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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 turn 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, crushing 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, I= Page 4 April 2004 <br />