Hauling.
—To haul away the material and take in supplies, tracks are laid on the invert masonry. Generally the permanent tracks are laid as fast as the lining is completed. A short section of temporary track is used to extend this permanent track close to the work of the advanced drift.
Advantages and Disadvantages.
—The great advantage of the English method of tunneling is that the masonry lining is built in one piece from the foundations to the crown, making possible a strong, homogeneous construction. It also possesses a decided advantage because of the simple methods of hauling which are possible: there being no differences of level to surmount, no hoisting of cars nor trans-shipments of loads are necessary. The chief disadvantage of the method is that the excavators and masons work alternately, thus making the progress of the work slower perhaps than in any other method of tunneling commonly employed under similar conditions. This disadvantage is overcome to a considerable extent when the tunnel is excavated by shafts, and the work at the different headings is so arranged that the masons or excavators when freed from duty at one heading may be transferred to another where excavation or lining is to be done as the case may be. Another disadvantage of the English method arises from the excavation of the full section at once, which in unstable soils necessitates strong and careful strutting, and increases the danger of caving. The fact also that the arch ring has to carry the weight of the crown bars, and their loading at one end while the masonry is green, increases the chances of the arch being distorted.
Conclusion.
—The English method of tunneling in its entirety is confined in actual practice pretty closely to the country from which it receives its name. A possible extension of its use more generally is considered by many as likely to follow the development of a successful excavating machine for soft material. The space afforded by the opening of the full section at once, especially adapts the method to the use of excavators like, for example, the endless chain bucket excavator used on the Central London Ry., and illustrated in [Fig. 11]. The method also furnishes an excellent opportunity for electric hauling and lighting during construction.
The English method of tunneling has been used in building the Hoosac, Musconetcong, Allegheny, Baltimore and Potomac, and other tunnels in America. The names of the European tunnels built by this method are too numerous to mention here.
AMERICAN METHOD.
In this country tunnels through loose soils are excavated according to the “Crown Bar” or American Method. This consists in opening the whole section of the tunnel before the construction of the lining as in the English Method. It differs from the English method, however, in that many timber structures are erected for the support of the roof, and that the excavation and construction of the lining are far apart, so allowing the miners and the masons to work continuously and without interfering with each other.
