1126 Subsurface Geologic Methods 



Exploration 



Subsurface exploration at a construction site is usually necessary to 

 furnish specific design information. The extent of this exploration is 

 determined by the size of the structure, the type of foundation present, 

 and the amount of overburden cover. The more complex the subsurface 

 geologic conditions, the more extensive the exploration program must be 

 to clarify fully all pertinent foundation conditions. 



An exploration program must specify not only the location of the 

 drill holes but also the type of data to be obtained from the holes. These 

 data include core recovery, an impertant factor in determining the sound- 

 ness of the rock, and water-test information, which is necessary to deter- 

 mine permeability and leakage. Locating, sampling, and determining the 

 extent of shattered zones, faults, and cavities that may transect good rock 

 are of primary concern during foundation exploration. These zones might 

 effect unequal or excessive consolidation, sliding, or large water losses 

 and might even cause complete failure of a structure. Frequently the 

 zones of primary interest are those where core is the most difficult to 

 obtain. Therefore, it is essential that every effort be made to recover 

 cores from such zones. 



Depending on the type of information desired, rotary core drilling, 

 churn drilling, wash boring, test pitting, tunneling, and geophysics have 

 been used to investigate subsurface geologic conditions. Geophysical ap- 

 plications will be discussed under specialized techniques. Churn-drilling 

 and wash-boring methods have a rather limited application, because core 

 is not recovered in a form amenable to testing. Wash boring, which has 

 the additional disadvantage of being unable to penetrate through large 

 boulders or heavy, gravelly soils, makes true bedrock determinations 

 difficult. 



Test pits may be used for foundation exploration when the overbur- 

 den cover is slight, or when large block samples are required for test 

 purposes. Test pits have the additional advantage of allowing visual ex- 

 amination of the materials in place. Recently large-diameter, 12- to 48- 

 inch power augers have been used to eliminate test pits in some tests. 



Exploratory tunnels have been used in numerous explorations, par- 

 ticularly where large concrete dams have been under consideration. These 

 tunnels are primarily used to obtain actual rock specimens for laboratory 

 testing and for performing field tests in strength and permeability. The 

 advent of more adaptable rotary-type core drills has minimized the use 

 of tunnels for foundation exploration. 



By far the largest percentage of exploration work has been done by 

 means of various types of rotary core-drilling equipment. A type of core- 

 drilling equipment widely used is a bottom-discharge core barrel that 

 cuts a 2|-inch core. The designation of this size core barrel in the X 

 series would be "NX." The X series is a standard size range, which will 



