A Study of the Propagation of Stress Waves in Sand

The propagation of near-failure, soft-fronted stress waves in 5-foot-long, 1-inchdiameter, unconstrained rods of dry 20-30 Ottawa sand is studied. The input is created by a shock tube, modified to yield soft-fronted, long-duration, airpressure loadings. Primary data are displacement-time histories from seven stations along the rod, read by light-sensing gages designed for negligible inertia and friction; secondary data are stress-time histories at the input and the rigidly held reaction ends. Static, dynamic, and additional static triaxial data are given. Variables are applied ambient stress and sand density. From findings that the material is strain-rate insensitive and follows Coulomb's failure law at subfailure stresses, the following observations of wave-propagation parameters for above-seismic stresses and a specific material condition have been made: (1) wave and particle velocities depend on the one-half power of ambient stress; (2) all energy densities depend on the first power of ambient stress; and (3) energy partitioning and reflection are independent of ambient stress. Effects of density on wave-propagation parameters for the sand and the variables studied are that (1) all increase with increasing density, (2) some are specifically linear on density, and all can be taken as such for engineering purposes, and (3) waveform parameters are much less sensitive to small errors in density than are static parameters. Also shown: (1) reflections of stress from rigid boundaries can be estimated for nonlinear materials; and (2) correlation between seismic velocity and triaxial strength exists, provided there is a power-function relationship between seismic velocity and ambient stress and the material follows Coulomb's law.