Fig. 31.—Cast-Iron Segment of Rziha’s Strutting and Centering.
Full Section Iron Strutting.
—The iron strutting devised by Mr. Rziha for full section work is shown by [Fig. 30]. Briefly described, it consists of voussoir-shaped cast-iron segments, which are built up in arch form. [Fig. 31] shows the construction of one of the segments, all of which are alike, with the exception of the crown segment, which has a mortise and tenon joint which is kept open by filling the mortise with sand. The segments are bolted together by means of suitable bolt-holes in the vertical flanges, and when fully connected form an arch rib of cast iron. This arch rib, A, [Fig. 30], carries a series of angle or T-iron frames bent into approximately voussoir shape, as shown at B, [Fig. 30]. Above these frames are inserted the poling-boards, running longitudinally, and spanning the distance between consecutive arch ribs. By removing the bent iron frames the cast-iron rib forms a center upon which to construct the masonry. Finally, to remove the cast-iron rib itself, the sand is drawn out of the mortise and tenon joint in the crown segment, which allows the joint to close, and loosen the segments so that they are easily unbutted.
The illustration, [Fig. 30], shows longitudinal poling-boards; more often longitudinal crown bars of railway rails span the space between connective arch ribs, and support transverse poling-boards. In building the masonry, work is begun at the bottom on each side, the bent iron frames being removed one after another to give room for the masonry. As each frame is removed, it is replaced with a sort of screw-jack to support the poling-boards until the masonry is sufficiently completed to allow their removal. The interior bracing of the arch rib shown at a a and b b consists of railway rails carried by brackets cast on to the segments. A similar bracing of rails connects the successive arch ribs. These lines of bracing serve to carry the scaffolding upon which the masons work in building the lining.
Fig. 32.—Cast-Iron Segmental Strutting for Shafts.
Iron Shaft Strutting.
—In soft-ground shaft work, the use of an iron strutting, consisting of consecutive cast-iron rings, has sometimes been employed to advantage. [Fig. 32] shows the construction of one of these rings, which, it will be seen, is composed of four segments connected to each other by means of bolted flanges. The holes shown in the circumferential web of the ring are to allow for the seepage from the earth side walls. The method of placing this cylindrical strutting is to start with a ring having a cutting-edge. By means of excavation inside the ring, and by ramming, the ring is sunk into the ground a distance equal to its height. Another ring is then fastened by special hooks on top of the first one, and the sinking continued until the second ring is down flush with the surface. A third ring is then added, and so on until the entire shaft is excavated and strutted. As in timber shaft strutting, the solid iron ring strutting is carried down only to the top of the tunnel section, and below this point there is an open timber or iron supporting framework.