=Strutting.=--The longitudinal method of strutting, with the poling-boards running transversely of the tunnel, is always employed in the Belgian method of tunneling. In driving the first center top heading, pairs of vertical posts carrying a transverse cap-piece are erected at intervals. On these cap-pieces are carried two longitudinal bars, which in turn support the saddle planks. As fast as part No. 2, Fig. 68, is excavated, the vertical posts are replaced by the batter posts _A_ and _B_, Fig. 69. The excavation of parts No. 3 is begun at the top, the poling-boards _a_ and _b_ being inserted as the work progresses. To support the outer ends of these poling-boards, the longitudinals _X_ and _Y_ are inserted and supported by the batter posts _C_ and _D_. In exactly the same way the poling-boards _c_ and _d_, the longitudinals _V_ and _W_, and the struts _E_ and _F_, are placed in position; and this procedure is repeated until the whole top part of the section is strutted, as shown by Fig. 69, the cross struts _x_, _y_, _z_, etc., being inserted to hold the radial struts firmly in position. The feet of the various radial props rest on the sill _M N_. These fan-like timber structures are set up at intervals of from 3 ft. to 6 ft., depending upon the quality of the soil penetrated. [Illustration: FIG. 69.--Sketch Showing Radial Roof Strutting, Belgian Method.] [Illustration: FIG. 70.--Sketch Showing Roof Arch Center, Belgian Method.] =Centers.=--Either plank or trussed centers may be employed in laying the roof arch in the Belgian method, but the form of center commonly employed is a trussed center constructed as shown by Fig. 70. It may be said to consist of a king-post truss carried on top of a modified form of queen-post truss. The collar-beam and the tie-beam of the queen-post truss are spaced about 7 ft. apart, and the posts themselves are left far enough apart to allow the passage of workmen and cars between them. The tie beam of the king-post truss is clamped to the collar-beam of the queen-post truss by iron bands. On the rafters of the two trusses are fastened timbers, with their outer edges cut to the curve of the roof arch. These centers are set up midway between the fan-like strutting frames previously described. They are usually built of square timbers. The tie beams are usually 6 × 6 in., and the struts and posts 4 × 4 in. timbers. The reason for giving the larger sectional dimensions to the tie beams, contrary to the usual practice in constructing centers, is that it has to serve as a sill for distributing the pressure to the foundation of unexcavated soil which supports the center. Sometimes a sub-sill is used to support the center upon the soil; and in any case wedges are employed to carry it, which can be removed for the purpose of striking the center. After the arch is completed, the centers may be removed immediately, or may be left in position until the masonry has thoroughly set.