160 philbrick. FOUNDATION PROBLEMS [Ch. 8 



Appalachian Basin the physical strength and intergranular permeabil- 

 ity decreases, in general, with decrease in grain size and increase in 

 clay content. 



Coarse-Grained, Insoluble Sedimentary Rocks 



These rocks include the sandstones, conglomerates, and sedimentary 

 breccias. Commonly these rocks have been well cemented. The bed- 

 ding varies from an inch or two in thickness up to over 20 feet. Cross 

 lamination is common. The rocks may be considered competent rocks 

 which have reacted to deformation with the development of joint 

 systems and fractures and, in the case of faulting, have produced brec- 

 cia and gouge zones containing granular material. Structurally the 

 coarse-grained insoluble sediments may be characterized by three sets 

 of planes referable to bedding, cross lamination, and deformational 

 jointing. On these may be superimposed faulting. From the stand- 

 point of physical characteristics, these rocks are among the hardest 

 and most durable materials upon which a structure may be founded. 

 With the cementation of the rock by silica the sandstones assume the 

 physical characteristics of a quartzite, and compressive strengths as 

 high as 34,960 pounds per square inch have been recorded (Eckel et at., 

 1940) . On the other hand, compressive strengths as low as 120 pounds 

 per square inch have been found in the poorly cemented sandstones. 

 In general, compressive strengths are of the order of 10,000 pounds per 

 square inch, and shearing strengths, in pounds per square inch, of 770 + 

 2.60p, where p equals intensity of load on the plane of shear, are found. 

 With these physical characteristics, the average sandstones, sedi- 

 mentary breccias, and conglomerates are commonly acceptable founda- 

 tion materials when considered within the bounding planes of the test 

 specimen. The chief difficulty with the sandstones lies in their brittle 

 character, which has caused them to fail under deformational forces 

 and produce not infrequent cracks and occasional zones of pervious 

 granular material along faulting planes and occasionally along the 

 major system of joints. Because of the high intergranular permeability 

 of some sandstones, uplift pressures may approach the theoretical 

 maximum. 



Mahoning Dam on Mahoning Creek, Armstrong County, Pennsyl- 

 vania, is a gravity dam about 176 feet high above the stream bed 

 founded on a sandstone which, in laboratory tests, indicated strengths 

 many times those required to support the load of the dam. The usual 

 small- and large-diameter core borings indicated no structural flaws 

 in the foundation and showed a low anticlinal nose plunging down the 

 dip of the regional structure. However, when the foundation was ex- 



