shock loading on the mast and rotation drive assem- bly. The roller stabilizers used on all drills are the tungsten-carbide studded roller type and are not normally repaired. Hole depth recorders are provided on both electric units. Records of drill bit life are kept to ensure that ade- quate life is being obtained from the tricone rotary tungsten-carbide bits. The grade of bit used is changed, depending on the rock formation being drilled (serpentine, footwall argillites, hanging-wall argil- lites/jade and volcanic formations). Bearing failure is the major reason for a bit being discarded — not loss of gauge or button failure — as the material being drilled is not highly abrasive. Some attempts have been made to replace bearings, without a great deal of success. Dust suppression is available at all times of the year for both operator and equipment intake air. The BE 40R drills were originally equipped with Rotoclone dust collectors, but these were removed and a water dust suppression system installed. The GD 80 drill was introduced in February, 1975. It has a fully pressurized operator’s cab and machine house, and was originally equipped with a pulse air- type bag collector. High air bailing velocity exiting from the drill-hole collar, coupled with drill holes that contained water, combined to render it ineffective due to dust particles freezing and blinding the filter bags in the baghouse. Water obtained from the hot water waste supply at the powerplant is trucked to the drills at the mine and kept heated in 600-gallon tanks fitted onto the drills. Water lines are insulated and wrapped with electric heat tape, and a blow-down valve is fitted on each drill allowing the operator to purge the water lines at shift change or prior to switching off power to the drills. Drilling patterns are varied depending on the rock competency, the depth being drilled and the frag- mentation required for loading. All drill patterns are laid out in accordance with the drill and blast fore- man’s instructions, with the assistance of the survey crew, who report to the short-term planning engineer. A first line of drill holes, adjacent to the back line of the previous blast, is laid out by instrument. This is normally 20 ft (6.09 m) to 25 ft (7.6 m) from the previous blast holes, depending on the bench height and backbreak. The drill line delineating the back line of the new blast is also laid out by instrument and the intermediate holes are tape measured and marked for drilling with the appropriate total depth of hole required, including subgrade drilling. Various patterns have been tried, including square and staggered patterns, with different burden and spacing ratios. However, a 22-ft (6.7-m) by 22-ft square pattern appears to give consistent fragmenta- tion results when drilling 45-ft (13.7-m) deep benches. This is reduced to 18 ft (5.5 m) by 18 ft on a 30-ft (9.1 m) deep bench and further reduced if drilling ramps. The drilling of a subgrade of 5 ft (1.5 m) is practised in waste rock on the hanging wall; in the softer argillite/serpentine rocks, a 3-ft (0.91-m) sub- grade is drilled. No sub-grade is normally required in the asbestos ore. In winter, the drill holes are covered with wooden discs or plastic bags filled with snow to prevent snow from entering the drill hole and causing a loss in depth. Redrilling of holes represents about 10% of total drilling and is due to broken ground, collars of holes collapsing or ingress of water. Secondary drilling is carried out by using a Gard- TABLE 1 — Equipment Details Unit Manufacturer Details Drills 1 Productivity GD80 — 9% in. hole RR10S — 9 in. hole 45R — 9% in. hole Gardner-Denver Robbins Bucyrus-Erie 45 ft/hr 40 ft/hr 45 ft/hr P&H 1900 AL — ll-yd 400 cu. yds/hr Wabco Wabco Euclid Haulpak — 50-ton Haulpak — 75-ton R85 — 85-ton Loaders 2 Caterpillar 992B — 10-yd 220 cu. yds/hr Tractors Caterpillar Caterpillar Caterpillar Caterpillar D8K — Ripper D8K — Winch DSH — Ripper 825 — RT Graders 2 16G — snow wings and plow Caterpillar ner-Denyer Airtrac equipped with a PR123 percussion/ rotary hammer, drilling a 21%4-in. hole and blasting with 2-in. by 16-in. Powerfrac. Secondary drilling on grade level is not significant except in the higher pit elevations, where the volcanic and jade intrusions are extremely difficult to blast effectively. Large oversized blocks of ore and waste are drilled with a Schram drill mounted on a rubber-tired tractor, utilizing 114-in. chisel-pointed drill steel. Primary Blasting The blast pattern is influenced by the pit configura- tion. The wider the bench, the more efficiently the blast can be designed to combine maximum yardage broken with good fragmentation and minimize the amount of moving that must be done by the electric shovels and drills. By ensuring that the drill pattern is correctly laid out, the holes are drilled to the re- quired depth and are charged properly with the ex- plosive; choke blasts of up to 400,000 cu. yds have been blasted with excellent results. All explosives are loaded in accordance with the drill and blast foreman’s instructions and loading of holes is carried out by only one team of blasters to ensure consistent results, with a high degree of safety. Ammonium nitrate is delivered to the mine by means of a bulk carrier, offloaded at the storage area into two 105-ton storage silos by the use of a low- pressure blower (7 psi), and then gravity loaded into the Amerind Mackissic 10-ton or 15-ton ANFO truck units for transportation to the open pit, 3 miles (4.8 km) away. Laboratory tests are carried out at regular intervals to ensure that consistent quantities of diesel fuel are being metered to obtain the required 5.7% diesel fuel addition to the ammonium nitrate prill. Maximum use is made of the ammonium nitrate fuel oil mixture (ANFO), in spite of water being encoun- tered in the holes. In the summer, wet holes can — 25 —