ABSTRACT ARSTRACT
simple dynamic calculations for internally loaded tunnels,
outline of proof-of-concept tests of proposed reinforcement,
and initial dynamic testing of reinforcement components in an underground high explosive (HE) test.
The concept of a reusable underground nuclear test facility, named HEDEF, has been under study for some time. This rock chamber would have to be engineered to sustain the repeated mechanical and thermal shocks of internal explosions. In this paper, we summarize preconceptual studies performed in support of HEDEF, including INTRODUCTION
The HEDEF Concept
The feasibility and desirability of constructing a High-Energy Density Experimental Facility (HEDEF) have been under consideration since 1981. The HEDEF would provide a capability for conducting nuclear tests with yields up to 300 tons, at intervals of a week or so. The tests would take place inside an underground chamber in rock, which would have to be engineered to sustain the repeated internal explosions. Adjacent galleries and shafts also would be included in the geological engineering design. A number of preconceptual studies have been performed to date. They cover the general concept of the facility [1], the response of the primary chamber which must contain the events [2-4], the mitigation of the mechanical and thermal shocks [5], matters related to decontamination of the facility [6,7], and design cost estimates [8]. These studies have indicated that the HEDEF can be built with current technology, but that numerous questions must be resolved regarding the actual engineering of the facility, and its cost. One design concept calls for a large, steel lined chamber sited in hard rock [9]. Externally serviceable reinforcement, i.e., rock bolts or rock tendons [10], would tie a steel lining to peripheral galleries surrounding the test chamber, which presumably would not be accessible for manned re-entry on a routine basis. For a yield of up to 300 tons and an equilibrium pressure of about 7 MPa the volume of the main HEDEF cavity would be about 35,000 m .
Blast Loading of Rock Structures
Even though the heart of the HEDEF would be a chamber subjected to repeated internal explosions, the adjacent rock structures would feel these explosions as external loading. Hence it was appropriate to consider both external and internal loading on rock chambers, as part of this study. Most of the information concerning the behavior of underground rock structures under blast loading has been obtained under external pulses; i.e., the structures being observed did not contain the explosive. Nuclear events for structural evaluations have taken place in two rock media: granite and tuff [11-15]. Detailed information was obtained both on models and on full size tunnels. Many different structures have been evaluated for the effectiveness of various types of reinforcements, structural liners, and shock mitigating schemes. In general, concrete and reinforced concrete did not perform well, rock bolts were effective in providing significant reinforcement in jointed rock, steel liners without backpacking could suffer serious damage from discrete block motion, and steel sections backpacked with cellular concrete usually performed very well. Design procedures and design charts have evolved from work concurrent with the above testing. They are available to guide the initial planning of externally loaded HEDEF structures [13-17].