Cone Penetration Testing

Seismic Measurements

Seismic measurements in geotechnical engineering built-up prevalence in the period from the mid-1960s through 2000. Advances in measurement methods and computing power have made seismic measurements even more amenable to routine practice. The method involves measurement of body waves (P-waves and S-waves) and/or surface waves which can be interpreted in the framework of soil dynamics to produce stiffness parameters over a more complete shear strain range, particularly as very low strains, which has a range of applications in deformation analyses such as settlement and ground movement assessments as well as soil liquefaction and soil profiling assessments.

The particularly type of seismic measurement we provide is referred to as downhole seismic velocity measurement which involves penetrating geophones (as part of a cone or dilatometer apparatus), stopping the push and introducing seismic shear waves at the ground surface by striking the ground at a source. The waves are then measured at the geophones.

Geophysical Testing

Seismic Measurements

Cone Penetration Testing

Seismic Measurements

Seismic measurements in geotechnical engineering built-up prevalence in the period from the mid-1960s through 2000. Advances in measurement methods and computing power have made seismic measurements even more amenable to routine practice. The method involves measurement of body waves (P-waves and S-waves) and/or surface waves which can be interpreted in the framework of soil dynamics to produce stiffness parameters over a more complete shear strain range, particularly as very low strains, which has a range of applications in deformation analyses such as settlement and ground movement assessments as well as soil liquefaction and soil profiling assessments. The particularly type of seismic measurement we provide is referred to as downhole seismic velocity measurement which involves penetrating geophones (as part of a cone or dilatometer apparatus), stopping the push and introducing seismic shear waves at the ground surface by striking the ground at a source. The waves are then measured at the geophones.

The seismic dilatometer (SDMT) is a combination of the flat dilatometer (DMT) with an add-on seismic module for measuring the shear wave velocity VS. First introduced by Hepton (1988), the SDMT was subsequently improved at Georgia Tech, Atlanta, USA (Martin & Mayne, 1997, 1998; Mayne et al., 1999). A new SDMT system has been more recently developed in Italy (Marchetti et al., 2008). The seismic module (Fig. 4.2) is a cylindrical element placed above the DMT blade, equipped with two receivers spaced at 0.50m. The shear wave source, located at ground surface, is an automatic hammer or a pendulum hammer (≈10 kg) which hits horizontally a steel rectangular plate pressed vertically against the soil (by the weight of the truck) and oriented with its long axis parallel to the axis of the receivers, so that they can offer the highest sensitivity to the generated shear wave.

When a shear wave is generated at the surface, it reaches first the upper receiver, then, after a delay, the lower receiver. The seismograms acquired by the two receivers, amplified and digitized at depth, are transmitted to a computer at the surface, which determines the delay. VS is obtained as the ratio between the difference in distance between the source and the two receivers (S2 – S1) and the delay of the arrival of the impulse from the first to the second receiver (Dt). The determination of the delay from SDMT seismograms, normally obtained using a cross-correlation algorithm rather than relying on the first arrival time or specific single points in the seismogram, is generally well conditioned. The true-interval test configuration with two receivers avoids possible inaccuracy in the determination of the “zero time” at the hammer impact, sometimes observed in the pseudo-interval one-receiver configuration. Moreover, the couple of seismograms recorded by the two receivers at a given test depth corresponds to the same hammer blow and not to different blows in sequence, which are not necessarily identical. Hence the repeatability of VS measurements is considerably improved (observed VS repeatability ≈1%, i.e. a few m/s). VS measurements are typically taken every 0.5m to 1.0m of depth (while the mechanical DMT readings are taken every 0.20m). Validations of VS measurements by SDMT by comparison with VS measured by other in situ seismic tests at various research sites are reported by Marchetti et al. (2008).

References

ASTM International. 2014. ‘Standard Test Methods for Downhole Seismic Testing’. ASTM D7400–14. West Conshohocken, PA. ASTM International, approved November 1, 2014.

Marchetti, S., Monaco, P., Totani, G. & Marchetti, D. 2008. ‘In situ tests by seismic dilatometer (SDMT)’. Proceedings. From Re-search to Practice in Geotechnical Engineering, ASCE Ge-otech. Spec. Publ. No. 180 (honoring J.H. Schmertmann): 292-311.