Underground
construction
Metro is high-speed urban transport, with lines laid in underground tunnels at a subsurface or deep level, on the surface, on overhead roads and flyovers. There are two major approaches to building metro tunnels and stations: shielding (underground) and cut-and-cover.
With shielding, all work is done underground — without disrupting traffic or affecting the town utilities in place. It is obvious that any major project in central Moscow can only be implemented by underground engineering. To bring people, machines and materials into the tunnel under construction, shafts are sunk, with hoists engineered. These are also used to bring out the excavated soil.
Soil excavation and lining are performed using a variety of methods. Drilling-and-blasting method is based on breaking soil with explosive charges placed in pre-drilled boreholes. After that, soil is removed from the working face by rock-loading machines and cast-iron or reinforced concrete lining is put in place by special-purpose erectors.
The cut-and-cover method is used for building subsurface lines. Work is done in a pit or trench, which after lining is covered with soil. In conditions of dense urban development, special structures are used to stabilize pits, such as a diaphragm wall or bored-secant piles with anchor or girder bracing.
Metro stations are most often built by the cut-and-cover technique; tracks between them by tunnelling.
Precast tunnel
casing of cast-iron
liners
Cast-iron liners
The number of components in a ring depends on its diameter and the structure of lining. Metro running tunnels have cast-iron liner rings of standard dimensions: width (measured lengthwise in tunnels) 1 m, outer diameter 5.5 m, inner diameter 5.1 m, rim height 0.2 m.
The New-Austrian procedure provides for primary tunnel lining to be engineered using shotcrete, and for secondary lining using cast-in-situ reinforced concrete with travelling formwork. The tunnel proper is built using drilling-and-blasting technique or combines.
When tunnels are built with tunnel-boring machines (TBM) using earth pressure balance, shields are used which have rotary tools, equipped with bits and cutters for earthwork.
In complex conditions, earth pressure balance TBMs are used. Bentonite slurry or crushed soil presses on the working face.
Watertightness high-precision reinforced concrete lining is put in place using an erector, which is part of the TBM.
When hydrogeological conditions in the area of future underground development make it impossible to use conventional techniques, special-purpose methods are used to reinforce soils.
The most widespread and time-proven procedure for stabilizing unstable soils is to freeze them. It was used as far back as when the first metro lines were built. Soils are frozen by bringing them into contact with a coolant circulating through freezing pipes, integrated into a single network. Freezing produces a stable body of soil.
The method of jet grouting of soils consists in impregnating and mixing soil with cement grout. The result is a body of soil and cement mixture, with structural performance enabling shielding work.
What a metro line is made of
A metro line comprises stations, running tunnels in-between, a depot, and technical facilities. The depots are on the surface, and trains enter tunnels through special entry points where tracks dive underground.
Tunnels are built single or double-track. To enable switching from a double-track section to two single-track sections, special-purpose crossover tracks are engineered. On the surface, two tracks are normally laid at the same time. Trains are powered by direct current running through the contact rail at 825 V.
Layout of facilities
on a metro
section
Platforms
A metro station may have up to three platforms. Most stations are single-platform, with tracks running along the sides of a wide central platform. Two-platform stations have tracks between the side platforms; three-platform stations, between the side platform and the central one.
Depth
Deep-level underground stations are at a depth of over 15 metres. They can only be built by tunnelling. The deepest station of the Moscow Metro, Park Pobedy, is 80 metres below ground level. Shallow, subsurface, stations are set up in custom pits. Surface stations are normally roofed platforms, while overground stations are set on overhead rails. If passengers need to go up or down more than 4.5 metres, stations are equipped with escalators.
Concourses
Most stations of the Moscow Metro are three-span, comprising a central space and two side naves. Two-span stations have columns running along the platform centreline or between the tracks if there are two platforms. Single-span stations (most often subsurface) feature a single-space concourse with a vaulted or flat ceiling. With three or more tracks running at the same level, the stations are multi-span. In this case, the concourses are connected via passages.
Design of deep-level stations
The metro station has one or several vestibules, which can be underground or on the surface. From the vestibule, the station can be reached through an inclined escalator tunnel.
The station consists of an open and closed space—with or without passages between the halls, respectively. Under the passenger platforms are service passages. The outer edges of station tunnels are covered with decorative track walls. Between the tunnels liners and track walls are laid cables and other services. To enable their inspection, the track walls have special-purpose doors every 50 metres.
All underground station installations are equipped with water-deflecting canopies. These are watertight ceilings, which direct leaking water into special-purpose gutters, from where it is pumped out.
Deep-level stations connect to the surface via escalators, which are set up in inclined tunnels. The centreline of an escalator tunnel nearly always matches the long axis of the station itself, and is tilted 30 degrees to the surface. Inside are three or four escalators.
The top section of the escalator tunnel houses the power compartment under the vestibule floor. Here are the motors that drive the escalators. At the bottom, below the level of the station platform, is the machine room. It houses machines that drive the escalator stairs.
As inclined tunnels cross a variety of soils, they are cased with cast-iron or concrete liners.
Three-vault pillar-type stations are engineered using cast-iron or concrete liners. Such a station comprises three parallel tunnels, running at the same level. The central tunnel with the station platform can be larger in diameter than the side tunnels, which house tracks. The tunnels are interconnected by passages. These are engineered by installing special-purpose frames into standard segments of the central and side tunnels.
Diagram of a pillar station
with reinforced concrete lining
and space bridging
with reinforced concrete girders
A station’s construction begins with tunnels; when all of these are completed, space frames are put in place (or passage vaults are set in concrete). This done, passages are opened between the tunnels.