Methods and Costs of Handling and Breaking Ore and Rock in Bulldozing Chambers

AT most mines where large tonnages are handled, "bulldozing" or secondary blasting is an important and costly operation. To reduce the large blocks from primary blasting operations or stoping so that they will be small enough to pass through the chutes into the cars at the haulage level, and not too large for the crushing plant, requires blasting. Often this is done by placing a few sticks of dynamite on top of a block with or without a capping of mud, but to consume less dynamite and to promote safety, most secondary blasting is now done by blockholing, or the drilling of holes 6 to 12 in. long in the blocks, using only a part of a stick of dynamite per hole. Regardless of the method of blasting, however, the term "bulldozing chamber" is applied to the room where this work is done. Bulldozing chambers permit secondary blasting at all times in a safe place without disturbance to the other mining operations. The chambers are usually high above the haulage level and well below the stope level or open-pit floor. In most instances the material is passed through a grizzly in the bulldozing chamber, but sometimes no grizzlies are used. One to four raises extend from the chamber to the stope floor and one or two from the haulage level to the chamber floor. Occasionally large slabs of ore or rock choke the raises above the bull-dozing chamber, and these have to be blasted by attaching a bundle of dynamite sticks on the end of a long pole and placing them up in the raise between the pieces causing the choke. This is called "chute-blasting." To give a picture of the progressive developments in the use of the bulldozing chamber, a few typical mines that show the application of this method to both metal and limestone mines have been selected for description here. Limestone mines have been included because many metal miners do not realize to what extent the nonmetallic, or rock, miners are now applying metal-mining methods successfully in their operations. The bulldozing chamber is the "neck of the bottle" for some mines.