Numerical study of piezoball dissipation test with penetration under partially drained conditions

Abstract

The piezoball, a full-flow penetrometer equipped with a pore pressure transducer, is increasingly being used in in-situ investigations due to its advantages of characterizing soft soils relative to the piezocone. The operative coefficient of consolidation, ch, can be estimated by monitoring the dissipation of excess pore pressures after piezoball penetration. However, the determination of ch from the dissipation test becomes complicated if the piezoball is penetrated under partially drained conditions. The performances of piezoball penetration and subsequent dissipation are investigated using a coupled large deformation finite-element approach. The numerical results are verified by comparing with existing centrifuge tests, against both undrained and partially drained conditions. A comprehensive interpretation method is then presented for inferring ch from piezoball tests against various drainage conditions. It is found that the variation of ch with drainage conditions is essentially dependent on the distribution of pore pressures before dissipation. Two backbone curves determining the nominal initial excess pore pressure at the equator of the probe and the coefficient of consolidation are obtained to assess the effect of partial drainage, which capture the influences of the piezoball geometry, stress level and over-consolidation ratio.