Fig. 78.—Sketch Showing Work of Excavating and Timbering Drifts and Headings.
Fig. 79.—Sketch Showing Method of Roof Strutting.
Two methods of strutting the soffit of the excavation are employed, one being a modification of the longitudinal system employed in the English method of tunneling described in a [succeeding chapter], and the other a modification of the Belgian system [previously] described. [Fig. 79] shows the method of employing the radial strutting of the Belgian system. At the beginning the center top heading is strutted with rectangular bents such as are employed for strutting the drifts. As this heading is enlarged by taking out the haunch sections, radial posts are inserted, as shown by [Fig. 79], which also indicates the method of strutting the side trenches when the excavation is carried downward from the center top heading instead of upward from bottom side drifts.
Masonry.
—Whatever plan of excavation or strutting is employed, the construction of the masonry lining in the German method of tunneling begins at the foundations of the side walls and is carried upward to the roof arch. The invert, if one is required, is built after the center core of earth is removed.
Centering.
—Tunnel centers are generally employed in the German method of tunneling, a common construction being shown by [Fig. 80]. It is essentially a queen-post truss, the tie beam of which rests on a transverse sill as shown by the illustration. The transverse sill is supported along its central portion by the unexcavated center core of earth, and at its ends either directly on the vertical posts or on longitudinal beams resting on these posts. The diagonal members of the queen-post truss form the bottom chords of small king-post trusses which are employed to build out the exterior member of the center to a closer approximation to the curve of the arch.
Fig. 80.—Sketch Showing Roof Arch Centers and Arch Construction.