The heading and bench method of excavating rock tunnels is not always followed in the manner just described but is employed with slight modifications. There is a large variety of modifications but only the two most commonly used in practical works are given here. The heading and bench method illustrated in Fig. 66 was used, among others, on the Gallitsin tunnel along the Pennsylvania R.R. at the summit of the Alleghenies near Altoona, Pa., and more recently in the tunnels constructed by the same company under Bergen Hill, N. J., for the entrance to New York City. The shape of the cross-section of these tunnels was semicircular arch on vertical side walls. The excavation was made in three consecutive cuts, viz., the heading marked 1 in the figure, the top bench 2, and the lower bench 3. A heading 7 ft. high and 10 ft. wide was attached near the crown of the arch and the rock was removed by means of a center cut and parallel side holes, the number of holes depending upon the consistency of the rock. The part No. 2 was excavated by drilling holes at each side to different depths and at different inclinations in order to reach the line of the profile as well as the springing line of the proposed tunnel. The central part of the top bench was excavated by means of holes driven vertically from the floor of the heading. The bottom bench No. 3, included between the springing line of the arch and subgrade, was removed by means of five vertical holes driven from the floor of the top bench. The three different working parts were kept nearly 10 ft. apart. Blasting was effected in reversed order to the figures marked in the diagram, viz., the bottom bench first and the heading last. [Illustration: FIG. 67.--Diagram Showing a Modification of the Heading and Bench Method.] Still another modification of the heading and bench method, commonly followed by American engineers, is the one shown in Fig. 67. This consists in dividing the tunnel section in three parts by horizontal lines. The resultant parts are first the heading excavated close to the roof, and as wide as the whole section of the tunnel; second, the top bench in the middle, and lastly the bottom bench excavated to the depth of the proposed tunnel floor. The excavation proceeds in the numerical order, beginning at the heading which was excavated, as usual, by means of a center cut and side holes to the full width of the proposed tunnel. First the top bench, then the bottom bench, are removed by means of vertical holes driven from the floor of the heading and the floor of the top bench, respectively. COMPARISON OF METHODS.