SMALL WATER MAINS BETWEEN STREET SURFACE AND SUBWAY ROOF, SUBSTITUTED FOR ONE LARGE MAIN—125TH STREET AND LENOX AVE.
SPECIAL CONSTRUCTION OF 6-1/2-FOOT SEWER, UNDER CHATHAM SQUARE
A feature of the construction which attracted considerable public attention while it was in progress, was the underpinning of a part of the Columbus Monument near the southwest entrance to Central Park. This handsome memorial column has a stone shaft rising about 75 feet above the street level and weighs about 700 tons. The rubble masonry foundation is 45 feet square and rests on a 2-foot course of concrete. The subway passes under its east side within 3 feet of its center, thus cutting out about three-tenths of the original support. At this place the footing was on dry sand of considerable depth, but on the other side of the monument rock rose within 3 feet of the surface. The steep slope of the rock surface toward the subway necessitated particular care in underpinning the footings. The work was done by first driving a tunnel 6 feet wide and 7 feet high under the monument just outside the wall line of the subway. The tunnel was given a 2-foot bottom of concrete as a support for a row of wood posts a foot square, which were put in every 5 feet to carry the footing above. When these posts were securely wedged in place the tunnel was filled with rubble masonry. This wall was strong enough to carry the weight of the portion of the monument over the subway, but the monument had to be supported to prevent its breaking off when undermined. To support it thus a small tunnel was driven through the rubble masonry foundation just below the street level and a pair of plate girders run through it. A trestle bent was then built under each end of the girders in the finished excavation for the subway. The girders were wedged up against the top of the tunnel in the masonry and the excavation was carried out under the monument without any injury to that structure.
THREE PIPES SUBSTITUTED FOR LARGE BRICK SEWER AT 110TH STREET AND LENOX AVENUE
