Centrifuge modelling of an instrumented free-fall sphere for measurement of undrained strength in fine-grained soils

Abstract

This paper describes centrifuge tests in which a model free-fall sphere was allowed to free fall in water before dynamically embedding within reconstituted samples of kaolin clay and two offshore natural clays. Instrumentation within the sphere measured accelerations along three orthogonal axes. The resultant acceleration was used to calculate sphere velocities and displacements. This allowed the penetration resistance acting on the sphere to be expressed in terms of a single capacity factor that captures soil resistance from both shearing and drag, and varies uniquely with the non-Newtonian Reynolds number. Undrained shear strength profiles obtained from a simple inverse analysis of the acceleration data show good agreement with those obtained using conventional push-in penetrometer tests.

Estimation of soil strength in fine-grained soils by instrumented free-fall sphere tests

“…6(b)). The operational strength, s uÀop , which is reflected in both the vertical and horizontal axes of Figs 6(a) and 6(b), was calculated using β = 0•07, consistent with interpretation of parallel centrifuge tests in three clays (Morton et al, 2016), and close to the lower end of β = 0•05-0•17 in Jeong et al (2009) and β = 0•05-0•15 in O'Loughlin et al (2013). β = 0•07 approximates to an 18% increase in undrained shear strength for every log cycle increase in shear strain rate, and is within the 10-20% range commonly reported (e.g.…”