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www.rockproducts.com ROCK products • March 2018 • 61 Figure 1 shows what this modern sinking cut looks like when shot with electric or electronic delays with each number representing a separate delay period. The first four holes (number 1s) are all placed a burden distance apart. These now all function as opening holes where the center of the shot is loaded to four times the normal explosive load. When these fire the middle area will be intensely radially fractured and then heaved up, allowing relief for the next few holes. This is similar to a cratering shot in the middle of the pattern. Typically, to ensure full depth is reached, these holes will be subdrilled to one half of the burden distance and stemmed to 100 percent of the burden distance. The second set of holes (number 2s) now use half the number of holes and half of the total explosive to break the same volume of rock as the number 1s. This is because these holes now have relief to both radially fracture and flexurally fail, assuming the number 1s function properly. The reason these break a still limited about of rock is because the opening is still small and the breakage angle (plan view) is compressed. Most of the remainder of the holes in the shot now have a full free face to work toward and function like normal blast- holes in a production blast. Because of this the remainder of the blastholes will have have a subdrill of 30 percent of the burden and a stemming of 70 percent of the burden. The timing of this pattern is extremely important where addi- tional time should be placed between rows firing. As with production blasting, the time should also be scaled according to the explosive diameter and burden distance. This pattern can also be fired with nonelectric caps as shown in Figure 2. This creates a spiral sinking cut which builds additional time between rows and has proven in hundreds of applications to be the most effective and efficient way of firing a sinking cut. The question then always comes up, what is the maximum depth that can be achieved with a sinking cut? Obviously infinite depth is not realistic as gravity begins to take effect and effect rock motion. There are two general rules of thumb for the depth of a modern sinking cut. The first is that the depth of the cut should not be greater than half the dimension of the pattern. For example, if the pattern was 60 ft. across then the maximum depth of the cut would be 30 ft. The second rule of thumb is the stiffness ratio, or the length of the borehole divided by the burden, should not be greater than 4. For example, if the burden was calculated to be 10 ft. for the blast, the depth should be less than 40 ft. It is import- ant to remember than the greater the depth of the sinking cut the greater the probability that the sinking cut will not function as intended. Let's look at an example for a sinking cut. Assume that a 6 in. drill is being used for blasting in limestone. The holes are dry, bulked loaded with ANFO, and fired with nonelectric caps. The burden for the shot (using the Konya burden formula) is Figure 2 - Nonelectric initiation sequence. Figure 1 - Modern sinking cut pattern with electric/ electronic delays. 13 ft. and the pattern will be drilled on a square. The goal depth for the bench is 40 ft., taking 40 ft. (bench height) and dividing that by the burden (13 ft.) gives a stiff- ness ratio of 3.1 which is below 4; therefore, the depth can reasonable be achieve with this drill hole size. This means that the pattern must be at least 80 ft. wide by 80 ft. in length. To fit this configuration with a 13-ft. burden, a 7-hole by 7-hole pattern must be used (49 holes in total). This now is a pattern that is 91 ft. by 91 ft. The design dimensions for the pattern are in Table 1.