Fig. 66.—Diagram Showing the Arrangement of Drill Holes in the Heading and Bench of the Gallitsin Tunnel.
Hauling.
—The excavated material was loaded at the foot of the bench in dump cars which were run by mule power to the portal or the shaft according to location, on 36 in. gauge-service tracks. Inclines at 159th Street were graded from the portal at 158th Street to the street surface. The cars were formed at this portal into a train and were taken up the incline to the dump at 162nd Street and the North River by construction locomotives. At the 168th Street and 181st Street shafts, the cars were hoisted to the surface in cages (elevators). In the former case, they were taken to the dump at 165th Street and the North River by mules and gravity; in the latter case, to various dumps by teams. At both shafts, stone crushers were located, therefore a great part of the material did not have to be hauled to the dumps or even taken to the surface as a great deal of stone was used in dry packing over the concrete arch. The material from the portal at Fort George was hauled by mules directly to the dump near by.
Lining.
—The entire tunnel was lined with concrete, consisting of a floor 4 ins. thick and vertical side walls 18 ins. thick and 25 ft. apart, which carried a semicircular arch 18 ins. thick except in the timbered portions where the thickness was increased to 21 ins. and to 24 and 27 ins. in some places. The springing line of the arch is 6 ft. 2 ins. above the concrete floor (5 ft. 6 ins. above the base of rail), hence the maximum clearance above the base of rail is 18 ft. The side walls and arch were built solid of rock to a height of 8 ft. above springing line and the space above that point between the concrete and the rock was packed by hand with small stones. The concrete of the arch was laid on timber centers erected for that purpose.
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.
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.