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• <br /> Review of Blasting Operations at Robinson Brick Company's Siloam Clay Mine <br /> 2.1.3 Potential for Damage to Buried Utility Lines <br /> When explosive charges detonate in rock, most of the energy is used to break and displace the <br /> blasted burden of rock between the charge and the nearest free-face. However, other energy is <br /> released in form of ground vibration, air overpressure (concussion), elastic strain waves and high- <br /> pressure gas pulses. When blasting occurs very close to buried utility lines (within about 10 ft) <br /> damage can occur if the line lies within ground that is physically ruptured by the blast. <br /> Ground Rupture Damage to Lines: <br /> Crushing usually occurs in the rock very near the charge. The extent of this compressive and shear <br /> failure zone is usually limited to one or two charge radii. Beyond the plastic crushing zone, the <br /> rock or ground is temporarily deformed by elastic strain waves. For some distance, tangential <br /> strain intensity exceeds the rock's strength and new fractures are created. The magnitude of strain <br /> wave energy and induced particle motion decreases as distance from the charge increases. Rock <br /> damage is a result of the induced strain, which is generally proportional to the particle velocity. <br /> i <br /> Tangential <br /> _ Tensile <br /> Stress <br /> Radial <br /> Compressive <br /> Stress <br /> Limited Blasthole <br /> Stress/Strain Crushing--in hard rock <br /> Wave Front <br /> Radial Cracks fromTangential Stress <br /> Figure 2.4--Process of Radial Fracturing around Explosive Charges <br /> GEOTEK& Associates, Inc. Page 10 January,99 <br />