206 terzaghi. SOFT-GROUND TUNNELING [Ch. 11 



is drilled, the water is likely to rise from different strata to different 

 elevations. The water table marked "principal water table" is the ap- 

 proximate locus of the points to which the water rises in drill holes 

 from most of the strata. 



A few years ago a municipal waterworks tunnel was built within the 

 city limits. The tunnel is 9 feet wide and high and about 1,500 feet 

 long. The bottom of the tunnel is located about 100 feet below the 

 street surface and 50 feet below the principal water table. The outer 

 parts of the tunnel, between the portals and the two ends of the middle 

 portion, with a length of about 500 feet, are located in water-bearing, 

 fine to medium sand with some clay. 



The tunnel was started as a free-air, hand-mined tunnel with timber 

 supports. However, the ground moved so rapidly into the tunnel and 

 the street surface settled so badly that air locks were installed. As 

 the ground was already badly disturbed, the loss of air was excessive. 

 Therefore the contractor did not succeed in drying up the bottom, and 

 it was necessary to use forepoling and to breast the working face. The 

 loss of ground was about 50 percent, and the buildings located above 

 the tunnel settled by amounts up to 8 inches. The middle part of the 

 tunnel, with a length of about 500 feet, is located in stiff, yellow clay. 

 No difficulties were encountered in this section. 



In view of the decisive influence of the position of the water table 

 on tunneling conditions, no guesswork regarding this position should 

 be tolerated. If the subsoil contains continuous layers of fairly clean 

 sand or gravel, sufficiently reliable information concerning the ground- 

 water conditions can be obtained by transforming the drill holes into 

 observation wells and by keeping a record of the variations of the water 

 level in these holes. If the subsoil consists of silty sand or silt, it is 

 necessary to install in the drill holes piezometric tubes with a diameter 

 of less than half an inch. Such tubes have been successfully used for 

 observing the rise of the piezometric surface for a silty stratum due 

 to the weight of a superimposed fill (Casagrande, 1949) . However, if 

 the ground surrounding the site of a proposed tunnel consists only 

 of fine silt or clay, no accurate information regarding the water table 

 is required, because the performance of these sediments in the tunnel 

 depends only on their water content and consistency. 



FUNCTIONS OF THE GEOLOGIST IN EARTH-TUNNEL JOBS 



The foremost duties of the geologist on an earth-tunnel job are the 

 construction of a geological profile along the center line of the proposed 



