GENERAL41266 - Laserfiche WebLink

Home Browse Search

• The log indicates that the total weight of explosives used in the blast was 19,714 lbs. The permit states that the maximum weight of explosives used per blast will be 54,7201bs. • The log indicates that the blast holes were drilled with a 7-7/8' drill bit. The permit states that the holes were drilled with a 7-7(8" drill bit. • The log indicates that the blast holes were drilled on a 16' X 18' pattem. The permit states that the blast holes will be drilled on a 16' X 18' pattem. • The log indicates that explosive material was placed in the holes no closer than 8 feet from the surface. The permit states that explosive material will be placed no closer that than 10 feet from the surface. • The log indicates that the blast was initiated with an electric initiation system. The permit states initiation will be with anon-electric system. • The log and blast sketch indicate that surface delays along rows were 17 ms. The permit states that surface delays along rows will be 9 ms. • The log and blast sketch indicate that surface delays between rows were 42 ms. The pemut . states that surface delays between rows will be 100 ms. • The log indicates that downhole delays were 100 ms. The permit states that downhole delays will be 600 ms. • Based upon my reconstruction of the blast, there were no instances of two or more blast holes detonating simultaneously. There were, however, 77 instances of two or more blast holes detonating within 8 ms of each other. The permit states that all blast holes will have a 9-ms delay between them, and indicates that each blast hole will detonated sequentially. C. Conclusions Mr. Leef s request for inspection of 5123/02 indicated a concern that the rockfall onto State Highway 12 that occurred on the morning of 5/23/02 was caused by blasting occurring at the Lorencito Canyon coal mine. My review focused on attempting to detemune whether the blasting at the mine caused or contributed to the rockfall by reviewing the records for the six blasts that were detonated during the seven days prior to, and on the day of, the rockfall. The Regulations of the Colorado Mined Land Reclamtion Board for Coal Mining do not prohibit ground vibration and airblast from occumng beyond blasting areas. The Regulations allow some level of ground vibration and airblast to occur at certain types of structures. The levels allowed depend on the type, location, and ownership of stnlcture, the arnounts of explosives detonated within an 8-millisecond interval during a blast, and, in the case of airblast, the sensitivity of monitoring equipment being used. Coal mines may demonstrate compliance with ground vibration limits by the appropriate use of either proper ground vibration monitoring equipment (so long as peak particle velocities do not exceed 1.0 ips) or the mathematical formula and table found in the Regulations that relate weights of explosives detonated within 8-millisecond intervals of blasts to peak particle velocities. Coal mines can demonstrate compliance with airblast limits only with the appropriate use of airblast monitoring equipment.