Marchetti Seismic Dilatometer Test (SDMT)


The seismic dilatometer is an instrument resulting from the combination of the DMT blade with a modulus measuring the shear wave velocity Vs.

The seismic modulus is an instrumented tube, located above the blade (see figure), housing two receivers at a distance of 0.50 m. The test configuration "two receivers" "true interval" avoids the problem connected with the possible inaccurate determination of the "first arrival" time sometimes met with the "pseudo interval" configuration (just one receiver). Also the pair of sismograms at the two receivers corresponds to the same blow, rather than at two successive blows - not necessarily identical. The adoption of the "true interval" configuration considerably enhances the repeatability in the Vs measurement.

The energization is obtained by a swinging hammer, hitting horizontally a steel parallelepipedal base, pressed vertically against the soil surface to insure a strong contact. The longitudinal axis of the base is oriented parallel to the axis of the receivers, to maximize sensitivity to the shear wave.

The shear wave velocity Vs is the ratio between the difference of distance between the energy source and the two receivers and the measured delay time of the pulse from the upper to the lower receiver.

Vs may be converted into the initial shear modulus Go. The combined knowledge of Go and of the one dimensional modulus M (from DMT) may be helpful in the construction of the G-Gamma modulus degradation curves. The Vs determinations are executed at 0.50 m depth intervals.

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