Deepwater soil investigation using a free fall penetrometer

Abstract

Free fall penetrometers (FFP) can improve the efficiency of deepwater geotechnical site investigations. This study investigates the derivation of geotechnical properties from piezocone FFP measurements in the upper 4 m of the seabed at a clayey site with water depths of ∼1500 m. Data analysis methods as commonly applied in cone penetration test (CPT) analysis were tested with strain rate normalization methods previously suggested for FFP analysis. The logarithmic, inverse hyperbolic sine, and power law approaches were applied for strain rate normalization. Strain rate effects appeared small for FFP cone resistance (with μ = 0.1 for the logarithmic and β = 0.06 for the power law approach), but were apparent. Undrained shear strength derived from FFP data matched overall the CPT results, using an N factor Nkt = 12. Undrained shear strength derived from excess pore pressure measured at the u1 position also yielded favorable results using N∆u _FFP = 6. Two soil behavior type classifications showed acceptable agreement between strain rate normalized FFP and CPT results.