1130 Subsurface Geologic Methods 



Specialized Techniques 



The mapping of the surface geologic features is a very important 

 and indispensable phase of engineering geology, but the fact should be 

 remembered that it is only the first in a series of investigations. If the 

 geologist is to obtain information that can be resolved into quantitative 

 terms of rock or soil behavior, he must apply every possible investiga- 

 tional tool. The specialized tools of the petrographic laboratory, the 

 materials laboratory, and geophysics may be required to provide the 

 necessary data. Frequently the final answers to an engineering geologic 

 problem represent the combined viewpoints of the geologist, the petro- 

 grapher, the soils technician, the geophysicist, and others whose specialties 

 can be applied to some phase of the problem. 



Application of Petrography to Engineering, Problems ^ 



Petrography serves the engineer in several ways: first, a detailed 

 study of rocks assists the engineering geologist in establishing the geologic 

 structure and interrelation of formations at construction sites; second, 

 petrography assists in determination of the engineering properties of the 

 rock materials in place at the site and in materials to be used in the 

 construction. 



The engineering geologist requires the application of petrography 

 to obtain the maximum geologic information from limited exposures or 

 relatively few drill cores so that preliminary estimates will be as valid 

 as possible, and so that future explorations might be the most appro- 

 priate. For example, the geologist may wish to know if the alteration 

 observed at the surface was caused by weathering and hence is limited 

 in depth or by hydrothermal processes and is likely to continue down- 

 ward. The decision would greatly influence preliminary estimates of ex- 

 ploration and construction costs, for if hydrothermal, deep-seated altera- 

 tions were indicated, the advisability of extensive preconstruction ex- 

 ploration would be established. At Anderson Ranch dam site the intense 

 hydrothermal alteration rendered the granite incoherent to depths in 

 excess of 300 feet below the original surface, and serious landsliding has 

 occurred during excavation of the site. 



Precise description and identification of rock formations aid the 

 geologist in problems of correlation. Thus, the presence or absence of 

 zones of shearing and faulting can be established by correlation of strata 

 across the site. For example, at Canyon Ferry dam site on the Missouri 

 River near Helena, Montana, the sedimentary formation constituting the 

 foundation and abutments contains thin sills of altered andesite, distin- 

 guishable only after petrographic analysis, continuity of which proved 

 the absence of significant faulting beneath the river alluvium. 



Petrography is a valuable tool for determination of the properties 



^ Mieilenz, R. C, Petrography and Engineering Properties of Igneous Rocks: U. St Dept, Interior, 

 Bur. Reclamation, Eng. Mon. 1, pp. 8-9, 1948. 



