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It has long been suggested that the one-dimensional or uniaxial strain response of most soils subjected to high-intensity transient loads differs from the response measured under static conditions. As the time to peak pressure decreases, most soils exhibit a stiffening of the loading stress-strain response. That stiffening is usually referred to as a time or loading rate effect. Some researchers have suggested that, as the time to peak pressure approaches the submillisecond range, a drastic increase (up to tenfold) in the loading constrained modulus occurs for partially saturated granular soils under unconsolidated-undrained conditions. The existence of this effect has been the subject of debate.
A test device was developed at the U.S. Army Engineer Waterways Experiment Station using explosives to obtain submillisecond loading times to peak pressures of approximately 138 MPa (20 000 psi). By using state-of-the-art measurement and acquisition systems, an accurate measurement of the uniaxial stress-strain response to submillisecond loadings was obtained. This paper presents the details of that device and the associated electronic measurement and acquisition systems, testing techniques, data interpretation, and typical test results used in assessing loading rate effects of a partially saturated carbonate sand. Results showed that a drastic stiffening did not occur for this material as the time to peak pressure approached the submillisecond range. Instead, a gradual stiffening occurred and the basic shape of the stress-strain curve was maintained.
Research civil engineer, Structures Laboratory, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS
Professor of civil engineering, University of Michigan, Ann Arbor, MI
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