The international reference test procedures on standard penetration testing (SPT) were proposed by the SPT working party of the ISSMFE (now ISSMGE) Technical Committee on Penetration testing in 1988.
In Calibration methods, a method using stress wave equation analysis was introduced for adjusting SPT N-values to account for the different magnitude of driving energy, as a means of quality control for the test.
In this method, the force-time record is obtained by measuring the stress wave produced by driving the SPT. The energy transmitted to the drill rods is computed by wave equation analysis of the first compression wave. The energy transfer ratio is given by the ratio of the transmitted energy to the normal kinetic energy at impact. The N-value can be adjusted to one corresponding to any energy transfer ratio, assuming it is inversely proportional to the energy transfer ratio.
The energy transfer ratio varies between 33% and 92%.An average of 67% (Kovac et al, 1983) for adjusting the N-value is recommended. The method is carried out in practice with increasing frequency. The N-value is generally adjusted to a corresponding energy transfer ratio of 60%, which is considered appropriate when applying the tables and diagrams giving the correlation between N-value and soil parameters proposed by Terzaghi and Peck (Mesri, 1996, and others).
From penetration-time and stress-time records, typically the hammer strikes the rod more than twice after the first blow. The penetration of the rod continues after the end of the first compression wave and increases with the hammer strikes. From the penetrationtime graph of each SPT blow, it is possible to calculate the stress-time record monitored until the end of the last compression wave.
Fujita (1997) proposed a diagram based on his suggestion of a fundamental equilibrium equation to represent the penetration, energy transfer ratio and dynamic resistance at each blow of the SPT. Figure 1 summarises a series ofexperimental SPT results on the dry Toyura Sand plotted in terms of the relative density.
The plot of penetration per blow and dynamic resistance varies with 300mm of penetration.
Based on SPT experiments intentionally applying a different energy transfer ratio, Nagasaki et al (1998) proposed that the Nvalue was inversely proportional to the square root of the energy transfer ratio and that the adjustment should be made accordingly.
Matsumoto et al (1992) applied the method of two-point strain measurement to the SPT successfully and the calculated data corresponded well with the measured records.
Abou-matar et al (1996) presented typical SPT records and the computed final set was equal to the observed one. Abou-matar et al (1997) also discussed the dynamic measurements and analyses related to the SPT and pointed out that the drill rod area may cause substantial changes in the measured N-value.
The time has come to discuss and reexamine the International Reference Test Procedures on SPT and calibration methods.
The application of dynamic measurements and wave equation analyses to the SPT should not be limited to obtaining the energy transfer ratio, it could determine or estimate the shear strength of soil insitu and design parameters.
Dr Keiichi Fujita, Science University of Tokyo