ACCLDKN IS AM) KKl'AIUS IN TUNNKLS 



271 



Any of the regular soft-ground methods of tunneling may be 

 employed, but it is usually better to select one which allows 

 the masonry to be built with as little excavation as possible at 

 tirst. For this reason the (ierman method of tunneling is par- 

 ticularly suited t<> repair work of this nature. The Belgian 

 method may also l>e used to advantage, pirticularly when the 

 caving extends to the surface of the ground above, and the 

 upper portion of the debris is, therefore, practically the same 

 material as that through which the original tunnel was driven. 

 The greatest defect of the Belgian method for making repairs 

 is that the roof arch is supported by a rather unstable mass of 



\'v^ ^fr^\T=7 V 

 Fio. 134. Tunneling through Caved Material by Heading. 



mingled earth, stone, and timl>er, which constitutes the bottom 

 layer of the fallen material. Tin- m.-tluxl of strutting the work 

 when the (ierman or Belgian method is used is shown by Fig. 

 l : -l. It sometimes happens that the fallen debris is so un- 

 stable that it will nnt carry safely the arch masonry in the 

 Belgian method or the strutting in the (ierman method, and in 

 these cases one of the full-section methods of excavation is 

 usually adopted. The nature of the strutting employed is 

 shown by Fig. 1 :> a. When the section has l>een opened and 

 the new masonry built, great can- should lx taken to fill the 

 cavity In-hind the masonry with timler or stone; and should 



