Interpreting strength parameters in soft clays from a new free-fall penetrometer

Abstract Free-fall penetrometers (FFP) are time-efficient and cost-effective tools for estimation of seabed strength parameters. However, the accuracy of strength parameters are coupled affected by the strain-rate effect, the strain-softening effect and the hydrodynamic drag on the FFP. A modified free-fall ball penetrometer with a booster (FFBPB) was put forward in the present study for the convenience of measuring the strength of soils with deep embedment depth. Numerical analyses based on the computational fluid dynamics (CFD) approach were conducted in the present study to investigate the complex ball-soil interaction, from which the expressions of the strain-rate factor, drag coefficient, and added mass were established. Then a back-analyses framework was developed to estimate the strength parameters, including the original undrained shear strength, strain-rate parameter, and strain-softening parameters, based on the measurements from the FFBPB. Finally, a series of model tests were performed to validate the back-analyses framework. The back-analysed strength parameters are considerably consistent with that from quasi-static tests, indicating the reliability of the back-analyses framework.