Geologic Techniques in Civil Engineering 



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take cores ranging in diameter from ^ inch for the EX size to 2| inches 

 for the NX size. (See table 35.) 



The need for obtaining cores on which laboratory tests may be 

 made has given an impetus to the development of core barrels of a larger 

 diameter than the NX size. A four-inch core is about the minimum size 

 that can be effectively used, but a six- to twelve-inch core would be pref- 

 erable. Calyx drills or shot drills ranging up to 84 inches in diameter 



TABLE 35 

 Nominal Dimensions of the X-Series Core-Drill Equipment 



* For a closer figure assume hole one thirty-second inch larger than bit. 



have been used. These drills usually are used in competent rocks and are 

 less expensive than excavating a shaft by conventional methods. 



It may be of interest to explain why large-diameter cores are re- 

 quired for the testing of undisturbed foundation samples. Two tests are 

 especially important, both requiring large-diameter cores from which test 

 specimens are cut. The first of these is the triaxial shear test, which is 

 used to determine the angle of internal friction (coefficient of friction) 

 and the cohesive properties. When fine-grained formations are being 

 tested, three or more If x 2f -inch cylindrical specimens are cut from a 

 single short section of the sample. After allowance for trimming away 

 the disturbed outer surface of the core and for the trimming loss between 

 each specimen, there is little leeway even with a six-inch core. The second 

 important test requires undisturbed specimens 4^ inches in diameter to 

 ■determine the rate and amount of consolidation that may be expected 

 under specific loading conditions. 



These and other tests also provide data on pressures developed by 

 the pore fluid during consolidation from superimposed loads and on 

 percolation rates of water through the material. Obviously, rather large 

 cores are required for these tests, and it is essential that the samples be 

 recovered with as little disturbance and change in water content as possible. 



Another method of taking undisturbed samples of overburden or 

 foundation materials is with the Denison sampler. The Denison sampler 



