1. Introduction
Of all the headings driven in hard rock mines, shafts are the most costly and time consuming. Moreover, the shaft sinking procedure is intricate and arduous. While a few shafts are advanced by big-hole drilling methods, the great majority use the traditional “drill and blast” cycle to which this chapter is devoted. In North and South America, shafts for smaller mines have traditionally been sunk rectangular and relied on timber for support.
Larger mines have typically employed circular shafts lined with concrete poured in place as the sinking advances. Today, independent mining contractors sink most shafts. While there have been significant technical advances, no world records have been broken for rate of shaft-sinking advance in hard rock since 1962. Part of the problem is that mining contractors have no discretionary funds to invest in research and development, while mining companies and government agencies have other priorities for what little resources are available.
Except at great depth, shafts sunk in hard rock mines do not normally require special considerations to maintain wall stability. A few shafts require the ground freezing method to traverse a waterbearing horizon (discussed separately in Chapter 12 – Collars and Portals).
2. Rules of Thumb
Schedule
• From time of award to the start of sinking a timber shaft will be approximately five months. A circular concrete shaft may take three months longer unless the shaft collar and headframe are completed in advance. Source: Tom Anderson
• The average rate of advance for shaft sinking will be two-thirds of the advance in the best month (the one everyone talks about). Source: Jim Redpath
Hoist
• The hoist required for shaft sinking needs approximately 30% more horsepower than for skipping the same payload at the same line speed. Source: Jack de la Vergne
• Without slowing the rate of advance, a single drum hoist is satisfactory to sink to a depth of 1,500 feet at five buckets per foot, 2,000 feet at four buckets per foot, and 2,500 feet at 3½ buckets per foot. For deeper shafts, a double drum hoist is required to keep up with the shaft mucker. Source: Jack de la Vergne
Bucket
• For sinking a vertical shaft, the bucket size should be at least big enough to fill six for each foot of shaft to be sunk; five is better. Source: Marshall Hamilton
• For the bucket to remain stable when detached on the shaft bottom, its height should not exceed its diameter by more than 50%. Source: Jim Redpath
• Tall buckets can be used safely if the clam is used to dig a hole in the muck pile for the buckets. Source: Bill Shaver
• A bucket should not be higher than 7½ feet for filling with a standard Cryderman clam (which has an 11-foot stroke). Source: Bert Trenfield
• A bucket should not be higher than 6 feet when mucking with a 630, which has a 6-foot-6-inch discharge height. Source: Alan Provost
• You can load a tall bucket using a 630 if you slope the muck pile so that the bucket sits at an angle from the vertical position. Source: Fern Larose
• In a wet shaft, the contractor should be able to bail up to 10 buckets of water per shift without impeding his advance. Source: Paddy Harrison
Water Pressure
• For any shaft, the water pressure reducing valves should be installed every 250 feet. “Toilet tank” reducers are more reliable than valves and may be spread further apart. Source: Peter van Schaayk
• Water pressure reducing valves may be eliminated for shaft sinking if the water line is slotted and the drill water is fed in batch quantities. Sources: Allan Widlake and Jannie Mostert
Compressed Air
• One thousand cfm of compressed air is needed to blow the bench with a two-inch blowpipe. Source: Bill Shaver
• Twelve hundred cfm of compressed air is needed to operate a standard Cryderman clam properly. Source: Bill Shaver
Shaft Stations
• The minimum station depth at a development level to be cut during shaft sinking is 50 feet. Source: Tom Goodell
• A shaft station will not be cut faster than 2,000 cubic feet per day with slusher mucking. It may be cut at an average rate of 3,500 cubic feet per day with an LHD mucking unit. Source: Jim Redpath
Circular Shaft
• The minimum (finished) diameter of a circular shaft for bottom mucking with a 630-crawler loader is 18 feet. Source: Tom Goodell
• For a circular concrete shaft, the minimum clearance between the sinking stage and the shaft walls is 10 inches. Source: Henry Lavigne
• A circular concrete lined shaft sunk in good ground will have an average overbreak of 10 inches or more, irrespective of the minimum concrete thickness. Source: Jim Redpath
• For a rope guide system in a shaft being sunk to a moderate depth, the minimum clearance between a conveyance (bucket and crosshead) and a fixed obstruction is 12 inches and to another bucket is 24 inches. At the shaft collar, the clearance to a fixed obstruction may be reduced to 6 inches due to slowdown, or less with the use of fairleads or skid plates. In a deep shaft, 18-24 inches is required to clear a fixed obstruction and 30-36 inches is required between buckets, depending on the actual hoisting speed. These clearances assume that the shaft stage hangs free and the guide ropes are fully tensioned when hoisting buckets. Various Sources
• When hoisting at speeds approaching 3,000 fpm (15m/s) on a rope guide system, the bonnet of the crosshead should be grilled instead of being constructed of steel plate to minimize aerodynamic sway. Source: Morris Medd
• The maximum rate at which ready-mix concrete will be poured down a 6-inch diameter slick line is 60 cubic yards per hour. Source: Marshall Hamilton
Timber Shaft
• For a timber shaft, the minimum clearance to the wall rock outside wall plates and end plates should be 6 inches; the average will be 14 inches in good ground. Source: Alan Provost
• For a timber shaft that encounters squeezing ground, the minimum clearance outside wall plates and end plates should be 12 inches. Source: Dan Hinich
• For a timber shaft, the blocking should not be longer than two feet without being pinned with rock bolts to the wall rock. Source: Jim Redpath
3. Tricks of the Trade
• For a circular concrete shaft, the bigger the bucket, the better. Source: Jim Redpath
• A bail of a shaft sinking bucket built from four chains and paired on each side permits smoother dumping and requires less overwind clearance in the headframe than does the typical three-chain bail. Sources: Fern Larose and Gabriel Juteau
• If the pull cord system fails, shaft signals can be transmitted between the collar and a sinking bucket or the Galloway stage at the shaft bottom by tapping on a steel pipe in the shaft. Source: Jack Brooks
• In an emergency, fresh concrete may be prevented from setting solid by mixing in several bags of ordinary sugar kept on site in case of such an event. Source: Jim Redpath
• To reduce segregation of concrete poured down a slick line, “grease” the line with half a readymix truckload of grout before starting the pour. Source: Bob Dengler
• The most practical way to handle the unavoidable segregation of concrete poured down a slick line is to wet the empty line and then direct the first concrete to the shaft bottom. After less than one cubic yard is wasted, the shaftmen can plainly see that segregation has ceased and direct the remainder of the pour into the forms. Source: Tom Goodell
• With the long shaft rounds used today, the average overbreak will be 12-14 inches, unless perimeter drilling and smooth-wall blasting is employed. Source: Jack de la Vergne
• A small circular concrete shaft can be slashed successfully on a raisebored hole as small as 4 feet diameter when hand-held sinker drills are employed. Source: Bob Hendricks
• A shaft of large diameter employing a long-steel drill jumbo is better slashed on a raisebored hole of 8-10 feet diameter. Source: Bill Shaver
• Horsepower demanded by the shaft-sinking hoist is greatest at the start of the sink and slowly decreases until a great depth has been reached. This is one reason not to be overly concerned about a hoist motor running hot at the onset of shaft sinking. Source: Bob Pronovost
• If an AC hoist motor overheats while sinking and equipping a concrete shaft in one pass, the problem is often traced to the long creep from the lower chairs to the galloway caused by leaving the shaft equipping too far behind. Source: Jack de la Vergne
• If a hoist motor stalls when lifting a full bucket off the bottom, the problem can often be traced to voltage drop. The torque of an AC motor varies with the square of the voltage. A 10% loss in voltage results in a 21% reduction in torque and hence rope pull. Source: Jarvis Weir
• A hoist satisfactory for shaft sinking will invariably have sufficient capacity to subsequently hoist the muck from pre-production development. Source: Jack de la Vergne
• One good location for the stage winches is in the sub-collar. The winches can be arranged to pay the rope directly down the shaft or, if necessary, deflected with a Teflon slide thereby removing the requirement for head sheaves and problems with fleet angles. The bucket cross heads ride on the return ropes (two parts of line), which are anchored high in the headframe. Source: Bert Trenfield
• A good way to ensure that the galloway stage hangs plumb is to employ four stage winches, each with one part of line. The four ropes are arranged offset at an angle from the shaft centerline to provide guides for the shaft buckets. Sources: Doug McWhirter and Vic Whalen
• The best way to ensure that the galloway stage hangs plumb is to employ three stage winches, each with two parts of line. The ropes are located at the stage in a manner that provides equal weight distribution to each rope. Source: Jim Tucker
• The doors on a galloway stage (required to pass the vent duct when hoisted to clear for a blast) are better built of aluminum to facilitate manhandling. Source: Tony Campbell
• Corrosion of stage ropes is inhibited by employing zinc bushings in addition to, or instead of, the normal HDP plastic bushings on the crosshead attachment. Source: OML (Ontario Ministry of Labour, Mines Branch)
• The best way to hang rigid vent duct when shaft sinking to moderate depths is to install an extra tugger hoist or small winch on the surface, of the rope from which hangs inside the string of duct already in place. It is then a simple matter to thread the rope down through a section of duct to be installed and hoist it neatly into place for fastening. Source: Bert Trenfield
• In a frozen shaft, the easy way to anchor a rock pin required to support screen on the shaft walls is to drill the pin hole slightly downwards and fill the annulus with plain water poured from an oil can after the pin is inserted. Source: Herb Fredrickson
• Deep shafts employing hydraulic drills only need compressed air for mucking, rock bolt holes, and cleaning the bottom. This means that a large compressed air plant is virtually idle throughout a large portion of the cycle time. This spare capacity can be put to use very easily by using air as a refrigerant and providing a standard expansion valve to cool service water required for drilling, etc. before resorting to air conditioning. Source: Jack de la Vergne
4. Types of Shafts
Shafts sunk today in hard rock mines are mostly limited to the standard three-compartment timber shaft and the circular concrete shaft. The standard three-compartment timber shaft has two hoisting compartments that measure six feet by six feet inside the timbers. The third compartment, used for a manway and utilities, is sometimes slightly shortened from the six-foot width. These timber shafts are still contemplated for exploration entries in general and production shafts for small hard rock mines. For remote sites, the shaft timber may be replaced with steel sets to save on weight and the cost of transportation. It is now widely believed that any savings thus realized are later lost in the shaft sinking costs and schedule, mainly due to the increased difficulty in installing blocking (which cannot be nailed), catch pits, and launders (water rings). In addition, omission of hanging rods makes the sets more difficult to hang and align.
Although a number of ingenious methods have been developed for timber shafts to successfully traverse bad ground conditions (jacket sets, pony sets, squeeze blocking, etc.), timber shafts are no longer considered when bad ground or highly stressed ground is anticipated. In North America, suitable timber has become scarce and expensive. For this and other reasons, shaft sinking by this method is now mainly confined to deepening existing timber shafts. The circular concrete shaft, sunk vertical, is invariably employed for large shafts and most often employed for any deep shafts. The circular shafts may be as little as 12 feet in diameter for shallow applications, but deep shafts are better tooled for shaft sinking if they are of larger diameter. At hard rock mines, only a few shafts of appreciable depth have been sunk larger than 26 feet in diameter outside of South Africa.
5. Hoist Selection for Shaft Sinking
Table 10-1 identifies the size of a double drum hoist satisfactory to sink a typical shaft at an acceptable rate of advance.
Table 10-1 Selection (diameter) of Double Drum Hoist for Shaft Sinking
10.6 Solved Problems
Overbreak Measurement
Example
Find the concrete volume.
Facts:
1. The shaft has a 20-foot inside diameter
2. The minimum lining thickness will be 12 inches
3. Overbreak will be 10 inches
Example
Find the average overbreak.
Facts:
1. 78 cubic yards of concrete was poured for a 16-foot lift
2. The shaft is 20-feet diameter
3. A minimum of 12 inches of concrete is required
