164 philbrick. FOUNDATION PROBLEMS [Ch. 8 



Fine-Grained, Insoluble Massive Sediments 



These rocks are fine-grained argillaceous sediments comparable in 

 almost all characteristics to the shales with the exception that the 

 bedding is infrequent. They seem to occur sporadically through the 

 geologic column from the Paleozoic into the Tertiary. They are vari- 

 ably cemented and, like the true shales, exhibit decrease in strength 

 generally with a decrease in intergranular cement. Field tests of such 

 materials, made in connection with Possum Kingdom Dam on the 

 Brazos River, Texas, to obtain the limiting data upon which to base the 

 criteria of design of a safe structure, are summarized by Niederhoff 

 (1940). The Bear Paw shale at the site of Fort Peck Dam, Montana, 

 is another of the massive argillaceous sediments which has been ex- 

 tensively investigated. However, at Fort Peck the foundation problem 

 was complicated by the presence of bentonitic beds, variable weather- 

 ing, and considerable faulting. The problems there involved the test- 

 ing of foundation rock in the laboratory by means of undisturbed 

 cores, the preservation of the rapidly disintegrating rock during con- 

 struction, and determination of permeability of bentonitic seams. As 

 most of these massive argillaceous rocks are poorly cemented and owe 

 their strength to compaction produced primarily by the weight of 

 the overlying sediments, it is not unusual to find that such rocks expand 

 with reduction in load resulting from removal of the overlying rocks. 

 At Fort Peck the Bear Paw shale has expanded in the spillway area 

 and heaved the spillway pavement. 



Foundation problems in these rocks are subject to closer determina- 

 tion by methods of direct testing than problems in the other argil- 

 laceous rocks as these rocks tend more toward homogeneity. 



Cyclic Sediments 



The concept of cyclic sediments has been applied only recently to 

 foundation problems (Philbrick, 1947), although structures have been 

 built on cyclic sediments for many years in the Carboniferous of the 

 Appalachian and Mid-continental coal basins of the United States. 

 Practically all the problems of both shale and sandstone foundations, 

 together with those of coal and limestone foundations, occur in a single 

 sequence in a cycle, and several cycles may be present at a site. For 

 a low structure the problems are usually those of the rock type which 

 happens to be the bearing bed. In a structure higher than the thick- 

 ness of the strata composing the sedimentary cycle, the problems of 

 the several rock types present in the cycle will be repeated. Difficulties 

 encountered in the design and construction of Tygart Dam and Lake 



