Ground Support at Craigmont A. J. PETRINA, Mine Manager, Craigmont Mines Limited, Merritt, B.C.

ABSTRACT

At Craigmont Mines Limited, Merritt, B.C., tests were begun in March, 1965 to evalua~e the use of shotcre!e as a method of ground support m development h~admgs; previously, ail drifts had. required suppo~t, wh1ch was accomplished mainly by tJmber and occasiOnally by the use of rockbolts. The tests demonstrated conclusively the effectiveness of shotcrete for supporting Craigmont's ground. Gradually, the use of timber for ground support was virtually eliminated in favour of shotcrete.

This paper describes the equipment used, the operation of the equipment, pertinent cast data and opera ting details. Also discussed are other uses developed for the shot­creting equipment, the use of reinforcing bar rockbolts, and a method used at Craigmont for lining long vertical ore passes with steel plate and concrete as part of the raising cycle.

DEFINITION OF SHOTCRETE

SHOTCRETE MA Y BE DEFINED AS FOLLOWS:

Mortar or concrete that bas been conveyed (by re­gulated air pressure or by a positive-displacement pump or screw) through a bose and discharged through a nozzle at high velocity onto a suitably pre­pared inflexible surface; the product, which bas been premixed either dry (water added at the nozzle) or wet (water added prior to entry into the bose) , is sufficiently · stiff at impaction to support itself with­out sagging from an overhead surface or sloughing from a vertical surface.

Rebound is the mixture of spent shotcrete material,

A. J. PETRINA, born in Sudbury, Ontario, started out in mining at MacLeod-Cockshutt Gold Mines Ltd. in 1951 and worked for various On­t ::trio mines and contractors prior to bein'l: admitted to Queen's University in 1956. He graduated from Queen's in 1959 with a B.Sc. in mining engi­neer in t;, spent fifteen months at Will­roy Mines Ltd. and then joined Craig­mont Mines Limited in 1960. He was a.ppointed mine superintendent at Cr aigmont in 1965, and mine manager in May of 1968.

THE P AFER W AS PRESENTED: at the 70th Annual General Meeting of the lnstitute, Vancouver, April, 1968.

leaner and coarser than the original mixture, that has bounced off the surface during impaction; it is usual­ly expressed as a percentage of the original mixture.

There are a number of proprietary terms such as Truecrete, Blastcrete, Blocrete, Gunite, Guncrete, Jet­crete, Spraycrete, etc. used to describe shotcrete; they are appropriate when the proprietary method or equipment is involved.

The term gunite in Canadian mining practice com­monly refers to that shotcreting process in which pre­mixed dry sand and cement is conveyed pneumatically through a bose, with water added at the nozzle. The term shotcrete, as used henceforth in this paper, re­fers to the practice of premixing aggregate, cement and water before conveying through a bose.

DESCRIPTION OF GROUND ,CONDITIONS

The main rock types encountered in underground development at Craigmont are altered limy sediments (greywacke and skarn), volcanics (andesite ) and in­trusives (diorite ) . There is not much evidence of pressure or squeezing in mine openings; support is required because of the high degree of faulting, frac­turing and jointing which affects all of the rock types to a greater or lesser extent - ranging from 20-ft-plus zones of clayey fault gouge to "blocky" ground with hairline fractures a foot or more apart. Sorne ground seems to deteriorate slowly with pro­longed exposure to air.

KEYWORDS IN THIS P APER: Ground support, Craig­mont Mines Ltd. , Shotcrete, Rockbolting, Drifting, Ore passes, Concr eting, Steel lining. Trackless shotcretiJ~g equipment in travelling position.

(CIM) Bulletin for December, 1968 1445

ORIGINAL ME1'HODS OF SUPPORT

Prior to the introduction of shotcrete, nearly all of the underground development was supported by timber sets or, on rare occasions, by rockbolts. Ordi­nary rockbolts were generally unsatisfactory for per­manent support, because the fractured ground would gradually deteriorate around the plate or else it would be difficult t o get the expanding shell to anchor pro­perly.

Drift sizes were 8 by 9 ft inside timber on secon­dary levels, and 10 by 9 ft inside timber on the main haulage leve!; the timber sizes used were 3 by 8 ins., 10 by 10 ins. and 12 by 12 ins., with 2 or 3 ins. of rough lumber lagging. Set spacing varied, according to ground conditions, from 10 or 12 feet to nil. In many instances, the sets were little more than a "shed" over the drift, and were required because the fractured grou nd could not be scaled to solid; in other cases, the ground would gradually, but steadily, slough away so that good sturdy sets at close spacing were required simply to support the weight of the caved mate rial.

The timber sets had major disadvantages:

(i ) rock openings had to be 40 to 50 percent larger than the act ua! clearances required;

(ii ) the back could not be arched; ( iii ) timber sets provided only point support; (iv) timber sets would not prevent slough, but

would only stop it from blocking the drift; in a great many cases, sloughing eventually caused sets to yield to the point where extensive retimbering WftS neces­sary;

(v) timber sets deteriorated badly due to dry rot, a lthough cuprinol treatment proved effective in delay­ing this;

(vi ) good timbermen are few and far between.

INTRODUCTION OF SHOTCRETE

Shotcrete was first used at Craigmont in March, 1965. It was proposed to first try a wet-mix machine (water added in mixer ) rather than a dry-mix ma­chine (water added at nozzle ) for these reasons:

(i ) better quality control; nozzleman's guesswork is eliminated; wet pre-mixing ens ures uniform product to nozzle;

( ii ) sand does not have to be perfectly dry;

NOZZLE

A IR VALVE

"' ,,_ e::: Figure 1.-Schematic view of the True Gun-all machine.

1446

( iii ) Jess dusting at nozzle; (iv ) coarser aggregate can be used ; (v ) lower air cons umption;

(vi ) easy to train operators. The machine chosen for the first trials (and which

subsequently became the standard basic shotcreting unit ) was the True Gun-all Mode! H pneumatic con­crete machine. This unit consists of a 30-inch-diameter mixing chamber (11 ft3

) , complete with mixing pad­dles rotated by air-motor drive, a batching hopper, a water measuring deviee, a 2-inch material bose and a nozzle (F i gur e 1).

OPE.RATION

Craigmont is fortunate in having an ample deposit of good sandy grave! a few hundred yards from one of the main portais. It was found that by simply screening the run-of-bank material through a 7/ 16-inch screen, the sand which resulted would produce, when used in the True Gun-all machine, shotcrete with a compressive strength ranging from 6,000 to 7,000 psi . This screened sand is taken into the mine in Granby cars and dumped into lined raises (shotcret­ed ) , from where it is drawn as required.

A typical screen analysis of the sand is as follows:

Screen Number

4 . . .. . 8 .. .

14.. .. .. 28 ... . .. .... . . . .. . . . 48 ............. .. . ....

100 ................. . 200 ... . . . .. .. . . . .. . . . .. . .. . Pan ... . .... . .. . . . . . . . ... ..... . . .. . . . .

Weight % On

16.3 14.4 16.7 17.9 17.2 12.4 3.4 1.7

100.0

To apply shotcrete using the Mode! H True Gun-all machine, screened sand is shovelled into the measur­ing hopper along with one bag of ordinary Portland cement. When full, the hopper contains 41/2 cubic feet of sand and one bag of cement, t o make one-sixth of a yard of shotcrete. Opening an air-cylinder-operated guillotine gate allows the material t o faU directly into the mixing chamber, which, after closing the gate, is pressurized to about 60 psi. Once the gate is closed, the next batch can be shovelled into the hopper. Along the axis of the mixing chamber is a rotating shaft with severa! paddles attached; the rotation of the shaft causes rubber blades on the ends of the paddles to wipe the entire inner surface of the drum, as weil as mix the ingredients.

A measured amount of water (3 t o 4 gallons, de­pending on the moisture content of the sand ) is in­troduced into the tank, and, after a minute or so of mixing, the material valve is opened, permitting the con crete to flow along the bose to the nozzle; more compressed air is introduced at the nozzle through a separate air line to give the material added velocity. The action of the rotating paddles forces alternating "slugs" of air and concrete into the bose in such a way that a very thick mix can be pushed through up to 250 feet of 2-in. bose. The nozzleman tries to keep the spray normal to the surface being gunned, and at a distance of 3 to 6 feet, to minimize rebound !osses. Cycle time per batch is usually five minutes or Jess.

The concrete produced by the True Gun-all process is, by virtue of its low water content, no-slump con­crete. It adheres weil to almost any rock surface and

The Canadian Mining and Metallurgical

Replacing timber sets with shotcrete.

can be applied up to 2 inches thick on a vertical wall and up to 1 1f2 inch es thick on a drift back. The rock requires no preparation ( other than scaling) bef ore gunning; the shotcrete will adhere to a dry dusty surface or a clean wet one as long as the surface is not making water.

When a quick set is required, calcium chloride, amounting to 2 per cent of the weight of the cement, is added to the mix. This gives a fair set in an hour, and after two hours a nail cannat be driven into the shotcrete. The thickness applied depends on ground conditions; the more fractured and friable the ground, the thicker the shotcrete application. Generally, a first coat would not be much more than an inch thick, with additional thicknesses built up by additional coats. The shift boss or shift foreman decides how thick the shotcrete is to be applied, and whether or not stand­ard rockbolts or re-bar rockbolts are necessary in ad­dition to the shotcrete. The range of thickness would be from Jess than an inch to 3 or 4 inches.

VARIOUS APPLICATIONS OF SHOTCRETING

Ground Support in New Development

After the initial test work commenced in March of 1965, additional shotcrete machines were put into use until finally the use of timber for ground support in drifting was virtually eliminated.

Shotcreting was incorporated into the drifting cy­cle in three ways, depending on grou nd conditions:

(a ) After blasting a round, the new back would be gunned bef ore mucking out; after mucking out, the walls would be gunned and the back regunned.

(b) After mucking out a newly blasted round, the walls and back would be gunned before drilling, with perhaps a second coat on the preceding round.

(c) Severa! rounds would be advanced before apply­ing any shotcrete at ali. Even when shotcrete is carried right to the face, back-break from blast­ing the next round is not significant.

When trackless development was undertaken on a large scale, the practice was adopted of drilling peri­meter holes not more than 2 feet apart and blasting them with Xactex, thereby reducing scaling time and generally improving conditions prior to shotcreting.

(CIM) Bulletin for December, 1968

Rail-mounted shotcreting equipment in the travelling position - note the self-unloading sand car.

Severa! short raises in very bad ground were shot­creted to a height of about 50 feet. The main difficulty in gunning small raises was in getting the nozzle far enough away from the surface to be gunned. There was a!so a problem in communications between the nozzleman and mixer operator.

The No. 1 ore body service winze, a vertical opening about 17 by 17 feet and 570 feet high, was supported by shotcrete during the sinking process. The proced­ure was to set up a Mode! FSM True Gun-all machine in the deepest completed station, and, with the mate­rial hose suspended in the pipe compartment, gun the walls after each bench was blasted; upon sinking to the next station, 90 feet lower, the Gun-all machine would be moved down. Sand was brought to the ap­propriate station by passing it down from the top deck through a 6-inch pipeline, which later became the drain li ne.

Ground Support after Removal of Deteriorated Timber

Where timber has begun to fail through dry-rot or excessive weight of slough in the main adits (which were driven prior to 1961), most of the repair work is now done with shotcrete. Faulty sets are toppled by pulling them over with a locomotive. Generally, a con­siderable amount of rubble collapses with the timber, and ali exposed rock surfaces are gunned before re­moving the debris; after clean-up, the walls are gunned right down to the floor.

Construction of Ventilation Bulkheads, Seals and Doors

Shotcreting has proved to be an ideal way to con­struct ventilation bulkheads. The bulkhead is first constructed of 2-inch rough lagging and 4- by 6-in. posts, and then gunned with 2 or 3 inches of shotcrete to make a perfectly air-tight seal. The shotcrete ad­heres weil to wood.

The procedure in building a large door is to first construct a light wood wall across the opening, place the channel iron door frame against the wall and gun around the frame until the shotcrete is as deep as the channel. If desired, reinforcing rods can be placed in the wet shotcrete during the gunning process.

1447

Whitewashing

By substituting an appropriate whitewash mixture for sand and cement, a fast and efficient job of wliite­washing can be done with the shotcreting equipment.

Filling Forms

The use of the True Gun-al! machine in filling forms in awkward places was tested by using the material hose instead of conventional wheel-barrow transport. Results were totally unsatisfactory due to air pockets and accumulations of loose rebound material.

VARIATIONS TO EQUIPMENT

F or different applications, the True Gun-all Mode! H blower is used in various combinations with other equipment:

( i ) Track-mounted blower; hopper filling by hand; nozzle held by hand; sand in Granbys or big flat-cars; transported by locomotives; three-man crew.

( ii ) Track-mounted blower; nozzle mounted on fully articulated hydraulic boom; hopper filled by screw conveyor out of self-unloading cars; trans­ported by locomotives; two-man crew.

( iii ) Rubber-tire-mounted blower; towed behind die­sel truck; sand and cement on truck; nozzle held by hand; hopper filled by hand; three-man crew.

( iv ) Rubber-tire-mounted blower; towed behind spe­cially built truck; nozzle mounted on fully arti- -cu lated hydraulic boom; hopper filled by screw conveyor out of truck box; two-man cre w.

COST OF SHOTCRETING

True-Gun-all ~Repair and Maintenance for 68,817 batches, $0.51 per batch, Shotcreting \in~Trackless Development (12 x 12)­

Shotcreting labour . . . . . . . . . . . . . . . . . . . . . $2.32 Repair and maintenance on Gun-all machines... .. . . . . . . . . . . . . . . . . . . . . . .51

Repair and!maintenance on remaining shot-f creting equipment. .. . . . . . . . . . . . . . . . . . . .63 Supplies (cement, calcium chloride , etc.) 1.67 Sand .. . .. .. .......................... .21

$5.34 per batch

12' >: 12' SHOTCRETEO

CRIFT

r "'

Approximately 2.9 batches are used per foot of 12 by 12 trackless drift; theoretically, this should re­suit in more than a 4-in. thickness of shotcrete on ali rock surfaces. In practice, the rebound !osses are from 20 to 30 per cent, the measuring hoppers are never quite filled and extra gunning jobs are included in the drifting statistics (vent doors and bulkheads, repair gunning), with the result that the average thickness of shotcrete on drift surfaces is from 1lj2 to 2 inches.

COST COMPARISON

1964 Costs for 8 x 9 Timbered Drift ttrack), based on 6,496 feet of drift

Direct labour and bonus ...... . .... . ... . Drills ............... . . . ... . .. . . . .. . Blasting ................ . .... . .. .. .. . . Mucking (Eimco 21) . . . . . .. . . .. . .... .. . Timber ............. .... . ............ . Rockbolting ........ . ... ... ..... . . . . . . . Track ..... . . . .. . . . . .. . ... . . ... . Haulage (to ore or waste pass) . . .. . ...... .

$21.14 per foot 3.32 4.48 1.30 8.97

.84 3.25 1.06

$44.36 per foot

In the above, roughly 25 ·per cent of the labour can be attributed to timbering, and the cost of timbering per foot of drift would then be $14.25 (see F igure 2).

1967 Costs for 12 x 12 Shotcreted Drift (trackless), based on 11,144feet

Direct labour and bonus (except shotcreting) ... . ............... . .. . . . .

Drilling . . ............................ . Blasting.... .. . . ........... . . . .... . Mucking (Wagner ST4A) ...... .. . ... .. . Rockbolting ..... . . .... ... . . .. . . .. .. .. . Shotcreting (labour included) . ........... .

$11.53 per foot 10.45 8.88 5.62 .93

15.62

$53.03 per foot

Haulage to the ore or waste pass is included in the mucking costs (see F igure 2).

*1968 Costs for 12 x 12 Shotcreted Drift (trackless), based on 5,658 feet

Drilling (labour included) ........ . ... .. . Blasting (labour included) . ... ... . Mucking (labour included) .... . Shotcreting (labour included) . .......... . Miscellaneous (rock bol ting, timber, etc. ).

*Costing system changed

$ 9.80 per foot 13.28 6.39

15.61 2.65

$47.73 per foot

Haulage costs to the ore or waste pass in trackless drifts are included in the ST4A mucking costs.

Theoretical Cost of 12- by 12-ft Timbered Drift (trackless) This comparison is based on current costs for wages,

timber and other materials and makes the following as­sumptions:

(i) 12 by 12 ft inside timber is big enough for track­less equipment;

(ii) rock opening must be 15 by 13 72 ft to get 12 by 12 ft inside timber;

(iii) 15-ft-wide drift can be supported by timber.

The comparison then becomes:

Drilling .. Blasting .. Mucking . . . . Shotcreting ..... . Timbering .. . . Miscellaneous . . ..

Shotcreted Drift Timbered Drift (actual costs) (estima ted costs)

$ 9.80 per ft 13.28 6.39

15.61

2.65

$47.73 per ft

$13.20 per ft 17.90 9.90

24.00

$65.00 per ft

Figure 2.-Comparison of an 8- by 9-ft timbered drift The estimated cost of using timber support is 36 with a 12- by 12-ft shotcreted drift. per cent higher than the cost of using shotcrete.

1448 The Canadian Mining and Metallurgical

SUMMARY AND RE-MARKS

(1) It is apparent from the preceding cost data that substantial savings have been effected at Craig­mont through the use of shotcrete in lieu of timber in development work. In addition to the direct saving, there is also a considerable cost advantage in being able to make the shotcreted opening much smaller than the timbered opening to get the same final clear­ance. Resistance factors for ventilation are lower in shotcreted drifts; experience to date has shown that the cost of maintaining a shotcreted drift will be sub­stantially Jess than that of a timbered drift.

(2) Not all ground can be supported by shotcrete. Ordinary shotcrete will not adhere to ground that is making water; sorne of the ground at Craigmont was so incompetent that the weight of the freshly blown shotcrete would pull off a layer of back. Badly squeez­ing ground can only be supported to a limited extent by shotcrete, as evidenced in drifts along the wall of the Craigmont open pit. Quite often, it is necessary to use, in addition to shotcrete, standard rockbolts, or reinforcing bar rockbolts, particularly when drifting through strong flat-lying slips.

(3) Failure of shotcrete rarely occurs without warning; small hairline cracks will appear days bef ore a piece will actually drop off. The appearance of cracks does not necessarily augur a fall of grou nd; the fail­ures usually occur in very friable ground when the layer of shotcrete is not thick enough. If the appear­ance of cracks is considered to be a warning of failure, or if a failure has already occurred, the area is re­scaled and regunned and is usually as good as new.

( 4) Surprisingly little resistance was evident at Craigmont with the introduction of shotcreting as a means of ground support. Both supervisors and work­ers seemed to recognize very soon the superiority of shotcreting in the particular conditions they had to deal with, and before long there was a decided resist­ance to timbering for support. Training shotcrete operators is relatively easy, as no special skills are re­quired; a man can be taught to opera te the machine in a few hours, and can become quite proficient in a few days.

Literature on Shotcrete

The writer recommends the paper "The New Aus­trian Tunnelling Method," by Prof. L. V. Rabcewicz, Water Power, November, 1964, and "Engineering Properties of Shotcrete," by William R. Lorman, May, 1966, Clearinghouse for Federal Scientific and Tech­nical Information. The latter publication contains an extensive bibliography on information published on shotcreting.

RE-BAR ROGKBOLTING

In particularly bad ground or under special circum­stances, reinforcing bar rockbolts are used for extra support in addition to the shotcrete. The boit consists of ordinary % -inch-diameter deformed reinforcing bar eut into 5- or 8-ft lengths.

(CIM) Bulletin for December, 1968

Figure 3.-A re-bar "pig."

When ground is bad enough to require re-bar bolts, no bolting is undertaken until the area has been weil shotcreted. The procedure followed is to drill the re­quired length of 1% -in.-diam. hole, fill the hole with mortar (consisting of half mortar sand, half cement and water) and emplace the re-bar boit either with a sledge hammer or a rockdrill. A small concrete placer (known locally as a "cement pig," Figure 3), with a length of l-in. flexible polythene pipe, is used to place the mortar in the hales; usual practice is to drill ali the holes before setting up the placer, and then set all the bolts without interruption. Calcium chloride is often added to the mortar to speed up setting time.

Performance on installing re-bar rockbolts is 6.53 bolts per manshift. In the trackless development, bolt­ing amounts to an average of one re-bar per 15 feet of drift and one conventional boit per 2.7 feet of drift.

Re-bar bolts continue to provide support even if the material around the collar of the hole sloughs away, whereas a conventional rockbolt is useless once the ground against the plate deteriorates. It is for this reason that the re-bar bolts are used at Craigmont in extremely friable ground, or to make a "tie" across a strong slip plane.

Numerous pull-out tests conducted on installed re­bar bolts showed that the boit would break before the concrete bond would fail. Chemically grouted bolts also have this property, but are considerably more ex­pensive than the re-bar system.

STEEL-LINED RAISES

The Need for Lined Raises

Experience at Craigmont has shown that an unlined raise, after passing 200,000 tons or Jess of ore or waste, will deteriorate to the point where it can no longer be used. Renee, an important part of the con­version to underground mining was the installation of lined ore passes able to withstand the abrasion and impact of large tonnages.

It was decided to provide circular vertical ore passes, lined with 1h-in.-thick CHT 360 steel plate and

1449

Steel-Lined Raise Costs, 1966-1967

Pilot hole - 8-in. diameter . .. . . .. . . ... . Po~:u~ing concrete (labour included) ..... .

$ 8.23 per foot 46.00

Rrusmg ................ . .......... · · · 53.20 Steel plates (ma teri al only) ............ . 104.30 Mise. shop hardware ............... . . . 6.92 Preparation for raising (7 alimak set-ups) 4.42

$223.07 per foot

Not included - access drifts and raise cut-outs - haulage leve! chutes - chain controls, transfer points, etc. - mine services

------------- ····~·

Figure 4.-Plan of a vertical steel-Jined ore pass.

fx ,~ NELSON~

SHEAR CONNECTOR ' ] rr rr n =

'1 , ..... ~

fABRICATEO FROM r C H.T. STEEL

0

0

s'.sf-

Figure 5.-0re-pass liner plate.

1450

0i~'·lj. ... r

or:·:

0

0

J

with approximately 9 inches of concrete backing to the rock. The inside diameter was to be approximately 7 feet, with the circle being made up of sixteen plates, 16 inches wide (Figure 4) . The plates are 2 metres high, weigh about 180 pounds each and have eight % -inch Nelson stud anchors on the concrete side. Three mild steel lugs welded to the bottom of each plate serve to anchor the plate during the concrete pou ring cycle (Figure 5) . One plate in sixteen bas two holes drilled near the top to provide a means of anchoring the Alimak monorails, which are also 2 me­tres high.

Description of Raising Method

The job was accomplished as follows: On the main haulage leve!, a permanent concrete and steel chute was constructed in the raise eut-out in such a way as to permit the Alimak raise climber to enter and leave the raise through the chute and yet permit the tramming of the raise muck out of the chute. From the leve! above, a 4-in.-diam. pilot hole was drilled down the center of the raise and reamed to an 8-in. diameter.

A 4-in.-i.d. heavy-duty pipe was suspended in the hole with a tugger at the top to raise and lower the pipe·; the pipe itself was in 2-metre lengths, with threaded couplings. To start the steel and concrete lining, a 2-ft-high base ring was installed in the raise just above the chute and concreted in. The base ring bas sixteen sides and in plan view is identical to the final raise configuration. At this point, the normal raising cycle was commenced.

The cycle is as follows: A-Shift - Assuming that a round bas been blasted

on the previous shift (Figure 6) , A-Shift trams the muck out of the raise and lowers the 4-in.­diam. concrete pipe into the raise by adding 2-metre lengths of pipe to the top end. The two-man raise crew climbs the raise with a 2-metre Ali­mak monorail section, and installs it (Figure 7 ) . Then they dismantle the blasting ring, taking it down the raise, and attach the portable tugger to the Alimak deck. One man goes up with sixteen angle brackets and one stays below and sends up the first liner plate (the one with the two holes ) . The first plate is installed and bolted to the mo­norail. The remaining fifteen plates are sent up and installed one at a time, using the angle clips and working outward from the first plate. After al! plates are installed, the raiseman descends to get his partner, sorne blocking, the concrete bose ("elephant trunk") and the center support ring. They then install the above items, descend the raise and remove the tugger from the platform (Figu1·e 8).

B-Shift - One rais eman ascends the rai se; the other, with two labourers, goes to the top of the pilot hole to pour concrete. They pour concrete to with­in 3 inches of the top of the plates, using an air­powered vibrator to distribute the concrete, and moving the "elephant trunk" whenever necessary. The center support ring prevents deflection of the plates due to the pressure of the concrete . Then, the man on the raise climber removes the "elephant trunk" and descends the raise. The crew on top cleans the mixer, flushes the pipe and hoists the concrete pipe three lengths out of the hole.

The Canadian Mining and Metallurgical

APPROX 15' TO 17'

1 LOOSE MUCK

4" ID PIPE

- SM OIAM PILOT

HOLE

ROUND

BLASTEO

Figure 6.-(1) Showing conditions immedia;tel'y after blasting a round. A­SHIFT (2 men) starts the cycle by drawing the broken muck out of the ra ise.

' !~

·~ ,- ·~

.~i~

MONORAIL

SECT~

-

~: '"tr -{}-

~:.1

1 ... ,

F"' .~

:: ~;

Figure 7.- (2) A-Shift (2 men) adds one 2-metre length of concrete p·ipe. (3) Enters raise with 2-metre monorail section. (4) Scales raise, cleans loose muck from top of concrete, and removes blasting header and blast­ing ring. (5) Installs monorail section. (6) Des­cends raise with blasting ring. (7) Mounts Pikrose tugger with Alimak; one man goes up with angle brackets, one stays down and sends up first liner plate. (8) Installs first liner plate, attaching to monorail.

C-Shift - Two raise miners ascend the raise, drill off a raise round at !east 2 metres long (Figure 9) , remove the center support ring and install the blasting header and the blasting ring which protects the top of the plate; they th en load and blast the round.

In arder that the raise round may be blasted 9 feet away from eight-hour-old concrete, Sikacrete is used; this is a liquid accelerator which is added in the ce­ment mixer in a ratio of about three gallons to the yard. In over 1,600 feet of raise, there were only two instances in which plates were dislodged from the fresh concrete by blasting.

One lap of a raise was driven 365 feet from the same Alimak set-up by leaving a window in the lining at a leve! halfway up and servicing from that point; the Alimak went below the window only when blast­ing.

(CIM) Bulletin for Oecember, 1968

Figure 8.-(9) Still on A­Shift, the remaining fif­teen liner plates are sent up and are installed. (Ht) Go down for center sup­port ring, sorne block and concrete hose. (11) Hang center support ring, hook up concrete hose, block plates into final position, taping wide joints if ne­cessary, and remove tug­ger from Alimak plat­f o r m. This completes A-Shift's work.

Drilling Pilot Holes

u

ll

BLASTING RING

~

....

n

li

0 '~

. ·.:t;t ~ -:.C:.,'f, -;-::".-:-:

~(~ • •• 11 '1

~r . ... m

' .. .. ... r,l

::. i

. ' ' .,

Figure 9.- (12) On B­SHIFT, one raise man as­cends the raise; the other raise man, with two la­bourera, goes to the top of the pilot hole to pour concrete. (13) Pour con­crete to within 3 inches of the top of the plate, using the air vibrator. (14) Re­move concrete hose. (15) Remove two 2-metre lengths of 4-inch concrete pipe. This completes B­Shift's work. (16) C­SHIFT (2 men) ascends the raise and drills off the round. (17) Remove center support ring, ins­tall the blasting ring and blasting header, and blast the round. This completes C-Shift's work.

Access to the raises was available from existing Jevels at vertical intervals not grea ter than 190 feet; it was from these levels that the pilot hales we're dril­led, so that the longest pilot hale was 190 feet. The pilot hales were ali dawn holes, collared on the center of the raise, and in ali cases, except one, broke through within the 9-ft-diam. raise opening. A Gardner-Denver 133 rockdrill mounted on an Air Trac drilled the pilot holes using 10-foot Series 1800 hex-rods, with a 4-inch T.C. four-wing bit on the first pass and an 8-inch T.C. four-wing bit on the second pass.

ACKNOWLEDGMENTS

The writer thanks those who assisted in the prepa­ration of this paper, and also the management and di­rectors of Craigmont Mines Limited for the opportu­nity to present the paper and for their encouragement in the development of the ideas described herein.

1451