8 TUNNELING 



others, like clay and shale, are impervious. Detrital soils 

 lying above clay are almost certain to carry water just above 

 the clay stratum. lu tunnel work, therefore, when the exca- 

 vation keeps well within the clay stratum, little trouble is 

 likely to be had from water ; should, however, the excavation 

 cut the clay surface and enter the pervious material above, 

 water is quite certain to be encountered. The quantity of 

 water encountered in any case depends upon the presence of 

 high mountains near by, and upon other circumstances which 

 will attract the attention of the engineer. 



A knowledge of the pressure of the water is desirable. 

 This may be obtained by observing closely its source and the 

 character of the strata through which it passes. Water 

 coming to the excavation through rock crevices will lose 

 little of its pressure by friction, while that which has passed 

 some distance through sand will have lost a great deal of its 

 pressure by friction. Water bearing sand, and, in fact, any 

 water bearing detrital material, has its fluidity increased by 

 -water pressure ; and when this reaches the point where flow 

 Tesults, trouble ensues. The streams of water met in the 

 construction of the St. Gothard tunnel had sufficient pressure 

 to carry away timber and materials. 



