The thirty years old history of underground nuclear tests gives a possibility to accumulate the sufficient amount of experimental data from instrumental and visual observations, which one can use to understand the influence of rock structure on excavation's stability. The analysis of results, received from investigations of the deformation and destruction processes in the close-in zone of underground nuclear explosions, is presented. It was shown, that the principle mechanism of failure of underground openings over important for practice dynamic loading amplitude range is the roof and wall breakage due to the fall of key blocks. The rock massif properties change exclusively due to relative displacements of individual rock blocks. Some empirical relations for estimations of failure extent were derived.
Most of the criteria of underground excavation stability applied to rock mechanics is based on the analysis of static and dynamic stresses acting in vicinity of opening as well as on comparison of stress level with shear, tensile and compressive strength limits of rock material. It should be readily apparent that the exceeding of the strength limits actually results in failure of material near the opening, however, it does not necessary bring to the large destruction of the excavation. The indispensable condition of excavation failure (here we ignore the falling of separate small blocks and thin rock layers) is apparently to be the achievement of some level of strain at which the significant volume of rock near the opening loses the stability or, in other words, become movable. In the rock massif divided into the blocks by the joints and faults of different scales the problem of providing the underground dynamic excavation stability cannot be properly solved on the basis of the traditional solution of mechanics of continua. Any engineering methods of providing stability and usually designed for averaged spaced stress may appear to be useless due to very large values of the discrete local rock loads acting on opening support even if the averaged stress level is relatively low. It is especially likely to be in the case of large - scaled dynamic loadings where besides the constant gravity acting on the massif there occur additional body forces - mertial forces - which are able to initiate the loss of rock block stability. The deformations which are originated in this case and are localised in the soft interblock gaps, i.e. faults, joints, and fractured zones can attain very considerable values. Several mechanisms of excavation destruction should be noted (here we confine our consideration by a sufficiently long - period wave so that we could avoid the spalling effect).
The underground nuclear explosions are considered to be an ideal tool for the investigations of rock deformation properties. A large scale of loadings, a wide range of amplitude and a lot of instrumental and visual observations which were carried out during many years of tests, make it possible to analyse the rock massif structure effect on the process of rock deformation and the destruction of rock.
