+-------------------+------------+--------+-------+----------+ | ORDER OF FIRING. | KINDS OF | DEPTH. |CHARGE.| KIND OF | | | HOLES. | | | DYNAMITE.| +-------------------+------------+--------+-------+----------+ |Bench { 1st round|4 grading |3′ to 5′|50 lbs.|40% climax| |Holes { |5 bench |9′ 6″ |45 „ |40% „ | | { 2nd round|6 trimming |3′ to 9′|42 „ |40% „ | | | | | | | |Heading { 3d round |8 center cut|9′ |56 „ |60% „ | | Holes { 4th round|8 side |8′ |48 „ |40% „ | | { 5th round|6 dry |8′ |36 „ |40% „ | +-------------------+------------+--------+-------+----------+ CHAPTER IV. GENERAL METHODS OF EXCAVATION: SHAFTS: CLASSIFICATION OF TUNNELS. A number of different modes of procedure are followed in excavating tunnels, and each of the more important of these will be considered in a separate chapter. There are, however, certain characteristics common to all of these methods, and these will be noted briefly here. [Illustration: FIG. 14.--Diagram Showing Sequence of Excavation for St. Gothard Tunnel.] [Illustration: 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.