142 



Tl NNELING 



then the two side parts No. 2 are excavated, opening up the 

 entire upper portion of the section in which the roof arch is 

 built, as in the regular Belgian method. The next step is to 

 excavate part No. 3, shoring up the arch 

 at frequent intervals. Between these sets 

 of shoring the side walls are built, resting 

 on planks on the floor of part No. 3, and 

 then the sets of shores are removed and re- 

 placed by masonry. Next part No. 4 is 

 excavated, shored, and filled with masonry 

 as was part No. 3. In exactly the same 



FIG. 67. Diagram Show- 

 ing Sequence of Excava- way parts 5, 6, 7, and 8 are constructed 



tion in Modified Belgian , , -, i_ j TI , r. 



in the order numbered. To prevent the 

 distortion of the arch during the side-wall 

 braced by horizontal struts, as described 



Method. 



construction it is 

 above in Fig. 65. 



Advantages. The advantages of the Belgian method of 

 tunneling may be summarized as follows : (1) The excavation 

 progresses simultaneously at several points without the differ- 

 ent gangs of excavators interfering with each other, thus secur- 

 ing rapidity and efficiency of work; (2) the excavation is done 

 by driving a number of drifts or parts of small section, which 

 are immediately strutted, thus causing the minimum disturb- 

 ance of the surrounding material ; (3) the roof of the tunnel, 

 which is the part of the lining exposed to the greatest pressures, 

 is built first. 



Disadvantages. The disadvantages of the Belgian method 

 of tunneling may be summarized as follows : (1) The roof arch 

 which rests at first on compressible soil is liable to sink ; (2) 

 before the invert is built there is danger of the arch and side 

 walls being distorted or sliding under the lateral pressures ; (3) 

 the masonry of the side walls has to be underpinned to the arch 

 masonry. 



Accidents and Repairs. One of the most frequent accidents 

 in the Belgian method of tunneling is the sinking of the roof 



