11 \\KI.> IHl;oi(;H SOFT GROUND 



135 



before the side walls and invert are built. The excavation, 

 therefore, begins with the driving of a top center heading 

 \\hii-h is enlarged until the whole of the section above the 

 springing lines of the arch is opened. Various modifications 

 of the method have been developed, and some of the UK in- 

 important of these will be described farther on, but we shall 

 begin its consideration here by describing first the original and 

 usual mode of procedure. 



Excavation. Fig. 61 is the excavation diagram of the Bel- 

 gian method of tunneling. The excavation is begun by open- 

 ing 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 



Fioa. 61 and 82. Diagrams Showing Sequence of Excavations in the Belgian Method. 



by excavating part No. 2, to a depth corresponding to tlu 

 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 descrilx-d, 

 the roof arch is built, with ite frrt supported by the unexca- 

 vated 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, 



