Simplified analysis of curvilinear penetration of free fall sphere projectile in soft soil

Abstract

Free fall projectiles have been used in offshore engineering to determine in-situ undrained shear strength profiles of near-surface soft soils from accelerations recorded during penetrating. In doing so, however, only the equation of motion in the vertical direction has been formulated with the vertical acceleration by assuming vertical impact and penetration. In this study, the governing equations of curvilinear penetration of a sphere projectile are established with consideration of an angle of impact with soil surface. By treating the penetration process as an initial value problem, an algorithm is proposed for computing the trajectory, velocity and linear acceleration of penetration. The effects of impact angle, impact velocity and soil strength non-homogeneity on the penetration behavior are analyzed using a set of parameters reported in literature. The computed results of the final embedment depth and linear acceleration of sphere are shown to be in reasonably good agreement with published field data. Moreover, a new explicit equation is derived for determining the undrained shear strength of soil based on both vertical and horizontal linear accelerations. The accuracy of the undrained shear strength estimated by assuming vertical penetration is assessed. The impact angle is shown to have important influence on soil strength determination.