Ch. 8] INSOLUBLE SEDIMENTARY ROCKS 161 



posed, a branching thrust fault carrying about 18 inches of gouge 

 was encountered just below the foundation level at one side of the 

 spillway and some 20 feet deeper on the other side. The fault re- 

 quired the excavation of the- overlying material to a depth of about 20 

 feet, where the gouge thinned and became sufficiently granular to permit 

 consolidation by shallow pattern grouting. 



The sandstone foundations are commonly acceptable, subject to 

 geologic structural weaknesses not reflected by laboratory tests and 

 related to the inherent characteristics of the rock en masse. 



Medium-Grained, Bedded, Insoluble Sedimentary Rocks 



These rocks include siltstones and some of the coarser shales. The 

 general characteristics of the siltstones are similar to those of the sand- 

 stones with the exception of permeability, which is commonly lower 

 in the siltstones. Physical tests of siltstones in the Appalachian Basin 

 indicate compressive strengths of 8,800 pounds per square inch and 

 shearing strengths, in pounds per square inch of 800 + 2Ap. The re- 

 action of siltstones to deformation is similar to that of sandstones. In 

 general, then, siltstones are similar to sandstones as foundation mate- 

 rials, although somewhat weaker. 



Fine-Grained, Laminated, Insoluble Sedimentary Rocks 



These rocks are the common shales, they vary greatly in character, 

 and they constitute some of the most difficult foundation rocks. The 

 shales are distinctively thin-bedded to laminar and composed of lam- 

 inar minerals which lie parallel to the bedding. Shales are character- 

 ized by an established plane of potential weakness on which may be 

 superimposed regional and local systems of joints, fractures, folds, 

 and faults. The bedding may be disturbed by prelithification dis- 

 tortion, as in some of the near-shore deposits, or it may remain almost 

 planar in its smoothness. 



The minerals are usually micas and clay minerals with varying 

 amounts of silica. The intergranular cement may be siliceous, cal- 

 careous, or, less frequently, ferruginous, or intergranular cement may 

 be lacking entirely. The first type of shale has been termed by Mead 

 (1938) a "cemented" shale, and the second type a "compaction" shale. 

 The physical properties of these two pure types differ considerably. 

 The difficulty of the foundation problems seems to increase with the de- 

 crease in cement and resulting decrease in strength and durability. 

 The cemented shales behave more like rocks and are variably elastic, 

 whereas the compaction shales behave more like soils in relation to 

 stress. There is no hard and fast line separating these two types of 



