SDMT  is a combination of the standard Flat Dilatometer (DMT) with a seismic module. The seismic module is a tubular element placed above the DMT blade, equipped with two receivers located 0.5 m apart. 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 PC at the surface, which is  used to measure the shear wave velocity V_s. From V_s one can determine the small strain shear modulus G₀

(a) DMT blade and seismic module; (b) Schematic layout of the seismic dilatometer test; (c) Example of seismograms as recorded and re-phased.

Applications of SDMT
The modern norms increasingly require seismic analysis, for which the basic parameter is V_s . SDMT provides profiles of Vs in a quick, precise, simple and economical way. Repeatability of measured Vs  is 1-2 % (see below).

According to Eurocode 8 all new constructions should be preceded by an analysis of the local seismic response, requiring Vs  from ground surface to 30 m depth.

SDMT provides, besides V_s , all the information obtained by traditional DMT.  In particular, SDMT provides at each depth two independent estimates of the liquefaction resistance, one derived from V_s and the other from K_d.   K_d is sensitive to factors almost unfelt by other tests, in particular, ageing, a factor that may increase liquefaction resistance by as much as 60% in loose sands.

SDMT is used worldwide. Important projects include barriers for protecting Venice, Barcelona harbour and airport, the new shuttle crawlerway at NASA Cape Kennedy, the San Andreas fault, big power plants and high speed railways etc.

Results from two nearby SDMTs showing high repeatability.
 

Further information on the SDMT can be downloaded by clicking on the link here: The Seismic Dilatometer for In Situ Soil Investigations