CHAPTER XIV.
THE FULL SECTION METHOD OF TUNNELING: ENGLISH METHOD; AMERICAN METHOD; AUSTRIAN METHOD.
ENGLISH METHOD.
The English method of tunneling through soft ground, as its name implies, originated in England, where, owing to the general prevalence of comparatively firm chalks, clays, shales, and sandstones, it has gained unusual popularity. The distinctive characteristics of the method are the excavation of the full section of the tunnel at once, the use of longitudinal strutting, and the alternate execution of the masonry work and excavation. In America the method is generally designated as the longitudinal bar method, owing to the mode of strutting, which has gained particular favor in America, and is commonly employed here even when the mode of excavation is distinctively German or Belgian in other respects.
Fig. 84.—Diagram Showing Sequence of Excavation in English Method of Tunneling.
Excavation.
—Although, as stated above, the distinctive characteristic of the English method is the excavation of the full section at once, the digging is usually started by driving a small heading or drift to locate and establish the axis of the tunnel, and to facilitate drainage in wet ground. These advance galleries may be driven either in the upper or in the lower part of the section, as the local conditions and choice of the engineer dictate. Whether the advance gallery is located at the top or at the bottom of the section makes no difference in the mode of enlarging the profile. This work always begins at the upper part of the section. A center top heading is driven and strutted by erecting posts carrying longitudinal bars supporting transverse poling-boards. This heading is immediately widened by digging away the earth at each side, and by strutting the opening by temporary posts resting on blocking, and carrying longitudinal bars supporting poling-boards. This process of widening is continued in this manner until the full roof section, No. 1, [Fig. 84], is opened, when a heavy transverse sill is laid, and permanent struts are erected from it to the longitudinal bars, the temporary posts and blocking being removed. The excavation of part No. 2 then begins by opening a center trench and widening it on each side, temporary posts being erected to support the sill above. As soon as part No. 2 is fully excavated, a second transverse sill is placed below the first, and struts are placed between them. The excavation of part No. 3 is carried out in exactly the same manner as was part No. 2. The lengths of the various sections, Nos. 1, 2, and 3, generally run from 12 ft. to 20 ft., depending upon the character of the soil.
Strutting.
—The strutting in the English method of tunneling consists of a transverse framework set close to the face of the excavation, which supports one end of the longitudinal crown bars, the other ends of which rest on the completed lining. The transverse framework is composed of three horizontal sills arranged and supported as shown by [Fig. 85]. The bottom sill A is carried by vertical posts resting on blocking on the floor of the excavation. From the bottom sill vertical struts rise to support the middle sill B. The top sill, or miners’ sill C, is carried by vertical posts or struts rising from the middle sill B. The vertical struts are usually round timbers from 6 ins. to 8 ins. in diameter; and the sills are square timbers of sufficient section to carry the vertical loads, and generally made up of two posts scarf-jointed and butted to permit them to be more easily handled. In firm soils the struts between the sills are all set vertically, but those at the extreme sides of the roof section are inclined. In loose soils, however, where the sides of the excavation must be shored, the V-bracing shown by [Fig. 85] is employed between one or more pairs of sills as the conditions necessitate. The manner of holding the transverse framework upright is explained quite clearly by [Fig. 85]; inclined props extending from the completed masonry to the sills of the framework being employed. Two props are used to each sill. Sometimes, in addition to the props shown, another nearly horizontal prop extends from the crown of the arch masonry to the middle piece of the strutting.