One‐Dimensional Loading‐Rate Effects

It has long been recognized that the one‐dimensional or uniaxial strain response of most soils subjected to high‐intensity transient loads (i.e., blast pulses) differs from the response measured under quasi‐static loading rates. Recent research has suggested that for submillisecond rise times, increases up to 10‐fold in the loading constrained modulus occur for some remolded partially saturated granular soils under undrained conditions. Parallel research has shown that, in contrast, loading‐rate effects can be ignored for a similar granular material tested under nearly identical boundary conditions. Stress‐strain curves from 60 uniaxial strain tests are summarized and presented herein depicting the behavior of three soils (two clean sands and a silty clay) to a variety of loading rates. Loadings are typically carried to 10,000 psi (69 MPa) with times to peak ranging from a few tenths of a millisecond to several minutes. These laboratory test results show that a dramatic increase in the loading constrained modulus does not occur for the rise times examined. Rather, a gradual stiffening occurs as the time to peak pressure decreases. The maximum ratio of the dynamic‐to‐static loading constrained modulus is observed to be about a factor of two for the soils tested. Based upon these test results, a strain‐rate and strain‐level dependent modulus stiffening model is developed. This model is implemented into a one‐dimensional plane wave propagation computer code to predict the results of field tests that are performed using two of the three soils tested in this study. A comparison between the laboratory‐based model predicted behavior and response obtained from the field tests is favorable.