Engineering Geology

E n g i n e e r i n g G e o l o g y
Figure 2.18
(a) Details of a core orientator. The probes take up the profiles of the core stub left by the previous drilling run and are locked
in position when the spring loaded cone is released. (b) Stages of the integral sampling method.
hole position. The probe is released by pressure against the core stub and, when released,
locks the probe in position and releases the clamping teeth to allow the instrument to ride up
inside the barrel ahead of the core entering the barrel. In the first stage of integral sampling, a
drillhole (diameter D) is drilled to a depth where the integral sample is to be obtained, then
another hole (diameter D¢) coaxial with the former and with the same length as the required
sample is drilled, into which a reinforcing bar is placed. The bar then is grouted to the rock mass.
Drilling is then resumed to obtain the integral sample. The method has been used with success
in all types of rock masses, from massive to highly weathered varieties, and provides information
on the spacing and orientation, as well as the opening and infilling of discontinuities.
Drillhole inspection techniques include the use of drillhole periscopes, drillhole cameras or
closed-circuit television. The drillhole periscope affords direct inspection and can be orientated
from outside the hole. However, its effective use is limited to about 30 m. The drillhole camera
also can be orientated prior to photographing a section of the wall of a drillhole. The television
camera provides a direct view of the drillhole, and a recording can be made on videotape.
These three systems are limited in that they require relatively clear conditions and,
hence, may be of little use below the water table, particularly if the water in the drillhole is
murky. The televiewer produces an acoustic picture of the drillhole wall. One of its advantages
is that drillholes need not be flushed prior to its use.
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