The Problem of Induced "Block Motion" Related to Deep Underground Strategic Facilities--A Summary; DNA 4078T Topical Report for Period June 1975--October 1976

The format is approximately 8 inches by 10.5 inches. 60 pages (per DD Form 1473). Figures/Maps. Footnotes. Tables. References. Bibliography. Ex-library with usual library markings. This report was prepared for the Director, Defense Nuclear Agency. Abstract: The block motion problem of explosion-induced differential displacement along geologic discontinuities must be better understood to adequately design and site underground strategic facilities that would survive wartime threats. The purpose of this investigation is to review pertinent literature, develop concepts addressing current understanding of explosion-induced block motions, and identify research that could quantify risks for particular sites. The block motion problem is concerned both with those motions driven or dominated by the explosion-induced stress transients and with those motions triggered by the explosions but dominated by the partial relief of pre-stress conditions. Emphasis is given to the problem of triggered motions because this is the aspect of greatest uncertainty and because the limited evidence is suggestive that differential slip can be triggered at considerable distance from the explosion, depending greatly upon in-situ stress conditions. Methods for avoiding, relieving, absorbing, and resisting explosion-induced block motions are addressed and it is concluded that quantifying the block motion problem appears difficult but not hopeless. The threat of block motion , defined to encompass all explosion induced differential displacement along geologic discontinuities , is an important consideration in siting and designing underground strategic facilities for maximum system survival. A Predictive capability does not presently exist that can satisfactorily assure avoidance of the block motion problem by site selection , relief from the problem by insitu stress alteration , or assure that the design of an underground facility can absorb a block motion of uncertain magnitude. The principal reason for this situation is that there is insufficient knowledge of insitu stress field conditions and failure criteria associated with faults and other possible weakness discontinuities that are the likely locations of block motion responses to explosively induced stress transients. It is clear that the focus of developing a predictive capability should be to set lower and upper bounds on the credible block motion displacements . The motions of concern are those directly induced by explosions interacting with 'imbalanced or excessive prestress conditions within the earth. The block motion problem also includes differential displacements induced by explosions in the rupture and spall regions of surface and near-surface bursts where induced stresses overwhelm either balanced or irnbalanced prestress conditions. Emphasis in this review has been directed at identifying the risk of explosions triggering block motions in prestressed media and examining ways of minimizing the problem. The triggering problem is of special interest because there is greater uncertainty of the risk and more distant displacements are credible for a given explosion threat than would be the case for conditions where stored tectonic stress is small. The major deficiency in assessing the risk of explosion triggered block motion is that the threats are surface or near-surface bursts , and all known triggered block motions are associated with contained explosions.…