Estimating consolidation parameters from field piezoball tests

The piezoball, a ball penetrometer featuring pore pressure measurements, is a relatively new device that is potentially superior to the more commonly used piezocone for profiling fine-grained soils. This is due to lower uncertainty in how to derive soil strength from the net penetration resistance and the option of measuring consolidation characteristics during pauses in the penetration, potentially more quickly than in a piezocone test. This paper presents results from a series of piezoball tests undertaken at a soft clay test site using a piezoball that measures pore pressure concurrently at the ball equator, tip and half-way between the tip and equator, the so-called mid-face position. Analysis of the test data provides strong arguments for measuring pore pressure at both the equator and mid-face positions. The coefficient of consolidation derived from piezoball dissipation data using recently developed numerical solutions is shown to be highly comparable to that deduced from a piezocone dissipation test. This paper shows that the penetration resistance varies significantly with the rate of penetration due to either viscous rate effects or increasing degrees of partial consolidation during penetration, both of which influence the estimation of undrained shear strength and hamper interpretation of dissipation data. Guidance on assessing the drainage response during a piezoball penetration test is provided. Finally, dissipation test data presented in the paper are added to a database formed from centrifuge and field tests that is used to form a new empirical method for estimating the coefficient of consolidation.