ABSTRACT
A study of ground vibrations in pink westerly granite specimens 12x12x6 in. (30.5 x 30.5 x 15.2 cm) resulting from explosive charges is described. The method of holographic interferometry was employed to obtain full-field pictures of the surface motion during propagation of the elastic waves. Explosive charges varying from 100 to 200 mg of PETN were detonated in the center of the large face and at several depths below the free surface. Through the use of a pulsed ruby laser, holograms depicting the ground motion were taken and the vertical, radial and transverse components of displacement were determined. This was done for several times after the detonation of the explosive. Identification of the primary (dilatational, P) secondary (distortional, S) and Rayleigh waves was made and their wave, speeds determined to be 13,120 ft/ sec (4.0 km/sec), 8,725 ft/sec (2.66 km/sec) and 7,800 ft/sec (2.38 km/sec) respectively. It was found that when the explosive is detonated on the surface the Rayleigh wave predominates. When the explosive was moved below the surface the P wave is more important in the region of r/h<1 where r is the radial distance on the surface and h is the depth of the source. In the region 1<r/h<2.5 the P and Rayleigh waves have about equal effects, with the effects occurring at different times. For r/h > 2.5 the Rayleigh wave is once again the dominating influence. The S wave system was weak due to the fact that no fragmentation of the models took place. Thus influences from the S wave were minor. The greatest amplitudes in all tests occurred in the radial and vertical directions.
INTRODUCTION
This work is concerned with a model study of explosively generated elastic waves in a half space of rock. In the last ten years, new model methods have been developed that will permit an improved visualization of the ground motions from a blast in actual three dimensional materials. These techniques yield full field data on the three orthogonal components of displacements at selected times after the blast. With this information one can better appreciate the relative importance of arriving wave systems, and how they interact with fissures, voids and flaws. It also is possible to examine 'the importance of three dimensional geometry changes on the propagation of the elastic disturbances, or how the wave systems are propagated through dissimilar materials. It may then be possible to develop improved scaling relationships, and to examine schemes for reducing the ground motions and for isolating structures. Many attempts have been made to quantify the propagation and interaction of elastic waves. Woods (1967) for example, examined the role of trenches in isolating a building from vibration. Dally and Lewis (1968) studied the effects of a slit on a propagating Rayleigh wave through the use of dynamic photoelasticity. Gupta and Kisslinger (1964) observed the surface displacements in an explosively generated Rayleigh wave, by modeling a half space with a plane and employing capacitor probes to measure the vertical and radial displacements.
