Pablo Segarra

Uncertainty in Measurements of Vibrations From Blasting

Ground vibrations from blasting induce environmental concerns near the blasting sites. National standards and recommendations (AENOR 1993; BSI 1993; DIN 1986; Singh and Roy 2010; SIS 1991; Siskind et al. 1980) define threshold values as function of the characteristics of the structures and also of the predominant frequency of ground vibrations. In order to assess compliance with these regulations, the engineer should monitor the resulting seismic field at the position of interest. For this purpose, monitoring devices (geophones and recording units) must meet some minimum characteristics and must be periodically calibrated (AENOR 1993; ISEE 2010; DIN 1995; Instantel 2011). Other important considerations for measuring vibrations are that the sensor does not lose contact with the ground in the vertical direction nor slip in the horizontal plane, and that it accurately follows ground motion (Adhikari et al. 2005; Drijkoningen et al. 2006; Hutchison et al. 2005; Krohn 1984; Wheeler 2005; Williams and Treleaven 2003). The preferred coupling methods are: for soil, attach the sensor at the top of a mount that is embedded into ground, and for rock, anchor, glue or cement the sensor to the ground surface (Blair 1995a). Other suggested coupling methods of the sensors to rock at low vibration levels, such as sensors freely laid on the ground surface or sensors held with sandbags (ISRM 1992; ISEE 2009), alter the amplitude of vibrations and should be avoided (Blair 1987, 1995b). Once vibrations have been measured, the estimated peak particle velocities and dominant frequencies must be written in a report. In many science and engineering fields, when a result of a measurement is reported, it is obligatory to show together with it, an estimate of its uncertainty (Miller and Miller 1993; Taylor and Kuyatt 1994; JCGM 2008). This is mandatory to qualify a laboratory in accordance with ISO regulations (AENOR 2005) and may have a bearing in lawsuits, but at present it is not made in vibration measurements from blasting. This prevents assessment of how well a reported value represents the value of the magnitude being measured or to study the quantitative effect of poor or good measuring practices. There are, nowadays, few studies that assess uncertainty in vibration measurements from blasting (Segarra et al. 2010). This work tries to fill this void, and reports and analyzes uncertainties for four measuring conditions that are representative of common field monitoring techniques. The implications of these uncertainties in the assessment of compliance with standards are also highlighted.

The influence of some blasting techniques on the probability of ignition of firedamp by permissible explosives

The ignition of firedamp by permissible explosives is assessed by means of gallery testing conducted by the Bruceton up-and-down method. Six test series were made in order to analyze the influence of several blasting practices in long-holes coal blasting, namely: use of slotted PVC pipes, detonating cord, salt cartridges and double (top and bottom) initiation. The parameters of the distributions of the probability of ignition are determined by the maximum likelihood method; normal, logistic, lognormal and Weibull distributions have been used. Confidence bands for the probability points are obtained both from the asymptotic standard errors of the parameters and by a bootstrap-like technique. The four distributions used give similar results in a rather ample probability range; discrepancies in the probability points are within 2% and in the confidence limits within 10% in a range of probability [0.1, 0.9] in most of the cases. The use of detonating cord is found to affect significantly the probability of ignition; the double initiation does also have an influence though not statistically significant at a 95% level; the use of salt cartridges, in the amount tested, has little effect in the ignition probability; the use of PVC pipe shows no effect.

Energy Efficiency in Rock Blasting

Ten production blasts and one single-hole confined blast were monitored in two quarries in order to assess the measurable forms in which the energy delivered by the explosive is transformed in rock blasting. The seismic wave energy, the kinetic energy, and the fracture energy transferred in the blasting process were determined using the seismic field from seismograph records, the initial velocity of the blasted rock face obtained from high-speed video footages, and the fragment size distributions from image analysis of the muck pile material, respectively. The maximum total energy measured accounts for not more than 26% of the available explosive energy, if this is rated as the heat of explosion, though lower figures are usually obtained. The values measured for each of the energy components range from 2 to 6% of the total energy available for the fragmentation energy, 1–3% for the seismic energy, and 3–21% for the kinetic energy. For the confined shot hole, the seismic energy was 9% of the heat of explosion.

Bestimmung der Korngrößenverteilung von Sprenghauwerk auf der Grundlage digitaler Bilder: Generelle Vorgehensweise, Grenzen der Anwendung

ZusammenfassungDer Beitrag behandelt die Bestimmung der Korngrößenverteilung von Sprenghauwerk in einem Steinbruchbetrieb auf der Basis der Analyse digitaler Bilder. Das zum Einsatz gebrachte Messsystem wurde an Hand aktueller Siebdaten kalibriert und über die Ermittlung eines Korrekturfaktors insbesondere für den Feinkornbereich mit der tatsächlichen Korngrößenverteilung abgestimmt. Das System wurde sowohl im manuellen als auch automatischen Betrieb eingesetzt. Dabei zeigte sich, dass ein gewisser Umfang an manueller Bildnachbearbeitung unerlässlich ist, um brauchbare Korngrößenverteilungen zu erhalten. Die Mindestzahl an erforderlichen Bildern für einen Hauwerkskegel nach dem Sprengen wurde ebenso untersucht wie der subjektive Einfluss des Bildbearbeiters auf das Korngrößenverteilungsergebnis. Das solcherart ausgelegte Korngrößensystem wurde über ein Jahr in einem Steinbruchbetrieb in Spanien eingesetzt. Dabei zeigte sich, dass Korngrößenmesssysteme auf der Basis der Analyse digitaler Bilder Informationen über die Korngrößenverteilung im Sprenghauwerk geben, welche die Grundvoraussetzung für die Optimierung der Sprengarbeit darstellen.AbstractThe method for measuring the fragmentation of the run-of-mine using image analysis software on a continuous basis in a limestone quarry is described. The system was calibrated by determining a fines correction factor that achieves a best fit of the measured size distribution curve to the actual one, obtained by sieving a sample of a muckpile. Size distribution curves from manual and automatic treatment of images are compared, from which it is apparent that a certain degree of manual correction to the images is required in order to adjust errors in particle delineation and fines detection. The minimum number of photographs to be analyzed, an important issue if human intervention is required, is addressed. Other sources of error are discussed, namely image sampling, outlier curves, and those connected with the subjectivity of the manual treatment. The overall performance of the system was monitored for about one year period; further tuning of the fines correction factor was required in order to overcome some inconsistencies of the fragmentation results, after which the system’s performance was deemed satisfactory.