Vertical Crater Retreat.pdf 1hu22

Chapter 11. Vertical Crater Retreat: an Important New Mining Method L.C. LANG INTRODUC'I'ION

The introduction of 165-mm (6% -in.) holes to underground mining operations has made possible the application of Canadian Industries Ltd's (CIL) vertical crater retreat (VCR) mining method, illustrated in the accompanying sketches. This unique and revolutionary new application of spherical charge technology (see the Appendix), when applied to primary stopes and pillar recovery, eliminates raise boring, slot cutting, and dilution of ore by backfill; greatly improves fragmentation; reduces labor and time requirements; eliminates uphole drilling and blasting; and minimizes or completely eliminates damages by blasts to the walls and retreating backs of the stope or pillar. If vertical large diameter holes are drilled on a designed pattern from a cut over a stope or pillar to bottom in the back of the undercut, and spherical charges of explosives are placed within these holes at the calculated optimum distance from the back of the undercut and detonated, a vertical thickness of ore will be blasted downwards into the previously mined area. Repeating this loading and blasting procedure, mining of the stope or pillar retreats in the form of horizontal slices in a vertical upwards direction until the top sill is blasted and the mining of a stope or pillar is completed. The VCR method is also applicable to drop raises and has the potential to replace all other raising methods under most circumstances. PILLAR RECOVERY Levack Mine

Inco Metals Co., Ontario Div., provided the first opportunity for the method in pillar recovery. The Levack mine's high grade ore pillar No. 4800 on the 975-m (3200-ft) level was used for the productionscale experiment (Figs. 1 to 3 ) . The pillar was about 49 m (160 ft) long, 6 m (20 ft) wide, and 20 to 26 m (65 to 85 ft) high. The mined area on both sides of the pillar was backfilled with a 1 :30 cement:sand mixture. The pillar was removed in two phases. In phase I ,

Fig. 1. Spherical charges placed in large diameter holes will blast downward a horizontal slice of ore into the previously mined area. This process can be repeated in a series of load and blast procedures, until the entire stope or pillar is mined. Metric equivalent: 1 in. x 25.4 = rnrn.

the standard uphole method was used to blast down the 18-m (60-ft) long section of waste from the bottom of the ore into the undercut. From the pillar's top sill, 165-mm (6%-in.) holes were drilled downward into the pillar, and by measuring the depth of the holes, the results of the uphole blast were determined and roof line 1 was established. The bottom of each hole was plugged, then filled with sand to place the center of gravity of each spherical charge (loaded from the top sill) at a predetermined optimum distance from the horizontal free surface. The charges were then detonated. After detonation, both draw drifts were full of extremely well-broken material. The depth of each hole was measured again, and the plot of these depths resulted in roof line 2. The same blasting procedure was repeated and the resultant new back elevation was marked by roof line 3. The poor results of the initial uphole blast at one location (notice the peak in area 1) appeared to influence the subsequent new backs. A third blast successfully evened the back, and resulted in roof line 4. An unblasted slab averaging 6.3 m (20.9 ft) thick remained below the pillar's top elevation as the final sill. In all three spherical charge blasts fragmentation of the blasted material was extremely fine. The backfill was fully exposed on both sides of the now-blasted pillar. The backfill remained undamaged and the ore was not diluted by sand. The remnants of all the 165-mm (6% -in.) holes remained clean and undamaged, and the holes had well-defined bottoms that could be easily measured and plugged. Each blast took down a 3.9 to 4.2-m (13 to 14-ft) thick horizontal slab of ore. Productivity was three times greater than that of the previously practiced cut-and-fill method. Since this was the first such experiment, blasting the remaining 6-m (20-ft) thick final slab was the subject of some deliberation. If the described method was repeated, we could have ended up with a 1.8-m (6-ft) thick sill unsafe to work on. It was therefore decided to blast the whole sill using two spherical charges properly placed in each hole, but with the application of

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Fig. 2. The carefully considered loading and delay pattern of phase 2 resulted in excellent fragmentation (see next

page) and caused no damage to the brows of draw drifts or to the sand-filled area of phase 1.

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delays in the vertical direction. In this way, the lower initial charges would provide a free face for the higher second charges. The fourth blast was also a success, with one exception: a layer of ore 0.6 m (2 ft) thick remained along the backfill on one side of the pillar, and the ends of rockbolts protruded into the cavity created. This was the result of more cautious loading of the side holes than was necessary. The layer was knocked down without any difficulties by the application of a few small ammonium nitrate fuel oil (ANFO) charges on the surface, and the backfill did not suffer any damage. Before proceeding with phase 2, the void created by blasting and drawing off the ore in phase 1 was

backfilled. Timber bulkheads were built on the top sill and in the No. 4800 drift to hold the sand. The first blast in phase 2 took down an average 2.9-m (9.6-ft) thick slab of waste immediately above the drift. In contrast to phase 1, the initial undercut was done by caving with spherical charges. The carefully considered loading and delay pattern resulted in excellent fragmentation and caused no damage to the brows of the draw drifts or to the sand-filled area of phase 1. The second blast advanced an average of 3.6 m (12 ft) upward, and the third blast averaged 5 rn ( 16.6 ft). The fourth blast removed an average of 3.8 m

UNDERGROUND MINING METHODS HANDBOOK

Fig. 2. Shows the excellent fragmentation.

(12.6 ft) and a top sill averaging 9.8 m (32.2 ft) thick remained in pIace. For safety reasons and for further experimentation, the thick slab was blasted down in one blast. Two charges, placed at optimum vertical distances in each hole, were vertically delayed by CIL's "l?IIE-DET" system. Blasting of the last sill was a complete success. The sand walls remained undamaged by the blasts, and fragmentation was excellent. The lesser advance in the initial undercut is explained by the lack of backfill on both sides of the blasted waste. The same advance should be expected in primary stoping because of the lack of free surface on both sides. If drawing of the blasted ore proceeds quickly to give sufficient void for swell and drop for secondary fragmentation, two lifts independently delayed may be dropped in a single blast in a pillar for greater efficiency. Drawing of the ore should be well synchronized with blasting and the void should be backfilled as soon as

Strathcona Mine

At Strathcona mine of Falconbridge Nickel Mines Ltd., Onaping, Ont., the first pillar has been successf ~ l l yrecovered by the VCR method. Pillar 25-D2-D4 was 61 m (200 ft) high and divided into two pafls of equal height by a sublevel. The bottom 15 m (50 ft) of the lower segment was developed into a cone for the drawpoints by using small diameter fanning holes and conventional blasting. The average width of the pillar was 6.7 m (22 ft), and only 30 m (100 ft) of its length was taken at this time. On both sides and on top the pillar was surrounded by backfill. Every 2.7 m (9 ft) a 0.3-m (I-ft) layer of 1 :8 cement:sand was poured while the remainder was a 1 :30 ratio. First the lower and then the upper section of the pillar was mined by the VCR method with great suecess. Further pillar recovery is planned at Strathcona mine by this method. PRIMARY STOPING Levack West Mine

Pillar recovery by the VCR method has now become an accepted mining method by Inco Metals Co., Ontario Div., and a substantial number of pillars have been mined out successfully at the Levack mine.

Following the successful extraction of pillars by this technology, it was considered more than appropriate to extend this new concept to primary stopes. With this objective, Inco Metals Co., Ontario Div., laid out a

SUBLEVEL on the hanging wall to enable ore flow into the undercut. This part of the operation required eight blasts. Following the initial phase of the blasting, the average vertical advance was 3 m (10 ft) per blast. The fragmentation of the blasted ore was satisfactory. Some large pieces of ore appeared in the drawpoints due to overbreak from the hanging wall and footwall s. Centennial Mine

Fig. 3. Vertical crater retreat in a high grade pillar at Inco's Levack mine. Metric equivalents: 1 in. X 25.4 = mm; 1 ft X 0.3048 = m.

stope between the 950 and 1080 levels at Levack West mine. The analysis of this VCR stope suggests that it is the safest method of mining known today, and is competitive with any other forms of stoping in of cost and production. What has been clearly proved over the duration of extraction is that the VCR method provides exceptionally fine muck, sound pillar walls, and drawpoints. In areas such as haulageways and drawpoints no wall or back deterioration was experienced. Sound pillar walls were very much in evidence as indicated by the 165-mm (6%-in.) borehole remnants along all of the remaining walls. It was observed then and now that in-the-hole drilling has to be set up accurately to obtain the designed pattern in the VCR method. Excess stretching of drill patterns causes substandard results. Although no actual study was performed on drilling accuracy it was learned through observation that initial alignment is vitally important. The extraction of the experimental stope was followed by others at Levack West with the same good results.

Hudson Bay Mining and Smelting Co.'s Centennial mine at Flin Flon, Man., has provided an opportunity for preliminary testwork in the sill of the 161 vertical pillar on the 165-m level. The study, conducted by CIL, indicated the feasibility of the VCR method despite the very high strength of this low grade ore body. The experimental stope presently in operation is about 43 m (140 ft) high, and has an average width of 6 m (20 ft). Because the ore body is dipping about 1.3 rad (7S0), all holes had to be inclined. While the drilling of the recommended pattern had been rather difficult due to the inclination of the holes, the stope has been retreating satisfactorily. The fragmentation is good, and no damage to the footwalls and hanging walls has occurred. Upon the extraction of the total quantity of ore available in this stope, the final results will enable the mine to evaluate the economic advantages or disadvantages of this method under these rather difficult conditions.

Birchtree Mine

The Manitoba Div. of Inco Metals Co. has recently initiated the use of the VCR mining method at the Birchtree mine 6.4 km ( 4 miles) south of the city of Thompson. The ore body at Birchtree is irregular both in strike and dip and has been mined by longitudinal cut-and-fill and sublevel blasthole methods. Peridotite bodies associated with the ore adversely affect dilution during mining. The VCR method provides the advantage of maintaining the stope full of broken ore during blasting, followed by rapid draw and filling. The 83 stope on the 1300 level was selected for the experiment (Fig. 4). The dimensions of the mining block were 38 m (125 ft) long, 33.5 m (110 ft) high, with width of 3 to 9 m (10 to 30 ft) at the top sill and 7.6 to 15 m (25 to 50 ft) at the undercut. Holes 152-m (6-in.) diam were drilled with the perimeter holes 1.5 m (5 ft) inside the footwall and hanging wall s. It was necessary to form a 0.78 rad (45") ledge

Fig. 4. Birchtree mine showing the 83 stope on the 1300 level, using the vertical crater retreat method. Due to the irregular shape of the Birchtree ore body, boreholes had to be drilled at different depths and angles. Scale: 1 in. = 50 ft. Metric equivalents: 1 in. X 25.4 = mm; 1 ft x 0.3048 = m.

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UNDERGROUND MINING METHODS HANDBOOK

Rubiales Mine Rubiales mine of Exploracion Minera Espana S.A. (EXMINESA) in northwestern Spain began to produce ore from the first stope in the upper ore blocks in midApril of 1977. It is estimated that at least 80% of the production will be extracted from open stopes averaging 30 m in width and 70 m in height. C10-512 is the first stope of a series opened on the top sill to gain access to the stope block. Inco's big hole blasthole method was considered for the effective mining of these stopes, and a Robbins 61R raise borer reamed a 1.8-m (6-ft) diam raise to start the slot. An Atlas Copco in-the-hole (ITH) drill was to drill the 165-mm (6V2-in.) holes that would be blasted into the slot. The Rubiales ores occur in the drag-folded limb of an anticline. Silicified limestone and argillite rock layers alternate in a complex ladder structure caused by the drag folding. Rock failures of different sizes were evident from the backs of development works, but a formidable failure in the experimental stope has caused considerable concern regarding cut-and-fill and bench-type mining methods. The extraction of ore from stope C10-512 by VCR began in mid-April 1977, with design and supervision of loading and blasting operations provided by CIL. The stope measures about 15 X 25 m and 70 m in height. A pillar was left in the center of the top sill to provide further to the back already secured by resin-anchored roof bolts. Several lines of 165-mm (6%-in.) diam vertical holes were drilled with an Atlas Copco ITH drill to a depth of about 55 m, five holes in each line thus establishing the recommended drilling pattern. The first step was the blasting of the cone-shaped undercut. This was done by conventional methods from the undercut with fanning small diameter holes up to about 15 m. The resultant new back was accessible for viewing from a sublevel when the VCR operation commenced. In six rounds of consecutive blasts the slope re-

Fig. 5. A crown pillar of ore left at the bottom of an open-pit mine is extracted by the VCR method.

treated about 15 m upwards. The fragmentation was fine and ideal for loading, and free of larger chunks requiring secondary blasting. A statistical 10 t/ft of borehole was produced at an explosives consumption of 0.34 kg/t. The walls and the back remained stable. Further advance in this part of the stope was temporarily suspended to allow time for drawing the blasted ore. During this period a constant check of hole depth indicated no slough from the back, which remained stable. About 70 000 t of ore was successfully extracted from this stope. The third VCR stope C10-517 is in operation at present with similar success. The good results obtained have confirmed all predictions and expectations, and there is well-founded reason to believe that the VCR method will be successful at Rubiales. Pamour Porcupine Mine This was the first gold mine to adopt the VCR method. When the small open-pit reached the most economical bottom, a decline was driven from the pit bottom and the ore body was undercut. The 36-m crown pillar was extracted by the VCR method. The concept, applicable to any ~ p e n - ~mine, it is shown in Fig. 5. Abminco N.L. At Ardlethan, N.S.W., Australia, a small offspring of the main tin ore body was found about 40 m below the surface just outside the open-pit mine. An adit was driven from the pit bottom and the Carpathia ore body was undercut (see Fig. 6 ) . All holes were drilled from the surface and the accuracy of these 80 to 90-m holes was of utmost importance. The cross section of the holes, the two VCR stopes, and the rib pillar are shown in Fig. 7 and the plan view of the mining operation in Fig. 8. The ore was mined out successfully without diluting its low-grade but precious tin content. DROP RAISING

The principles described previously also apply in drop raising. The VCR method in drop raising is quite

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Fig. 6. The Carpathia ore body is drilled

from the surface for the VCR method.

different from the Boliden experiment and is more productive. Two 36.5 m (120 ft) long, 3 X 3-m (10 X 10-ft) drop raises were completed for Agnew Lake mine, Ont. Although drilled off to apply conventional drop raising techniques, these raises were advanced most of the way using spherical charge technology. Depending on hole configuration and good blasting practices, 3-m (10-ft) advances per blast are possible. With adequate ventilation, two 3-m (10-ft) advances per shift, or at least three per day, are quite feasible. In conventional raising, setup time takes up a large portion of the total required time. Scaling, always a necessity, absorbs time as well. Drop raising eliminates these two practices and in doing so it provides greater advance and a safer working environment. Post-blast air shock must be vented through drawpoints to eliminate collar eruptions. It is therefore essential to continually muck drawpoints to keep them open. All the walls of these raises were rather even and free of any damage. It is appropriate to point out that it is better to

, Approx Surface Profile

Fig. 8. Plan view of the VCR operation at the Carpathia

operation. design larger drop raises where the VCR method is employed. The larger the surface blasting area the better the advance. Experience indicates that a 3 X 3-m (10 X 10-ft) area represents a practical minimum.

Fig. 7. Cross section of the Carpathia ore body shows the development work and the sequence of the VCR mining.

SUMMARY The breakage mechanism of a spherical charge greatly differs from that of the cylindrical charge used in underground mining. The advantages of the spherical charge could not be utilized until Inco Metals Co., Ontario Div., introduced large diameter holes to its operation. Production-scale experiments were carried out with spherical charges in stope-and-pillar mining with success. Then the new blasting method developed into a new mining method called vertical crater retreat, and is gaining acceptance by the industry. Its application in pillar recovery and in primary stopes has resulted in the elimination of raise boring and slot cutting. Dilution of the ore by backfill material