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Blasting Impacts and Rock Slope Stability Assessment for Cottonwood Quarry in Grand County,CO <br /> 3.0 PHYSICAL CHARACTERISTICS OF BLAST EFFECTS <br /> Before proceeding to analyze potential impacts of blasting at the Cottonwood site,the following <br /> pages and subsections 3.1 through 3.5 set forth a review of the physical characteristics and forces <br /> involved in the detonation of explosive charges used in mining applications. When explosive <br /> charges detonate in rock, they are designed so that most of the energy is used in breaking and <br /> displacing the rock mass. However, some of the energy can also be released in the form of <br /> transient stress waves, which in tern cause temporary ground vibration. Detonating charges also <br /> create rock movement and release of high-pressure gas, which in turn induce air-overpressure <br /> (noise),airborne dust and audible blast noise. <br /> In the very-near zone, crashing usually occurs in the rock around the charge. The extent of this <br /> compressive and shear failure zone is usually limited to one or two charge radii. Beyond the <br /> plastic crushing zone, the rock or ground is temporarily deformed by elastic strain waves. For <br /> some distance, tangential strain intensity exceeds the rock's strength and new fractures are <br /> created. The magnitude of dynamic strain and particle motion decreases as distance from the <br /> charge increases.From the crushed zone out to about 26 charge radii,radial cracks are created as <br /> a result of the strain that exceeds the rock's tensile strength. <br /> 3.1 Vibration Ground Waves <br /> Within and beyond the cracking zone, stress waves spread through the rock mass and along the <br /> ground surface. Some waves pass through the "body" of the rock mass. Primary compression <br /> waves and shear waves are examples of body waves. Other surface vibration waves travel along <br /> the ground surface similar to the way waves travel along the surface of water. In an ideal <br /> isotropic and homogenous rock mass, wave energy would travel evenly in all directions. <br /> However, most rock masses are far from ideal, so wave energy is reflected, refracted and <br /> attenuated by various geological and topographical conditions. The elastic properties of rock <br /> greatly influence vibration magnitude and attenuation rate.When seismic waves pass through the <br /> ground, ground particles oscillate within three-dimensional space. Soon after blasting has <br /> stopped,vibration energy dissipates and the ground particles become still. <br /> REVEY Associates,Inc. Page 4 April 2004 <br />