Effect of shaft on resistance of a ball penetrometer

The influence of the shaft on the resistance of a ball penetrometer was examined using a large deformation finite-element approach. A range of ball–shaft diameter ratios between infinitely large (no shaft) to 1 were examined in terms of resistance during monotonic penetration and extraction. It was found that the steady-state resistance decreased with decreasing ball–shaft diameter ratio, and that greater displacement (as a proportion of the shaft diameter) was required to reach a steady state. The finite-element results were matched closely using an upper bound approach, treating the shaft and outer part of the ball separately. It was also found that the rigidity index of the soil affected the penetration resistance of the shafted ball, but the influence was much less than for a cone penetrometer, being essentially proportional to the shaft–ball area ratio. Finally cyclic penetration and extraction tests were modelled comparing results for the no-shaft ball and a shafted ball with ball–shaft diameter ratios of 2 and 3. The presence of the shaft was found to have minimal influence on the resistance and cyclic degradation curves, although at the later stage of cycling the penetration resistance becomes greater than the extraction resistance. The corresponding resistance ratio of extraction to penetration for each cycle reduces with decreasing ball–shaft diameter ratio and increasing soil sensitivity. In terms of the change in mean total stress, however, the presence of the shaft introduces a pronounced gradient with respect to displacement during cyclic penetration and extraction, while increasing the changes in mean stress significantly during initial penetration.