The bottom is excavated so as to conform to the exterior lines of the masonry. The excavation follows so closely to the outer lines of the normal section of the tunnel that very little loss occurs, even in bad material; and there is no loss where sufficient bond exists in the material to hold it in place until the poling-boards are in position. [Illustration: ~Bracing.~ ~Arch Construction.~ ~Longitudinal Section.~ FIG. 111.--Sketch Showing Pilot Method of Tunneling.] In the Brooklyn sewer tunnel work, previously mentioned, the pilot was built of steel plates ³⁄₈ in. thick, 12 ins. wide, and 37¹⁄₂ ins. long, rolled to a radius of 3 ft. Steel angles 4 × 4¹⁄₂ ins. were riveted along all four sides of each plate, and the plates were bolted together by ³⁄₄-in. machine-bolts. The plates weighed 136 lbs. each, and six of them were required to make one complete ring 6 ft. in diameter. In bolting them together, iron shims were placed between the horizontal joints to form a footing for the wooden braces for the shell, which radiate from the pilot. The shell plates of the 15-ft. section of the tunnel were of No. 10 steel 12 ins. wide and 37 ins. long, with steel angles 2¹⁄₂ × 2¹⁄₂ × ³⁄₈ ins., riveted around the edges the same as for the pilot, and put together with ⁵⁄₈-in. bolts. These plates weighed 61 lbs. each, and eighteen of them were required to make one complete ring 15 ft. in diameter. The plates for the 12-ft. section were No. 12 steel 12 ins. wide with 2 × 2 × ¹⁄₄-in. angles. Seventeen plates were required to make a complete ring. CHAPTER XVI. OPEN-CUT TUNNELING METHODS; TUNNELS UNDER CITY STREETS; BOSTON SUBWAY AND NEW YORK RAPID TRANSIT. OPEN-CUT TUNNELING. When a tunnel or rapid-transit subway has to be constructed at a small depth below the surface, the excavation is generally performed more economically by making an open cut than by subterranean tunneling proper. The necessary condition of small depth which makes open-cut tunneling desirable is most generally found in constructing rapid-transit subways or tunnels under city streets. This fact introduces the chief difficulties encountered in such work, since the surface traffic makes it necessary to obstruct the streets as little as possible, and has led to the development of the several special methods commonly employed in performing it.