Spalling of the Earth's surface by underground nuclear explosions

Spalling of the near-surface earth layers caused by contained underground shots detonated in tuff, granite, halite, and alluvium was investigated. From the arrival-time data obtained from surface accelerometers, certain aspects of the kinematics of spalling were deduced by means of simple ballistic and plastic-diaphragm concepts. It was shown that the shape of the spall gap is conical with sides concave upward (the shape of a coolie hat) and not a dome as is commonly supposed. From the arrival-time data it was possible to estimate the radial extent of the spall and to compute the thickness of the spailed layer at ground zero. The maximum width of the spall gap at ground zero was smaller than the surface displacement at that point, indicating the existence of additional spall gaps at other depths and perhaps fluffing of the surface formation under the action of spalling forces.