excavations, temporary plank decks
are placed and maintained during the drifting and tunneling
operations, and after the permanent subway structure has been erected
up to the time when the street surface is permanently restored. The
roof of the subway is about 5 feet from the surface of the street,
which has made it necessary to care for the gas and water mains. This
has been done by carrying the mains on temporary trestle structures
over the sidewalks. The mains will be restored to their former
position when the subway structure is complete.

From Bowling Green, south along Broadway, State Street and in Battery
Park, where the subway is of reinforced concrete construction, the
"open cut and cover" method is employed, the elevated and surface
railroad structures being temporarily supported by wooden and steel
trusses and finally supported by permanent foundations resting on the
subway roof. From Battery Place, south along the loop work, the
greater portion of the excavation is made below mean high-water level,
and necessitates the use of heavy tongue and grooved sheeting and the
operation of two centrifugal pumps, day and night.

The tubes under the East River, including the approaches, are each
6,544 feet in length. The tunnel consists of two cast-iron tubes
15-1/2 feet diameter inside, the lining being constructed of cast-iron
plates, circular in shape, bolted together and reinforced by grouting
outside of the plates and beton filling on the inside to the depth of
the flanges. The tubes are being constructed under air pressure
through solid rock from the Manhattan side to the middle of the East
River by the ordinary rock tunnel drift method, and on the Brooklyn
side through sand and silt by the use of hydraulic shields. Four
shields have been installed, weighing 51 tons each. They are driven by
hydraulic pressure of about 2,000 tons. The two shields drifting to
the center of the river from Garden Place are in water-bearing sand
and are operated under air pressure. The river tubes are on a 3.1 per
cent. grade and in the center of the river will reach the deepest
point, about 94 feet below mean high-water level.

The typical subway of reinforced concrete from Clinton Street to the
Flatbush Avenue terminus is being constructed by the method commonly
used on the Manhattan-Bronx route. From Borough Hall to the terminus
the route of the subway is directly below an elevated railway
structure, which is temporarily supporte