- The seismic dilatometer SDMT is the combination of the flat dilatometer with an add-on seismic module for the measurement of the shear wave velocity Vs.
- Thanks to the true-interval test configuration (two receivers), the repeatability of the VS measurements is very high, approximately 1 %, i.e. a few m/s (table 1 Fig. 3).
- The Vs values are calculated automatically no need of an expert and displayed in real time, at a cost and time considerably lower than Downhole or Crosshole.
- SDMT provides the small strain modulus Go and the working strain modulus MDMT, i.e. two points of the G-γ The availability of two points is helpful while selecting the design G-γ curve (Amoroso et al. 2014).
- Based on SDMTs executed in many different soils and geographical regions, a chart (Fig. 5, Marchetti 2008) has been constructed permitting estimates of Vs from just mechanical DMT results.
- A Vs profile can be obtained in impenetrable soils by executing the test in a borehole backfilled with fine gravel (Fig. 7, Totani 2009).
The SDMT is the combination of the flat dilatometer with an add-on seismic module for the measurement of the shear wave velocity. The seismic module (Fig. 1a) is a tubular element placed above the DMT blade, equipped with two receivers located at 0.5 m distance. When a shear wave is generated at 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 PC at the surface, that automatically calculates the delay using the Cross Correlation algorithm. VS is obtained (Fig. 1b) as the ratio between the difference in distance between the source and the two receivers (S2 – S1) and the delay Dt from the first to the second receiver. The true-interval test configuration with two receivers avoids possible inaccuracy 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. The repeatability of the VS measurements (see examples in Table 1 and Fig. 3) is remarkable (observed VS repeatability ≈1 %, i.e. a few m/s). The Vs values are calculated automatically and no need of an expert and displayed in real time, at a cost and time considerably lower than Downhole or Crosshole.
Fig. 1c shows an example of seismograms obtained by SDMT at various test depths at the site of Fucino.