The distribution of peak particle velocity from construction blasting appears to be approximately log-normal so that the probabilities of exceeding specified levels should be approximately predictable. Separate models of Vmax distribution are presented for presplit (confined) and delayed shots. The preferred scaling factor operating on charge weight per delay is n = 1/3.
BACKGROUND AND INTRODUCTION
The careful synopsis by Nicholls, et al, (1971) of findings in the previous two decades (e.g., Crandell, 1949; Langefors et al, 1958; Edwards and Northwood, 1960; and continued efforts by the U.S. Bureau of Mines) confirmed that a relatively simple and highly useful criterion exists for relating structural damage to the peak particle velocity (Vmax) of ground motion. As the criterion has taken form and is presently used, it is a threshold below which the blasting design is kept for a reasonable assurance of no structural damage. Any means of assigning a probability of exceeding a prescribed ground Vmax when specifying blasting designs would be a useful refinement. It is towards this end that the present paper is written. The paper includes discussions of the statistical distribution of Vmax values (vertical component only) generated at an intermediate distance from blasting. In addition, the scaling factor, n, is investigated for the general propagation equation for particle velocity components (mathematical equation)(available in full paper)
The scaled distance, D/Wn, is one important quantity used in designing a blasting round, but in the V distribution analyses of this paper, D/Wn is heldmax more or less constant.
SOURCE OF DATA
The source of all data for this study was the main contract for excavation of an approach canal (Figure l) to the new Bankhead Lock on the Black Warrior River, Alabama. The canal excavation was planned by and contracted from the U.S. Army Engineer District, Mobile, and was noteworthy in having thorough monitoring of vibrations on the old lock and other structures. Preliminary test shots and an exploratory excavation contract (Phase I in Figure l) of 0.28 x l06 m 3(0.35 x l06 yd3) established blasting and monitoring techniques that assured that vibration and settlement of the old lock would not be excessive. During the main contract, involving excavation of 3.8 x 106 m3 (5 x 106 yd3) of rock, over 1000 shots were fired on nine working levels. More than two-thirds of the shots were combined by millisecond delays with other shots scheduled at the same time. Since most of these combined shots were not adjacent to one another, their superimposed vibrational effects were complex and not separable for analysis. Therefore, only the remaining shots, fired individually, were analyzed. In all, 92 presplit shots, 194 nine-millisecond delay shots, and 34 five-millisecond delay shots were used in the analysis. The particular shots available from one level are shown in Figure I as an example along with the locations of the pickups on and near the structures. See Table I for accompanying shot details for a group of six closely related shots.