Geotechnical Characterization of a Tidal Estuary Mudflat Using Portable Free-Fall Penetrometers

This study investigates the geotechnical characteristics of a soft tidal mudflat in the Great Bay Estuary, New Hampshire. Laboratory testing of surficial sediment samples of the upper 10 cm and field observations from a portable free-fall penetrometer (PFFP) were used to characterize soil strength properties (coefficient of consolidation and undrained shear strength). Pore pressure measurements from the PFFP were analyzed using a square-root of time method to estimate t50 and empirical correlations from the literature to estimate the coefficient of consolidation. Undrained shear strengths were estimated from the PFFP deceleration using cone factors of 10-12 and a strain rate correction factor of 0, resulting in undrained shear strengths of 0.5-1.1 kPa. Laboratory testing using miniature vane shear testing found the undrained shear strength to be 0.6-1.8 kPa. Both techniques indicate soft soils. The PFFP deceleration profiles also suggest the presence of layering within the seabed, a situation at the field site that could arise from environmental factors, such as winter ice formation and seasonal erosion. The coefficients of horizontal consolidation estimated with the PFFP (5.4x10-6-1.7x10-5 m2/s) were 1-2 orders of magnitude greater than the coefficients of vertical consolidation measured during oedometer testing (9.51x10-8-1.78x10-6 m2/s across all samples and load steps). Effects impacting the coefficient of consolidation such as to mismatches between sample depths and the resting depth of the penetrometer, sample disturbance, anisotropy, differences in loading conditions, and limitations associated with deriving values from literature charts are discussed. The results suggest that PFFPs are able to well-characterize the geotechnical properties of tidal mudflat soils and contribute to improving the understanding of the geotechnical parameters of undrained shear strength and coefficients of consolidation.