On the Park Avenue section from 34th Street to 41st Street two separate double-track tunnels were driven below a double-track electric railway tunnel, one on each side. The work was done from four shafts, one at each end of each tunnel. At first, top headings were employed at the north ends of both tunnels and at the south end of the west tunnel; at the south end of the east tunnel a bottom heading was used. Later, a bottom heading was also used at the south end of the west tunnel. The rock was very irregular and treacherous in character, and the strata inclined so as to make the danger of slips a serious one. The two headings of the west tunnel met in February and those of the east tunnel in March, 1902, and the widening of the tunnels to the full section was immediately begun. Despite the adoption of every precaution suggested 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_]