A new instrument for high-resolution in situ assessment of Young’s modulus in shallow cohesive sediments

This paper describes a new, miniature, instrumented flat dilatometer (mIDMT) designed to assess variations in nearly continuous compressive stress–strain behaviour with depth in shallow cohesive sediments. The instrument was tested both in situ in the Bay of Fundy, Nova Scotia, Canada, and in cored samples from Willapa Bay, Washington, USA. Comparisons between probe and laboratory uniaxial assessments for other elastic materials—gelatine and foam rubber specifically—show strong agreement over the range of strains induced in the experiments. Observed values of Young’s modulus (E) for the gelatine and ethylene-vinyl acetate foam ranged from 6–343 kPa. Sediment stress–strain curves were distinctly linear for the overconsolidated fine-grained sediments of the Minas Basin, and values of E were found to increase with depth from near zero to 500–1,300 kPa at 20 cm depth. At the Willapa site, the sandy tidal flat sediments also behave elastically but E tended to increase more strongly with depth than for sediments from the Minas Basin. Young’s modulus was inversely correlated to porosity at all sites tested, and linearly related to shear strength in the Minas Basin. The newly designed instrument has much finer resolution than for other, similar methods of determining E in situ, and it provides data at a resolution sufficient to assess small-scale processes such as gas bubble growth and infaunal locomotion, for which elastic constants are needed for modelling and prediction.