Ch. 8] CYCLIC SEDIMENTS 165 



Lynn Dam in West Virginia (Crosby, 1941) were primarily the result 

 of cyclic sedimentation. 



A cycle in the Coal Measures usually, but not always, is composed of 

 the following rocks, from the base upward: sandstone, shale, fresh- 

 water limestone, under clay, coal, shale, marine limestone, and shale. 

 Dissimilarity of physical properties of contiguous rocks is the dominant 

 characteristic of the cycle from the standpoint of foundations. The 

 sandstones may be hard massive rock. The shales and under clay may 

 be soft to medium hard and fissile to massive. The limestones range 

 from pure limestones to nodular limy shales or clays. The coal is 

 brittle. To even minor geologic deformation the several rock types 

 react differently. The sandstones, sandy shales, and carbonaceous 

 shales behave as competent beds and fracture. The weaker members 

 fail by plastic flow with the development of numerous, random, slicken- 

 sided surfaces. The coal develops a blocky structure, and the lime- 

 stones usually develop a fracture system. None of these systems neces- 

 sarily parallels the system of a contiguous member. The rate and 

 degree of weathering vary with each rock type. Localized zones of 

 weathering are found at the base of the pervious members. Inherently 

 soft materials are found at the top of the impervious members under- 

 lying pervious members. The result is that, from the standpoint of 

 design, the foundation is heterogeneous. Sampling of each bed or type 

 of bed is required to produce data upon which to base the design of 

 that portion of the structure controlled by the characteristics of that 

 bed. The individual portions of the structure may require individual 

 design. During construction, certain rocks must be protected from 

 weathering and disintegration, whereas other members can be left com- 

 pletely unprotected. The plan of excavation is dependent upon the 

 extent of weathering in the several rock types and the quantity of 

 material that may be removed without permitting deleterious expan- 

 sion of the underlying compaction shale members. Bond between con- 

 crete and foundation rock may be virtually unobtainable in some of 

 the compaction shales and the under clay without very careful prepara- 

 tion and cleaning. It is usually safer to place foundations only on the 

 cemented shales and sandstone. This causes the foundation to be 

 cut on nearly vertical slopes at monolith joints and requires the ex- 

 ercise of great care in excavation and protection of these riser faces. 



The cyclic sediments require as much care in foundation investiga- 

 tion, sampling, testing, design, treatment, and construction as any of 

 the sediments, because almost all the problems of the other sediments 

 are found in the cyclic sediments. Although the problem of extensive 

 natural leakage is not commonly met, artificial leakage channels may 



