Geologic Techniques in Civil Engineering 1133 



the properties of rock formations in place. For example, the petrographer 

 and geologist may be called upon to decide to what extent the fractures, 

 joints, and planes of shear in the rock in place should be cause for re- 

 ducing below the measured strength of rock specimens the strength of 

 the rock mass; or to what extent the swelling clays might remain stable 

 by virtue of their impermeability; or to what degree discontinuities such 

 as joints, bedding planes, and faults would augment the known permea- 

 bility of the rock itself. Not infrequently, the results of tests of rock speci- 

 mens, however carefully selected, are more deluding than instructive. Only 

 experience, geologic and petrographic skill, and good judgment will per- 

 mit translation of test results to the data required for engineering design. 

 The petrographer can assist the engineer in design, construction, and 

 maintenance problems. By working with the engineering geologist the 

 petrographer facilitates selection, exploration, and subsurface geologic 

 investigation of construction sites. Through application of petrography 

 to materials testing, specific tests to be applied and samples to be tested 

 can be selected with minimum hazards of inefficiency. Petrography is 

 effective in predicting the engineering properties of rocks because those 

 properties are determined by the texture, structure, and composition of 

 the rocks, characteristics that can be discerned by petrographic methods. 



Soils Mechanics 



The never-ending demand for exact information concerning the physi- 

 cal properties of foundation materials has introduced the application of 

 soils mechanics and earth-testing procedures into geologic investigations. 

 In the design of his structures the engineer is confronted with the problem 

 of stability of the material upon which he must place his structure. The 

 principal properties to be determined in a foundation study are consolida- 

 tion, shearing resistance, and permeability. 



Settlement of a foundation may effect a structure in several ways. 

 Most engineering works are constructed to precise measurements. Thus, 

 even a small settlement may throw various features out of line or grade 

 with each other. Differential settlement may actually cause failure of cer- 

 tain key parts of a structure. Settlement may result from direct vertical 

 consolidation, lateral deformation or shear, or a combination of the two. 



Consolidation tests must be made on poorly cemented, friable sand- 

 stones and poorly indurated shales or claystones. These materials while 

 in their dry state as found exposed appear to be highly competent and 

 capable of withstanding extreme loads. However, one must consider that 

 in the construction of a dam or other water-diverting structure these appar- 

 ently competent materials will probably become saturated and tend to 

 lose their properties as rock and behave more like natural soil types. 

 Conversely, some shales that appear to be incompetent on the surface or 



