One of the most stimulating research topic of SDMT testing is the possibility to estimate the in situ stiffness decay with strain level (G-strain amp. curve). The current practice is to determine the G-strain amp. curve by fitting “reference curves” based on two points, i.e the initial shear modulus G0 and a working strain modulus GDMT, whose location is not a priori known. This fact poses some uncertainties in the application of the method, especially for organic and soft cohesive soils. In this note, starting from SDMT and cyclic laboratory data collected in organic and soft alluvial clays of Tiber River deposits in Rome, an alternative approach to derive the in situ stiffness decay curve is presented and discussed.