Experimental observations from cyclic cone penetrometer testing in clay

Cone penetrometer tests (CPTs) involve a monotonic penetration and extraction phase, sometimes with pauses to measure the dissipation of excess pore pressure. This paper explores the merit of including cyclic episodes to measure the change in tip and sleeve resistance during cycling. Tests were carried out in a geotechnical centrifuge to explore how factors such as overconsolidation, cyclic amplitude and time affect the cyclic response of kaolin clay. The data indicate that the cone sleeve mobilises the remoulded undrained shear strength during the initial penetration, but that with continued cycling, the mobilised interface shear strength reduces to around one-third of the remoulded undrained shear strength before increasing. The initial reduction is considered to be due to consolidation around the cone sleeve, whereas the subsequent increase is attributed to consolidation-induced strength hardening. The timescales associated with these competing processes were seen to be a function of the cyclic amplitude, as this controlled the amount of shear strain accumulation in a given time period. The findings suggest that cyclic CPTs may be a convenient means of gathering data for input to geotechnical design of problems where the cyclic response at the clay–structure interface is of interest.