Block-caving With Branch Raises is a method followed at sev eral large copper mines in the United States and in South America. The method is shown in figs. io and II. From the haulage-level main inclined chutes (also called transfer raises or simply trans fers), I, are driven to the grizzly level, where drifts, G, are used to connect a line of chutes. The tops of the chutes are covered with a grizzly made of rails to catch the large pieces of ore which would choke the chutes. From the grizzly level inclined chutes, K, are carried up and terminate in a square set, C, called a control set, from which two or four inclined raises, F, called "finger raises" are driven to the undercutting level at the bottom of the block. The tops of the finger raises are spaced 12.5 ft. apart each way, thus offering an ample number of points for drawing off the caved ore.
The ore is undercut by running a series of drifts, D, 25 ft. apart to the limits of the block. At one end of the block the ends of the drifts are widened. Holes are then drilled in the intervening ribs or pillars and in the back. Blasting these generally starts the caving. Isolating shrinkage stopes, called "boundary stopes," are carried up across the ends or sides of the block before the ore is undercut to separate it from the adjoining ground and thus facilitate the caving. This method is suitable for ore that is soft or of medium hardness in which there is little or no waste. Hard ores which break in large blocks are better suited to shrinkage stoping. The capping should follow down freely. The ore should break into fragments small enough to run well into the chutes. Ores containing much fine material are likely to stick in the chutes, especially if wet. Details of the method vary at different mines. The undercutting level may be inclined to follow the bottom slope of the ore. Finger raises may vary in
number from two to four and in slope from to 45° or more. Chutes may have a slope as great as 70°, but vertical chutes tend to choke more easily than inclined ones. The spacing of the series of branch raises along the haulage way may vary from 25 to ioo feet.
The height of ore (called lift) in a block may be from 3o to 30o feet. A thick body of ore may be worked in more than one lift. For a given amount of development work, the higher the lift taken the less is the cost per ton for the development work, since the cost is spread over all the ore in a block. However, other factors such as the horizontal extent of the ore and the economic life of the workings must also be considered. Since the method is applied to low grade copper ores, between i and 2% copper, care must be taken to keep the diluting of ore by waste to a minimum. The line of contact between the broken ore and the cappings (see fig. I 1) is usually between 40O and 7o° with the horizontal. The flatter the angle the greater is the amount of broken ore in the stope, and trouble from packing of the ore is increased. The more temporary workings must be kept open a longer time, entailing a heavy expense. At the Ruth mine an angle of resulted in a minimum dilution with capping; at the Inspiration mine an angle greater than 6o° caused a tendency of the capping to break through into the chutes.
Results at different mines show a lowering of the estimated grade of the ore by only a small fraction of i%, under favourable conditions less than 0.1%. The tonnage recovered from a block is generally slightly higher than the calculated volume of the block due' to included capping. Some ore is lost. The actual re covery of the copper contents of blocks ranges from 83 to 99%.