Chapter 11 



GEOLOGIC ASPECTS OF SOFT-GROUND TUNNELING 



Karl Terzaghi 



Professor of the Practice of Civil Engineering 



Harvard University 



Cambridge, Massachusetts 



A geological report on the site of a proposed earth tunnel should 

 contain full information on all those known geological details and con- 

 ditions which may conceivably influence the behavior of the ground 

 in the tunnel during construction. From a scientific point of view, 

 most of these details are likely to be irrelevant. Therefore a geologist 

 will hardly be able to prepare an adequate report on the site of a pro- 

 posed tunnel unless he is familiar with the factors that determine the 

 tunneling conditions. This chapter contains general information con- 

 cerning these factors. In the discussion of tunneling conditions it is 

 necessary to make use of various elementary concepts which are asso- 

 ciated with soil mechanics. (See Terzaghi and Peck, 1948; and Chap- 

 ter 5 in this symposium.) 



EFFECT OF STAND-UP TIME ON TUNNELING CONDITIONS 



In connection with soft-ground tunneling, the difficulties and costs 

 of construction of a tunnel with given dimensions depend almost ex- 

 clusively on the stand-up time of the ground. The term stand-up time 

 indicates the time that elapses between the exposure of an area at the 

 roof of the tunnel and the beginning of noticeable movements of the 

 ground above this area. The factors that determine the stand-up time 

 of an unsupported roof area with given dimensions depend to a large 

 extent on the position of the water table. 



Above the water table, the stand-up time depends essentially on 

 the tensile and shearing strength of the ground. Below the water table 

 it depends not only on the strength of the soil but also on its average 

 permeability and, to a large extent, on the degree of continuity of the 

 most permeable members of the formation. A lens of water-bearing 

 sand in a silt stratum may be perfectly harmless; a continuous layer 



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