ested by experience in such work, some disturbance of
the surface above the east tunnel resulted, and several house fronts
were damaged. The portion of the tunnel affected was bulkheaded at
each end, packed with rubble and grouted with Portland cement mortar
injected under pressure through pipes sunk from the street surface
above. When the interior was firm, the tunnel was redriven, using much
the same methods that are employed for tunnels through earth when the
arch lining is built before the central core, or dumpling of earth, is
removed. The work had to be done very slowly to prevent any further
settlement of the ground, and the completion of the widening of the
other parts of the tunnels also proceeded very slowly, because as soon
as the slip occurred a large amount of timbering was introduced, which
interfered seriously with the operations. After the lining was
completed, Portland cement grout was again injected under pressure,
through holes left in the roof, until further movement of the fill
overhead was absolutely prevented.

[Illustration: COMPLETED ARCH AT MANHATTAN STREET]

As has been said, the tunnel between 157th Street and Fort George is
the second longest two-track tunnel in the United States. It was built
in a remarkably short time, considering the fact that the work was
prosecuted from two portal headings and from two shafts. One shaft was
at 168th Street and the other at 181st Street, the work proceeding
both north and south from each shaft. The method employed for the work
(Photograph on page 56) was similar to that used under Central
Park. The shafts at 168th Street and at 181st Street were located at
those points so that they might be used for the permanent elevator
equipment for the stations at these streets. These stations each have
an arch span of about 50 feet, lined with brick.

[Sidenote: _Steel Viaduct_]

The elevated viaduct construction extends from 125th Street to 133d
Street and from Dyckman Street to Bailey Avenue on the western branch,
and from Brook and Westchester Avenues to Bronx Park on the eastern, a
total distance of about 5 miles. The three-track viaducts are carried
on two column bents where the rail is not more than 29 feet above the
ground level, and on four-column towers for higher structures. In the
latter case, the posts of a tower are 29 feet apart transversely and
20 or 25 feet longitudinally, as a rule, and the towers are from 70 to
90 feet apart on centers. The top