Ch. 11] FUNCTIONS OF THE GEOLOGIST 207 



tunnel, based on the results of the test borings; selection of suitable 

 sites for supplementary borings which are needed for eliminating the 

 most objectionable uncertainties involved in the construction of the 

 profile ; and correlation of the results of the ground-water observations 

 with the geologic structure of the site. 



On the basis of the results of a general geologic survey of the region 

 surrounding the tunnel site, of a visual inspection of undisturbed 

 samples, and of the soil tests performed by the engineer, the geologist 

 ascertains the origin of the strata which were encountered in the test 

 borings. In accordance with his findings he designates the strata as 

 river, flood-plain, shore, lake, marine, glacial, fluvioglacial, wind-laid, 

 or composite deposits. The next step is to construct a geological pro- 

 file which is compatible with the geological origin of the strata. On 

 account of his knowledge of the structure of sedimentary deposits, the 

 geologist will be in a position to pass competent judgment on the 

 degree of accuracy of the profile. In the report attached to the profile 

 he will also inform the engineer about the possibility or probability of 

 significant deviations from the profile such as the existence of pockets 

 of water-bearing sand or of buried channels which were not encountered 

 during the boring operations. 



Finally, the geologist should carefully examine and comment on the 

 data that have been secured regarding the ground-water conditions and 

 correlate them with the geological structure of the site. If there are 

 indications of the existence of one or more perched water tables or of 

 artesian conditions, not clearly revealed by the records, he should insist 

 on the installation of all those supplementary observation wells or 

 piezometric tubes which are necessary to clarify controversial issues. 



The effects of geological details on tunneling conditions depend en- 

 tirely on the physical properties of the sediments and on the interaction 

 between solid and water. Therefore the geologist will hardly be able 

 to render satisfactory services on a tunnel job unless he knows the 

 fundamental principles of soil mechanics. The geologist should also 

 be acquainted with the techniques of boring and sampling and with the 

 sources of the manifold errors that may enter into test boring records. 

 On the other hand the engineer will hardly realize the uncertainties in- 

 volved in the construction of geological profiles and the benefits he can 

 derive from competent geological advice unless he is familiar with the 

 elements of physical geology. Hence an active interchange of informa- 

 tion between the two professions is urgently needed. 



