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-" WATER WELLS, PIPELINES AND OTHER UNDERGROUND STRUCTURES <br />The vibration response characteristics of domestic water wells, pipelines, and other underground <br />structures; and thus the potential for damage to such structures; must be considered totally different <br />from residential structures located above ground. <br />In the preceding discussion of building response we dealt with structures that aze relatively free to <br />respond in a manner different from the ground upon which they aze founded. They could amplify, <br />attenuate, or otherwise -change the characteristics. <br />This is not the case for well bores, buried pipelines, or similaz below-grade structures. Such structures <br />are totally constrained by the surrounding soil or rock and, therefore must move in exactly the same <br />manner as the surrounding material during a vrbration episode. Well casings and pipelines will <br />withstand vibrations well in excess of the OSM and USBM recommendations. The limits <br />recommended by the OSM and USBM aze designed to protect the brittle plaster and dry-wall interiors <br />found in residential structures and take into account that resonance is likely to occur in these structures <br />when low frequency vibrations are involved. <br />The primary concern of homeowners when blasting occurs, is the effect of the blasting on various <br />structures comprising the homeowners residence. Atypical concern is the effect of blasting on <br />• the integrity (i.e. quality and quantity) of their water supply system. Homeowners envision <br />blasting as the fracturing of rock and usually assume that since they felt the vibrations at a great <br />distance the fracturing of rock must also occur at this distance. Most homeowners then worry <br />about a diminished water supply or contamination to then water supply. <br />Furthermore, it has been found that the measurement of ground surface vibration is of limited value in <br />determining the safety of underground structures such as wells. What is of fundamental importance, <br />however, is whether there has been permanent or inelastic deformation of the rock at or in the vicinity <br />of a borehole. <br />The only important mechanism by which permanent movement can occur as a result of the detonation <br />of explosives, is through a phenomena known as cratering. <br />Cratering has been studied extensively by the USBM as well as by various military agencies. When an <br />explosive charge is wnfined in the ground and detonated, the volume which is permanently deformed <br />(cracked or moved) is ideally a conical solid with the base (point) of the cone in the area at the bottom <br />of the charge and the open end of the cone along the ground surface, or what is known as the free face. <br />The radius of the open end of the cone is approximately equal to the depth of the explosive. <br />Outside of this conical volume no pemiarrent deformation takes place. Thus, a pipeline run or a well <br />drilled just outside this volume would theoretically sustain no damage. <br />• <br />u <br />16 <br />