152 philbrick. FOUNDATION PROBLEMS [Ch. 8 



countered in the foundations. However, it would be unusual now to 

 attempt the design of anything other than a very minor structure 

 without core borings. The cores would probably be no larger than a 

 nominal 2 inches in diameter, and the logs would be based on the 

 driller's idea of what the rock is, supplemented by the engineer's or 

 architect's interpretation of what that means with respect to the clas- 

 sification in the building code or in some handbook. In general the 

 results of this approach to loadings under small structures, and in some 

 cases under much larger structures, has been entirely satisfactory be- 

 cause of the conservatism inherent in building codes and handbooks. 



There have been sufficient examples of the unfortunate results of 

 this approach in connection with structures of ordinary size to make 

 it advisable and almost mandatory to consider with some care the 

 foundation reactions under larger structures. As a result, emphasis 

 is placed upon determining geologic conditions more clearly and 

 exactly. Drilling tools are available with which almost 100 percent 

 recovery can be obtained in nearly all types of sedimentary rocks. 

 This equipment ranges from the diamond-bitted, modified standard, 

 double-tube core barrels operated by hydraulic-feed core drills for 

 rock drilling to ingenious soil samplers used in the sampling of uncon- 

 solidated soils and sediments. There are the large cores recovered 

 from borings ranging from 6 inches to over 48 inches in diameter. 

 These larger cores, formerly considered only awkward by-products 

 of the drilling of exploration shafts, provide a source of samples for 

 laboratory tests superior to those from smaller holes. Samples and 

 check samples may be drilled from the large cores in the plane normal 

 to the axis of the core, as well as parallel to the axis of the core. It 

 would be possible to evaluate roughly the sliding friction along bedding 

 planes with some degree of similarity to conditions underground with 

 the use of contiguous samples of the larger cores. In other words, 

 samples of almost any type of foundation rock can be recovered, even 

 very soft ones. 



But, the samples having been recovered, there remains the evaluation 

 of the representativeness of these samples. In a massive foundation 

 the samples are probably representative. But most sediments are not 

 massive. Or, if they are massive geologically, they may not be massive 

 in the engineering sense that the entire column of foundation rock sub- 

 ject to reaction to the imposed load will react as a homogeneous mass. 

 Bedding planes in a horizontally bedded rock will affect the foundation 

 reaction, and the closer the spacing of the bedding the more the re- 

 action will be affected. The shear-friction factor can well be the con- 

 trolling factor in the foundation design of a structure founded on hor- 



