Evaluation of a logarithmic-law strength rate parameter using full-flow penetrometers

The relation between the strength law rate parameter and the observed rate effect from full-flow penetrometer soundings performed at different penetration velocities is discussed and examined herein through an analytical investigation. It is shown, using numerical results of steady-state (continuous) penetrations, that the shear strength and global penetration resistance follow a similar law in which the shear strength and penetration resistance increase linearly with the logarithm of shear strain and penetration velocity, respectively. However, the strain rate parameter that is associated with the in situ penetration is found to be different from the conventional laboratory-based strain rate factor when considering the logarithmic relation between strength and rate. Consequently, previous suggestions to estimate the laboratory-based strength rate parameter as equal to the in situ rate parameter can only be considered a first-order approximation. Analytical examination of the energy terms involved in the plasticity solution, under the relaxing assumption of a constant plastic flow field, has led to the development of a simplified expression that resulted in an in-situ-based rate parameter of µ/(1 + 5µ) (where µ is the conventional laboratory-based strength rate parameter). Comparison of the numerically derived values with the simplified analytical expression shows good agreement, suggesting that this relation may help establish the soil rate strength parameter from full-flow penetration tests.