TUNNEL hold it in place, until the permanent brick or stone arching is built. Loose rock, as its name indicates, is rock either so seamy and broken by folding or compression, or so disintegrated, as to require an arch, generally much lighter than those necessary in soft ground. Accord- ing to the method generally adopted in driving a tunnel through soft ground, the first step is, if practicable, to open out a small bottom head- ing or adit, for the double purpose of drain- ing the ground above and making an opening through which to carry away the material sub- sequently excavated; this heading also is re- quired for passing hi the materials used in arching. Often, however, owing to long and heavy cuttings necessary in the outside ap- proaches to a tunnel, it is deemed advisable to begin with a top heading before the bottom bench of the open cut is brought up to the face of the proposed work. If a bottom head- ing has been driven (and it is generally best to do so when practicable in soft ground, while the opposite rule holds in tunnelling through FIG. 3. hard rock), one of the methods of subsequent enlarging that may be used is shown in figs. 3, 4, and 5. These represent the English plan, so called, it being the one generally adopted in England. For a full description of this method of enlarging, see the " Engineering and Mining Journal," vol. xix., p. 392 ; also Simms's " Treatise on the Blechingly and Salt- wood Tunnels." Fig. 3 shows the bottom heading driven, with a section excavated and ready for arching. The enlarging and arching of a tunnel to its full size is generally done in lengths or sections. If there is no top head- ing previously driven, 15 or 20 ft. of an ad- vanced heading is excavated at the top of the proposed work (shown in figs. 3 and 4). Heavy longitudinal bars of timber are then succes- sively put in, beginning with those num- bered 3, 6, and 7. The miners gradually work down, putting in a temporary arch of timber. When this is done, and foundations have been dug for the succeeding masonry, the masons take the place of the miners, and run up an arch under the timber, which is then with- drawn during the excavation o.f the next sec- tion, and the spaces left are securely blocked FIG. 4. up with pieces of timber or stone. In some methods of tunnelling, it is deemed more secure to brick the timber in and leave it in place, though at a considerable cost, especially when it is necessary to bring all the heavy timber down a shaft or slope, and through a log dis- tance underground. Shafts are often sunk, and sometimes slopes, so that the work may be attacked from several points at once. Fig. 5 shows the arch built, and is divided into two portions : that on the left shows the completed tunnel, with the ballast in place and the track laid ; that on the right shows the arch in place, and the supporting timbers struck, but still un- drawn. Where the ground is very treacherous, and much water is encountered, an inverted arch is often put in across the bottom of the tunnel, to withstand the pressure from below. Other methods are in vogue on the continent of Europe. A description of a new system of tunnelling by the use of iron centres, in place FIG. 5. of timber, devised by himself, may be found in Kziha's work, cited below. Tunnelling through Rock. One of the methods of tunael-
