the City Hall station. The road at this point is through an arched
tunnel. In order to secure consistency in treatment the roof of the
station is continued by a larger arch of special design. (See
photograph on this page.) At 168th Street, and at 181st Street,
and at Mott Avenue stations, where the road is far beneath the
surface, it has been possible to build massive arches over the
stations and tracks, with spans of 50 feet.




CHAPTER II

TYPES AND METHODS OF CONSTRUCTION


Five types of construction have been employed in building the road:
(1) the typical subway near the surface with flat roof and "I" beams
for the roof and sides, supported between tracks with steel bulb-angle
columns used on about 10.6 miles or 52.2 per cent. of the road; (2)
flat roof typical subway of reënforced concrete construction supported
between the tracks by steel bulb-angle columns, used for a short
distance on Lenox Avenue and on the Brooklyn portion of the Brooklyn
Extension, also on the Battery Park loop; (3) concrete lined tunnel
used on about 4.6 miles or 23 per cent. of the road, of which 4.2 per
cent. was concrete lined open cut work, and the remainder was rock
tunnel work; (4) elevated road on steel viaduct used on about 5 miles
or 24.6 per cent. of the road; (5) cast-iron tubes used under the
Harlem and East Rivers.