Perry Drilling Limited . Geotechnical and Environmental Drillers

By: Perry Geotech  06-Dec-2011

Seismic CPT testing

To further enhance Perry Drillings status as the leading CPT operator in New Zealand we own and operate a GeoMil ( )Seismic Cone Penetration Testing (SCPT) system. The SCPT is a very valuable geotechnical tool used in determination of low strain in-situ compression(P) and shear (S) wave velocities. The P-wave and S-wave velocities are directly related to the soil elastic constants of Poisson’s ratio, Shear Modulus, Bulk Modulus and Young’s Modulus.
The seismic CPT acquisition is conducted with a 10cm2 compression CPT(U) cone allowing both CPT data and SCPT data to be collect at the same time. It is designed such that the seismic sensors record the soil profile’s response to low strain seismic disturbances. SCPT testing is conducted using accelerometers for good definition of the frequency spectrum of the recorded signal. To avoid smearing, the GeoMil seismic adapter is equipped with accelerometers (1 Hz - 10 kHz) and not with geophones (typically 1 - 300 Hz).
For quality assessment and proper data analysis, the frequency filtering of data is done after acquisition and not during the recording. The measured values are transferred as voltage signals by means of a cable connected on surface to the "Signal Conditioning System" where CPT data is separated from the seismic data.
The seismic source used generates dominant SH waves (e.g. hammer beam). The hammer beam comprises of applying a hammer blow laterally to the sides of special designed plates fixed to the penetrometer. The hammer beam generates excellent polarised SH wavelets and it is standardly applied in reverse polarity analysis (Baziw, 1988).
The seismic data is then processed by software from Baziw Consulting Engineers ( ). BCE provides both data acquisition and the data reduction, analysis and visualisation software packages.
Accurate in-situ P-wave and S-wave velocity profiles are essential in geotechnical foundation designs. These parameters are used in both Static and Dynamic Soil Analysis. Another important use of estimated shear wave velocities in geotechnical design is in the liquefaction assessment of soils. Since the shear wave velocity is influenced by many of the variables that influence liquefaction (void ratio, soil density, confining stress, stress history, and geologic age), it is an excellent index of liquefaction.