges allowing for the measurement of the pressure force under a cone (qc ) and friction of a friction sleeve (fs). Piezocones CPTU are additionally equipped with a pressure sensor (u2) allowing for the in-situ measurement of the pore pressure during the test. All these parameters are measured continuously and simultaneously. Pressure sensors can also be used for tests of dissipation of an excess pore pressure. This method is characterised by a high accuracy of the measurement and sensitivity to changing properties of the substratum.

A qualitative interpretation of CPT probing and CPTU probing includes specification of:

  • homogeneity of the substratum structure,
  • boundaries between lithologically different layers,
  • a boundary between man-made fills and native soils,
  • a preliminary assessment of the cohesive soils consolidation degree.

A quantitative interpretation, in turn, includes specification of:

  • soil type (mostly on the basis of the Robertson chart),
  • condition (ID) of non-cohesive soils and consistency (ID) of cohesive soils (on the basis of correlation dependencies),
  • deformation (M0, E0) and strength (Ф', cu) parameters of soils,
  • bearing capacity of foundation piles (direct methods).