Book Description

Three large rock-bolted underground cavities were constructed at the Nevada Test Site. Two of the cavities, approximately 100 feet in diameter and 140 feet high, were constructed at a depth of 1300 feet in a very weak tuff of excellent rock mass quality. The third cavity, approximately 60 feet in diameter and 80 feet high, was constructed at a depth of 350 feet in a jointed granite of high intact strength and fair to good rock mass quality. The stability of the cavities was monitored throughout construction by measuring rock displacements, observing fractures in near-surface rock, and observing the behavior of the rock-bolt support system. Supporting field and laboratory tests were performed in order to evaluate intact and in-situ properties of the rock mass. Radial movement of the cavities was measured using extensometers anchored at various depths in holes drilled from the cavity surface. Displacement versus depth profiles were used to determine the depth at which rock movement was concentrated. By comparing rock movement with excavation and support progress, a continual estimate of the cavity stability was obtained. Large displacements, or large rates of displacement, were indicative of potentially unstable behavior. Displacements were compared with displacements predicted from elastic theory, using a finite element solution and simple closed solutions. (Author).