LINER PLATES FOR HYDRAULIC TUNNEL
Steel Liner Plates, which Eliminate the Necessity for Tunneling Shield—Tunneling Can Be Made Continuous on Account of Maximum Amount of Clearance Obtainable Unnecessary for Gangs to Wait for Each Other
Steel liner plates for lining a soft soil tunnel exavation, and which eliminate the necessity of a tunnelling shield, are now gaining popularity in water works tunnelling projects. These pressed steel plates are left in place and the brick or concrete lining built inside them. They do not afford a permanent shell protection and are not so intended as steel rusts much more quickly than cast iron which is sometimes used for this purpose. They do, however, protect the concrete or brick from any load until sufficient time has elapsed for same to become hard enough to attain its required strength.
In the use of these steel plates the excavation should be made as closely as possible to the outside diameter of the steel plates, so as to produce an even pressure around the surface. If this is carefully done it is figured that plate 1/8-inch thick will support any loads likely to be encountered, and can be safely used at any depth. The chief advantage in the use of the plates is in the economy of construction, which is accounted for by the rapidity of construction permitted by the light plates. A plate 12 x 1/8 x30 inches weighs only 18 pounds. The plates as purchased are exact duplicates of each other, both as to size and perforations, so that no reaming, fitting or drifting is necesssary.
By following up closely behind the excavation with plate construction the costly shield can be omitted. This does away with piping, pumps and shield, and the delay in operating the shield. The operation of tunnelling when the plates are used can be continuous.
Pressed steel forms of open hearth steel and to be used in connection with the plates are manufactured for any curvature or combination of curves and tangents likely to be met with in sewer or hydraulic tunnel construction. They are of light but strong construction. In view of the ease and rapidity with which these forms can be erected and taken down, a carrier is in most cases unnecessary, and by using the forms without a carrier a contractor can make his tunnelling or open sewer work continuous on account of the maximum amount of clear space obtainable. When these forms are employed it is usually unnecessary for one gang to wait for another. The forms are quick acting and only a hammer and pinch bar are needed to erect and remove them.
Figure 1 shows the Union Depot sewer at Chicago, Ill., with the forms in place and concreted. The space available for removing braces is quite noticeable. Figure 2 gives an idea of the manner in which the places are bolted together.