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48 • ROCK products • August 2016 www.rockproducts.com Precision Presplitting This is the third in a three-part article about Precision Presplitting. –Ed. I n the past two articles the authors have thoroughly dis- cussed precision presplitting design and applications. To recap, a precision presplit is a specialty way of presplitting rock to avoid overbreak of the final wall and to create just enough energy to crack the rock. In the past issue, the design of a precision presplit on a 24-in. spacing was discussed. A 24-in. spacing is the common spacing often used for design- ing a precision presplit in construction blasting with smaller drilling rigs. The mining industry typically utilizes larger drill rigs which create a larger drill-cuttings cone surrounding the blast hole making it difficult to use 24-in. spacing. In other situations, a spacing less than 24 in. can be used for better control and less overbreak. The goal is then to determine what explosive load (in grains per ft. of detonating cord) will mimic the results of 24-in. spacing. For calculating the explosive load based on spacing, each rock will be given a unique presplit constant (K) which is related to the Young's Modulus of the rock. These are shown in Table 1 below for some example rocks, others can be interpolated based on their Young's Modulus. Table 1 - Presplit Constant (K) Rock Constant (K) Young Modulus (GPA) Granite 76 40 Limestone 86 32 Shale 116 20 Sandstone 127 15 Siltstone 201 8.5 From this a model can be made analyzing the stresses around adjacent boreholes with the explosive energy at a set spacing. This results in unique equations for each rock type based on the presplit constant (K). For these five example rock types, a graph has been created for the explosive load at variable spacing, which is shown below in Figure 1. These graphs are for dry holes and loading with a ½-lb. cast booster or chub of emulsion at the bottom of the borehole with either delayed or instantaneous firing. For wet holes this graph can also be used, with slight adjustments during test blasting. It is not recommended to go beyond a 60-in. spacing, and the larger the spacing the higher the chance for radial cracking on the back wall. Figure 1 - Konya Presplit Load Prediction This graph shows the equations for the competent, aver- age rock sample of each group. From this, along with rock mechanics reports, the values can be adjusted for an "optima"' explosive load; however, the best way to use this graph would be to find the desired spacing and approximate rock type. Then find the explosive load to use and through test blasting analyze the performance of the final wall. Once the explosive load and spacing have been designed, one must analyze the environmental affects the precision presplit will have. The two major concerns from this are air overpressure and ground vibration. In normal blasting operations, air overpressure is not a large concern and regulations are rarely exceeded. However, in the United States a new form of regulating is emerging where structural or physical damage is not the concern but the annoyance of neighbors can be the limiting factor on air Precision Presplitting – Spacing Variations Calculating the Explosive Load Based on Spacing. By Anthony Konya and Dr. Calvin Konya