Underground Structures

structures built beneath the earth’s surface. Such components of underground construction as architectural planning, construction methods, structural elements and their shoring, and waterproofing and air conditioning are determined by the structure’s purpose and the properties of the surrounding rock or earth.

Areas of use. The construction of underground structures is increasing in most of the industrially developed countries owing to greater economy in comparison with surface structures, technical or industrial necessity, urban development conditions, and military considerations. Underground location of structures is advisable in regions with such unfavorable climatic conditions as abrupt decreases in air temperature, hurricane winds, lengthy downpours, and mudflows or in areas with steep terrain. The construction of underground structures has developed significantly in the ore-mining industry.

Underground structures fall into several groups: transportation and hydraulic-engineering tunnels; subway facilities; power plants, chiefly hydroelectric; warehouses and cold storage; such municipal installations as pedestrian underpasses, garages, and municipal conduits; reservoirs for drinking water; facilities for oil and gas storage; containers for burying harmful industrial wastes; industrial enterprises; medical institutions; and military installations. A special group consists of the underground structures of mines located at the shaft bottom: underground power plants, car parks, and pumping and medical stations. This group also includes facilities connecting the surface structures with the working faces: mine shafts, main drifts, and galleries.

Underground power plants are more economical to operate than surface ones because of reduction in the length of delivery water lines, in volume of concrete work, and in materials consumption. When a large underground hydroelectric power plant is built, several million cubic meters of rock are removed. For example, 3.2 million m³ of rock was removed for the Inguri Hydroelectric Power Plant in the USSR, with a capacity of 1,400 megawatts. The machine shops of power plants have large cross sections of hundreds of square meters in area and reaching dozens and hundreds of meters in length. There are three types of underground hydroelectric power plants: terminal (the building is located at the end of the diversion channel), head (the building is close to the intake), and intermediate (the building is located at the middle of the diversion channel). Thermal and atomic power plants are also built underground, in Sweden and Switzerland. By the mid 1970’s there were 350 underground hydroelectric power plants in the world in operation and under construction, with a total capacity of 4 × 10⁴ megawatts.

Underground warehouses are economical because existing mining works may be accommodated beneath them and because of stable humidity and temperature and protection from fire. In addition, above-ground space is freed and security is easily maintained. Underground warehousing may be used for dead or live storage. With live storage, there is continous operation and the handling of a large daily volume of products and materials. Well-planned, large loading docks are essential, as are direct connections between the warehouses and rail facilities.

For live storage it is efficient to use drifts cut through limestone from the sides of worked-out quarries. Such a warehouse, with a usable area of about 5 hectares, is located near Kansas City in the USA. Part of the warehouse is used for storing a 25,000-ton quantity of frozen foods at a temperature reaching -32°C. The cost of building the warehouse was approximately 10 percent of the cost of above-ground cold storage of the same capacity. In Inkerman in the Crimea, an underground wine storehouse was built using mining works 10–12 m high and 200m long remaining after the extraction of shell limestone. For dead storage, it is convenient to use worked-out mine shafts, utilizing the vertical shaft for access. The capacity of such warehouses is 10⁵–10⁶m³ The chief outlay in the construction of underground warehouses is for the building of access and transport routes.

Areas beneath cities are undergoing increasing development. The integrated development in large cities facilitates efficient utilization of areas above ground, helps regulate transportation services, decreases traffic accidents, reduces street noise and air pollution from vehicle exhausts, and raises the aesthetic level of the urban environment.

Urban underground structures are of several types. Transportation-engineering structures include pedestrian and traffic tunnels, parking lots and garages, and railroad-terminal installations. Underground facilities in the service sphere include stores, cafés, movie theaters, exhibit halls, book repositories, archives, cold storage, vegetable storage, and automatic telephone exchanges. Underground industrial and power facilities include machine shops, laboratories, boiler rooms, and thermal plants. Among underground utility installations are gas and supply lines, boiler and heating networks, and transformer and gas-distributing plants. There are also underground installations for civil defense. Underground structures are an integral part of large cities.

Underground construction frees much of the usable area of developing regions. Garages, often built on several levels, are of special importance in the municipal underground system. Their capacity can reach several thousand vehicles, and the depth of the lowest level may reach 15–25 m. Also promising are the built-in garages in basement and underground stories of apartment buildings. Plans are in progress (1974) for a unified city-wide network of underground garages and parking lots in such cities as Stockholm, Paris, and Budapest. A major urban-development project is the plan developed from 1971 to 1973 for organizing and utilizing Moscow’s underground space.

Underground storage for petroleum products, natural gas, and drinking water is greater in capacity than above-ground storage and can accommodate up to several million cubic meters of products. The underground reservoirs are made of concrete, reinforced concrete, or metal. When petroleum and other combustible substances are stored underground, the savings from reduced evaporation soon compensate for the additional expenses incurred in building the reservoir. Underground storage is the most effective means of burying harmful industrial wastes from the atomic, chemical, and metallurgical industries. Repositories for such wastes are existing brine cavities, abandoned mining works, and reservoirs built in clayey rock. Industrial waste and waters are also pumped down apertures into aquifers that are unsuitable for use.

Such underground industrial installations as pumping and compressor stations, blast-furnace pits, and the jackets of open-hearth furnace regenerators are built at shallow depths. Underground plants, first constructed abroad in the 1930’s, were built at great depths. Many such plants were constructed during World War II, chiefly in Germany and Japan; by 1945, Germany had 143 underground plants.

Underground medical facilities are located in worked-out mines, chiefly salt mines. Works with a large cross-section, or chambers, are adapted for wards and for doctors’ offices. Underground medical facilities are advantageous because of unchanging air pressure, humidity and temperature, and the absence of bacterial flora, solar radiation, and noise. Inhalation is natural owing to the saturation of the environment with chemical elements, and the effect of the magnetic field is limited. This creates a microclimate particularly favorable for the treatment of lung diseases. For example, in the USSR there is an underground hospital for patients with bronchial asthma located at a depth of 200 m in a salt mine near the settlement of Solotvina in the Transcarpathian Oblast.

Construction and operation. The method selected for constructing an underground structure depends on the depth of the foundation, the structure’s purpose, and the mining conditions of the construction site. Shallow underground structures are built by the open cut-and-cover method and prefabricated cover method, or in trenches under thixotropic suspensions. Deep underground structures and such shallow ones as the drainage tunnels of subways or municipal conduits are built by the closed, or underground, method.

When the open cut-and-cover method of construction is used, the trenches or foundation pits are generally shored. Horizontal reinforcing with cross bracing is used in dry soil and in soil with a natural moisture content, and interlocked reinforcing is used in unstable, saturated soil. Construction in open pits is effective to depths of 7-10 m in ensuring dependable water drawdown.

Of the construction methods using a sinking structure, the caisson method has predominated. In the USSR, 60–70 caissons are built annually (1973), with an area of 100–13,000 m² and a placement depth of 10–55 m. An advanced method of building underground structures is with a caisson in a thixotropic housing; this method makes it possible to build caissons with large diameters. A successful means of controlling the caisson sinking has been developed that uses a system of jacks located around the caisson’s periphery. The caisson method is used to build underground garages with several levels as well as underground structures at metallurgical plants.

The slurry trench method of constructing underground structures is based on the ability of thixotropic suspensions to keep earthen walls from collapsing. This method consists of erecting the vertical walls of the underground structure in slot trenches before removing the earth inside the structure. This method is advisable if there are complex hydrogeological conditions since it obviates the need for lowering the water level or for freezing. This method is also effective in built-up areas when constructing underground structures of a moderate size and with a considerable depth, usually about 20 m; such structures include transportation tunnels and pedestrian underpasses.

The construction of underground structures may be effected by means of blasthole drilling and by using such mechanized complexes as mining combines and tunneling shields and such drilling methods as underground leaching and blasting compaction of soil.

The cavities formed by drilling methods are used as storage for petroleum products and compressed gases; therefore, the surrounding rock should be impenetrable, uniform in composition, and chemically neutral in relation to the stored products.

The adaptation of worked-out mine shafts with stable surrounding rock involves cutting to even out and broaden the excavations and to build new ones. In strong stable rock, the underground structures are generally not shored, although in certain cases temporary supports are used, including prestressed reinforced-concrete supports. Also built are permanent structures made of cast concrete, reinforced concrete, precast reinforced concrete, and cast-iron tubing.

Operation of underground structures mainly involves maintaining the necessary microclimate and providing artificial illumination and a power supply. Regulation of the air parameters is usually achieved by means of air-conditioning units. Waterproofing is carried out by caulking the construction materials or by altering them by means of chemical additives. In addition, waterproof covering is used to protect the structure’s outer and inner surfaces. Lighting is generally fluorescent, interiors are painted in light colors, and decorative windows are installed. When an outside source of electric power is used, emergency units are installed to meet the minimum requirements for power and illumination. Water removal is effected by laying piping in the excavation walls or drainage pipe in the earth; the water is drawn off through these pipes to water receptacles and pumps.