In what is sometimes called the Belgian system, the plan is followed of finishin,2-_, the upper half of the tunnel first, the arch being supported by timbering erected on the floor of the excavated portion. The lower half of the tunnel is gradually excavated to the proper depth and finished in short sections at a time, the finished roof meanwhile receiving such tem porary support as may be required.
Figures 16 to IS the different plans of timbering employed are exhibited with sufficient clearness to render further de scription needless. Figures 2o and 21 are views showing the details of a sectional iron framework designed by Engrineer Riiha of Brunswick as a substitute for timbering in tunnel construction where artificial support is called for. The cast-iron segments interposed between the girder and the walls of the excavation are removed one by one as the masonry lining is advanced (fig. 19); and when an entire arch-section at the rear may be removed, it is transferred to the front. This plan of operation presents several advantages, and is favorably spoken of.
Subaqueous a tunnel is to be made under the bed of a river special precautions must be taken to guard against disaster, since the irruption of water constitutes a serious danger. In excavating the first tunnel under the Thames at London the English engineer Brunel, after several failures, adopted the plan of using at the heading a movable bulk head or shield, which was held in place by beams parallel with the axis of the tunnel and bearing- against the finished masonry (fi/. 26, fig. 15). One portion of the bulkhead was removed at a time, the earth behind it excavated to a certain distance, and the section of bulkhead replaced at the advanced position. Tbe bulkhead was thus advanced section by sec tion. The plan of using the movable bulkhead is commonly designated the English system. More recently rotary cutters and other mechanical devices operating in advance of a shield have been proposed Go/. 26, fig-.
22; /5/. 27, jig-s. 1, 2).
Worthy of notice in this connection is the system of subaqueous tun nelling devised by the American engineer Hall, which is claimed to be a great advance, in point of simplification, over the methods hitherto employed. At the present time (1888) it is proposed to lay a tunnel between Prince Edward Island and the mainland by this plan. Figure
3 (fi/. 27) illustrates the method. The inventor employs a movable cais son, which is advanced little by little. This is made of heavy iron plates of the form shown in the illustration. Its interior is large enough to accommodate a complete section of the tube of which the shell of the tunnel is to be built, and as each section is put in place the caisson is advanced one length by means of hydraulic jacks placed inside tbe caisson and push ing against the rim of the last-finished section. The rear of the caisson is furnished with an orifice of circular (or other) section to conform to the determined shape of the tunnel, and is provided with a stuffing-box having a strong but elastic packing, by which means a water-tight joint is formed around the unfinished end of the tunnel. This mode of procedure in sub aqueous tunnel construction does away with the necessity of costly caissons and the use of compressed air and air-locks for entrance and egress for the workmen and for removal of the materials excavated, and likewise with the use of the movable bulkheads at the headings, where the material to be penetrated is earth.
Machinery and Compounds.—Within the past twenty-five years great advances have been made in methods and machinery employed in tunnelling. The discovery and application to blasting purposes of ex plosive compounds much more powerful than gunpowder, such as nitro glycerine and the numerous mixtures into which it enters as a constituent (known variously under the names of dynamite, giant-powder, explosive g-elatine, etc.), and others of different character (rackarock), were probably the incentives that induced engineers to improve their tools and machinery so as to permit these powerful agents to be used to the best advantage. Consequently, the discovery of high explosives has led to great improve ments in the construction and efficiency of the tools and machinery used in rock-drilling. The old-time practice of hand-drilling, slow and labori ous, has been snpplanted by improved power-drills operated by steam (fig% 4), or, preferably, in all extensive tunnel works, by compressed air (pi. 26,fig. 22), increasing perhaps twenty-fold the rapidity of rock excavation and relieving- the workmen of the severest part of their labor.