Ch. 11] 



POSITION OF THE WATER TABLE 



205 



rived at the first wet pocket, the sand exhibited the character of 

 flowing ground. Large quantities of sand mixed with water invaded 

 the tunnel. Sinkholes appeared on the ground surface above the tunnel. 

 When attempts were made to continue the mining operations in the 

 tunnel under compressed air, the disturbance of the ground surround- 

 ing the tunnel was already so far advanced that the attempts failed, 



2,450 



-* 2,400 



"■§ 2,350 



2,300 







500 



1,000 



1,500 



2,000 



2,500 ft 



^ Stiff clay 



\v*m Fine, silty sand 



f$£#i Coarse, clean, silty, or argillaceous sand 



Fig. 6. Geological section through a valley in the city of Sao Paulo, Brazil. The 



strata are part of a Tertiary delta formation and the relief is believed to be 



post-Pliocene. (After E. Pichler.) 



and it was decided to finish the job by means of a series of compressed- 

 air caissons which were carried down from the ground surface to the 

 level of the bottom of the tunnel (Schweizer Bauzeitung, 1894-1895). 

 Figure 6 is a simplified geological section through one of the dis- 

 tricts of the city of Sao Paulo in Brazil. The city is located on 

 Tertiary deposits which appear to have been formed by an aggrading, 

 meandering river. As a result of a lowering of base level, the river 

 deposit was dissected to a depth of at least 100 feet below the original 

 elevation of the valley floor. The sediments consist of stratified layers 

 of clay, silt, and silty sand and gravel. In horizontal directions there 

 are imperceptible transitions from one stratum into another. There- 

 fore the position of the water table is not well defined. When a hole 



