Cone penetrometer test (CPT/CPTu)

Essential aspects of the equipment and test procedure: measured parameters

The cone penetrometer test was first developed in the Netherlands in the 1930s and is one of the most commonly used field tests. It has the obvious advantage over SPT that it is completely automated, so that its results are fully reproducible, that is, independent of the operator. However, it does not allow for sample collection. Therefore, the CPT is strictly an in situ test, and it is often complemented by borehole drilling for visual identification of the soil stratigraphy. However, in cases where stratigraphy is already clearly established in the context of previous investigation campaigns in the vicinity, the CPT is sometimes used on its own (Lunne et al., 1997).

The test consists of the continuous driving of a penetrometer into the soil by means of a hydraulic system at a rate of 20 mm/s, as shown in Figure 1.16. The penetrometer comprises a conical tip (tip angle of 60° and base of the cone area equal to 10 cm2) and a friction sleeve (length of 134 mm, area of 150 cm2).

Since the 1980s, the version of the device known as piezocone or CPTu has been generalized, allowing the measurement of pore water pressure near the tip during pushing. As shown in Figure 1.16a a ring filter (consisting of a porous metal) is located immediately

CPTu tip

Figure 1.16 CPTu tip: a) details and measured parameters; b) tip resistance correction for the device with the filter element above the cone.

above the cone, which allows the transmission of pore water pressure to a pressure transducer housed inside the nozzle tip.[1]

The measured parameters are:

  • (i) the cone tip resistance, qc, the ratio of the vertical reaction force of the ground (measured in a load cell housed at the conical tip) to the area of the base of the cone;
  • (ii) the sleeve resistance, fs, the ratio of the friction force developed along the sleeve (measured by another load cell next to the sleeve) to its surface area;
  • (iii) the pore water pressure (measured on the inner transducer in the filter element).

Besides the three measured parameters, the equipment software allows for the calculation of the friction ratio, Rf, by the following equation:

shows an image of the system

Figure 1.17 shows an image of the system. In the foreground, the set of rods (each 1 m in length) is visible, rods being sequentially added as penetration advances; inside the rods runs a cable, with the set of electric wires connecting the tip transducers to the power source and to the acquisition box, both located at the surface; the latter is connected to a computer. In the background, the structure of the pushing equipment can be seen, anchored to the ground by a set of augers, in order to achieve the necessary reaction for the hydraulic jacks to drive the cone and the rods through the soil. An alternative reaction system can be achieved using concrete blocks (heavy weight).

It is desirable that the reaction structure allows the application of a driving force of at least 10 tonnes. Otherwise, the tip will not be able to cross relatively resistant layers, and the test may be limited to shallow depths in many situations. In any case, this test is not appropriate to characterize very hard soils nor those with large particles, such as medium and coarse gravels.

Image of the pushing equipment (anchored drill-rig) of the piezocone, with the string of rods in the foreground (photo

Figure 1.17 Image of the pushing equipment (anchored drill-rig) of the piezocone, with the string of rods in the foreground (photo: Carlos Rodrigues).

The use of a piezocone requires a correction of the values taken for the cone tip resistance, and the parameter q, is used instead of qc. As shown in Figure 1.16b, since the ring-shaped filter is located immediately above the cone, pore water pressures will be exerted downwardly on the top of the conical tip, more precisely on a circular crown of inner diameter d and outer diameter D. Thus, the total resistance of the soil to the penetration of the conical tip, q„ is equal to qc added to the ratio between the resultant of the water pressures on that circular crown and the area of the base of the cone.

This correction is only relevant in soft fine-grained soils, in which very low values of qc, combined with high values of pore water pressure (due to the excess pore pressure induced by pushing the cone), are observed. For stiff clays and sandy soils, this correction is negligible, and qc and q, are practically identical. In any case, the latter parameter will be considered in the following sections.

  • [1] In certain devices, the point of pore pressure measurement is located elsewhere in the tip, namely in the face ofthe cone.
 
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