=Strutting of Headings.=--The method of strutting the heading that is employed depends upon the material through which the heading is driven. In solid rock strutting may not be required at all, or only for the purpose of preventing the fall of loose blocks from the roof, then vertical props are erected where required, or horizontal beams are inserted into the side walls, as shown by Fig. 19. These horizontal beams may be used singly at dangerous places, or they may be placed from 2 ft. to 3 ft. apart all along the heading. In the latter case they usually carry a lagging of planks, which may be placed at intervals or close together, and filled above with stone in case the roof of the excavation is very unstable. Planks used in this manner are usually called poling-boards. Where the side walls as well as the roof require support, vertical side posts are employed to carry the roof beams, as shown by Fig. 20; and, when necessary, poling-boards are inserted between these posts and the walls of the excavation. [Illustration: FIG. 19.--Ceiling Strutting for Tunnel Roofs.] [Illustration: FIG. 20.--Ceiling Strutting with Side Post Supports.] [Illustration: FIG. 21.--Sill, Side Post and Cap Cross Frame Strutting.] [Illustration: FIG. 22.--Reinforced Cross Frame Strutting for Treacherous Materials.] _Frame Strutting._--In very loose soils not only the roof and side walls, but also the floor of the heading require strutting. In these cases frame strutting is employed, as shown by Fig. 21. It consists simply of a rectangular frame; at the top there is a crown bar supported by two vertical side posts setting on a sill laid across the bottom of the heading. These frames are spaced at close intervals, and carry longitudinal planks or poling-boards. The sill of the frame is sometimes omitted when the soil is stable enough to permit it, and in its place wooden footing blocks are substituted to carry the side posts. In soils where the pressures are great enough to bend the crown bar, a secondary frame is employed, as shown by Fig. 22, the two inclined roof members, or rafters, of which support the crown bar at the center. [Illustration: FIG. 23.--Longitudinal Poling-Board System of Roof Strutting.] [Illustration: FIG. 24.--Transverse Poling-Board System of Roof Strutting.]