Ch. 8] MATERIALS TESTING 153 



izontally bedded, average shale. In considering this factor it is neces- 

 sary to obtain quantitative information on the shearing strength and 

 sliding friction along bedding planes. If the samples obtained from 

 the borings are approximately the lengths of the intervals between 

 the major bedding planes, the laboratory tests will not produce these 

 data. The samples therefore would not be representative. The crite- 

 rion, then, of whether a sample is representative of foundation condi- 

 tions is not whether the sample came from a given elevation in a boring 

 in the foundation but whether that sample contains the critical ele- 

 ment — be it structural, stratigraphic, or lithologic — which is present 

 at that elevation. 



MATERIALS TESTING 



It is common practice to investigate the physical properties of the 

 foundation materials by laboratory tests in order to obtain data to 

 be included in the computations relating to the stability of the struc- 

 ture and to define the characteristics of the foundation rock. The 

 stability calculations require the determination of the shearing and 

 compressive strengths. It is desirable to include tests to indicate the 

 coefficient of friction between concrete and the proposed bearing sur- 

 face of the rock, as well as between rock surfaces on either side of the 

 planes of weakness on which sliding friction may be a factor in the 

 design. More recently there has been added to these three tests a 

 test for the purpose of measuring the unit tensile strength of the rock. 

 By means of this test, in conjunction with the unit compressive strength 

 of the material, the angle of internal friction or cohesion of the rock 

 may be calculated. Certain routine tests to determine the total ab- 

 sorption, specific gravity, void ratio, and porosity are primarily de- 

 signed to provide quantitative data on the general characteristics of 

 the rock. These do not enter into the calculations involving stability, 

 but they may be used in indicating consolidation under loading or ex- 

 pansion with decrease in loading. A test to determine the soundness 

 and durability of the rock, and its susceptibility to weathering when 

 exposed, is called the weathering test and is of considerable use in in- 

 dicating the relative cohesion and the necessity for protecting the 

 rock from deterioration during the period of construction. This test 

 also provides an indication of the reliability of the bond that may be 

 obtained between concrete and rock. Some compaction rocks weather 

 rapidly and in a matter of a few hours may develop a thin coating 

 of fine, discrete particles which would form a film between the rock 



