Mining Excavation

Mining Excavation

 

the artificial creation of cavities in the earth’s crust through the extraction of rock; the cavities, or excavations, that are created are used to open mineral deposits, transport minerals, and ventilate mines. Excavations include shafts, galleries, crosscuts, and horizontal and inclined excavations (drifts, chutes, inclines).

There are several ways to excavate a mine. The choice of method depends on the function of the excavations; mining, geological, and hydrological conditions; and the level and extent of mechanization in the excavation operations.

In rock where the subterranean-water inflow is up to 5 m3/hr, mine shafts are constructed by conventional methods. Special methods are used when the inflow is greater than 5 m3/hr; determining factors include not only the subterranean-water inflow but also the firmness of the rock. For example, when creating horizontal and inclined excavations in sands and sandy loams, special methods are used if the water inflow is less than 5 m3/hr; in firm rock, however, water inflows of as much as 50 m3/hr may not require the use of special methods. In contrast to conventional methods, special methods involve prior reinforcement of the rock in which the mining excavation is driven; this is done to increase stability and reduce water inflow. Special methods include the caisson method, the injection method, and the frozen-rock method. Another special method involves lowering the level of subterranean water by means of wells and draining devices. In another special method, the rock along the contour of the mining excavation is reinforced with sink or sheet piles, which protect the mining face from rock slides, reduce water inflow into the mining excavation, and significantly improve the conditions for removing rock at the mining face and erecting the permanent support.

Experimental work is being conducted on the fusion of loose, moist rock by electric current. In the construction of vertical shafts and inclined excavations in quicksands, the frozen-rock method is generally used along the contour of the mining excavation, whereas horizontal excavations are created by completely freezing the rock mass from the surface or by driving under compressed air (caisson method).

In water-bearing rock, preparatory drying and reduction of the water inflow can be accomplished by pumping the water with submersible or artesian pumps both from wells bored into the water-bearing horizon from the surface and through mining-face filters in the excavation.

Vertical mining excavations (shafts) are generally constructed with workers at the excavation face. As of 1975, large-scale industrial experimental work was under way to study the possibility of boring mine shafts. In the construction of mine shafts, a sinking platform is placed 10–20 m from the face; a temporary lift is constructed with buckets that automatically unload on the surface, thus providing a transport link between the face and the surface. Hard and medium rocks are broken by boring and blasting, and soft rocks are broken mechanically, with picks and pneumatic hammers.

In the USSR, most processes for constructing mine shafts are mechanized. Blastholes are drilled to a depth of 5 m with remote-controlled multispindle drilling machines, which are moved along the face by a loader telpher. Loaders with mechanized grippers are used to load the rock. The permanent support of most shafts is cast-in-situ concrete, which is brought from the surface in pipes and placed behind the metal support moving directly behind the face. Rock loaders have been used as the basis for the construction of several types of cutting units; the units allow maximum integration of major cutting operations and permit record cutting speed and labor productivity. In 1969, a world record of 401.3 m/month was established in the 17–17b shaft in the Donets Coal Basin; the shaft is 6.2 m in diameter.

Cutting machines have been used successfully to construct shafts in soft and medium rock. The machines allow mechanization of the processes by which the rock is broken down and removed from the shaft face and also permit simultaneous installation of the permanent cast-in-situ concrete support. The construction of shafts by such cutting machines in the Karaganda Coal Basin has resulted in increased labor productivity (13.7 m3 of prepared shaft per miner) and driving speed (133 m/month).

Horizontal and inclined mining excavations are driven by drilling and blasting or by driving machines. The selection of the driving technology and equipment depends both on the size of the shaft cross section—which depends on the function of the shaft—and on the hardness and firmness of the rock.

When excavations are cut by drilling and blasting methods, loaders or scrapers are used to load the broken rock.

Driving machines are used to cut horizontal and inclined (to 35°) excavations in soft rock. The machines break the rock and load it into cars or onto a conveyor. This method is safer and more efficient than the drilling and blasting method, and the surrounding rock is left intact. When the face advances, workers install supports, new rail lines, conveyors, pipelines, ventilation pipes, and fire-prevention water-supply lines.

In the USSR, the maximum driving speeds that have been attained are 2,000 m/month with driving machines, 800 m/month with the drilling and blasting method, and 4,700 m3/month with the creation of excavations of large cross section in collecting stations underground. Average speeds are much slower, however, because of the complex mining and geological conditions of actual sites and the necessity of suspending other operations while placing the timbering in transport tunnels and elsewhere.

Underground chambers are usually located in hard, firm rock. Depending on the dimensions of the cross section and the firmness of the surrounding rock, the chambers are constructed either with a continuous face or by dividing the face into segments, with consecutive developing of the rock mass in each segment.

The permanent timbering of the chambers is usually made with cast-in-situ concrete or reinforced concrete. Sometimes use is made of combined timbering, in which, for example, the roof is supported by cast-in-situ reinforced concrete and the walls are supported by an anchoring support and spray concrete on a metal mesh.

To ensure a reliable contact of the timbering with the surrounding rock mass, the remaining gaps between the timbering and the rock mass are filled with inert, incombustible material or slurry. In hard, firm rock, tunnels are constructed by drilling and blasting with equipment and technologies analogous to those used to cut excavations. In soft and medium rock, tunnels may be constructed with tunneling shields or cutter-loaders. When mechanized tunneling shields are used in constructing tunnels, a cast-in-situ pressed support is used as the permanent support; the support is produced by pressure on a concrete mixture from the shield itself or from another device. A pressed cast-in-situ concrete casing is placed tightly against the surrounding rocks, eliminating the need to force the solution behind the support.

In the USSR, the technology and methods of cutting mining excavations are being improved so as to allow the construction and introduction of driving machines that provide for maximum mechanization of driving operations.

Long tunnels, including those with large cross sections and in hard rock, are constructed with special driving machines or mechanized shield units.

REFERENCES

Pokrovskii, N. M. Proektirovanie kompleksnykh vyrabotok podzemnykh sooruzhenii. Moscow, 1970.
Mel’nikov, L. L. Sooruzhenie vyrabotok bol’shogo secheniia v krepkikh porodakh. Moscow, 1974.

D. I. MALIOVANOV

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