CPTu Probes

The Geotech CPT probes are equipped with individual sensors for point resistance (qc), sleeve friction (fs), pore pressure (u) and a tilt sensor. The readings from the three channels are corrected for temperature drift by a temperature sensor and a processor in the electronic part of the probe. The unique cordless system makes the CPT sounding quicker and easier. Saving time from the trouble with the cord, just put the rods together.

Seismic CPT

A seismic piezocone is pushed progressively into the ground. Each metre or so it is stopped. A hammer is used at the surface, to produce seismic waves. For shallow saturated soil the compressional (P) wave velocity is normally primarily a function of the bulk modulus of the pore water, and is not sensitive to changes in the stiffness of the soil skeleton. Shear waves are usually used, and are typically generated by placing a wooden sleeper under the wheel of the penetrometer truck and striking it horizontally with a large hammer.

The striking of the block with the hammer triggers an armed seismograph, and this records the arrival of the seismic waves at the cone tip. If the site is noisy, the signal- to-noise ratio can be improved by repeating the process and stacking the signals. The travel time from the surface to each cone position is determined from the seismograph traces, and the time taken for the wave to travel between each cone position is determined by subtraction. The so-called ‘interval velocity’ is then determined by dividing this difference in travel time by the distance between the two cone positions. Better results can be obtained by using a cone with two sets of geophones, mounted a metre or so apart, in it. Finally, the very small strain shear modulus of the soil can be determined from the equation.