Geologic Techniques in Civil Engineering 1129 



was developed at the United States engineer office at Denison, Texas, by 

 H. L. Johnson. This sampler is basically a double-tube core barrel, which 

 cuts a core approximately six inches in diameter. (See fig. 593.) Five 

 advantageous features of the Denison sampler are as follows: 



1. The size of the sample is satisfactory for testing. 



2. Provision is made for a sample container of sheet metal, which 

 acts as a removable inner-barrel liner. 



3. The inner nonrotating barrel trims the core as the sampler pene- 

 trates into the ground and thus anchors the inner barrel to prevent rota- 

 tion and erosion of the core. 



4. A spring core catcher retains noncohesive fine-grained materials 

 as the sampler is withdrawn from the hole. 



5. A check-valve assembly vents fluids or water from the inner bar- 

 rel as the core is received. This valve also seals the inner barrel at the 

 top when the core barrel is lifted. 



The Denison sampler is by no means an all-purpose core drill. 

 Coarse materials or those finer grained materials that contain gravel can- 

 not be successfully sampled. Materials most adaptable to sampling by 

 this method are fine-grained cohesive materials and extremely fine damp 

 sand. 



The Denison core barrel is forced into the material by a combina- 

 tion of pressure and the rotary cutting action of the core bit. The drilling 

 mud is pumped through the drill rod between the inner and outer barrels 

 and then upward along the exterior of the outer core barrel to remove the 

 cuttings. This drilling mud forms a skin on the drill hole, and thus holes 

 can usually be drilled without casings. Holes drilled below the ground- 

 water surface should not be pumped or bailed dry because the external 

 ground-water pressure on the mud casings will very likely cause caving; 

 furthermore, the flow of water through the bottom of the hole is likely 

 to disturb the structure of the material. 



Numerous types of drive-sampling devices have been developed for 

 obtaining undisturbed type samples in cohesive and plastic materials. 

 This sampling procedure is not suitable for brittle, highly compacted, 

 cemented materials or cohesionless materials. Drive sampling is not used 

 as a means of boring or drilling an exploration hole but rather to obtain 

 representative samples while the main drilling is made by other means. 

 Drive sampling consists in forcing a sampling tube or barrel into the 

 material without rotation. Usually a quick, steady drive for the full 

 length of the sample is the best means of obtaining an undisturbed type 

 sample. The most common method is by means of repeated blows of a 

 driving head. Samplers with thick walls in general increase disturbance 

 by causing more displacement and compaction of the samples. Thin walls 

 and sharp cutting edges that taper on the outside are very important. 



