The German method of tunneling was first used in 1803 in constructing the St. Quentin Canal. In 1837 the Königsdorf tunnel of the Cologne and Aix la Chapelle R.R. was excavated by the same method. The success of the method in these two difficult pieces of soft-ground tunneling led to its extensive adoption throughout Germany, and for this reason it gradually came to be designated as the German method. Briefly explained the method consists in excavating first an annular gallery in which the side walls and roof arch are built complete before taking out the center core and building the invert. [Illustration: FIG. 76.--Diagrams Showing Sequence of Excavation in German Method of Tunneling.] =Excavation.=--The excavation of tunnels by the German method is begun either by driving two bottom side drifts or by driving a center top heading. Fig. 76 shows the mode of procedure when bottom side drifts are used to start the work. The two side drifts No. 1 are made from 7 ft. to 8 ft. wide, and about one-third the total height of the full section; the width of each heading has to be sufficient for the construction of the masonry and strutting, and for the passage of narrow spoil cars alongside them. These drifts are increased in height to the springing line of the arch by taking out the two drifts No. 2. Next the top center heading No. 3 is driven, and finally the two haunch headings No. 4 are excavated. The center core No. 5 is utilized to support the strutting until the side walls and roof arch are completed, when it is broken down and removed. In case of very loose material, where the first side drifts cannot be carried as high as one-third the height of the section, it is the common practice to make them about one-fourth the height, and to take out the side portions of the annular gallery in three parts, as shown by Fig. 76. [Illustration: FIG. 77.--Diagram Showing Sequence of Excavations in Water Bearing Material, German Method.] The top center heading plan of commencing the excavation is usually employed in firm materials or when a vein of water is encountered in the upper part of the section. In the latter contingency a small bottom drift _A_, Fig. 77, is first driven to serve as a drain; but in any case the excavation proper of the tunnel consists in first driving the center top heading No. 1, and then by working both ways along the profile parts, Nos. 2, 3, 4, and 5 are removed. Part No. 6 is left to support the strutting until the side walls and roof arch are built, when it is also excavated.