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i 1 <br />I§ I <br />I~I <br />Southwestern Portland Cement <br />Mining Impacts on Montgomery School <br />SHB AGRA Project E93-7038 <br />Page 2 <br />are therefore of critical concern when the effects of Southdown's contemplated mine are <br />assessed. <br />1 The school is not within the primary impact area (mine and overburden disposal area), however, <br />it is in a secondary impact area that will be subject to ground vibrations and air overpressure <br />' caused by blasting operations at the mine. As presently outlined in the mine plan, mining <br />operations will progress from south to north. Accordingly, the effects of the blasting at the <br />school site will decrease over time as the distance between the blast sites and the school <br />~ , increases. <br />The school building's construction is of coursed stone, locally quarried, and mortar. Buildings <br />' of this type can suffer both cosmetic and structural degradation from excessive ground <br />vibrations, resulting in wall cracks and in severe cases, structural damage can occur. The <br />' school's National Register value as discussed above and the potential for degradation of that <br />value as a result of blasting therefore has become a concern addressed by Southdown through <br />its consultants. <br />' Southdown recognized the historic value of the Montgomery School early in the planning process <br />and has undertaken examinations of studies and alternatives for protection against blast damage <br />' at other sites. Particular emphasis was placed on studies of the types and severity of impacts <br />to historic structures. The goal is to understand the types of vibrations and what should be done <br />to both limit them to safe levels and monitor the structural condition of the school during the <br />' construction and mining activities. <br />2.0 BASELINE GROUND VIBRATIONS <br />' The Montgomery School building is currently subjected to background vibrations levels <br />associated with several sources. The structure is located approximately 190 feet from Colorado <br />' Highway 66 which is utilized by a wide range of vehicles including heavy trucks. Monitoring <br />of ground vibration associated with traffic on Highway 66 was conducted on September 16, <br />1993. Ground vibrations were monitored at various distances from the edge of the highway at <br />the intersection of County Road 47. A Sprengnether VS-1200 engineering seismograph was used <br />for the monitoring which records ground vibrations in three mutually perpendicular orientations. <br />Ground motion was recorded in terms of peak ground velocity which has been found to provide <br />' the best correlation to structural damage levels associated with vibration. As indicated by the <br />summary of monitoring results presented in Table 1, peak resultant ground vibrations at the <br />school building location are in the range of 0.003 to 0.006 inches per second (in/sec) as a result <br />' of existing traffic on Highway 66 and County Road 47. <br />While not observed during the field monitoring, the structure would also be subjected to <br />vibrations associated with aircraft overflights. Monitoring reported by King, et.al. (1988) <br />indicates low flying aircraft can induce vibrations in a structure that is of the same general <br />' magnitude as operation of vehicles adjacent to the building. <br />L, A G R A <br />' forth&EnvironmeMe/Group <br />