[SUBJECT TO REVISION]

DISCUSSION OF THIS PAPER IS INVITED. It should preferably be presented in person at the New York meeting, February, 1916, when an abstract of the paper will be read. If this is impossible, then discussion in writing may be sent to the Editor, American Institute of Mining Engineers, 29 West 39th Street, New York, N. Y., for presentation by the Secretary or other representative of its author. Unless special arrangement is made, the discussion of this paper will close Apr. 1, 1916. Any discussion offered thereafter should preferably be in the form of a new paper.

Underground Mining Methods of Utah Copper Co.*

BY THOMAS S. CARNAHAN, B. S.,† BINGHAM CANYON, UTAH

(New York Meeting, February, 1916)

THE mining property of the Utah Copper Co. is situated in the West Mountain mining district, Salt Lake County, Utah, in the Oquirrh Range of mountains.

GEOLOGY

In a general way the rock formation of the district consists of a series of beds of quartzite and limestone intruded by a body of monzonite porphyry roughly elliptical in shape, with an east-west axis over a mile in length and a north-south axis of about 3,000 ft.

This porphyry intrusion, accompanied by strong mineralizing action and fracturing, resulted in the formation of orebodies in the adjacent sedimentary rocks, and was itself sufficiently mineralized to make it one vast orebody. The Utah Copper Co.'s mining property comprises within its boundaries practically the entire outcrop of the monzonite mass, of a commercial grade.

GENESIS OF PORPHYRY ORE

Following an intricate system of fracturing, mineral solutions circulated freely through the porphyry, depositing small quantities of copper and iron, and resulting in a considerable silicification of the monzonite. The quantity of copper originally deposited was undoubtedly too small to have ever given the porphyry a commercial value, had not secondary enrichment, due to the leaching of the copper from the surface of the mass and re-deposition in the sulphide zone, been active over a long period of years.

A large portion of the leached material has probably been removed by erosion, but there still remains a blanket of leached and oxidized porphyry of varying thickness covering the sulphide ore, known as capping. Over certain sections of the orebody, this capping contains commercial quantities of copper carbonates, but most of it contains little or no copper.

* Originally presented at the annual meeting of the Utah Section, Salt Lake City, Oct. 4, 1915.

† Mine Engineer, Utah Copper Co.

To Jan. 1, 1915, a total of 377,690,000 tons of ore had been developed, of which 342,500,000 tons averaging 1.45 per cent. copper still remained to be mined. The average thickness of the developed ore was 465 ft., while the layer of capping covering the ore averaged 115 ft. Further development will undoubtedly show an increase in the average thickness of the ore, with a corresponding increase in the tonnage of developed ore.

UNDERGROUND MINING AUXILIARY TO STEAM-SHOVEL OPERATIONS

As is generally known the Utah Copper mine is primarily a steamshovel operation, and it will perhaps surprise many that up to April, 1914, a considerable tonnage of ore was obtained by underground mining methods.

During the early years of steam-shovel mining the amount of ore available was naturally limited, since most of the shovels were working in capping, and it was necessary to stope a large tonnage underground in order to keep the mills at Garfield running at capacity.

During the 3-year period from 1911 to 1913 inclusive, a total of 102,719 ft. of drifts, raises, etc., was driven on the property. Most of this development served the double purpose of proving the shape and value of the orebody, and providing the necessary openings for stoping operations. The output of ore from underground operations in these 3 years amounted to 3,071,719 dry tons, of which 247,280 tons came from development and the rest from stopes.

SHRINKAGE STOPING SYSTEM ADOPTED

Realizing that underground mining was to be but an incident in the mining of the orebody as a whole, a system of stoping was adopted which would not affect adversely future steam-shovel operations. In order to fulfill this requirement it was essential that the surface should not be caved, that no large openings be left unfilled, and that the capping should not be mixed with the ore.

The system as finally adopted and successfully operated, consisted in starting stopes on three separate levels or tunnels. The first of these tunnels, at an elevation of 6,733 ft. driven 7 by 7.5 ft. in the clear, was the main or motor-haulage level. The second, at an elevation of 6,883 ft. or 150 ft. vertically above the main level, was equipped for hand tramming only, so all drifts, crosscuts, etc., were driven 5.5 by 6.5 ft. in the clear. The third, at an elevation of 6,983 ft., or 100 ft. vertically above the second, was also a hand-tramming level and driven 5.5 by 6.5 ft. in the clear. These three levels were connected by many manways and raises for dropping the ore from the upper tunnels to the motor-haulage level. An underground shaft centrally located, equipped with a com

SECTION 1-1

Motor Haulage Level-*

pressed-air hoist, was used to hoist supplies from the motor-haulage level to the upper levels, but no ore was handled through it. Each level had one or more surface connections, affording good natural ventilation to all parts of the workings (Figs. 1 and 2).

OREBODY WORKED ON THREE LEVELS

By means of these three levels, the orebody was divided into blocks for stoping, the central block, 500 ft. wide (see Fig. 1), was bounded on the main level by two parallel motor drifts, and on the upper levels by two main parallel tramming drifts, directly over those on the haulage level. At intervals of 120 ft. along these motor drifts, raises 5 by 6 ft. were put up on a 55° pitch to the level above and afterward extended to the third level.

In order to make the stopes as safe as possible, to minimize the amount of timber required, and to leave substantial walls for the safety of future steam-shovel operations, it was decided that the standard width of stope should be 16 ft., with 44-ft. pillars between stopes.

The method of starting stopes on the motor-haulage level was somewhat different than on the upper levels although the size of stopes and pillars was the same.

Main or Motor-Haulage Level

Motor drifts were driven on the motor-haulage level, spaced at 50ft. centers, and parallel to the main drifts forming the boundaries of the block. At intervals of 60 ft. along these drifts the surveyor marked the center of the stopes as the drifts were driven; if the ground required timbering the tunnel sets were spaced to be suitable for stope-chute sets later on. After the stope-chute sets were completed, a man with a stoping machine drilled both sides directly over the chutes, nearly horizontally and on the center line of the stope, to form a pocket at this point. The next round on each side pointed strongly upward, and from that point on, the raise was extended on a 60° pitch until the face was 31 ft. vertically above the top of rail. The stoping machine was then taken out, and a No. 9 Leyner machine set up near the top of the raise, and a drift started each way. These drifts were run horizontally following the center line of the stope, and were made 8 by 8 ft. Since their maximum length from any ore chute was only 18 ft., little shoveling was necessary to get the ore to the chute. After this drift was completed for the full length of the stope, the Leyner machine took 4 ft. off each side of the drift, to bring the stope to the standard width of 16 ft. The ore broken in all this work was drawn from the chutes, so that when this stage of operation was completed, an excavation 450 ft. long, 16 ft. wide and 8 ft. high was ready for stoping.

Chute timbers were constructed as follows: Three tunnel sets of 12 by 12-in. timbers were set up, spaced at 51⁄2-ft. centers. Posts 8 ft. long

were set in hitches cut in the floor deep enough to make the bottoms of the caps 7 ft. above the top of rail. Caps were cut 10 ft. long, so as to extend 6 in. beyond the side of each post, and blocked tightly against the walls. Planks, 2 by 12 in. by 7 ft. long, nailed under the cap, acted as spreaders for the posts, but no sills were used. The sides were lagged with 2 by 12in. planks. Enough ground was then broken above the tunnel sets to make room for a short set. The posts for this set were cut 3 ft. 9 in. long of 12 by 12-in. timber. Only two short sets were put in, one on each side of the chute mouth, the top and sides being lagged with split round poles (see Fig. 3).

Manway Raises and Drifts. In alternating drifts, that is, at intervals of 100 ft., manway raises were put up in the pillars, midway between the stopes. The raises were started in offsets 6 ft. from the center of the track, and driven on a 50° pitch. They were made 4 by 6 ft. and divided

FIG. 3.-STOPE CHUTE TIMBERS ON MOTOR-HAULAGE LEVEL.

into a chute and a manway by means of stulls and 3 by 12-in. planks. The manways were equipped with ladders, and the chutes equipped with gates, so that the ore broken in the raises could be loaded direct into motor cars. When the manways reached an elevation 31 ft. vertically above the top of rail, manway drifts were started both ways at right angles to the raises, or parallel to the motor drifts. The bottoms of these drifts were 25 ft. above the top of rail, or on a level with the bottom of the stopes. These drifts, at 19 ft. from the manways, broke into the sides of the stopes. At the junction of the drifts with the stopes, 8 by 8-in. sills 8 ft. long were laid 3 ft. in the stope and 5 ft. in the drift and 6 by 6-in. cribbing built upon them, dapped 1 in. so as to leave a space of 4 in. between cribbing. The crib timbers were 4 ft. long, making the manways 3 by 3 ft. in the clear. The cribbing on the drift side was left off for the first 5 ft. to form an entrance into the manway. The manway timbers projected 2 ft. into the stope and 2 ft. into the pillar, thus placing them