Ch. 11] SUBSOIL EXPLORATION 203 



enters the tunnel through the portholes shown in Fig. 4/. This pro- 

 cedure is very economical because it permits rapid progress and it 

 practically eliminates tunneling hazards. Therefore, nowadays, long 

 tunnels through soft, flowing ground are seldom constructed without the 

 use of a shield. 



SUBSOIL EXPLORATION 



Reliable information on the subsoil conditions can be obtained only 

 by means of test borings. The methods for making the borings were 

 developed in connection with the exploration of coal measures, and 

 they are more than a hundred years old. However, up to the beginning 

 of the twentieth century, soil samples were secured by means of tools, 

 such as augers or mudpumps, which destroy the structure of the soil 

 and change its consistency. In tunnels the behavior of sediments 

 with similar grain-size characteristics and mineralogical composition 

 varies greatly with their porosity and water content and with the 

 details of stratification. A varved clay, for instance, is likely to soften 

 much more rapidly than a homogeneous clay with equal average water 

 content. Hence, in connection with the subsoil exploration for earth 

 tunnels, relatively undisturbed samples should be secured by means 

 of one of the many sampling devices developed during the last decades. 

 A complete report on the present status of the technique of sampling 

 is being prepared by the Soil Mechanics Division of the American 

 Society of Civil Engineers. 



The amount of information that can be derived from the results of 

 the test borings is limited by the fact that it is seldom practicable to 

 make more than one boring for every hundred feet of the length of the 

 tunnel. Furthermore, if compressed air is to be used, borings should 

 only be made well beyond the horizontal boundaries of the space 

 to be occupied by the tunnel. Thus the test borings furnish informa- 

 tion only on the sequence of the strata along a few vertical lines, widely 

 separated and located beyond the boundaries of the tunnel. On the 

 basis of this fragmentary information, the investigator is compelled 

 to construct a continuous geologic profile through the center line of 

 the tunnel. The results of this operation can be very misleading, 

 unless the geologic structure is very simple or the profile is prepared 

 by an experienced geologist and is accompanied by a report dealing 

 with the possible differences between profile and reality. A thorough 

 investigation of the unpredictable variations of the soil properties be- 

 tween drill holes was made during the construction of the subway of 

 Chicago (Terzaghi, 1943). 



