Strain Rate Effects in Soft Estuarine Soils using Portable Free Fall Penetrometers

Portable free fall penetrometers (PFFP) have previously been used as an economical, fast and robust tool to characterize the upper subsurface sediment in marine environments by providing initial information on sediment type, strength and stratification. The penetrometer impacts and advances through the soils at a high velocity and consequently the soil is sheared at high strain rates. For most soils, it can be assumed that high strain rates lead to higher shear resistance, and thus, a strain rate correction is applied when analyzing the soil properties from free fall penetrometers and assessing the shear strength or state of consolidation. Here, the applicability of strain rate corrections for very soft and cohesive estuarine soils is investigated. PFFP measurements were conducted at Clay Bank located in the York River, Virginia. Gravity core samples were obtained at approximately the same location to a depth of 75 cm, and geotechnical laboratory tests such as laboratory miniature vane shear, Atterberg Limits, and grain size distribution were carried out. The sediments at the site are classified as clay of high plasticity according to unified soil classification system. The natural water content (126-140%) along the profile is greater than the liquid limit (92%), i.e., it is expected that the seafloor surface exhibits the behavior of a viscous fluid rather than of a plastic soil. The undrained shear strength (S_u) was estimated from the PFFP and matched to derived from the vane shear. The results showed best fits at a strain rate multiplier of k = 0.1-0.2 using a logarithmic strain rate correction (Fig. 1) and at a strain rate parameter of β = 0.01-0.03 using a power-law strain rate correction (Fig. 2). These values indicate minimal to negligible strain rate effects for this soil. Thus, the soil may be described better in terms of its rheological behavior than in terms of a traditional soil behavior, which is a finding relevant for engineering applications as well as the understanding of sediment transport and deposition processes.