Estimation of undrained shear strength in rate-dependent and strain-softening surficial marine clay using ball penetrometer

The spherical penetrometer (known as ball) has been widely used in laboratory tests and in-situ investigation to determine the undrained shear strength of fine-grained sediments in offshore geotechnical engineering. The conventional interpretation of the ball penetration tests suffers from either neglecting the effect of the strain softening and strain rate for steady state penetration or inaccurate accounting for the evolving bearing factor during the shallow failure stage of soil. In this study, the continuous penetration of the ball is simulated using the “Remeshing and Interpolation Technique with Small Strain” (RITSS) with a strain softening and rate-dependent soil model. Three stages from shallow failure mechanism to deep flow-round mechanism are clarified. The strength distribution of ball penetration into softening and rate-dependent soil is also revealed. A parametric study is performed to investigate the effect of strain rate and softening parameters on the failure mechanism and the evolved bearing factor profile. The effects of the dimensionless soil strength sum /γ′D and the frictional condition of the soil-ball interface are explored. Empirical formulas are developed to estimate the critical depths of the ball penetration test. A practical interpretation framework, incorporating the effects of strain rate and softening, is developed to assess the soil undrained shear strength using the ball penetrometer.