Chapter 07. Methods of Timbering to Support the Street Surface
The New York Rapid Transit Railway Extensions · Engineering News, 1914
The specifications for the construction of the subways in Manhattan and in most of the streets in Brooklyn require that the work be "carried on under covered roadways." This practically means that the paved surface of the street and generally also of the sidewalks has to be taken up and replaced by a timber deck, under which the excavation and construction may proceed with little or no interruption of the ordinary street traffic. Openings for shafts to give access to the excavations are permitted at intervals of 300 to 500 ft. in the upper part of the city, but are about 1000 ft. apart in the lower section.
In nearly all the streets in Manhattan where the subways are being or are to be built, there is a double-track street railway with underground contact system which has to be supported. In Brooklyn, the Bronx and Queens, the overhead trolley is used, which makes the problem of track support somewhat easier, though of course poles have to be taken care of.
The usual method of procedure is first to excavate about 3 ft. of the street surface on one side of the tracks, putting in the decking and track supports in the form shown in the accompanying views. When one side of the street is decked over, the other side is taken care of in the same way.
Excavation is then carried on under this decking, the first lift being from 10 to 15 ft. in depth, practically the depth of the ordinary cellars and basements, the walls of which usually form the sides of the excavation. In very wide streets the full width is, of course, not taken out but where the additional width beyond the neat lines is not excessive, the whole width is excavated in this first lift, as this permits easy access to the buildings for the underpinning operations. Where the full width to the cellar or vault walls at the sides is taken out, no sheeting is required on this first lift, but if this is not done sheeting must necessarily be driven from the surface.
Below this first lift, the ordinary form of timbering, using rangers and braces (see Fig. 35), may be continued in much the same manner as for the excavation of any trench, though, of course, on a larger scale, or one of the many special forms hereinafter described and illustrated may be used. There are two general types, one for earth and one for rock, the former well illustrated on the left and the latter on the right in Fig. 35 and in Fig. 36 and the accompanying drawings.
The essential differences are the necessity in earth excavation of supporting the side sheeting as well as the decking and in rock the provision of a clear working space and to guard against disaster by the loosening or destruction of one or more supports by the blasting operations or by slides in the very unstable New York rock.
Fig. 38. Tower system of timbering in rock excavation, Lexington Ave., section 13.
One of the most interesting of the methods used for support in a deep rock excavation is that developed on Section 13, Lexington Ave., by Messrs. McMullen, Snare & Triest and illustrated quite clearly in the photograph, Fig. 38, and the sketch, Fig. 39.
Fig. 39. Sketch showing steel beam supports for tracks over rock excavation, Lexington Ave. section 13. (Click image to enlarge.)
On this section the concrete troughs which support the street-railway tracks are first supported longitudinally by the three or more 6x12-in. timbers laid flat (more than three when there are ducts to be taken care of). These, as the drawing, Fig. 39, shows, are held up by 12x12-in. cross-beams 18 ft. long, which are blocked up from the "needle beams" (F), which are 12-in. 31.5-lb. I-beams 30 ft. long spaced 10 ft. apart. It may be noted that it was not usually possible to put these needle beams directly under the blocking of the troughs of the street-railway tracks, on account of the presence of various gas and water pipes, etc., at about that level, a condition which obtains quite generally.
These needle beams have two pairs of 6x12-in. yellow-pine blocks about 5 ft. long bolted to them, one on each side and spaced so that they will come directly under the tracks, as shown on the sketch. These wooden blocks have their corners cut away so that they fit tight against the web and under the flanges of the I-beams, making at these blocks solid points of support for the longitudinal I-beam stringers underneath or for any temporary blocking or posts which may be required, and tending to prevent any overturning of the needle beam. Long X-braces and turnbuckles are also used between the needles.
At the end of the needle beam, holes are drilled so that 6x12-in. struts to the sides can be bolted to it, the 6x12's being fitted tight to the I-beam the same as the needle blocking.
The needle beams are then supported on the timber towers shown in the photograph, Fig. 38, by two pairs of 20-in. 65-lb. I-beams (A in Fig. 39), which are bolted together by long plates to develop full strength at the joints, making them equivalent to a continuous beam the whole length of the work. On either side of these two pairs of what might be called permanent longitudinal supports, are two pairs of the same size I-beams, bolted together the same way, the outer ones (C) 80 ft. long, and the inner ones (B) 120 ft. long, these latter being used as supports from the last timber tower over the face of the excavation to give a clear span of 50 to 60 ft. over the working space.
The inside 120-ft. pair is supported on the tower and on blocking just back of the working face, but also projects back of the tower and beyond the blocking, and these overhanging portions are wedged down tight from the decking, making it act as a cantilever (see sketch, Fig. 39). The 80-ft. pair spans from the tower to blocking ahead of the working face.
The towers are spaced 40 ft. c. to c., and as the excavation progresses, and space is cleared for a new tower, these pairs of I-beams are moved ahead for another space.
In the timbering to support the street decking and the electric-car tracks the plan adopted on Section 9, giving continuous support to the street-car tracks by means of I-beams spliced so as to develop full strength, is worthy of note. As shown in Fig. 40, there are three pairs of these beams directly under the decking. The cross-timber on which the tracks are supported is suspended from the I-beams. The side struts or diagonals also give additional arching support so that the danger due to the displacement of any of the posts in the excavation is reduced to a minimum. Fig. 40 shows an effective method of obtaining clear support over the rock excavation for the construction of the lower-level tracks.
Fig. 40. Timbering for rock excavation for double-deck subway, Lexington Avenue between 78th and 79th Streets, section 9. (Click image to enlarge.)
On Sections 10 and 11, after some trouble with slipping and sliding rock, an additional means of supporting the street decking was adopted. Continuous girders about 4 ft. high and 150 to 200 ft. long were erected on top of the decking near the edge of the sidewalk and the timbering was virtually suspended from them, as is shown in Fig. 41, and the photograph, Fig. 36. These girders were generally intended to be supplementary to the system of supports beneath the deck and to be necessary only in case of the displacement of these latter. As a matter of fact, however, they proved useful in spanning the spaces where work was actually being carried on where changes and replacements of these lower supports were frequendy necessary. Their disadvantage is that they occupy space on the street surface.
Fig. 41. Timbering supported from longitudinal girders on street surface, Lexington Ave., section 10. (Click image to enlarge.)
Where the regular system of cross-bracing and rangers are required, as in Section 14, Lexington Ave. (McMullen & Hoff contractors), where there is earth nearly to subgrade, some very elaborate systems of timbering are necessary. The type of timber construction on this section is shown in the photographs, Fig. 35; there are seven sets of braces in a depth of about 40 ft. below the deck and the bents are about 10 ft. apart. Tension rods are put in to help hold up the bottom braces during excavation. The sheeting on this section is mostly Lackawanna steel, 14-in. arched-web, 41-lb., but some 6-in. plank (plain) is used. The steel piling mostly showed up quite well, although there were numerous boulders; and of course, in some places where they were struck, the piling was more or less out of line toward the bottom.
Fig. 35. Timbering in earth excavation at left and road excavation at right, on Lexington Ave.
On this section careful additional horizontal cross-bracing was carried diagonally from the center at each of the shafts through several bents to the sides, which added materially to the rigidity of the whole structure.
On the down-town sections, nearly all of which are in earth, more or less similar types of timbering are used. The sides are necessarily held by sheeting, generally wood, and the timbering is the usual system of rangers, braces, and posts, though varied in detail by each of the several contractors.
The street decking is, of course, laid first, in some cases with a heavy wearing surface of 6-in. planks, in others, 2-in. planks on closely spaced 6x6-in. timbers or 6-in. I-beams. The first lift of the excavation is then taken out to a depth of 10 or 15 ft., the street-railway tracks and pipes secured and generally one brace is carried across the whole width of the excavation, this first long brace being usually not less than 5 or 6 ft. below the surface. Great care, of course, is taken in all these sections to arrange the timbers so that they will clear the steel when the latter is erected.
The two principle variations seem to be in making either the posts or the braces continuous. On Section 2, the first operation was to sink the posts in sheeted pits to subgrade. The material was sand, and many men in New York now have become quite expert in sinking these 4x4-ft. pits in soft material by the use of horizontal sheeting (which will be described in more detail under the heading of underpinning), so that the sinking of this large number of pits is not so serious an operation as might be imagined. No water, of moment, was encountered on this section above subgrade. Once the posts were down, of course, the deck was held in safety for any operation and the continuance of the excavation and timbering was a matter of routine.
A method used on part of Section 3 is clearly shown in Fig. 42, which needs little explanation. The building of the concrete sidewall was not followed throughout the section. The method of framing the timbers to get an arching effect is, however, to be noted, as this principle was applied also on Section 1.
Fig. 42. Concrete sidewalls and arched timber supports with hollow steel piles used on Broadway, section 3, route 12. (Click image to enlarge.)
The features of this system as developed on Section 1, by J. C. Meem, who was the engineer in charge for the contractors, are the combined arching and bracing effect secured and the use of the continuous girder throughout the length of the excavated section, supported on posts or hollow piles.
At a convenient distance below the decking, continuous 12x12-in. cross-braces spanning the whole width of the excavation were put in, 10 ft. apart. These were usually made up of two 6x12-in. timbers bolted together in as long lengths as it was practical to use. The short 12x12-in. cap, with the ends cut as shown, was then placed nuder the cross-braces and supported temporarily while the next lift of 5 or 8 ft. was excavated to the level of the next cross-brace.
The hollow-steel piles were then driven, a 10x10-in. post fitted inside of them, the longitudinal girders extended, and the arch legs and posts put in position. The appearance of the lowest level is shown in Fig. 37.
Fig. 37. Street decking and timbering carried on hollow steel piles under Church St., section 1, route 5.
The use of continuous I-beams for the support of the street decking or for tying together the timbering or supports is a feature of quite a number of the sections, differences in detail, of course, having been developed by each individual contractor, but in general the principle is that of a group or series of groups of I-beams splice together with long plates to obtain 100% strength at the joints and running the whole length of the work under the decking and often supporting directly the street-car tracks. The advantage of this method of course, is in its general security in case of accident to any one or even more of the supporting columns or bents and also to permit easy removal or change of supports during erection.
Fig. 36. Steel girders on street surface carrying timbering and decking, Lexington Ave., section 11, route 5.
An interesting and unusual example of the method of timbering by rangers and braces applied to the excavation of a large area in water-bearing material is that of Sec. 2a at the intersection of Broadway and Canal St., which, on account of its difficulty, was let as a separate contract. A double-deck structure is being provided to permit the future Canal St. cross-town line to pass under the Broadway line. The excavation at this point is some 55 to 60 ft. deep and for four-track lines approximately 250 ft. long on Broadway and 150 ft. on Canal St., joined by a connecting curve in the northeast corner. The extremely heavy floor of steel girders and concrete placed in the bottom of this section to resist the water pressure, as shown in Fig. 11, will convey an idea of the conditions which were successfully overcome.
The normal water level on this section is about 15 ft. below the surface of the street, but water was actually not found in quantity until a depth of about 23 ft. was reached. This left, however, a depth of 30 ft. to be excavated in water-bearing sand and gravel. The entire area is surrounded by buildings and there is a very heavy travel to support on both streets, so that it was necessary to exercise extreme care to avoid losing ground.
The area excavated is surrounded by 6-in. tongued-and-grooved sheeting driven in three lifts, and great care was necessary in spacing the elaborate system of cross-bracing to permit the erection of the steel. There is nothing particularly novel in the layout of this latter, except its great extent over such a large area; special care, of course, being necessary to keep the long line of rangers and braces in place with proper cross-bracing, and to so design the whole layout that the steel could be erected with the minimum amount of interference with the timbering.
Fortunately, the large amount of water coming into the excavation seems to be quite clear, and the most careful levels, carried out for a considerable distance in every direction from the excavation and continually checked, indicate little, if any, settlement of the adjacent ground or buildings. The total capacity of the pumps is about 20 million gallons daily, and while the whole capacity has only been infrequently required, a large proportion of it has been needed continuously.