he unit costs of the work. These are
given in two parts: The first is called the unit labor cost, and is the
cost of the labor in the tunnel directly chargeable to the thing
considered. It does not include the labor of operating the plant, nor
watchmen, yardmen, pipemen, and electricians. The second is called "top
charges," a common term, but meaning different things to different
contractors and engineers. Here, it is made to include the cost of the
contractor's staff and roving laborers, such as pipemen, electricians,
and yardmen, the cost of the plant and its operation, and all
miscellaneous expenses, but does not include any contractor's profit,
nor cost of materials entering permanent work.

The contractor's plant is to be described in a paper by Henry Japp,[B]
M. Am. Soc. C. E., and will not be dealt with here.

The contractors carried on their work from three different sites. From
permanent shafts, located near the river in Manhattan, four shields were
driven eastward to about the middle of the river; and, from two similar
shafts at the river front in Long Island City, four shields were driven
westward to meet those from Manhattan. From a temporary shaft, near East
Avenue, Long Island City, the land section of about 2,000 ft. was driven
to the river shafts.

[Footnote A: Presented at the meeting of December 15th, 1909.]

[Footnote B: _Transactions_, Am. Soc. C. E., Vol. LXIX. p. 1.]

TUNNELS FROM EAST AVENUE TO THE RIVER SHAFTS.

The sinking of the temporary shaft at East Avenue was a fairly simple
matter. Rough 6 by 12-in. sheet-piling, forming a rectangle, 127 by 34
ft., braced across by heavy timbering, was driven about 28 ft. to rock
as the excavation progressed. Below this, the shaft was sunk into rock,
about 27 ft., without timbering. As soon as the shaft was down, on
September 30th, 1904, bottom headings were started westward in Tunnels
_A_, _B_, and _D_. When these had been driven about half the distance to
the river shafts, soft ground was encountered. (See Station 59, Plate
XIII.) As the ground carried considerable water, it was decided to use
compressed air. Bulkheads were built in the heading, and, with an air
pressure of about 15 lb. per sq. in., the heading was driven through the
soft ground and into rock by ordinary mining methods. The use of
compressed air was then discontinued. West of this soft ground, a top
heading, followed by a bench, was driven to the soft ground at about
Station 66.