CHAPTER VII.

TYPES OF CENTERS AND MOLDS EMPLOYED IN CONSTRUCTING TUNNEL LININGS OF MASONRY.

The masonry lining of a tunnel may be described as consisting of two or more segments of circular arches combined so as to form a continuous solid ring of masonry. To direct the operations of the masons in constructing this masonry ring, templates or patterns are provided which show the exact dimensions and form of the sectional profile which it is desired to secure. These patterns or templates will vary in number and construction with the form of lining and the method of excavation adopted. Where the excavation is fully lined on all four sides, the masonry work is usually divided into three parts,--the invert or floor masonry, the side-wall masonry, and the roof-arch masonry. At least one separate pattern has to be employed in constructing each of these parts of the lining; and they are known respectively as ground molds, leading frames, and arch centers, or simply centers. In the following paragraphs the form and construction usually employed for each of these three kinds of patterns is described.

=Ground Molds.=--Ground molds are employed in building the tunnel invert. They are generally constructed of 3-inch plank cut exactly to the form and dimensions of the invert masonry as shown in Fig. 38. To permit of convenience of handling in a restricted space, they are generally made in two parts, which are joined at the middle by means of iron fish-plates and bolts. Either one or two ground molds may be employed. Where two molds are used they are set up a short distance apart, and cords are stretched from one to the other parallel to the axis of the tunnel, by which the masons are guided in their work. Extreme care has to be taken in setting the molds to ensure that they are fixed at the proper grade, and are in a plane normal to the axis of the tunnel. Where only one ground mold is employed, the finished masonry is depended upon to supply the place of the second mold, cords being stretched from it to the single mold placed the requisite distance ahead. The leveling and centering of the molds is accomplished by means of transit and level.

Two modifications of the form of ground mold shown by Fig. 39 are employed. The first modification is peculiar to the English method of excavation, and consists in combining the ground mold with the leading frame for the lower part of the side walls, as shown by Fig. 39. The second modification is employed where the two halves or sides of the invert are built separately, and it consists simply in using one-half of the mold shown by Fig. 38. When the last method of constructing the invert masonry is resorted to, extreme care has to be observed in setting the half-mold in order to avoid error.

=Leading Frames.=--As already stated, leading frames are the patterns, or molds, used in building the side walls of the lining. Like the ground mold they are usually built of plank; one side being cut to the curve of the profile, and the other being made parallel to the vertical axis of the tunnel section. The vertical side usually has some arrangement by which a plumb bob can be attached, as shown by Fig. 40, to guide the workmen in erecting the frame. The combined leading frame and ground mold shown in Fig. 39 has already been described. The use of this frame is possible only where the masonry is begun at the invert and carried up on each side at the same time. This mode of construction is peculiar to the English method of tunneling; in all other methods the form of separate ground frame shown by Fig. 40 is employed. The ground frames are lined in and leveled up by transit and level; and, as in setting the ground frames, care must be taken to secure accuracy in both direction and elevation.

=Arch Centers.=--The template or form upon which the roof arch is built is called a center. Unlike the ground molds and leading frames, the arch centers have to support the weight of the masonry and the roof pressures during the construction of the lining, and they, therefore, require to be made strong. Owing to the fact that the pressures are indeterminate, it is impossible to design a rational center, and resort is had to those constructions which past experience has shown to work satisfactorily under similar conditions. In a general way it can always be assumed that the construction should be as simple as possible, that the center should be so designed that it can be set up and removed with the least possible labor, and that the different pieces of the framework and lagging should be as short as possible, for convenience in handling.

Tunnel centers are usually composed of two parts,--a mold or curved surface upon which the masonry rests, and a framework which supports the mold. The curved surface or mold consists of a lagging of narrow boards running parallel to the tunnel axis, which rests upon the arched top members of two or more consecutive supporting frames. The supporting frame is built in the form of a truss, and must be made strong enough to withstand the heavy superimposed loads, consisting of the arch masonry during construction, and of the roof pressures which are transferred to the center when the strutting is removed to allow the masonry to be placed. The framework of the center is supported either by posts resting upon the floor of the excavation, or upon the invert masonry when this is built first, as in the English and Austrian methods, or it may be supported directly upon the ground where the arch masonry is built first, as in the Belgian method of tunneling.

In describing the various methods of tunneling in succeeding chapters, the center construction and method of supporting the center peculiar to each will be fully explained, and only a few general remarks are necessary here. Centers may be classified according to their construction and composition into plank centers, truss centers, and iron centers.

One of the most common forms of plank centers is shown by Fig. 41. It consists of two half-polygons whose sides consist of 15 in. × 4 ft. planks having radial end-joints. These two half-polygons are laid one upon the other so that they break joints, as shown by the figure, and the extrados of the frame is cut to the true curve of the roof arch. The planks commonly used for making these centers are 4 ins. thick, making the total thickness of the center 8 ins. Plank centers of the construction described are suitable only for work where the pressures to be resisted are small, as in tunnels through a fairly firm rock, although there have been instances of their successful use in soft-ground tunnels.

Where heavy loads have to be carried, trussed centers are generally employed, the trusses being composed of heavy square beams with scarfed and tenoned joints, reinforced by iron plates. Different forms of trusses are employed in each of the different methods of tunneling, and each of these is described in succeeding chapters; but they are generally either of the king-post or queen-post type, or some modification of them. The king-post truss may be used alone, with or without the tie-beam, as shown by Fig. 42; but generally a queen-post truss is made to form the base of support for a smaller king-post truss mounted on its top. This arrangement gives a trapezoidal form to the center, which approaches closely to the arch profile. Owing to the character of the pressures transmitted to the center, the usual tension members can be made very light.

The combined center and strutting system devised by Mr. Rziha has already been described in a previous chapter. In recent European tunnel work quite extensive use has also been made of iron centers consisting of several segments of curved I-beams, connected by fish-plate joints so as to form a semi-circular arch rib. The ends or feet of these I-beam ribs have bearing-plates or shoes made by riveting angles to the I-beams. Centers constructed in a similar manner, but made of sections of old railway rail, were used in carrying out the tunnel work on the Rhine River Railroad in Germany. The advantages claimed for iron centers are that they take up less room, and that they can be used over and over again.

_Setting Up Centers._--According to the method of excavation followed in building the tunnel, the centers for building the roof arch may have to be supported by posts resting on the tunnel floor; or where the arch is built first, as in the Belgian and Italian methods, they may be carried on blocking resting on the unexcavated earth below. Whichever method is employed, an unyielding support is essential, and care must be taken that the centers are erected and maintained in a plane normal to the tunnel axis. To prevent deflection and twisting, the consecutive centers are usually braced together by longitudinal struts or by braces running to the adjacent strutting. Only skilled and experienced workmen should be employed in erecting the centers; and they should work under the immediate direction of the engineer, who must establish the axis and level of each center by transit and level.

_Lagging._--By the lagging is meant the covering of narrow longitudinal boards resting upon the upper curved chords of the centers, and spanning the opening between consecutive centers. This lagging forms the curved surface or mold upon which the arch masonry is laid. When the roof arch is of ashlar masonry the strips of lagging are seldom placed nearer together than the joints of the consecutive ring stones, but in brick arches they are laid close together. Besides the weight of the arch masonry, the lagging timbers support the short props which keep the poling-boards in place after the strutting is removed and until the arch masonry is completed.

_Striking the Centers._--The centers are usually brought to the proper elevation by means of wooden wedges inserted between the sill of the center and its support, or between the bottom of the posts carrying the center and the tunnel floor. These wedges are usually made of hard wood, and are about 6 ins. wide by 4 ins. thick by 18 ins. long. To strike the center after the arch masonry is completed, these wedges are withdrawn, thus allowing the center to fall clear of the masonry. Usually the center is not removed immediately after striking, so that if the arch masonry fails the ruins will remain upon the center. The method of striking the iron center devised by Mr. Rziha has been described in the previous chapter on strutting.