=Excavation.=--Fig. 68 is the excavation diagram of the Belgian method of tunneling. The excavation is begun by opening the center top heading No. 1, which is carried ahead a greater or less distance, depending upon the nature of the soil, and is immediately strutted. This heading is then deepened by excavating part No. 2, to a depth corresponding to the springing lines of the roof arch. The next step is to remove the two side sections No. 3, by attacking them at the two fronts and at the sides with four gangs of excavators. The regularity and efficiency of the mode of procedure described consist in adopting such dimensions for these several parts of the section that each will be excavated at the same rate of speed. When the upper part of the section has been excavated as described, the roof arch is built, with its feet supported by the unexcavated earth below. This portion of the section is excavated by taking out first the central trench No. 4 to the depth of the bottom of the tunnel, and then by removing the two side parts No. 5. As these side parts No. 5 have to support the arch, they have to be excavated in such a way as not to endanger it. At intervals along the central trench No. 4, transverse or side trenches about 2 ft. wide are excavated on both sides, and struts are inserted to support the masonry previously supported by the earth which has been removed. The next step is to widen these side trenches, and insert struts until all of the material in parts No. 5 is taken out. When the material penetrated is firm enough to permit, the plan of excavation illustrated by the diagram, Fig. 68A, is substituted for the more typical one just described. The only difference in the two methods consists in the plan of excavating the upper part of the profile, which in the second method consists in driving first the center top heading No. 1, and then in taking out the remainder of the section above the springing lines of the arch in one operation, while in the first method it is done in two operations. The distance ahead of the masonry to which the various parts can be driven varies from 10 ft. to, in some cases, 100 ft., being very short in treacherous ground, and longer the more stable the material is.