Distant Plain Events 6 and 1A, Project 3.02A, Earth Motion and Stress Measurements

Abstract : The objectives of this study were to measure and analyze earth motions produced by Events 6 and 1A of Operation Distant Plain. Event 6 was a 100-ton TNT surface-tangent sphere, and Event 1A a 20-ton TNT sphere detonated at a height of burst of 29.5 feet. Acceleration, particle velocity, and soil stress gages were installed for both events to measure ground motion. Time histories obtained by this instrumentation are included in Appendixes A and B. For Event 6, ground shock arrival times indicated development of outrunning ground motion at about the 30-psi level. Shock propagation velocities were calculated to be 1,100 ft/sec above the water table and 5,500 ft/sec below it. The ratio of acceleration to overpressure for the 1.5-foot depth for both events was about 0.4 g/psi at the 1,000-psi level, and increased to 1.0 g/psi at the 20-psi level. At depths of 5 and 10 feet, accelerations averaged 20 and 7.5 percent of the shallow data, respectively. At the 1.5-foot depth the ratio of velocity to overpressure varied from 0.03 ft/sec/psi at 1,600 psi to 0.08 ft/sec/psi at 10 psi. Velocities attenuated less sharply with depth than did accelerations. Velocities at the 5-foot depth were about 40 percent of those at 1.5 feet.