Noise and Ground Vibration Monitoring Related to Dragline Operations in Phosphate Mining
The primary objective of this study was to determine levels of noise and ground vibration as a function of distance from draglines operating under various meteorological conditions. The goal was to provide a reliable, statistically sound data base of dragline noise and ground vibration for future reference. To accomplish the goal, noise and ground vibration readings were taken from four distinct draglines mining in Polk and Hardee counties. The draglines monitored were the Bucyrus Erie #llSOB (at Noralyn), Bucyrus Erie #1250B (at Phosphoria), Bucyrus Erie #1260 (at Clear Springs), and Page Model 752 #16 (at Fort Green).
Field monitoring was conducted during each of the four seasons of the year with the last measurement concluding in December 1994. Noise monitoring was conducted using a Type I precision 2236A (Bruel & Kjaer) integrating sound level meter while the vibration monitoring was conducted using a piezoelectric accelerometer Type 4378 (B & K). Both the noise and ground vibration signals were recorded in real-time using a portable, digital audiotape recorder (Sony PC 204A) via the sound level meter and charge amplifier respectively. Noise and ground vibration readings were collected continuously and simultaneously over a duration of approximately two minutes for each increment of distance from the dragline. The measurements were collected two times by day and two by night during each site visit with one reading taken between midnight and 6 a.m..
Immediately, after completion of the field monitoring program, equivalent continuous sound levels, both LAV5 and Leq9 were read directly from the sound level meter. The L109 L909 and the sound exposure level presented in this report were also taken directly from the sound level meter. Regression analysis performed on the Leq readings revealed a discernible gap between the average sound levels of the different draglines. Specifically, the average sound levels resulting from the 1260W dragline at Clear Springs were noticeably higher than the sound levels from the other monitored draglines.
In the laboratory, the ground vibration signals were transferred from the recorded tapes to a PC computer whereby a FORTRAN program was utilized to calculate peak particle velocities. All of the measured peak particle velocities were below 0.106 inch per second which is classified by NAVFAC DM 7.3 (1983) as easily noticeable to persons. Comparison amongst the different draglines revealed that the 1250B dragline at Phosphoria produced the highest peak particle velocities.
After the noise parameters and the peak particle velocities were determined, the recorded noise and vibration signals from the tapes were further analyzed by the FORTRAN program. Fast Fourier Transform (FFT) analysis was applied to the noise signals to show how the sound pressure level varied as a function of frequency. The analysis revealed that the frequency distribution of the prominent sound pressure levels occurred within the 0 to 2000 Hertz range. It was also found that the 1250B and the 1150B has consistent dominant frequencies of approximately 1617 and 1400 Hz respectively.
FFT analysis was also applied to selected vibration measurements to show how particle velocity varied as a function of frequency. The resulting vibration spectrograms showed that the frequency ranges of the ground vibrations extended from 0 to just over 60 Hz. Most of the significant spectral energy, however, was contained between approximately 5 to 25 Hz.
The noise results were further analyzed to determine the influence of various factors on the sound levels. Since the operations of the draglines were not controlled in any manner, it was difficult to differentiate between the causes of the variability in the measured dragline noise. Thus, no definite conclusions could be drawn as to the effect of seasonal variations in the weather or the time of day, on the measured sound levels. However, background noise levels during the early morning measurements did seem slightly lower than other times of day.
In addition, an assessment of the dragline noise was made, and acceptable noise levels were determined from the literature. Finally, a range of distances from each dragline where mining noise is considered as acceptable or unacceptable was established.
In summary, field monitoring and laboratory analysis of noise and vibration measurements were successfully carried out to achieve the project objectives. A reliable data base of noise and ground vibration levels was established to provide a better understanding of dragline effects on the physical environment. This study may provide industry and the public with: (1) a quantitative measure of sound and ground vibration levels caused by dragline mining; (2) an indication of what noise levels are reasonable and can be expected; and (3) actual measured sound levels instead of subjective estimates of loudness. The study may also provide local officials with a better understanding of reasonable noise levels, so that any future regulations to be considered may be realistic.