Fig. 14.—Diagram Showing Sequence of Excavation for St. Gothard Tunnel.
Fig. 15.—Diagram Showing Manner of Determining Correspondence of Excavation to Sectional Profile.
Division of Section.
—It may be asserted at the outset that the whole area of the tunnel section is not ordinarily excavated at one time, but that it is removed in sections, and as each section is excavated it is thoroughly timbered or strutted. The order in which these different sections are excavated varies with the method of excavation, and it is clearly shown for each method in succeeding chapters. As a single example to illustrate the proposition just made, the division of the section and the sequence of excavation adopted at the St. Gothard tunnel is selected ([Fig. 14]). The different parts of the section were excavated in the order numbered; the names given to each part, and the number of holes employed in breaking it down, are given by the [table] on [page 35]. Whatever method is employed, the work always begins by driving a heading, which is the most difficult and expensive part of the excavation. All the other operations required in breaking down the remainder of the tunnel section are usually designated by the general term of enlargement of the profile. The various operations of excavation may, therefore, be classified either as excavation of the heading or enlargement of the profile.
Excavation of the Heading.
—There is considerable confusion among the different authorities regarding the exact definition of the term “heading” as it is used in tunnel work. Some authorities call a small passage driven at the top of the profile a heading, and a similar passage driven at the bottom of the profile a drift; others call any passage driven parallel to the tunnel axis, whether at the top or at the bottom of the profile, a drift; and still others give the name “heading” to all such passages. For the sake of distinctness of terminology it seems preferable to call the passage a heading when it is located at the top of the profile, and a drift when it is located near the bottom.
Headings and drifts are driven in advance of the general excavation for the following purposes: (1) To fix correctly the axis of the tunnel; (2) to allow the work to go on at different points without the gangs of laborers interfering with each other; (3) to detect the nature of material to be dealt with and to be ready in any contingency to overcome any trouble caused by a change in the soil; and (4) to collect the water. The dimensions of headings in actual practice vary according to the nature of the soil through which they are driven. As a general rule they should not be less than 7 ft. in height, so as to allow the men to work standing, and have room left for the roof strutting. The width should not be less than 6 ft., to allow two men to work at the front, and to give room for the material cars without interfering with the wall strutting. Usually headings are made 8 ft. wide. The length of headings in practice varies according to circumstances. In very long tunnels through hard rock the headings are sometimes excavated from 1000 ft. to 2000 ft. in advance, in order that they may meet as soon as possible and the ranging of the center line be verified, and so that as great an area of rock as possible may be attacked at the same time in the work of enlarging the profile. In short tunnels, where the ranging of the center line is less liable to error, shorter headings are employed, and in soft soils they are made shorter and shorter as the cohesion of the soil decreases. When the material has too little cohesion to stand alone, the tops and sides of the heading require to be supported by strutting. To prevent caving at the front of the heading, the face of the excavation is made inclined, the inclination following as near as may be the natural slope of the material.
Enlargement of the Profile.
—The enlargement of the profile is accomplished by excavating in succession several small prisms parallel to the heading, and its full length, which are so located that as each one is taken out the cross-section of the original heading is enlarged. The number, location, and sequence of these prisms vary in different methods of excavation, and are explained in succeeding chapters where these methods are described. To direct the excavation so as to keep it always within the boundaries of the adopted profile, the engineer first marks the center line on the roof of the heading by wooden or metal pegs, or by some other suitable means by which a plumb line may be suspended. He next draws to a large scale a profile of the proposed section; and beginning at the top of the vertical axis he draws horizontal lines at regular intervals, as shown by [Fig. 15], until they intersect the boundary lines of the profile, and designates on each of these lines the distance between the vertical axis and the point where it intersects the profile. It is evident that if the foreman of excavation divides his plumb line in a manner corresponding to the engineer’s drawing, and then measures horizontally and at right angles to the vertical center plane of the tunnel the distance designated on the horizontal lines of the drawing, he will have located points on the profile of the section, or in other words have established the limits of excavation.