Numerical investigation of dynamic penetration factors for a free falling penetrometer

Free falling penetrometers (FFPs) are being used increasingly to determine the geotechnical properties of the soil at sites where the static cone penetration test (CPT) cannot be conducted easily, if at all, due to inaccessibility. Examples include many seabed sites, particularly in deep water. FFPs can provide the total time and depth of penetration as well as the deceleration characteristic of the penetrometer, which can then be used to infer some fundamental soil properties. This study employs a validated finite element technique based on the Arbitrary Lagrangian-Eulerian method to investigate numerically the dynamic penetration factor, N_dp for a free falling penetrometer, and to find the relation between this factor and the geotechnical properties of a layer of cohesive soil. It is demonstrated that the dynamic soil resistance can be obtained as the product of its undrained shear strength and the dynamic cone penetration factor, and the latter depends on several parameters such as the rigidity index of the soil, the rate of loading and the rate of shear strength increase with depth of the soil. A wide range of numerical predictions is discussed here and a closed-form expression is derived for the dynamic penetration factor. Comparisons are also made between the deduced values of N_dp and published values of the conventional cone factor, N_c.