Use of dynamic photoelasticity to investigate seismic stress fields from blasting

Abstract

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...