154 



philbrick. FOUNDATION PROBLEMS 



[Ch. 8 



surface and the concrete, thus preventing the formation of a good bond 

 between the concrete and the rock. 



The shearing test consists in mounting a test specimen in two cast- 

 iron blocks in such a manner that a single shearing load may be freely 

 applied to the required shearing plane of the test specimen. In a core 

 of horizontally bedded rock drilled normal to the bedding plane, a 

 load is applied parallel to the axis of the core by means of a cal- 

 ibrated spring, and the load parallel to the bedding is then applied 



to one of the shearing blocks 

 through a compression machine at 

 a rate of approximately 500 pounds 

 per square inch per minute. The 

 test is continued until the specimen 

 fails, and the unit shearing strength 

 is then computed by dividing the 

 total load by the area of the plane 

 of failure. 



The unconfmed compression test 

 is conducted similarly to the com- 

 pression test of concrete cylinders, 

 with the load applied to the speci- 

 men at the rate of 1,000 pounds per 

 square inch per minute. Where 

 possible, the angle a, which the sur- 

 face of the compression break 

 makes with the horizontal, is measured, and the angle of internal 

 friction <f> is computed by the formula </> = 2 (a — 45). 



Mr. R. R. Philippe (1941, p. 3), Director, Ohio River Division 

 Laboratories, Corps of Engineers, U. S. Army, has developed a ring 

 test for measuring the unit tensile strength of the rock. 



The test consists of applying a line load to a ring specimen of the rock. 

 The specimen is prepared by cutting a narrow section of the rock core with 

 the cutting machine, and then finishing the cut surfaces on a lapping stone 

 with a carborundum grain to make them true, after which a hole is drilled 

 through the center. The rings are so proportioned that the thickness of the 

 ring and the diameter of the drilled hole are from 10 to 15 percent of the 

 diameter of the specimen. [The method of applying the load and the re- 

 quired dimensions to be measured are shown in the sketch of Fig. 1.] 

 • The dimensions of the ring and the total load at failure allow an evaluation 

 of the unit tensile strength which, together with the unit compressive strength 

 of a similar material, is used to calculate the angle of internal friction and 

 cohesion of the rock. The equations used are as follows: 



kf*l 



P= total load 

 p 



-777-r =unit load on section A- A 



f t = unit tensile strength at r 



Fig. 1. Diagrammatic sketch of ring 

 test. (After Philippe, 1941.) 



