CAIT project no.: 88 04 RU932
Fiscal Year: 2000/2001
Rutgers-CAIT Author(s): Thomas Bennert, Ali Maher, Nenad Gucunski
External Author(s): Anthony Chmiel
Sponsor(s): NJDOT, FHWA-USDOT
This report is an evaluation of the seismic piezocone for geotechnical design parameters. The seismic piezocone is a “direct-push” device that is hydraulically pushed into the ground at a constant rate of 2 cm/sec. During penetration, the device measures the resistance at the tip and along the side. The device also measures the water pressure due to the soil penetration. Based on these measured parameters, a number of correlations have been developed to determine a vast number of soil properties that include; undrained shear strength, stress history, compressibility, and soil classification. Correlations have also extended to be used to estimate the SPT N-values (blowcounts). The device can also be used to conduct seismic testing in a similar manner to traditional downhole testing. This only requires a stop in penetration and approximately 30 seconds to create a shear wave source, measure the time for the shear wave travel, and save the data on the computer. Therefore, a device that can classify the soil, determine design parameters (such as the SPT N-value), and also be used to conduct seismic testing would be an extremely valuable tool in geotechnical engineering.
The objective of the study was to briefly introduce the NJDOT to the capabilities of the seismic piezocone (SCPTU). The testing results from a number of sites across New Jersey, and also New York, show that the device is extremely accurate at providing a soil profile and determining layers within the subsurface. However, difficulties in determining the differences between silt mixtures (clayey silt, silty clay, silty sand, sand with silt) are shown when comparing to laboratory soil classification procedures. The SCPTU N-value comparison to actually measured N-values from an SPT drill rig show good agreement, however, since the SCPTU eliminates operator error from the test, results from individual sites show better consistency than the drill rig values. It must also be noted that the SPT determined N-values must corrected to 60% applied energy. The SCPTU shear wave profile compared quite favorably to traditionally used crosshole and downhole testing, proving that the SCPTU can be utilized as a seismic testing tool. Developed correlations, based on CPT penetration data, were also evaluated and shown to be quite accurate at estimating the shear wave velocity. However, the equations are soil based, either sand or clay type soils, and therefore must be used as such.