Evaluation of the undrained shear strength profile in soft layered clay using full-flow probes

The undrained shear strength profile of soft clay deposits are routinely estimated with a piezocone (CPTu) and the field vane test (FVT). Though suitable for a majority of soil types, both devices require significant geometric and/or empirical corrections that are inherent to the device design and/or testing method. These corrections become problematic for soft soil and offshore deposits with extreme ambient pore pressures and/or overburden stress. Recently, "full-flow" probes, which provide a direct measure of the force required to induce soil flow, have been proposed to measure the undrained and remoulded undrained strengths of soft clays in situ. While analyses and tests have been performed with these probes, minimal full-scale field studies have been performed. Moreover, their suitability to anisotropic soft clays has not been investigated. A series of full-flow soundings were performed at the University of Massachusetts Amherst test site, which consists of a continuous deposit of Connecticut Valley Varved Clay (CVVC) to a depth of 30 in. CVVC consists of alternating layers of sand-silt and clay, which together comprise a varve. Below the 6 in thick crust, the peak and remolded residual shear strength values determined with the FVT are uniform with depth and are approximately equal to 35 kPa and 4 kPa, respectively. The penetration resistance determined from the continuous penetration full-flow soundings are compared with that measured with the CPTu. Probe factors for both the overconsolidated crust and the lightly overconsolidated thick deposit are developed to estimate the undrained shear strength profile, using the FVT undrained shear strength as the reference. Finally, remolded undrained shear strength values determined from cycling the probes over selected intervals are compared with residual undrained shear strength values determined from the FVT.