The first step is to chart the subway precisely, taking note of all underground utilities, existing subways, foundations, and soil and rock conditions.
If the route happens to coincide with the path of a street, or if it lies beneath clear areas above ground, a “cut and cover” method of digging trenches at the surface several blocks at a time may be used.
If, on the other hand, the surface is occupied by buildings, plowing through them for the sake of constructing an underground tunnel may not be the wisest approach. When the streets and buildings of a bustling city cannot be disturbed, subways are made by digging very deep shafts from which the tunnels are bored.
If the cut and cover method is possible, the construction crew marks out on the street the exact dimensions of the trench and inserts heavy vertical members on each side of the street. Steel beams are placed to span the vertical members.
The crew then proceeds to dig along the designated path of the tunnel. Twelve inch thick wooden beams or steel plates are placed across the street to provide a temporary road and a platform from which to lower material into the trench.
It is essential to divert all utilities with temporary pipes and ducts, or to support them as the digging progresses with horizontal beams, or shoring. Any water seeping into the trench is channeled into sewers. The floor of the tunnel is constructed first, then the sides, with the aid of supporting rods. Electrical cables for power, signals, and emergency systems are pulled through ducts in the wall to vaults sited every few hundred feet along the tunnel.
When the form work for floor and walls has been erected, the crew pours concrete through long funnels and constructs the ceiling. The tunnel structure thus complete, tracks and lighting are installed and the outside of the tunnel coated with waterproofing. The crew then fills the trench to street level with compacted gravel and soil.
If a deeper tunnel is required, or if the route of the tunnel passes under city buildings, the second method of construction is used. Deep vertical shafts are dug at regular intervals along the charted path of the subway. Steel lining supports the shafts to prevent mud slides and collapse. It is through these shafts that the “muck” (soil and rock) from digging is removed and fresh air pumped in through ventilator pipes.
Deep down in the earth, workers bore away with high powered drills and boring machines. When they face hard clay or solid rock, they drill small holes into which they insert explosives. The ensuing rubble is carried away in carts.
The sides of the hole (slightly larger than the size of the finished tunnel) are covered with a steel tube called a shield. In solid rock this shield may not be necessary. Segments of the shield are lowered down the shaft and erected a piece at a time; jacks are used to advance the pieces through the earth.
The segment at the top and front of the advancing tunnel always protrudes somewhat to obstruct any rock and soil that might fall. An inner face for the shield is then constructed out of cast iron or precast concrete. Crews can work simultaneously in different shafts, since precise measurements have of course been made in advance so that the tunnels will meet.
Ground containing a great deal of water presents another set of problems. The air pressure at the face of the tunnel must be increased considerably to prevent water from rushing in.
This involves construction of two parallel concrete walls within the tunnel; high pressure is maintained between one wall and the face of the tunnel. Between the two walls is a compression chamber, where the workers must remain for a while to adjust when passing back and forth between the normal and high pressure areas. Steel doors with air locks are constructed in the walls to permit passage without loss of pressure.
After completing the tunnel, the workers construct tracks and string cables along its cylindrical sides. The original construction shafts may be left in place to be used for stairways and elevator shafts, and through which to install ventilating fans to propel fresh air into the tunnel.