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<br />Rietspruit, South Africa <br />The following diagrams demonstrate this: <br />NEW PRESPLI7 HIGHWALL <br />r~ <br />HIGMY <br />ORES'. <br />PREVIOUS <br />BLAST <br />• • • • . <br />i - JP <br />\. a0 • • • • tE / <br />Up 9/ <br />^~8 m . (a0~ I • .0 <br />~J <br />OLD PRESPLI7 HIGHWALL <br />FIG. 9. Possible Tie-up 8m x 7m pattern. <br />NEW PRESPLI7 HIGHWALL <br />. \ ~ . <br />' ~\. [i0° TIE-UP •/• <br />\ ~ <br />m~• 1 • •60° •~• PARA~ LEL TIE- ~P <br />.ALL.. _L_ <br />7m <br />OLD PRESPLI7 HIGHWALL <br /> <br />TOE-____._______ <br />FIG. 8. Typical Hole Layout and Blast tie-up <br />sequence as used in August, 1980. <br />INITIATION <br />xEY <br />- 40 Ms Relays <br />m (FORA <br />+7m+Bm <br />m PATTERN <br />fore being handled by the dragline. Only [hen can the <br />"digability" of the blast be evaluated. <br />Before the introduction of presplitting, the mine had more <br />or less settled on the following blasting method. <br />Of note is the double row of holes on the highwall and low <br />wall sides oC the blast. It was felt that the highwall double <br />row was necessary to minimize tltc difficult digging of <br />the key cut and the double row on the low wall side was <br />necessary due to the additional toe burden. <br />The main problem with this system of blasting is the close <br />proximity of these holes to each other. This resulted in nu- <br />merous misfires due to cut ofl-s. <br />Due [o the buffer blasting previously employed, it tool: the <br />mine about six months to arrive a[ the stage where the <br />entire length of the pit had a presplit highwall. It was dur- <br />ing this period [hat the initial experimentation of presplit- <br />ting was undertaken. <br />Having successfully introduced presplitting throughout the <br />entire length of the pit by early 1981, [he mine was now <br />faced with designing a pattern and blasting sequence to <br />best suit the presplit technique. <br />The 60° Tie-up <br />The author's had had some experience with these tech- <br />niques elsewhere, where a square pattern was blasted on <br />the second chevron. This resulted in holes denotating in <br />rows at an angle oC 45° to the presplit highwall and a pro- <br />nounced power trough. Volumetric surveys of blasts indi- <br />cated [hat blasting results in a swell of the order of 25% of <br />[he original volume. <br />Bearing the above facts in mind, it was thought that [he <br />ideal pattern for a single seam operation should be blasted <br />in rows at some angle between 90° and 45° to the high- <br />wall. It was thought that this would minimize the power <br />trough. 1C the expansion oC the blasted material could be <br />controlled then the pad preparation for the draglines would <br />also be minimized. With this in mind, the drill pattern was <br />rota[ed by 90° which on the equilateral pattern results in <br />[wo me[hods oC tying in the second chevron at 60° to the <br />highwall or parallel to the highwall. <br />FIG. 10. 90° Rotation of previous 8m x 7m pattern <br />possible Tie-ups. <br />It was decided that a 1000m section of overburden would <br />be blasted in [his manner, in order to give the system a fair <br />trial. It was necessary to rotate the pattern by 90° in order <br />to be able to tie the ti0° pattern. The resulting profile is <br />shown in Figure I I. <br />ROLL BACK (pad building) f <br />FILE BEFORE BLAST f <br />/ f// ACTUAL PROFILE j <br />_, / <br />~ i/( DESIRED <br />\I~ ~ PROFILE <br />~ \ <br />I \ <br />I ~~ <br />NO. 2 SEAM / ~ <br />- / ~ <br />FIG. 11. Section through Blast for 60° Tie-up. <br />E20 <br />