Difficult Engineering in the Subway (1902)

Supporting The Columbus Monument.

Century Magazine · October, 1902 · pp. 908-911.

By Frank W. Skinner, C.E., Author of "Triumphs of American Bridge-Building," in the June Century, and Associate Editor of the Engineering Record.

Under The Columbus Monument. Near where the subway swings around the southwest corner of Central Park it passes through and under the foundations of the Columbus monument. The slender stone shaft, surmounted by its heroic statue, is seated on a molded pedestal with extended base, which altogether rises seventy-five feet above the street and weighs nearly a million and a half pounds. It has a masonry foundation forty-five feet square and fourteen feet deep, which was built partly on rock, but mostly on earth. Its east corner overhangs the subway nearly forty feet, and the position of the latter is so near the surface of the ground that its walls and roof cut a wide and deep section out of the masonry.

This made it necessary to support the monument so that its tall shaft should neither lean nor settle a hair's-breadth, nor the thin, accurately fitted pedestal stones be cracked, or their polished joints open, under the great strains developed when the masonry was cut out to a mere shell and the support removed from under a third of its base and almost up to the center, reducing its stability to a slender margin. This would have been a delicate and hazardous task under any circumstances, but was made more difficult and dangerous by the unknown conditions and the known character of the soil.

Supporting The Columbus Monument.

It was uncertain whether the interior of the foundation masonry was sound and strong enough to resist the great strains which might be safely imposed on the best stonework, and great potential peril lay in the fact that only one corner of the foundation, that diagonally opposite to the subway, stood on the rock, the rest being built on earth and sand. The surface of the rock slopes down very steeply toward the subway and below it, so that when the excavation was made there and the equilibrium of the compressed earth was destroyed, the unbalanced pressures, especially in wet weather, might well cause the earth to slide out from under the foundation and produce a serious disaster. Safety alone was not sufficient: there could not be tolerated even a slight or harmless disturbance of the monument. The lofty shaft is like a sensitive needle, quick to quiver and diverge with an almost imperceptible displacement at the base, and to magnify many times the smallest unequal settlement, so as to deflect its graceful lines from the perfect vertical and emphasize even a trivial deviation to the appearance of an offensive blemish. These exacting conditions called for the work to be executed with an excess of solidity, and at the same time the commercial requirements demanded rapidity, simplicity, and economy.

It was determined first to extend the foundations under the center of the monument to a greater depth, so that they would reach below the subway excavation and beyond the base of the shaft, and thus carry most of the load directly and prevent any danger of slipping down the sloping rock surface. Afterward the wide corner of the foundation was to be first supported, then undercut and undermined, so as to allow the excavation to be made under, through, and alongside, and the subway to be built and eventually carry the overhanging part of the old foundation.

Supporting The Columbus Monument.

First, shafts fifty feet apart were sunk about twenty-five feet deep on the north and south sides of the old foundation, and their bottoms were connected by a small tunnel which was roofed by the base of the old foundation and had its floor well below the bottom of the subway, and its east wall where the west wall of the subway was to be built. A solid bed of concrete was laid on the floor of the tunnel, and vertical timber posts were set on it and wedged up against the under side of the foundation to support it. The tunnel was then filled solid with stone masonry, beginning at the middle, working out to both ends, and permanently inclosing the timber posts. This virtually made a massive stone beam supporting the foundation from side to side and seated below the level which would be disturbed by the subway construction.

A trench ten feet deep was dug around the east side of the monument, exposing the upper part of the foundation where it extended over the line of the subway. From this trench a gallery, or slot, six feet high was cut about twenty-five feet horizontally into the face of the foundation masonry, and as it advanced, vertical timber posts were set on its floor and wedged up to support its roof. When the slot extended about thirty feet through the corner of the foundation, two solid steel girders, like beams in a railroad-bridge, were set in it between the rows of posts.

Supporting The Columbus Monument.

A pit was dug close to the foundation at each end of the slot, and the bottom was covered with concrete, which afterward formed part of the subway floor. On this concrete were set braced wooden posts to carry the ends of the girders, which were thus lifted clear of the floor of the slot. Pairs of steel wedges were driven between the tops of the girders and the roof of the slot until they lifted the whole mass of masonry a fraction of an inch and transferred the weight of the overhanging portion to the girders. Then the roof posts were removed, and the outer edge of the foundation and all that portion below the slot were cut away, the excavation completed, and the subway built in it, under the overhanging foundation and around the posts which supported the girders.

Under the edge of the overhanging foundation, outside of the girders, a wall was built on the concrete roof of the subway which is very strong, with steel beams and columns. A course of cut stone was laid in the upper part of the wall, and on it many pairs of steel wedges supported a loose course of cut stone carefully fitted in under the overhanging masonry of the foundation. The wedges were driven up, and developed an enormous pressure, which lifted the monument again, transferred part of its weight to the new wall, and released the girders. They were removed, and the spaces they had occupied were filled in solid with masonry, built and wedged up from the center outward in the same manner as the wall. Liquid cement was forced into the interstices between the wedges, and solidifying as hard as flint, perfected the support of all the overhanging foundation on top of the finished subway.

In doing this work one portion had to be completed before another could be begun, and as but few men could work at once, and the operations were conducted with great care and accuracy, it took about six weeks to complete it in a manner which was highly creditable to the able engineers who designed and approved it and the experienced contractors who skillfully executed an undertaking unlike any previously recorded.

Relocating A Long, Thin, High Wall. An ordinary derrick will handle compact loads of three, five, or even ten tons; a hundred-ton load is about the limit of the capacity of the heaviest steel-ordnance cars drawn by powerful locomotives, or of the largest hydraulic jacks, which will lift it a few inches so slowly that the motion is scarcely perceptible. A building weighing five hundred tons may be carefully braced and lifted up or moved laterally with rollers on smooth level tracks by the help of scores of powerful jacks. It would require immense power to push along even a fifty-ton boulder resting on the ground, and be yet more difficult to move a long, thin, high wall several feet transversely without cracking, tipping, or twisting it.

Generally, when such a wall is to be relocated, it is taken down and rebuilt; but such was not the case on the subway above One Hundred and Thirty-fifth street, where, at the entrance to a tunnel section, walls nearly two hundred feet long retain the bank on each side of the cut. After the structure was completed it was decided to widen it eleven feet to receive a third track, and although it was at first intended to tear down the masonry and build new, it was finally decided to move it bodily, and this was successfully accomplished at a saving of several thousand dollars.

The walls are of concrete and brick, thirteen feet high at one end, three feet thick on top, and weigh about four hundred thousand pounds each. The earth was dug away behind them for a width of six feet, and to a depth a little below their foundations. In the bottom of each trench a concrete floor was laid just below the level of the foot of the wall. Small holes were tunneled under the wall a few feet apart, and in them were laid transverse timbers reaching to the floor of the trench and having both ends supported on cross-sills. Narrow, thin, greased steel track-plates were inserted under the walls, on top of the timbers, and extended across the trench floors. Small steel bearing-plates were set on the track-plates under the front and rear edges of the walls, and pairs of oak wedges, driven between the cross-timbers and their sills, lifted the whole wall on the steel plates.

Horizontal five-ton jack-screws were set close together against the face of the wall at the base for its whole length, and being simultaneously operated, the wall in a few hours was moved back five and a half feet on to the floor in the trench. The projecting ends of the track-plates were cut off, and the spaces between the plates under the wall were filled with liquid cement. The work on each wall was done by twenty men in ten days, and the walls were not distorted a sixteenth of an inch.

Moving A Tunnel. The north ends of these walls join the tunnel section of the subway, which was a solid, rectangular concrete tube about twenty-eight feet wide, seventeen feet high, three hundred feet long, and weighed about six million pounds. It was built in an open trench, which had not yet been refilled with earth above the tunnel roof. It had a framework of steel columns and roof-beams five feet apart, which were bedded in the concrete, and, like the approach, had been built for two tracks. When it was determined to provide for a third track, it was decided to widen the old structure by moving its walls out both ways five and a half feet from the center, and building in between them new strips of roof and floor to complete a larger tube on the same center line.

A trench seven feet wide was dug down to the bottom of the tunnel along each of its walls, and a concrete bed was laid in it to form a part of the new tunnel floor and side-wall foundation. As the tunnel had very little strength except to resist exterior pressure, it was thoroughly braced with timbers and wire ropes, inside and outside, to stiffen and bind it together to resist the temporary stresses and distortion of moving. Horizontal cuts were made from end to end of the tunnel through the bottom of the east wall and the top of the west wall, and the beams and columns were disconnected there so as to divide the structure into two nearly equal parts, one comprising the roof, east wall, and center columns, the other the west wall and floor.

Moving The Subway Tunnel Structure.

The west ends of the roof-beams were lifted a few inches with jack-screws, tipping the roof and east wall about the foot of the east wall as a pivot, and raising the center columns enough to place steel track-plates under their bases. Then the east ends of the roof-beams were similarly lifted, rocking the roof back again around the feet of the center columns as pivots, and lifting the east wall and columns high enough to insert under them track-plates which extended across the concrete floor in the bottom of the east outside trench.

Fifty five-ton jack-screws were set against the ends of the horizontal cross-timbers in the bottom of the tunnel, bearing on the east wall and center columns, and twenty-five men, turning the alternate screws quarter revolutions simultaneously on signal, gradually pushed the roof, east wall, and center columns five and a half feet east in two days, although the speed was half an inch a minute when they were actually moving. In order to keep the motion regular, a piano wire was stretched from end to end of the tunnel, one inch from the wall, and each man had a one-inch gage with which he tested this distance every time he turned his jack-screws.

A slot was cut from end to end of the west wall, separating it from the floor, stiffening-timbers were clamped to it, and horizontal cross-timbers were braced to the foot of it in such a manner as to project halfway across the tunnel, forming an extended base wide enough to give it great stability. Jack-screws under its braces lifted the wall enough to allow the insertion of track-plates under it and the base timbers; then it was pushed away from the undisturbed floor five and a half feet west on to the new floor in the trench by jack-screws set against horizontal braces from the inner face of the wall at its foot. Additional columns and roof-beams were set in the gap between the old parts of the tunnel, the extended roof and floor surfaces were closed up with concrete, the earth filled in on top of the roof up to the street surface, the braces removed, and the work successfully completed. The east wall and roof, as moved, weighed about three million pounds, and the west wall alone about seven hundred thousand pounds.

The work was done by forty men, at an estimated saving of six thousand dollars over the expense of tearing out the roof and walls, and is probably the first instance of moving a tunnel. The method was planned by the contractors, who executed it at their own risk, with the approval of the engineers. They were not tunnel-builders, but many years' experience in the erection of great bridges, roofs, and tall steel buildings had qualified them safely to undertake difficult and unusual heavy work requiring skill, ingenuity, and experienced judgment, and the safe handling of enormous forces and masses.

Underpinning The Columbus Monument. Engineering Record, July 27, 1901.