CONSTRUCTION. Tunnels may be classified ac cording to the general nature of the materials penetrated, into hard-roek tunnels and soft ground tunnels. Whatever the material pene trated, the general methods of construction are the same, but a variety of differences in detail exist. In certain kinds of tunnels, such as sub aqueous tunnels and tunnels under city streets, other conditions than the hardness of the mate rial penetrated exercise a controlling influence. Fo• the purpose of description, therefore, it is common to classify tunnels into hard-roek tun nels, soft-ground tunnels, open-eut tunnels. and submarine tunnels. In open-cut tunnels and sub marine tunnels the material penetrated may be either rock or earth. When it has been decided to construct a tunnel the first task is to construct a geological map which will show as accurately as may be determined the character and inclina tion of the earth's strata to he penetrated, the probability of water being encountered, etc., all of which exercise an important influence upon the difficulties and cost of the work. The next step is to establish exactly the centre line of the tunnel; this is done on the surface of the ground and its purpose is to find the exact length of the tunnel and to furnish a reference line by which the excavation is direeted. The determination of the centre line is a simple problem in surveying, requiring only skill and exactness for its success ful solution ; the longer the tunnel, and the higher and more inaccessible the mountain is above it, the more difficult is the work. So per
fect, however, has the skill of the surveyor be come that it is a common thing to work within a small fraction of a foot in alignment in driv ing a long mountain tunnel.
The centre line being established, the next task is to establish the form and dimensions of the cross-section of the hole o• passageway which it is proposed to excavate. The form and dimen sions of cross-section adopted are determined by the purpose to which the tunnel is to be put and by the diameter of the material penetrated. The best one for the majority of conditions, and the one most commonly employed, is the polyeentrie figure (Fig. 1), in which the number of centres and length of the radii are fixed by the engineer to meet the particular conditions which exist. The dimensions to be given to the cross-section of a tunnel depend upon the purpose for which it is to be used, as will he observed in the succeed ing descriptions of prominent tunnels. In all eases the form and the dimensions are those of the inside of the completed tunnel, which is of course the inside of the lining in all eases except those of unlined hard-roek tunnels. where it is the inside of the original excavation.