Chapter 09. Underpinning Buildings Along the Line
The New York Rapid Transit Railway Extensions · Engineering News, 1914
Under the general heading of "protection of adjacent buildings" the specifications provide for three classes of work.
A. Buildings "which are supported on firm soils" and bearing such a relative position in regard to the subway structure that a slope "represented by 1 ft. vertical to 2 ft. horizontal, inclined downward from the bottom outer edge of the building foundation, passes beneath the bottom outer edge of the completed subway structure" have to be taken care of by the contractor, and such cost as there may be is included in the price for excavation.
Fig. 53. Sketches illustrating various typical methods of underpinning buildings. A. Underpinning by needles, ordinary form. B. Underpinning by pile and short cross I-beams. C. Underpinning by piers and I-beam between. D. Underpinning by cantilever. E. Underpinning by caisson and I-beams. (Click image to enlarge.)
B. When necessary to secure adjacent buildings or to prevent bringing an unusual pressure on the subway structure when completed, the contractors are required to "safely and permanently underpin adjacent buildings the foundations of which are above the bottom of the adjacent subway excavation." This work is paid for at a price bid per front foot of the building. The latter are classified according to height, less than seven stories, seven to twelve stories, and over twelve stories. The prices for this work on contracts let so far range from $50 to $100 per front foot for 7-story buildings and from $75 to $200 with an average of about $150 for buildings 7 to 12 stories. The prices are, however, very irregular and there is little distinction in the bidding between uptown and downtown. Bids for underpinning buildings over 12 stories range from $90 to $400 per front foot and are too few and erratic to be of any use as a guide to the cost of the work.
C. In certain cases where underpinning is not considered necessary but where buildings have to be secured and maintained during construction, there is a price bid per front foot for "maintaining, protecting and securing." These prices range from $15 to $60 and average about $40.
It is generally required that underpinning be carried down to solid rock or to at least 2 ft. below the lowest excavation for the subway, if rock is not encountered before that depth is reached.
Often when the necessary excavation for the subway structure takes a fairly large proportion of the width of the street, the excavation of the "first lift" is carried out to the full width of the street between building lines and to the depth of the cellar, thus providing easy access for working, as is shown in Fig. 53 (A). The excavation of this first 10 ft. is generally first carried ahead the whole length of the section, or for a considerable portion of it, and this is followed by the underpinning before any further excavation is undertaken.
Probably the most common way of supporting ordinary buildings is to temporarily carry the walls to be supported on needles, generally I-beams, as shown in Fig. 53 (A) and extend the foundation walls down to rock if this is not too deep.
In cases where the depth to rock or to the required bottom of foundation is deep, piers (from 2 to 3 ft. square for ordinary buildings 5 or 6 stories high) are usually sunk at intervals of 10 to 15 ft. The spacing of the piers depends, of course, on the position of the piers or columns of the building which require direct support, and the front wall of the building between the piers is supported on two or more I-beams spanning the space between these piers. The excavation of the pits for the piers is usually made by hand, the well being sheeted, and the piers are built of concrete. In some cases, instead of the concrete piers, 12-in. pipes in short lengths are sunk either by driving with a weight or with a water jet, the subsequent procedure being the same.
To avoid supporting the buildings on needles, two methods have been used, somewhat similar in principle. Concrete piers 2 ft. square are sunk to rock in pairs, one inside and one outside the wall, each pair 10 to 12 ft. apart, or instead of the piers, 12-in. wrought-iron pipes in lengths of about 4 ft. with inside sleeve couplings are sunk and filled with concrete, the tops of these piers or pipes being 20 to 30 in. below the cellar floor. I-beams of suitable size (50 to 60 lb. generally for ordinary buildings) are then laid on top of these piers or pipes parallel and adjacent to the walls of the buildings; small holes are then broken through the wall and smaller I-beams put through spanning the first two and the walls caught up on these, as shown in the sketch in Fig. 53 (B).
On one section where a building of moderate size had to be taken care of, piers were sunk to rock (or to the required depth) and then the wells were widened out at the top in the direction of the line of the building, as shown in Fig. 53 (C) and filled with concrete, in which reinforcing rods were placed. These piers were 30 to 40 ft. apart and the space between them was spanned by two 30-in., 200-lb. Bethlehem beams, which were placed in niches cut in the wall to receive them.
On Section 14, Lexington Ave., W. Melvin, Superintendent for the McMullen & Hoff Co., devised a form of shield on the principle of the tunnel shield, but reduced to its simplest elements, to sink 3-ft. 6-in. cylinders vertically for underpinning. The apparatus is shown by the drawings, Fig. 55, and, as will be seen, consists simply of a cylinder of 1/2-in. boiler iron about 3 ft. long with a 3x4-in. angle bent to circular form to fit inside just back from the lower end. Four segments, as shown in the drawing, form an 18-in. ring, which is added to the bottom of the caisson inside the tail of the shield as this latter is shoved down. The shield is "shoved" by four jack bolts, bearing against the 3x4-in. angle of the shield and reacting against the last ring.
Fig. 54. Underpinning buildings with steel-pipe piles filled with concrete. (Click image to enlarge.)
The material is removed by means of small buckets which are raised by a hand winch and taken out of the top through the air lock. On account of the small working compartment only one man can work at the excavation, but usually there is another puddling the joints and sealing the shaft.
The shield, of course, was left in the bottom when the caisson was concreted. These caissons were sunk about 20 to 25 ft. apart, directly under the front walls of the buildings, the location depending on the location of the main columns or piers which it was desirable to support directly. The space between them was spanned by two 26-in., 150-lb. Bethlehem beams on which the wall was supported directly, as shown in Fig. 53 (E).
At one point on Section 13, Lexington Ave., where the rock was quite near the surface and the necessary excavation close to the building line, a slip occurred in the rock, endangering the front of the building. In order to avoid blocking up in the excavation, long I-beams were used as cantilevers, blocked up in the cellar just back of the front wall and running back under the back wall, against which they were blocked and which afforded the necessary reaction, as shown in the sketch in Fig. 53 (D).
On Sections 8, 9, 10 and 11, Lexington Ave., three methods were used: First, the common one of supporting the buildings on needles while the walls were carried down in trenches to rock, this being usually adopted when the rock was not deep. Second, piers of concrete about 5 to 6 ft. wide and 8 to 10 ft. long were sunk to rock or subgrade and spanned by two or sometimes three I-beams on which the building wall was carried. This method was usually used where the rock or subgrade was deep, say 10 ft. or more below the basement floor of the building. Third, 10-in. iron pipes in lengths of 3 to 5 ft. with inside sleeve couplings were sunk under the walls and capped with I-beams, as shown in Fig, 54. The pipes were forced down by jackscrews reacting against the walls of the building, and were put down under columns or sections of the wall which carried the load. They were sunk dry and the material inside excavated with small orange-peel scoops. The small boulders encountered were taken out by a sort of net on the end of a pole called a "snare." They were supposed to be sunk to rock, sealed to it by a rich cement mortar, and filled with concrete in which were embedded two 3/4-in. square steel rods.
Fig. 55. Section caissons of cast iron sunk with shield. (Click image to enlarge.)
This method of underpinning is quite effective and safe in many cases and, of course, is very much cheaper than either of the first two methods, if the foundations have to be carried down to any great depth. The defects are that it is sometimes difficult to tell if the pipes are on solid rock or on a boulder, and if the excavation for the subway or other purpose comes close to the building line, the front of the pipes may be uncovered, leaving them as practically unsupported columns. So far as could be learned, however, where they have been used so far, they have served their purpose and no actual difficulty or failure has been encountered. This work was done by the Underpinning & Foundation Co., under a subcontract from the principal contractors.
Fig. 57. Underpinning with needles, building at 459 Broadway. Fig. 58. Underpinning with a latticed girder.
A method which obviates the necessity of supporting the buildiugs on neeedles was developed and used on Sections 1 and 3 of the Broadway line. It consists essentially in tying the foundation columns together with a reinforced-concrete mattress or girder, as shown in Fig. 59, then sinking pits or piles under it to the required depth. Built-up steel girders or I-beams are first laid along on either side of the columns and parallel to the face of the building, at about the level of the basemeut floor, or just below it, one outside and one inside and tied to each column and to each other. These girders are made up of short sections, on account of the confined space in which they have to he handled, and a convenient form is one made up of four angles latticed together, and riveted so as to be continuous for the length of the front of the building (see Fig. 58), though I-beams are used in some cases. Light hitches are cut in the piers to get a firm bearing, and the girders or I-beams are tied together firmly with rods or sometimes with steel-wire ropes, the whole being then concreted, making a continuous reinforced-concrete girder supporting the whole front. The photograph, Fig. 58, shows in the foreground the latticed two girders and behind that the completely concreted beam or girder.
Fig. 59. Sketch illustrating underpinning with reinforced concrete girder. (Click image to enlarge.)
Rectangular pits are then sunk at intervals under this girder, as shown in the sketch. Fig. 59, and in spaces between the column footings so that the ground under these latter remains undisturbed. The pits are sheeted with horizontal sheeting, and are sunk to the necessary depth, i.e., to 2 to 3 ft. below the subgrade of the subway structure, and filled with concrete. Care is taken not to have the open pits close together. About two at a time, some distance apart, are put down, filled and blocked under the concrete girder before others are started. If water is encountered, hollow steel piles in short sections are sunk from the bottom of the pits below the water level and filled with concrete. All piles are tested by hydraulic pressure to take up any slight settlement.
Fig. 56. Putting down hollow steel pile with hydraulic jack reacting against building.
There are two methods of sinking those hollow steel piles which are quite extensively used, one by the use of hydraulic or screw-jacks reacting against the building above, as shown in the photograph, Fig. 56, and the second, to drive them by a hammer. The first is, it is stated, a patented process. Where the hammer is used it generally consists solely of a weight, about 300 lb., suspended from a rope passing over a single block attached to the floor above tho pit, and running to a small single-drum hoist. The fall is usually only a few feet and the hammer is guided by hand by a man standing near the pile being driven. A square cast-iron cap is used on top of the pile.
Fig. 60. Horizontal sheeting for sinking pits. (Click image to enlarge.)
An interesting detail of the horizontal sheeting so generally used in sinking the pits for underpinning and other purposes is the method of chamfering the corners of the boards so as to allow of packing the ground solid behind them as they are placed (see Fig. 60). The boards on two opposite sides are cut so that they fit inside those on the other sides, making a brace, and short blocks are spiked on to hold the short sides as shown in the plan. The long sides are placed first, and made to give a firm driving fit to the short ones. Each width is firmly packed as it is placed.
The pits are filled with concrete up to within about 12 or 15 in. of the bottom of the concrete girder. When the concrete in the pit has set, short sections of I-beams are placed on top of it and wedges are firmly driven between these I-beams and the bottom of the foundation girder. This holds the latter while the other pits are being put down, the shrinkage in the concrete in the pits being taken up from time to time by the wedges and the whole finally completely filled in, after all shrinkage and settlement of the new foundation have taken place.
The foundations of the Havemeyer Building (14 stories) are on spread brick piers on wooden piles, the bottom of the brick and the top of the piles being at approximately the level of the floor of the subway. To protect these foundations, additional hollow steel piles were sunk under the front edges of the building piers and then a double row of steel sheet piling 10 or 12 ft. long and with a space of about 3 ft. between the rows was driven as additional protection. This acts as a coffer-dam and not only will tend to prevent any disturbance of the ground around the piles, but also to retain the level of the ground water.
In turning from Church St. through Vesey St. to Broadway, it was necessary to obtain easements under private property at each of the corners in order to get around. One of these is under Trinity Parish House and the other under the old Astor House. That portion of the latter under which the tunnel passes was dismantled and taken down, the city agreeing to provide foundations for a new building along each street line. Open trenches were sunk through the sand to a depth of about 30 ft. and from the bottom of these trenches pneumatic caissons are being sunk to the required depth. The sinking of the caissons is done under a special subcontract by the Foundation Company.
The Trinity Parish House is a four-story brownstone building about 30 ft. wide and 160 ft. long. The tunnels pass under it as shown in the sketch, Fig. 61. Rectangular pits were sunk as shown by the drawing, there being 115 of these pits. Part of these pits supported the building directly; at other places the walls are supported on cross-girders over the tunnel, as indicated on the sketch. With the exception of part of the ground floor and basement the building has been continuously in use during the whole operation.
Fig. 61. Location of subway tunnels under Trinity Parish House. (Click image to enlarge.)
A method of underpinning was developed on Sec. 3 of Route 5 (see Fig. 42) by the Underpinning & Foundation Co., which consists essentially of the construction of a retaining-wall the face of which is practically at the neat line of the structure. This is made practicable by reason of the fact that along most of the route the space underneath the sidewalks is occupied by vaults used by the owners of the adjacent buildings, under revocable permits from the city. The width of the subway structure makes it necessary to occupy part of these vaults, though any remaining portion is afterward restored for the use of the abutting property owners. Most of the excavation of this section is in sand and the depth is comparatively shallow. The contractor, therefore, took advantage of the situation to build his sidewalls as retaining walls, working from the bottom of the vaults before commencing the main excavation. This effectually prevented any disturbance of the ground under the building and lessened the amount of timber, as no bracing for the sides was required. The retaining wall was built in sections by sinking 4 ft. square pits or wells (using the horizontal sheeting) separately, and some distance apart, then intermediate pits were put down and finally the whole closed up to make a continuous wall.
At the lower end of this section, thc two middle tracks are depressed for the Canal St. connection, and as this is mostly below ground-water level aud quite deep, the method above described did not wholly apply. It served, however, for the upper level and steel sheeting was then driven between tho outer tracks and the inner pair, to enable these latter to be taken down to the required depth.
The six-story brick building at the corner of Broadway and 17th St. was held on needles while it was underpinned, and is a good example of what this method involves for a heavy building. The length of tho front which was supported is about 25 ft.. aud eighteen 24-in. I-beams each about 25 ft. long were required to hold the weight. There were two piers between the corners and the three spaces between them were filled with heavy timber bracing and blocking before operations were commenced, as is shown in the sketch, Fig. 62. The I-beams were used in groups of three, supported on a continuous grillage built up on the cellar floor inside and on the vault floor outside. Hitches were cut, one at a time in the sides of the columns to take the three I-beams, and the whole load finally transferred to them while the foundations were carried down.
Fig. 62. Underpinning with needles, 6-story building, Broadway and 17th Street. (Click image to enlarge.)
Elevated Railway Columns. The columns of the elevated railway as originally built were generally supported beneath the surface of the street on spread brick footings, the removal of which is made necessary by the construction of the subway. Temporary supports as shown in the photograph, Fig. 63, are built to hold the elevated structure during construction and considerable care is necessary to prevent any settlement and to provide as nearly as possible absolute safety.
Fig. 63. Temporary support for elevated-railway columns. Fig. 66. Timber a-frame with eye-bar clamp supporting elevated-railway columns, Church St.
On Sec. 1, Route 5, which is under Church St. and the Sixth Ave. Elevated, the A-frame supporting the cross-girder above the column is supported on timber bents resting on steel piles. The spread brick footings are uncovered and spaces are cleared at the two sides of the structure in each of which three 14-in. steel piles are sunk to below suhgrade and to a firm bearing; these are capped on each side by a reinforced-concrete beam on which a perpendicular timber bent is erected to about the level of the street surface as shown in the sketch, Fig. 64. These bents are about 16 ft. apart and an A-frame is then erected on them. The legs of the A-frame are held together by eye-bars and pins or timber bracing, as shown in Fig. 66. When the final support of the columns is to be below or at subgrade, clusters of hollow steel piles are driven and capped with reinforced concrete, to form the new footing, as shown in the photograph, Fig. 65. In many cases, however the new column footings are carried directly on the roof of the completed subway structure.
Fig. 64. Support of elevated railway columns on Church Street. (Click image to enlarge.)
The method of support adopted where the construction of a new sewer, on Third Ave., required temporary support of one pair of columns, is shown in the photograph, Fig. 66, the structure being blocked up from two pairs of girders laid direct on blocks on the paved surface of the street.
Fig. 65. Hollow piles for footing of elevated-railway columns.
On West Broadway, at the lower end of the Seventh Ave.-Varick St. line, the elevated columns are supported above the street surface by a timber tower. Immediately below this and supporting it are two heavy 30-in. Bethlehem beams about 20 ft. long, laid on either side of the brick footing parallel to the street line. Both ends of these are in turn supported by a pair of 24-in. I-beams which rest on timber blocks on the ground outside of the brick footings.