Fig. 85.—Sketches Showing Construction of Strutting, English Method.
Referring to [Fig. 85], it will be observed that the longitudinal crown bars are above the extrados of the roof arch. When, therefore, the lining masonry has been completed close up to the transverse framework, the latter is removed, leaving the crown bars resting on the arch masonry; and excavation, which has been stopped while the masonry was being laid, is continued for another 12 ft. to 20 ft., and the transverse framework is erected at the face, and braced or propped against the completed lining as shown by [Fig. 85]. The next step is to place the crown bars, and this is done by pulling them ahead from their original position over the masonry of the completed section of the roof arch. It will be understood that the crown bars are not pulled ahead their full length at one operation, but are advanced by successive short movements as the excavation progresses, their outer ends being supported by temporary posts until the transverse framework is built at the face of the excavation.
Centers.
—Two standard forms of centers are employed in the English method of tunneling, as shown by [Figs. 86] and [87]. Both consist of an outer portion, constructed much like a typical plank center, which is strengthened against distortion by an interior truss framework. The elemental members of this truss framework take the form of a queen-post truss, as is shown more particularly by [Fig. 86]. In [Fig. 87] the queen-post truss construction is less easily distinguished, owing to the cutting of the bottom tie-beam and other modifications, but it can still be observed. The possibility of cutting the tie-beam as shown in [Fig. 87], without danger, is due to the fact that the lateral pressures on the haunches of the center counteract the tendency of the center to flatten under load, which is usually counteracted by the tie-beam alone. The object of cutting the tie-beam is to afford room for the props running from the completed masonry to the transverse framework of the strutting as shown by [Fig. 85].
Figs. 86 and 87.—Sketches of Typical Timber Roof-Arch Centers, English Method.
Generally four or five centers are used for each length of arch built. They are set up so that the tie-beams rest on double opposite wedges carried by a transverse beam below. This transverse beam in turn rests on another transverse beam which is supported by posts carried on blocking on the invert masonry. It is usually made with a butted joint at the middle to permit its removal, since it is so long that the masonry has to be built around its extreme ends. The lagging is of the usual form, and rests on the exterior edges of the curved upper member of the centers.
Masonry.
—In the English method of tunneling, the masonry begins with the construction of the invert, and proceeds to the crown of the arch. The lining is built in lengths, or successive rings, corresponding to the length of excavation, which, as previously stated, is from 12 ft. to 20 ft. Each ring or length of lining terminates close to the transverse strutting frame erected at the face of the excavation. Work is first begun on the invert at the point where the preceding ring of masonry ends, and is continued to the transverse strutting frame at the front of the excavation. As fast as the invert is completed, work is begun on the side walls. In very loose soils the longitudinal bars supporting the sides of the excavation are removed after the side walls are built; but in firmer soils they may be taken out one by one just ahead of the masonry, or in very firm soils it may be possible to remove them entirely before beginning the side walls. In all cases it is necessary to fill the space between the masonry and the walls of the excavation with riprap or earth. To build the roof arch the centers are first erected as described above, and the crown bars are removed as previously described by pulling them ahead after the arch ring is completed. As with the side walls, the vacant space between the arch ring and the roof of the excavation must be filled in. Usually earth or small stones are used for filling; but in very loose soils it is sometimes the practice not to remove the poling-boards, but to support them by short brick pillars resting on the arch ring and then to fill around these pillars.