Characteristic features of the seismic effect of massive blasting in highly stressed block masses

Abstract

It is demonstrated within the framework of a new scientific trend "Geomechanical reaction of geomaterials to powerful dynamic effects" currently being developed at the Institute of Geosphere Dynamics, Russian Academy of Sciences, and the Mining Institute, Siberian Branch, Russian Academy of Sciences that actual rock masses are intricately constructed media, which are broken by surfaces and regions of weakness into individual structural components of a different level, while discreteness and inhomogeneity are imprescriptible properties of the geophysical medium [1]. The reaction of a block medium to powerful tecnogenic explosions differs significantly from that of a continuum and is manifested in the particular response pattern of structural media to external dynamic effects. The conclusion concerning the phenomenon of an alternating-sign reaction of rock to dynamic effects, which consists in the fact that changes in various signs between the blocks of rock in the structural hierarchy of masses occur during the formation of cavities within the rock mass as a result of powerful blasts in their vicinities, is drawn as a result of experimental investigations conducted by Kurlenya and Oparin [2]. Their oscillatory motion with respect to one another is governed by translational and rotational movements of the different levels of rock blocks, depending on the dimensions of the cavities that are formed, the mine pressure, and the blast energy. According to this phenomenon, a large part of the blast energy is expended not only in crushing the rock mass in the epicentral zone and its immediate vicinity, but is also largely transmitted in the form kinetic energy to structural blocks of geomaterials of a different hierarchical level; this is manifested as deformation of the medium due to relative slippage of the blocks, their rotation, and translational motion, tn that case, the existence of pendulum-type waves is observed experimentally in block models [3]. Their formation is explained by the transformation of the initial seismic pulse with distance from subblock to subblock in a wave with a line spectrum due to the addition of the resonant frequencies of each subblock. The results obtained undoubtedly deserve mention, because the variation in the pattern of oscillations may exert a significant influence on both the "remote action" of the blast, and also the response of the structural medium to blast effects. Experimental observations made under field conditions made it possible to continue this research as applies to production conditions of short-delay massive explosions. The experiments were conducted at the +252-m horizon of the Kirovsk Mine operated by the Production Union "Apatit," during which observations embraced sections beneath the hanging wall of the deposit, in the block pillar, and in the footwall. Data were recorded and processed by an DS-477 seismograph manufactured by the Canadian firm Instantel Inc. in the frequency range from 0 to 250 Hz. Massive explosions with the same drilling-blasting parameters and delay intervals (t = 23 msec), but with different total amounts of explosive were recorded. Figure la-d presents the most characteristic recording of the oscillations of internal points in the mass, which are represented in the form of plots of the time dependence of displacement rate and spectra of the rate of displacement due to blasts set off in the footwall of the deposit (Fig. la), beneath the hanging wall in strips cut out from the block pillar (Fig. tb and c), and in the footwall in the immediate vicinity of a clearly expressed tectonic fault (Fig. Id). Comparing the results of observations of parameters of the seismic waves generated from these blasts, it is possible to visualize their significant difference. The blasts in the footwall (see Fig. la) therefore generate oscillations in all three components, which are close to sinusoidal, and the basic part of the energy is transferred through the x and z components at a frequency of --- 23 Hz. The spectrum of oscillations differs somewhat with respect to the y component (perpendicular to the break