162 philbrick. FOUNDATION PROBLEMS [Ch. 8 



shale, nor is one possible inasmuch as most shales are partially 

 cemented and partially compacted rocks. The simple weathering test, 

 which is the behavior of the rock during five successive cycles of wet- 

 ting and drying with water or 100 N ammonium oxalate, seems to be 

 the most revealing of the coherent quality of the shale. Those shales 

 which are reduced by this process to uncohering aggregates of ap- 

 proximately grain-sized particles are compaction shales. Those which 

 are entirely unaffected or reduced only to flakes are cemented shales. 

 This test also indicates the behavior of the shale upon exposure to 

 atmospheric conditions during construction. 



The chief problem with a shale is the determination of the physical 

 characteristics of the rock from the standpoint of design criteria. Such 

 a shale has been described by Rountree (1940, p. 1539) as follows: 



The shale underlying the easterly portion of the Watts Bar Dam is a part 

 of the Rome formation and is classified as a cemented, clay and silty shale. 

 It is fairly compact and, in general, unweathered. The shale is characterized 

 as "fissile" and is comprised of very thin layers which are easily separated; the 

 surfaces of these layers are smooth and glossy in appearance. The shale is 

 interbedded with thin layers of hard sandstone which vary from % inch to 2 

 inches in thickness and which are normally from 1 inch to 24 inches apart. 

 In general the shale has a dip angle of from 20° to 30°, but occasionally, in 

 limited areas, the bedding planes are horizonal. The apparent weakness of 

 the material against movement parallel to the bedding prompted these tests. 



The field load tests were conducted on six blocks of concrete poured 

 on the shale which had been trenched to a depth of 2 inches around the 

 base of the blocks. The shearing strength of the shale was appraised 

 at a value in tons per square foot intermediate between 0.9 + OASq 

 and 0.8 + 0.35(7, where q is the intensity of load on the plane of shear. 

 The value of the coefficient of friction of shale sliding on shale, parallel 

 to the bedding planes, was determined as something less than 0.53 

 (Rountree, 1940, pp. 1542-1543). 



If it is assumed, then, that in a shale foundation the physical proper- 

 ties of the shale are known, what effect do its geological properties have 

 upon the design of the foundation? If the materials are homogeneous 

 and infinite in extent, the geological properties are defined by the 

 physical properties. But in practically all cases the shale is not homo- 

 geneous and infinite in extent. Therefore one is forced to consider 

 the inherent geological properties. The most important are: the inter- 

 bedding of softer, thin layers; dip; fracture systems; and durability 

 upon exposure. Usually these can be defined only by visual investiga- 

 tion of the rock in place through preconstruction shafts and tunnels, 



