Shaft sinking

• 1. Introduction

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  Shaft sinking is the act of excavating a mine shaft from the top down, beginning from the surface with no access to the bottom whatsoever. Right now, the deepest continuous single-lift mine shaft in the world sits at South Deep Mine in South Africa, owned by Gold Fields Limited. It reaches 2,991 meters into the earth. Along with its twin ventilation shafts, it took ten years to sink and equip. That number invites a question: what does it actually take to drive a tunnel straight down into the planet, through groundwater and unstable rock, for nearly three kilometers? The answer involves a cast of specialized workers called sinkers, a sequence of construction phases that each feed into the next, and an industry that is only now beginning to mechanize what has for most of its history been done by hand and explosive. What makes shaft sinking so distinct from other forms of mining development? And how does a team manage to build something permanent while descending through ground that has never been disturbed?

• 2. What Shafts Actually Do

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  A mine shaft is not a single-purpose hole. It is a vertical corridor that carries people, materials, broken rock, compressed air, water, backfill, power, communications, fuel, and ventilation air, sometimes all at once. Depending on the mine's design, different portions of the shaft are divided into compartments by structures called shaft sets, which can be made of either timber or steel. The vertical members inside a shaft set are called guides; the horizontal members are called buntons. The largest compartment typically houses the mine cage, a conveyance that functions much like an elevator, suspended from a hoist on steel wire rope. Cages may be single-, double-, or in rare cases triple-deck, and they always carry multiple redundant safety systems against unexpected failure. A second compartment holds one or more skips, which hoist ore to the surface. A third compartment serves as an emergency exit, either through an auxiliary cage or a ladder system. An additional compartment carries mine services: high-voltage cables and pipes for water, compressed air, and diesel fuel. The distinction between shaft, winze, and raise matters here: when the top of the excavation is the ground surface it is a shaft; when it begins underground it is a winze or sub-shaft; when it is excavated upward from within an existing mine it is called a raise.

• 3. The People Behind the Drill

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  Historically, shaft sinking has been among the most dangerous of all mining occupations. The workers who specialized in it were called sinkers, and the work was the preserve of dedicated mining contractors. Today those contractors are concentrated in four countries: Canada, Germany, China, and South Africa. The difficulty of the work is structural, not incidental. Restricted space, the constant pull of gravity, groundwater intrusion, and the need for specialized procedures combine to make shaft sinking one of the most formidable development methods in mining. Experience matters enormously on a shaft sinking project. It is typical for progress, measured as a "sinking rate," to follow a learning curve as the team repeats the same cycle of activities over and over. Using experienced shaft sinkers shortens that learning curve and reduces the overall duration of the project. Three things are considered key to a successful project: getting the components of the sinking setup correct and installed at the right time, involving experienced people as early as possible, and using the available time on the shaft bottom as efficiently as possible. All activities at the shaft bottom sit on the critical path for the project schedule, meaning any delay there cascades through everything else.

• 4. Safety and the Second Way Out

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  The Hartley Colliery disaster left a permanent mark on how mines are designed. After the single shaft at that mine became blocked, the United Kingdom made single-shaft mines illegal in 1862. The law established that all underground mines must have a second means of egress. Many other mining jurisdictions around the world adopted the same rule, which is why shafts are so often found in pairs, though multiple alternative methods of providing that second exit also exist. The shaft lining is a central safety feature. After excavation and the installation of temporary ground support, many shafts are lined to prevent loose or unstable rock from falling into the working space. The material of choice is mass concrete, poured behind shaft forms in lifts of six meters as the shaft advances deeper. Shotcrete, brick, cast iron tubing, and precast concrete segments have all been used at various times. In extreme cases, particularly when sinking through halite, composite liners made of two or more materials may be required; specific circumstances have even called for bitumen and squash balls. The lining always lags behind the advancing shaft bottom by a fixed distance, so temporary ground support is installed below it. Installing that temporary support, a process called bolting, is among the most physically demanding parts of the sinking cycle, as bolts must be driven using pneumatic powered rock drills.

• 5. Building Down, Phase by Phase

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  Constructing a new shaft follows a sequence of distinct phases, each with its own infrastructure and logic. The first is the box cut: a large square or rectangular excavation down to bedrock, completed with earthmoving equipment, inside which the shaft collar will be built. Transferring the loads of the collar, headframe, and hoisting plant into solid rock prevents subsidence. Where bedrock lies too deep or is too weak, civil engineering techniques such as diaphragm walls or concrete piles are used instead. Next comes the pre-sink, excavating and supporting the first 60-100 meters of the shaft barrel, often using smaller pneumatic equipment and mini-excavators. Hoisting during the pre-sink is typically done by a mobile crane on the surface. The pre-sink must reach sufficient depth before the Galloway, a multi-level working platform suspended by winches from the surface, can be installed safely above the blasting zone. The Galloway is usually constructed off to one side of the shaft and lowered into the pre-sink by crane in as complete a state as possible. Once the collar is built and the Galloway is in place, the headframe and hoist house are constructed and the hoist and winch ropes are installed. Only after the full sinking setup is commissioned does the main sinking phase begin, working through the cycle of excavation, temporary ground support, lining, and extending shaft services, all the way down to the final design depth.

• 6. The Shaft at Its Core

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  At the top of any traditionally built shaft stands the headframe, also called the winding tower, poppet head, or pit head. Depending on the type of hoist used, the top of the headframe houses either a hoist motor or a sheave wheel with the motor mounted on the ground below. The headframe typically contains ore storage bins for material being transferred to the processing facility. At ground level beneath it sits the shaft collar, also called the Bank or Deck, a massive reinforced concrete structure with more than one level. It provides the foundation for the headframe's weight and the point through which workers, materials, and services enter and exit. Beneath the collar, the shaft barrel descends into the ground. At points where the barrel meets horizontal workings there is a shaft station, or inset, where men, materials, and services can transfer between the vertical shaft and the horizontal tunnels, which can extend toward the ore body for many kilometers. The lowest shaft station is usually where broken rock leaves the mine levels and transfers to the shaft; a loading pocket is excavated there to accommodate the transfer equipment. Below the lowest station the shaft continues as the shaft bottom, connected to the rest of the mine by a ramp that also houses the water handling facility called the sump, since water naturally collects at the lowest point in any mine.

• 7. The Push Toward Mechanization

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  Raise boring technology has already changed the calculus on certain shaft projects. Where access exists at the bottom of a proposed shaft, raise borers have been used to create a pilot hole first; the sinking phase then enlarges that pilot hole to full diameter in a process called slashing. Muck from slashing falls down the pilot hole and is handled by existing mine infrastructure below, removing it from the critical path entirely. Full-face shaft boring, which works like a vertical tunnel boring machine, has shown promise as a further step. As of 2019, that method was not yet widespread. Some shafts have already moved away from rigid guide beams inside the shaft, using steel wire ropes kept in tension by massive counterweights at the shaft bottom called cheese weights, named for their resemblance to a truckle or wheel of cheese. Research and development in temporary ground support is focusing on robotic application of shotcrete and the commercialization of thin sprayed polymer liners. The industry is also integrating more of the permanent shaft setup into the sinking phase itself rather than building temporary infrastructure first. When service piping used during sinking does not need to be stripped out to make way for permanent piping, the overall project duration shortens. Shafts in the Canadian Shield illustrate a regional variant: bedrock there is both strong and close to the surface, reducing the time and complexity required to establish the shaft collar.