| Seismic Cone Penetrometer Testing
(SCPTu)
Gregg Drilling & Testing, Inc. uses a modified CPT cone that contains a built in seismometer to measure compression and shear wave velocities in addition to the standard piezocone parameters (qc, fs, and u2). Therefore, four independent readings are compiled with depth in a single sounding. The standard CPT parameters are recorded continuously while the seismic test is usually performed at 5-foot intervals.
Shear waves are generated by striking a seismic beam coupled to the ground surface by a hydraulic cylinder under the CPT rig, Figure SCPTu. Compression waves are generated by striking an auger in the ground. The sledgehammer that strikes the beam/auger acts as a trigger, initiating the recording of the seismic wave trace. Before measurements are taken, the rods are decoupled from the CPT rig to prevent energy transmission down the rods. 
Geophones in the body of the piezocone recognize the arriving waves generated at the ground surface, Figure Seismic. Any waves received by the geophones on the cone penetrometer are sent back up to the truck to be displayed on an oscilloscope. On site software then plots the wave amplitude versus time to calculate wave velocities. At least two waves are recorded for each test depth so the operator can check consistency of the waveforms. Shear wave data is sampled at a frequency of 20kHz (20,000 samples per second) and compression wave data is sampled at 40kHz (40,000 samples per second). To maintain a desired signal resolution, the input sensitivity (gain) is increased with depth. |  | Figure Seismic | Offset distances of the beam from the cone and the location of the geophone are all taken into account in calculations. The shear wave velocity (Vs) provides information about small-strain stiffness while the penetration data provides information about large-strain failures. From interval shear wave velocity (Vs) and the mass density (ρ) of a soil layer, the dynamic shear modulus (Go) of the soil can be calculated in a specific depth interval. The dynamic shear modulus (G0) is a key parameter for the analysis of soil behavior in response to dynamic loading from earthquakes, ice, vibrating machine foundations, waves and wind. For a detailed reference on seismic CPT, refer to Robertson et. al., 1986. |
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