[Illustration: ~Section G-H.~ ~Longitudinal Section.~ FIG. 92.--Transversal and Longitudinal Section of a Tunnel Excavated and Strutted According to the American Method.] =Strutting.=--The longitudinal strutting is used in connection with the American method of tunneling. In fact, the strutting consists of a series of longitudinal bars supporting planks laid transversally to the axis of the tunnel and abutting against the roof of the excavation. These crown bars during the excavations and immediately after are temporarily supported by radial timbers forming almost a fan-like structure, but this is soon substituted by a permanent one composed of a polygonal timber frame of five or seven segments which are cut to dimensions. The batter posts of the heading, the radial posts of the temporary timber structure and the crown bars are all round timbers from 10 to 12 ins. in diameter. All the other timbers are square edged, the usual dimensions being 10 × 10 ins. or 12 × 12 ins. with the exception of the wall plates which are 14 × 14 ins. The dimensions of the various members of the strutting and the distance apart of the different frames vary with the quality of the soil. For instance, in ordinary loose soils the frames are placed between 4 to 6 ft., but in very soft soils they are erected only 3 or 3¹⁄₂ ft. apart. Chiefly in the southwest, in tunnels excavated according to the American method, the timbering has been left as regular lining and it was only after many years when this temporary structure had decayed or was burned down, that the tunnels were lined with masonry. But in many instances the whole timber structure was left in place even when the tunnel was lined with masonry immediately after the excavation had been made. This was usually done when the tunnel was lined with concrete masonry. In such a case the timbering was left to support the pressures of the roof while the concrete was plastic and before it hardened. =Centers.=--In the American method the whole section of the tunnel is open before the construction of the lining, thus the masonry can be built from the foundations up. The centers are designed so as to support only the weight of the masonry during its construction and not the pressures of the tunnel as in the other methods and consequently they are of light construction. The centers described in the Murray Hill tunnel, page 123, may be advantageously used in building the concrete lining in tunnels through loose soils excavated by the American method.