Project Pre-Gondola I was a series of chemical explosive, single-charge cratering experiments in weak, wet clay shale conducted by the U.S. Army Engineer Nuclear Cratering Group as a part of the Joint Atomic Energy Commission-Corps of Engineers nuclear excavation research program. The four 20-ton (nominal) charges were detonated during the period 25 October to 4 November 1966, near the edge of Fort Peck Reservoir approximately 18 miles south of Glasgow, Montana. Motions of the ground surface were measured by high-speed photography of surface ground zero. (Author).

Project Pre-Gondola I was a series of chemical explosive, single-charge cratering experiments in weak, wet clay shale conducted by the U.S. Army Engineer Nuclear Cratering Group as a part of the Joint Atomic Energy Commission-Corps of Engineers nuclear excavation research program. The four 20-ton (nominal) charges were detonated during the period 25 October to 4 November 1966, near the edge of Fort Peck Reservoir approximately 18 miles south of Glasgow, Montana. Motions of the ground surface were measured by high-speed photography of surface targets which had been positioned as far as 96 feet from each surface ground zero (SGZ). The results indicated that the maximum SGZ velocities recorded for these detonations were larger than those for cratering detonations in other media at comparable scaled burial depths. The four Pre-Gondola I detonations produced maximum SGZ velocities of 255, 200, 167, and 139 ft!sec for scaled burial depths of 152, 167, 187, and 202 ft/kt1/3, respectively. Maximum resultant surface velocities for the three shallowest detonations varied approximately as the -2.2 power of the radial distance from the charge. The maximum resultant surface velocities for the deepest detonation varied approximately as the -2.9 power of the radial distance from the charge.

The report presents the results of preshot and postshot geologic and engineering properties investigations performed in conjunction with the Project Pre-Gondola I cratering experiment series. The Pre-Gondola I series consisted of the detonation of four 20-ton HE charges at varying depths of burst in highly over-consolidated Bearpaw clay shale at Fort Peck, Montana. Detailed surface and subsurface postshot investigations were conducted at the Bravo crater. Surface postshot investigations only were conducted at the other crater sites. (Author).

The clouds resulting from four 20-ton nitromethane cratering explosions in a wet clay shale medium were studied by photographic analysis and lidar (laser-radar) tracking. A technique for detecting tracers in future events in the same medium was investigated. (Author).

Measurements of intermediate range ground motions and of structural response were made during the Pre-Gondola high explosive cratering experiments at Fort Peck, Montana. Liquid nitromethane charges (1000-lb and 20-ton), emplaced at various depths of burst, and a 140-ton row charge were detonated in the Bearpaw shale, which is a weak, wet clay-shale medium. An additional experiment to validate a charge emplacement concept designed to decouple seismic energy proved inconclusive. All seismic measurements were of particle velocity. Using an inverse power law equation to describe the attenuation of seismic amplitudes with distance, it is found that the amplitudes from the single charges decayed as approximately R( -2.4), and amplitudes from the row-charge decayed as R( -1.7). A dependence of amplitudes on depth of burst exists, and a yield scaling exponent near 0.8 appears to be appropriate. It is suggested that a variation of particle velocity with distance as R( -A)e( -kfR), where f is the signal frequency, is a more physically realistic description of attenuation than is the inverse power law. The preliminary data from the 140-ton row charge appear to fit this type of attenuation law, and indicate that A = 0.5 and k = 0.015 sec/km. An estimate is made of the near-source variation of peak seismic amplitude with frequency for the row charge, and predictions are made for possible future row-charge cratering experiments at the site.