4 TUNNELING 



annular hole, and take away a core for the whole depth of the 

 boring, thus giving a perfect geological section showing the 

 character, succession, and exact thickness of the strata. By 

 making such borings at different points along the center line 

 of the projected tunnel, and comparing the relative sequence 

 and thickness of the different strata shown by the cores, the 

 geological formation of the mountain may be determined quite 

 exactly. Where it is difficult or impracticable to make dia- 

 mond drill borings on account of the depth of the mountain 

 above the tunnel, or because of its inaccessibility, the engineer 

 must resort to other methods of observation. 



The present forms of mountains or hills are due to 

 weathering, or the action of the destructive atmospheric influ- 

 ences upon the original material. From the manner in which 

 the mountain or hill has resisted weathering, therefore, may be 

 deduced in a general way both the nature and consistency of 

 the materials of which it is composed. Thus we shall gener- 

 ally find mountains or hills of rounded outlines to consist 

 of soft rocks or loose soils, while under very steep and crested 

 mountains hard rock usually exists. To the general knowl- 

 edge of the nature of its interior thus afforded by the ex- 

 terior form of the mountain, the engineer must add such 

 information as the surface outcroppings and other local evi- 

 dences permit. 



For the purposes of the tunnel builder we may first classify 

 all materials as either, (1) hard rock, (2) soft rock, or (3) 

 soft soil. 



Hard rocks are those having sufficient cohesion to stand 

 vertically when cut to any depth. Many of the primary rocks, 

 like granite, gneiss, feldspar, and basalt, belong to this class, 

 but others of the same group are affected by the atmosphere, 

 moisture, and frost, which gradually disintegrate them. They 

 are also often found interspersed with pyrites, whose well- 

 known tendency to disintegrate upon exposure to air intro- 

 duces another destructive agency. For these reasons we may 



