Agenda - 10-06-1981 - Laserfiche WebLink

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685 - � . � ~°'7�� ' `''�!��`��. f; � Page 5 Such vibrations attenuate very rapidly with increasing distances from the source pquipment, As far as the ground motion from blasting is concerned, it is of a short-term or transient nature. When a blast is detonated, vibrations are produced which radiate outwards through the ground in all directions. The situation is analogous — to what happens when a rock is dropped into a body of water. In the latter, the ripples radiate outwards in all directions on the surface of the water. There ".are two important differences: 1)� the water ripples can be seen whereas the ground movcment is generally limited to a few thousandths of an inch at most, and there.- fore is not visible; 2) in the case of the water ripples, the distance between successive waves (wave length) is measured in inches or a few feet at most, depending upon the size of the rock hitting the water, while in the case of the eround waves the wave length is measured in hundreds of feet. In the case of the ground vibrations, we are concerned with wha happens when they reach the location of a building. The vibrations cause the ground at a given point or a building on top of the ground at that point to oscillate briefly back and forth, up and down, and side to side. Research in blasting seismology er the past 50 years has shown that the occurrence of damage is related not to the amount of oscillation or mo nt to which a building is subjected (this is called "particle displacement"), but it is related instead to the speed or rate at which a structure is vibrated. The latter quantity is called "particle ' velocity". In short, therefore, it is how fast a structure is vibrated rather t|:an how uch it is vibrated that determines whether or not damage will ensue, The research mentioned above has been conducted by governmental agencies such as the United States Bureau of Mines and the National Research Council of Canada, by insurance cdmpanies, and by individual seismological consultants. O:e limit on blasting vibrations, widely accepted in the past as a result of this research, is a particle velucity Of 2.0 'inches per sedond. The foregoing does not mean that a building moves 2'0 inches; it means that rate of movement at any instant is limited to 2.0 inches per second. With ``'/esting vibrations, movement does not continue in the same direction and at a constant rate for more than a few thousandths of a second. Movement of 2.0 inJ/cs could only be produced by continued displacement of'the ground in one &iractfnn and at the rate of 2.0 inches per second over a full second of time. This discussion can be understood by noting that a car, traveling at a rate of 60 miles per hour, will cover a map or straight-line distance of 60 miles only if it continues in one direction at this speed for a full hour. There- fore, just asthere is no direct relation between car speed and the distance traveled without taking other factors. Such as time of travel into consideration, :imflarly there 1ono s1mplafxe1ati*nohYu �mty��n ground rmundmpY�m� t'cd] unent). end particle velocity:" ' ` ��'�_� ~ ~' ` The vibration limit cited above applies to any single nnent of ground motion (longitudinal, vertical, or transverse) with the vibrations being measured with a three-component seismograph. In lay terms, the limit would apply to the particle velocity in any one of three directions, back and forth, up and down, / cr side to side. ;