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Mine supports
permanent structures erected in underground mines to prevent the surrounding rocks from collapsing and swelling, to keep the cross-section at the correct size, and to absorb and manage rock pressure.
Mine supports should ensure a safe working environment, be economical, portable and easy to maintain, and should not interfere with or complicate production processes. In permanent and preparatory mine-workings, supports are classified by material, by type (hard, malleable, hinged or combined), by lifespan (permanent or temporary), by shape (trapezoidal, open or closed arch, ring-form, elliptical or polygonal) and by mine type (horizontal, inclined and vertical). In permanent excavations (shafts and pit-bottom workings, tunnels, permanent crosscuts, chambers, etc) with a long lifespan, concrete or reinforced concrete supports are used alongside prefabricated roof supports (tubing) of reinforced concrete and steel and metal frameworks.
Ring-form concrete supports lie flush to the surrounding rocks; it may be 20-25 cm thick or more, depending on the magnitude of rock pressure and the diameter of the excavation. Concrete vaulted roof supports are used in horizontal and inclined (up to 30-35 degrees) excavations for (non-swollen) medium and hard rock formations; the wall linings are curved to deal with any lateral pressure. Reinforced concrete supports differ in that they are reinforced by flexible steel rods or rigid metal bars that allow them to absorb the tensile forces from large and uneven rock pressure. In permanent horizontal mine-works, precast concrete linings (solid tubing, arch, ring-form or elliptical) are used alongside solid concrete supports. In permanent mine-works, hard supports are commonly used that can absorb a certain amount of elastic strain to avoid having to change the shape or size of the support or the excavation.
Arch and ring-form yielding supports are the most commonly used in preparatory excavations. Yielding supports give under rock pressure as a result of displacement or strain, consequently reducing the size of the excavation, but maintain their carrying capacity and efficiency of construction. Overlapping frame components made of a pliable, specially produced metal are connected by clamps and bolts; yielding roof supports allow their components to "slip" at their connecting points. Mixed-type supports consist of hollow reinforced concrete bars (tubular or rectangular) and metal beams connected by hanging brackets. Roof bolting is used as a standalone support, particularly in ore mining, in combination with frame supports. Also used (mainly in ore mines) are hard metal supports (trapezoid, arched and ring-form) consisting of I-beams and repurposed railway rails with a mix of lining elements, nuts and bolts, or more rarely using special "shoes". Reinforcement frames are usually set 0.5-1 m apart. Between supports, the roof and walls of the mine are protected by concrete slabs, metal bars, nets of various types, boards and saw-cuts.
Wooden frame supports are used mainly in mines that have a small cross-section and short lifespan (three to five years maximum). Longer-lifespan mines are usually reinforced using non-flammable materials (metal, concrete, etc). Trapezoid frames are usually set 0.5-1 m apart, with wooden reinforcements in between. Closed (or complete) frames using sleepers are used in the event of pressure from below. Wooden square sets in preparatory vertical and inclined (more than 45 degrees) small-section mine-works (prospecting holes, dummy shafts, ramps, joining sections, etc) are solid when beams are placed on top of each other and on supports measuring 0.5-2 m between beams. Beams are usually made of logs or boards, and are connected by dovetail joints; they usually slot into a groove in the supports.
At working faces, supports are designed to protect the area where equipment is located and where excavation work takes place. Such arrangements are known as individual supports and consist of a frame of metal or wooden racks and roof bars. Frames are laid out regularly along the working face and moved accordingly; roof supports are installed differently depending on whether the coal seam is shallow or steep. In modern coal mines, more modern mobile mechanical supports have become popular. Special supports known as waste-edge chocks are used to control the roof (manage rock pressure) in excavation areas. Advances in the design of su
pports for permanent and preparatory excavations include: reductions in the amount of material used in roof supports per unit of carrying capacity; the introduction of polymer materials; the creation of prefabricated sections and reusable supports; mechanised mobile roof supports for use in both preparatory digging and work at the face; temporary supports for linking sections; the use of polymer resins and quick-setting mineral bindings to fix roof bolts in place; and screw anchors.
