The U. S. Geological Survey is investigating the mechanism of collapse of nuclear explosion cavities for its value in containment, with the hope of eventually determining those geologic environments most susceptible to early collapse or most adaptable to the incitation of collapse. This is a progress report on a part of these studies.
The investigations to date have been confined largely to the subsidence of the ground surface in the desert alluvium of Yucca Flat by collapse of volcanic rocks and (or) alluvium into the large cavities formed by nuclear explosions.
The sinks thus formed are as much as 60 m (meters) deep and 250 m in radius; they have forms that are commonly variations of inverted cones or sectors of spheres. The outer half of each sink is an inward slide of slightly disoriented but relatively undisturbed alluvium. The major physical disturbance within the slide mass is by subsidiary spoon-shaped cracks and slide surfaces along which individual blocks have adjusted within the glide mass. Each individual block moves a net distance that is small relative to the total movement of the main slide. The cracks or glide surfaces are assumed to flatten with depth within the slide and to join the major surface of failure at the base of the main slide mass. The spoon-shaped surfaces, where they intersect the top surface of the slide, dip steeply to vertically. In plan they make a pattern of curving, intersecting cracks.
The depth of burial of the explosion cavity in alluvium relative to its size affects the profile of the sinks, the height of chimneys for collapses not extending to the surface, and, to an extent, the likelihood that later surface subsidence will occur.
A working model of collapse includes: (a) maintenance of the explosion cavity until main collapse occurs; (b) a two-phase collapse taking place in several seconds or several tens of seconds, culminating in surface subsidence if the cavity is sufficiently close to the surface. The first phase of collapse in alluvium, at least, consumes 70-90 per cent of the collapse period; collapse propagates upward at rates of 50-80 ft/sec. The second phase shows faster rates of upward propagation, a lesser degree of particulation of the collapse material, and ends with the drop of a central mass of alluvial material that initiates formation of the sink. The shape, size, and distance of vertical drop of this alluvial plug control the size and form the resulting sink. Acceleration of this mass ranges from 0.35 to 0.8 g.