Ch. 8] INSOLUBLE SEDIMENTARY ROCKS 163 



although their presence may be indicated by surface exposures. Not 

 infrequently shales carry variably open bedding planes along which 

 there may be very soft veneers of clay. With these conditions it is 

 almost impossible to define physical properties exactly, and large 

 safety factors are considered in the calculations of stability and built 

 into the structure. As an example, the base of the foundation may be 

 inclined so that the angle of the resultant of all forces tends to be 

 more normal to the foundation surface, with the result that the founda- 

 tion rock is engaged to a greater depth, thus reducing the influence 

 of weaker zones immediately below a level, higher foundation surface. 

 Similar results are obtained by the construction of shear walls suitably 

 anchored to the structure and extending below the base of the re- 

 mainder of the foundation of the structure. But in general the tend- 

 ency in overcoming geological weaknesses in shale foundations is to re- 

 duce the unit loading by broadening the base of the structure or re- 

 ducing its weight and founding it well below the surface of sound 

 bedrock. The concrete of the foundation is placed directly against 

 the rock enclosing the excavation. Advantage is taken, thus, of the 

 shearing resistance of the rock mass into which the foundation is 

 keyed. 



The foundation problems of the "immature" shales of the Fort 

 Union formation represent the other end of the transition between the 

 foundation problems which are controlled by the geological character- 

 istics of shales and those which are controlled by the physical char- 

 acteristics of shales. Golder, in a personal communication dated Feb- 

 ruary 9, 1949, has described these materials at the site of Garrison 

 Dam, North Dakota, as follows: 



The Fort Union is composed of nearly flat-lying fine sands, silts, and clays 

 (with clayey phases predominant), limestone and lignitic horizons of Paleocene 

 age, ranging from thin partings to beds over 15 feet thick. These sediments 

 have been described both as clays and shales, but we feel that they are some- 

 thing in between these two, and should be called immature shale. There is 

 little cementing material in the Fort Union and most of the standard soils 

 laboratory testing procedures are applicable to the sediments of this formation. 



Shear tests of undisturbed samples show that the cohesive strengths 

 range from practically to almost 2 tons per square foot with angles 

 of internal friction ranging from 15° to 34°. The foundation problems 

 arising from the necessity to construct massive structures on these sedi- 

 ments include the determination of the safe bearing values by test pro- 

 cedures and the preservation of the rock during construction and 

 prior to the placement of concrete. 



