=Strutting.=--The side drifts were strutted by means of frames composed of two batter posts resting on boards, and having a cap-piece extending transversely across the roof of the drift. These frames were spaced about 4 ft. apart. The excavation was advanced in the usual way by driving poling-boards at the top and sides, with a slight outward and upward inclination, so that the next frame could be easily inserted leaving space enough between it and the sheeting to permit the next set of poling-boards to be inserted. These poling-boards were driven as close together as practicable so as to prevent as much as possible the inflow of water and earth. [Illustration: FIG. 81.--Sketch Showing Method of Excavating and Strutting Baltimore Belt Line Tunnel.] The center top heading was strutted in the same manner as were the side drifts. The arrangement of the strutting employed in enlarging the center top heading is shown clearly by Fig. 81, which also shows the manner of strutting the side drifts and face of the excavation, and of building the masonry. =Centers.=--Both wood and iron centers were employed in building the roof arch. The timber centering was constructed of square timbers, as shown by Fig. 82. This construction of the iron centers is shown by Fig. 83. Each of the iron centers consisted of two 6 × 6 in. angles butted together, and bent into the form of an arch rib. Six of these ribs were set up 4 ft. apart. They were made of two half ribs butted together at the crown, and were held erect and the proper distance apart by spacing rods. The rearmost rib was held fast to the completed arch masonry, and in turn supported the forward ribs while the lagging was being placed. [Illustration: FIG. 82.--Roof Arch Construction with Timber Centers, Baltimore Belt Line Tunnel.] =Masonry.=--The side walls of the lining were built first in the bottom side drifts, as shown by Fig. 81. They were generally placed on a foundation of concrete, from 1 ft. to 2 ft. thick. As a rule the side walls were not built more than 20 ft. in advance of the arch, but occasionally this distance was increased to as much as 90 ft. The roof arch consisted ordinarily of five rings of brick, but at some places in especially unstable soil eight rings of brick were employed. The arch was built in concentric sections about 18 ft. in length. All the timber of the strutting above the arch and outside of the side walls was left in place, and the voids were filled with rubble masonry laid in cement mortar. It required about 125 mason hours to build an 18-ft. arch section. Figs. 82 and 83 show various details of the masonry arch work. [Illustration: FIG. 83.--Roof Arch Construction with Iron Centers, Baltimore Belt Line Tunnel.]