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The need to diminish the seismic effect of blasting on nearby structures frequently arises in performing blasting operations.One method of reducing dynamic stresses and strains due to the action of seismic-blast waves may be the use of slitlike shields so that the object being protected is in the zone of the slit's geometric shadow.To evaluate the expediency and effectiveness of these shields, it is necessary to investigate the laws governing the formation aid distribution of wave fields in a mass beyond the slit and the influence exerted by the geometric and acoustic parameters of the shield on the characteristics of the stress--strain state of the zone of the geometric shadow.The present study is devoted to an experimental model investigation of the distribution of dynamic stresses beyond a free shielding slit as a longitudinal wave from a blast source strikes the shield.The studies were conducted by the method of photoelasticity on a dynamic polarization apparatus in the stress-study laboratory at the Moscow V. V. Kuibyshev Civil Engineering Institute [i].The problem was solved in the plane elastic statement on models in the form of plates made of an optically sensitive epoxy-resin-base material.The pulse effect was created by the detonation of a cylindrical lead azide microcharge.The dynamic stressed state at internal points of the model was evaluated from the magnitudes and distribution o~ maximum tangential stresses, and on the free perimeter of the model from the magnitudes and distribution of the normal stress parallel to the perimeter.We investigated two basic cases: i) positioning of the slit in an infinite plate; and 2) the zone being shielded beyond the slit as a free surface along which variation in dynamic stresses is considered.The distance from the source to the shield 1 = (2-4.5)~I, where %~ is the length of the first phase of the incident pulse; the front of the incident wave is parallel to the plane of the shield.One half of the length of the slit was adopted as the characteristic dimension a of a slit in positioning the shield in the infinite medium, while its height from the surface to the head was adopted in the presence of a free surface beyond the shield.The dimension a was caried within the limits a = (0.2-0.5)~i.The incident pulse is characterized by the following parameters the ratio of the length of the segment of stress accumulated to the total length of the first phase %a/l~ ~ 0.6; the ratio of amplitudes in the compression and tension phases Tmax. Tmax-9 + = 2.5:1.When the cylindrical longitudinal wave interacts with a single free slit in an infinite medium, the stress field in the zone of the geometric shadow is determined by the diffracted longitudinal Pg and transverse Sg waves that propagate from the ends of the slit. The characteristic pattern of the wave field of stresses beyond the slit can be observed from the motion picture frames (Fig. i).Comparison of wave-field patterns in models beyond slits of different lengths and a similar pattern in a homogeneous model indicates tha...

At great distances from rock blasting various types of seismic stress waves propagate: longitudinal P waves, transverse S waves, surface Rayleigh R waves, etc. Near exposures or holes in the rock a complex pattern of dynamic stress arises owing to reflection of the waves from the free surface and their diffraction and interference.Even in the elastic formulation, theoretical investigations of these wave patterns are very complicated, and at present we have solutions only for infinite regions and simply shaped (e.g., circular) obstacles.A very effective experimental method for investigating problems concerning the propagation and interaction of stress waves in the remote zone of a blast is the method of dynamic photoelas~icity [i, 2], which is based on the use of optical polarization to investigate the stresses with the aid of hlgh-speed klnematography.At the Problem Laboratory of Photoelasticity of the V. V. Kuibyshev Moscow Civil Engineering Institute we have used this method to investigate a wide range of wave problems [3][4][5] involving the concentration of dynamic stresses near free and supported holes of various shapes (tunnels, underground cavities, and extraction rooms), the interaction of seismic blast waves with exposed surfaces of ledgerock excavations, and the propagation of waves in stratified media (screening interlayers of rock crushed by blasting).The dynamlc-photoelasticity method is very effective when the stress-wave fields are distributed near the free surface during the blasts.From the viewpoint of the dyD~m~c stresses, the explosion of charges at shallow depths has a number of special features.It is known that if the charge is set at a considerable depth the explosion can be interpreted with fair accuracy as the action of a source of the expansion-center type, for which the wave fields can be dehermined in the depth of the rock and at the free surface [6, 7]. On the other hand, a surface blast creates in the remote zone a wave field identical to that due to a different type of source --an impulsive point force normal to the surface [4]. If the charge is close to the free surface, the character of the action will obviously correspond to an intermediate type of source in which the ratio between the vertical and horizontal components of the impulsive load will vary according to the depth of the charge.In determining the seismic effect of shallow-burled charges, when the distance to the investigated region is much greater than the depth of burial of the charge, we must also take account of the fact that waves reflected from the free surface soon (in a short time compared with the time of propagation of the waves from the source to the zone of observation) undargo a secondary reflection from the source; this process is repeated many times, and the reflected waves, interfering with the direct waves from the source, complicate the resulting wave pattern.The difficulty of making a quantitative estimate of the influence of these factors means that it is better to make an experimental determination of...

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