The excavation of the upper half of the tunnel section is strutted as in the Belgian method, with radial struts carrying longitudinal roof bars and transverse poling-boards. On account of the enormous pressures developed by the treacherous soils in which only is the Italian method employed, the radial strutting frames and crown bars must be of great strength, while the successive frames must be placed at frequent intervals, usually not more than 3 ft. After the masonry side walls have been built in the lower part of the excavation, longitudinal planks are laid against the side posts of the center bottom drift frames, to form an enclosure for the filling-in of parts No. 2. The object of this filling is principally to prevent the squeezing-in of the side walls. [Illustration: FIGS. 101 and 101A.--Sketches Showing Construction of Centers, Italian Method.] =Centers.=--Owing to the great pressures to be resisted in the treacherous soils in which the Italian method is used, the construction of the centers has to be very strong and rigid. Figs. 101 and 101A show two common types of center construction used with this method. The construction shown in Fig. 101 is a strong one where only pressures normal to the axis of the tunnel have to be withstood, but it is likely to twist under pressures parallel to the axis of the tunnel. In the construction shown by Fig. 101A, special provision is made to resist pressures normal to the plane of the center or twisting pressures, by the strength of the transverse bracing extending horizontally across the center. [Illustration: FIG. 102.--Sketch Showing Invert and Foundation Masonry, Italian Method.] =Masonry.=--The construction of the masonry lining begins with the invert, as indicated by Fig. 100, and is carried up to the roof of parts No. 2, as already indicated, and is then discontinued until the upper parts Nos. 4, 5, and 6 are excavated. The next step is to sink side trenches at the ends of part No. 7, which reach to the top of the completed side walls. This operation leaves the way clear to finish the side walls and to construct the roof arch in the ordinary manner of such work in tunneling. Since this method of tunneling is used only in very soft ground which yields under load, the usual practice is to construct the invert and side walls on a continuous foundation course of concrete as indicated by Fig. 102. The lining is usually built in successive rings, and the usual precautions are taken with respect to filling in the voids behind the lining. The thickness of the lining is based upon the figures for laminated clay of the third variety given in Table II.