Fig. 19.—Ceiling Strutting for Tunnel Roofs.
Fig. 20.—Ceiling Strutting with Side Post Supports.
Fig. 21.—Sill, Side Post and Cap Cross Frame Strutting.
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.