Numerical investigation of dynamic free-fall penetrometers in soft cohesive marine sediments using a finite difference approach

Rapid assessments of undrained shear strength of marine sediments are often of great interest in various branches of offshore civil and petroleum engineering, as well as the navy. Dynamic free-fall penetrometers offer a potential for such rapid strength assessment, but have typically been employed in conjunction with mostly empirical relationships to investigate the undrained shear strengths of sediments. In this work, we analyze the behavior of STING (sea terminal impact naval gauge) using the deceleration record of sediment impact. We modify an existing numerical technique for extraction of the undrained shear strength from the deceleration history of the probe. We utilize a finite-difference code for the analysis of continua (FLAC3D) for solution of the penetrometer impact and penetration into the heterogeneous marine sediments. The numerical solutions are described in detail and address many aspects of modeling penetration resistance in cohesive materials. Analysis was performed into a number of numerical aspects of attaining the solution in FLAC3D, including the indeterminate contact geometry after discretization, and incompressibility of the deforming material and the influence of the Poisson´s ratio on the solution. Undrained deformation of the water saturated cohesive sediments is characterized by nearly incompressible response, leading to some difficulties in achieving accurate solutions. These effects were investigated and stable solutions were attained. Numerical predictions thus accomplished were compared with the values of the undrained shear strength, as determined in standard laboratory tests on specimens recovered from several locations in the Gulf of Mexico and showed good accuracy and large improvement over the original STING processing algorithm.