Question

You are the project engineer on a bridge project in the Pacific NW that requires you to blast a ledge that is only 10 ft in width and 18 ft in height. The rock to be blasted is weak limestone with many cracks and fissures and the rock is dipping away from the cut (“downhill”). The bedrock is exposed to water. You have ANFO and gel emulsions to choose from. The ANFO has a specific gravity of 1.0 and the gel emulsions have a specific gravity of 0.95. This blast will occur in one propagated shot with no delay. You have a 3” and 5” drill bits to choose from. You need to make sure that rock fragments and other debris do not interfere with an adjacent roadway. You will use crushed limestone for your stemming material. Design the parameters of the blasting system.

Relevant Tables and Information: TABLE 13.2 Density by nominal rock classifications Rock classification Basalt Diabase Diorite Dolomite Gneiss Granite Gypsum Hematite Limestone Marble Quartzite Sandstone Shale Slate Trap rock Specific gravity 2.8-3.0 2.6-3.0 2.8-3.0 2.8-2.9 2.6-2.9 2.6-2.9 2.3-2.8 4.5-5.3 2.4-2.9 2.1-2.9 2.0-2.8 2.0-2.8 2.4-2.8 2.5-2.8 2.6-3.0 TABLE 13.3 Burden distance correction factors Rock deposition Bedding steeply dipping into cut Bedding steeply dipping into face Other cases of deposition 1.18 0.95 1.00 Rock structure Heavily cracked, frequent weak joints, weakly cemented layers Thin, well-cemented layers with tight joints Massive intact rock 1.30 1.10 0.95

Answer 1

Length = 18ft Width = 10ft instantaneous intiation, with the SR greater than 1 but less than H. S = L + 2B/3 S = 18 + 2 times 10/3 S = 12.66 ft instantaneous intiation, with the SR equal to or greaterthan H. S = 2B S = 2 times 10 S = 20ft delayed intiation, with the SR greater than 1 but less than H S = L + 7B/8 = 18 + 7 times 10/8 S = 11ft delayed intiation, with SR lessthanorequalto 4 S = 1.4 times 10 S = 14 ft