204 



terzaghi. SOFT-GROUND TUNNELING 



[Ch. 11 



POSITION OF THE WATER TABLE 



At the beginning of this chapter it was shown that the working con- 

 ditions in tunnels through sediments of any kind other than silt and 

 clay depend primarily on the position of the water table with reference 

 to the bottom of the tunnel. Therefore the second prerequisite for a 

 reliable forecast of tunneling conditions consists in securing conclusive 

 information regarding the position of the water table. 



Even today it is by no means uncommon that the position of the 

 water table is inferred from the height to which the water rises in the 

 drill holes between shifts. A conclusion arrived at on such a basis can 



121 Moraine 



□ Moist, fine sand 



■I Saturated.fine sand 



Caisson section 

 H-6 00'-H 



Fig. 5. Geological section through a terminal moraine in northern Switzerland, 

 along the center line of the Emmersberg Tunnel. (After Schweizer Bauzeitung, 



1895, p. 135.) 



be utterly misleading, because in sand it may take only a few hours 

 whereas in silt or clay formations it may take several weeks or years 

 for the water level in the drill hole to arrive within a few inches of the 

 position of the water table. The practical consequences of erroneous 

 conceptions regarding the position of the water table are illustrated in 

 Figs. 5 and 6. 



Figure 5 is a section across a terminal moraine along the center 

 line of the Emmersberg Tunnel in northern Switzerland. The tunnel 

 has a length of about 2,600 feet, and the crest of the moraine is located 

 at an elevation of about 70 feet above the roof of the tunnel. The 

 moraine consists of a dense mixture of rock fragments, grit, and rock 

 flour. It contains large, irregular lenses of very fine sand which do 

 not communicate with each other. Since the permeability of the sur- 

 rounding moraine material is very low, the water table in the sand 

 lenses is located at very different elevations. Some of the lenses do 

 not contain any free water at all. In the first lenses that were en- 

 countered, the sand was only moist. As a consequence it was so stable 

 that no breasting was required. However, as soon as the heading ar- 



