Vibration and structure response from dade county quarry blasting

摘要

Vibrations from Dade County (Miami area) quarry blasting and structure responses were measured at 11 locations between February and April, 2000, and 10 homes were inspected to analyze the characteristics of their cracking and other damages. These results were combined with information collected by the County and other studies done in south Florida for a blasting impact assessment specific to this area. In addition over 40 quarry blasts in the nine-week monitoring period, about 800 measurements were available from previous monitoring in Dade and neighboring Broward counties. An overall worst-case analyses based on the highest individual amplitudes suggested that vibrations in some areas could have been as high as 0.35 in/s. While amplitudes in the study period were below this value, they were found to be relatively high for the distances andcharge sizes. Attenuation with distance is less in Dade County than found elsewhere with quarries having to use scaled distances several times higher than similar sites in the north. Consequently, the vibrations are perceptible at very large distances. Vibrations were also found to be of long durations at the homes (some over 17 seconds) and a mixture of frequencies including "low" frequencies of about 8 Hz, which are close to house resonant frequencies. and very low frequencies of 2 to 4 Hz. These low frquencies are particularly noticeable to persons and are understandably alarming~* The responses of south Florida structures are sufficiently differnet from frame structures studied elsewhere to justify some concenr. Walls of concrete blocks with concrete caps and extenisve openings, and sometimes higher than standard 8-ft ceilings, respond as if they have low effective damping. The highest dynamic superstructure amplification exceeded 6x and there were several-blasts and structures above 3.6x. More structure response measurements are needed to establish exactly how serious and widespread are these high responses. The combination of high structural response and full-height CMU walls with stucco coating creates the possibi8lity of visible vibration-produced cracks at relatively low amplitudes. An initial cracking threshold of 100 muepsilon for such walls could be exceeded if vibrations approached the current Dade County limit of 0.75 in/s, depending on response frequency. Five of the 10 houses inspected for damage characteristics had some cracks, mostly exteriro, which could be from dynamic sources. "Dynamic" here is used for short-period or transient forces which cause superstructure racking and shear forces in the planes of the walls with the most obvious examples being blasting and gusty winds. As a general conclusion, blasting in local quarries does not appear responsible for cracks and other damages existing in the Dade County residences examined. This is based on vibration amplitudes and frequencies, structure responses, theoretical analyses of material strength and strains, and the nature and degrees of the existing damages in the homes inspected. However, thecurrent Dade County blast vibration regulatory limit of 0.75 in/s PPV needs to be revisited and, based on the few responses obtained here, a reasonable cautionary maximum level of 0.50 in/s is justified.