A comparative field study of free-fall cone and sphere penetrometers in soft sediment

This paper describes the field performance of two instrumented free-fall penetrometers (FFPs), which offer the possibility of low cost rapid strength profiling without requiring the equipment associated with conventional CPTs or drilling and sampling. The devices considered are a free-fall cone penetrometer (FFcP) and an instrumented free-fall sphere (IFFS). The performance of the FFPs in assessing soil undrained shear strength was investigated and compared through field trials in the soft sediments of the Swan River, Perth, Western Australia. The penetrometers achieved impact velocities of 1.4–5.8 m/s from drop heights of 0.7–3.0 m above the riverbed, and embedded to depths of up to 1.6 times the FFP length (1.45 m) and 4.5 times the IFFS diameter (0.89 m). The FFP data provided the shear strength indirectly using the acceleration measured from an onboard inertial measurement unit. The accuracy depends on the assumptions made regarding the magnitude of dynamic bearing and shaft frictional resistance. Uncertainty associated with dynamic bearing resistance causes a ±10% and ±13% variation in the calculated strength for the IFFS and the FFcP, respectively. However, the equivalent uncertainty in dynamic shaft resistance can cause a variation in the calculated strength of up to ±76% for the FFcP. The FFcP has higher embedment potential than the IFFS, but the IFFS has the advantage of a simpler interpretation without needing to account for dynamic shaft resistance. •A free-fall cone penetrometer (FFcP) and an instrumented free-fall sphere (IFFS) were developed for offshore SI.•Their field performance in measuring the shear strength of soft sediments was compared.•The FFcP has higher embedment potential than the IFFS, but IFFS has a simpler interpretation.