208 terzaghi. SOFT-GROUND TUNNELING [Ch. 11 



TOPICS FOR FUTURE RESEARCH 



The most important subjects for future research activities in the 

 field of soft-ground tunneling are the details of the structure of sedi- 

 mentary deposits, the behavior of different sediments at tunnel head- 

 ings, and the loads on temporary and permanent tunnel supports. 



A great deal of time and labor has been devoted by geologists to an 

 investigation of the different processes of sedimentation, but the 

 products of sedimentation have received relatively little attention. We 

 need systematic collections of geological profiles showing the structural 

 patterns which are produced by different processes of sedimentation. 

 These patterns can be seen only on the slopes of open cuts, immedi- 

 ately after excavation, on the slopes of sand, gravel, and clay pits, and 

 at the headings of tunnels during construction. Familiarity with such 

 patterns would discourage the engineer from constructing geological 

 profiles without the assistance of a geologist, and it would help the 

 geologist in the interpretation of test boring records. 



The behavior of different sediments at tunnel headings can be learned 

 only from personal experience, which is inevitably limited, and from 

 case records accompanied by an adequate description of the geology 

 of the tunnel site. Yet, up to this time, very few records of earth tun- 

 nels have been published, and in most of the published ones adequate 

 geological information is conspicuous by its absence. 



The measurement of the load on tunnel supports is needed as a check 

 on our methods of estimating the loads in advance of construction. 

 Extensive observations of this kind were made during the construction 

 of the subway in Chicago (Terzaghi, 1942, 1943). Equally important 

 is quantitative information about the pressure exerted by swelling 

 ground on rigid and yielding tunnel supports, because this pressure 

 can be estimated only on the basis of precedents. Measurements of 

 swelling pressures were carried out in various tunnels in the "Argile 

 plastique" of the region of Paris (Langer, quoted by Terzaghi, 1936) . 

 At present, similar measurements are being made in coal mines in 

 southern Holland (according to a personal communication received by 

 the author from Mr. J. M. Hermes, Staatsmijn Emma, Treebeek, Hol- 

 land), and, by the U. S. Army Engineers, in an experimental tunnel 

 at the site of the Garrison Dam in North Dakota. 



REFERENCES 



Casagrande, A. (1949). Soil mechanics in the design and construction of the 

 Logan Airport: Jour. Boston Soc. Civ. Engrs., vol. 36, pp. 192-221. 



