Excavation.
—The excavation was carried out according to the German method of tunneling. Bottom side drifts were first driven, and then heightened to the springing line of the roof arch. Next a center top heading was driven, and the haunch sections taken out. The object of beginning the excavations by bottom side drifts, was to drain the soil of the upper part of the section. The center core was removed after the side walls and roof arch were completed, its removal being kept from 50 ft. to 75 ft. to the rear of the advanced heading. The dimensions of the side drifts proper were about 8 × 8 ft., but they were often carried down much below the floor level to secure a solid foundation bed for the side walls.
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.
Fig. 81.—Sketch Showing Method of Excavating and Strutting Baltimore Belt Line Tunnel.
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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.