Numerical Analysis of FFP Impact on Saturated Loose Sand

• Main Author: Yalcin, Fuat Furkan
• Other Authors: Civil and Environmental Engineering
• Format: Others
• Published: Virginia Tech 2021
• Subjects: Free-Fall Penetrometer; Numerical Model; Material Point Method; High Strain Rate Loading
• Online Access: http://hdl.handle.net/10919/106512

Free-Fall Penetrometer (FFP) testing is an easy and rapid test procedure for seabed sediment characterization favorable to conventional geotechnical testing mainly due to its cost-effectiveness. Yet, FFP testing results are interpreted using empirical correlations, but difficulties arise to understand soil behavior under the high-strain rate (HSR) loading effects during rapid FFP penetration. The numerical simulation of FFP-soil interaction is also challenging. This study aims to numerically analyze FFP testing of saturated loose sands using the particle-based Material Point Method (MPM). The numerical analysis was conducted by simulating calibration chamber FFP tests on saturated loose quartz sand. The numerical results using quasi-static properties resulted in a reaction of the sand softer than the actual calibration chamber test. This implied the necessity of considering HSR effects. After performing parametric analyses, it was concluded that dilation plays an important role in the response of sand-water mixtures. Comparison of dry and saturated simulations showed that FFP penetration increases when the soil is dry and tends to develop a general bearing capacity failure mechanism. This is because the pore water increases the stiffness of the system and due to the increased strength that develops in saturated dilative sands when negative pore pressures develop. Local bearing failure mechanism is observed in all saturated simulations. Finally, numerical CPT (quasi-static) and FFP tests were used to examine the strain rate coefficient used in practice (K); and a consistent range between 1 to 1.5 was obtained. === Master of Science === Accurate characterization of seabed sediments is crucial to understand sediment mobilization processes and to solve nearshore engineering problems such as scouring around offshore structures. Its portability, low testing effort, and repeatability make FreeFall Penetrometer (FFP) testing a highly cost-effective sediment characterization test. Nevertheless, due to the complex penetration mechanism of FFPs in soils (e.g., high-strain rate effects due to rapid FFP loading), converting FFP output into practical information is complicated, and it heavily relies on empirical correlations. This thesis presents a numerical analysis of FFP testing on saturated sand using the Material Point Method. First, the simulation results were compared with laboratory tests. Later, a parametric study was performed to understand the effect of different material parameters on the FFP response and to highlight in a simplified manner the effects of rapid loading on the sand behavior. Additional simulations in dry sand (without water) revealed that dry conditions provide larger FFP penetrations than saturated ones for the same material parameters. Lastly, the strain rate coefficient, which is a parameter required in one of the most common empirical methods for converting FFP output into geotechnical parameters, was back-calculated. The results were consistent with values used in practice for similar conditions.