Soil strength characterisation using free-falling penetrometers

Free-falling penetrometers (FFPs) are rapid and cost-effective tools that can be used in seabed strength characterisation. To date, FFPs have not been widely used, because of the lack of reliable interpretation methods and well-controlled experimental studies. This paper presents an interpretation method for a thin-shafted FFP based on the results of extensive laboratory model FFP tests and triaxial compression tests on normally consolidated kaolin. In the model tests, a thin-shafted FFP with circular plate tip has been dropped into claybeds with a range of undrained shear strengths at various impact velocities. The FFP has also been pushed into the claybeds at slow constant-penetration rate to provide a comparison between the static and dynamic penetration resistances. Triaxial compression tests have been conducted to provide the critical state parameters, and to study the rate-dependence behaviour of the kaolin. Using these test results, an empirical model is proposed to estimate the undrained shear strength from a thin-shafted FFP test in clay. The applicability of the proposed model is compared with existing methods and numerical solutions.