Soil Exploration - Soil Properties: Testing, Measurement, and Evaluation - page 9

Figure 2-4
Cutting Bit for Rock
Coring.
and reliable in situ testing and sampling. Another advantage of test pits
is that they allow the detection and removal of larger soil particles
(gravel or rocks, for example) for identification and testing; this may not
be possible with boring samplers. On the other hand, test pits are gen-
erally limited by practical considerations as to depth; they generally do
not extend deeper than 10 to 15 ft, whereas auger boring samplers can
extend to much greater depths. Also, a high water table may preclude or
limit the use of test pits.
Often, the presence of subsurface rock at a construction site can be
important. Many times, construction projects have been delayed at con-
siderable expense upon encountering unexpected rock in an excavation
area. On the other hand, the presence of rock may be desirable if it can
be used to support the load of an overlying structure. For these and
other reasons, an investigation of subsurface rock in a project area is an
important part of soil investigation.
Core borings are commonly used to drill into and through rock for-
mations. Because rock is invariably harder than sandy and clayey soils,
the sampling tools used for drilling in soil are usually not adequate for
investigating subsurface rock. Core borings are performed using a core
barrel, a hardened steel or steel alloy tube with a hard cutting bit con-
taining tungsten carbide or commercial diamond chips (see Figure 2-4).
Core barrels are typically 5 to 10 cm (2 to 4 in.) in diameter and 60 to
300 cm (2 to 10 ft) long.
Core borings are performed by attaching the core barrel and cut-
ting bit to rods and rotating them with a drill, while water or air, serv-
ing as a coolant, is pushed (pumped) through the rods and barrel,
emerging at the bit. The core remains in the core barrel and may be
removed for examination by bringing the barrel to the surface. The rock
specimen can be removed from the barrel, placed in the core box (see
Figure 2-5), and sent to the laboratory for testing and analysis. The
(empty) core barrel can then be used for another boring.
A wealth of information can be obtained from the laboratory test-
ing and analysis of a rock core boring. The type of rock (such as granite,