Civil Engineering Reference
In-Depth Information
Down-Hole Logging.
For geologic hazards such as landslides, a common form of sub-
surface exploration is large-diameter bucket-auger borings that are down-hole logged by
the geotechnical engineer or engineering geologist. Figure 5.5 shows a photograph of the
top of the boring with the geologist descending into the hole in a steel cage. Note in Fig.
5.5 that a collar is placed around the top of the hole to prevent loose soil or rocks from being
accidentally knocked down the hole. The process of down-hole logging is a valuable tech-
nique because it allows the geotechnical engineer or engineering geologist to observe the
subsurface materials as they exist in place. Usually the process of excavation of the boring
smears the side of the hole, and the surface must be chipped away to observe intact soil or
rock. Going down-hole is dangerous because of the possibility of a cave-in of the hole as
well as “bad air” (presence of poisonous gases or lack of oxygen) and should only be
attempted by an experienced geotechnical engineer or engineering geologist.
The down-hole observation of soil and rock can lead to the discovery of important geo-
logic and seismic hazards. For example, Figs. 5.6 and 5.7 provide an example of the type
of conditions observed down-hole. Figure 5.6 shows a knife that has been placed in an open
fracture in bedrock. The open fracture in the rock was caused by massive landslide move-
ment. Figure 5.7 is a side view of the same condition.
Trench Excavations.
Backhoe trenches are an economical means of performing subsur-
face exploration. The backhoe can quickly excavate the trench, which can then be used to
observe and test the in situ soil. In many subsurface explorations, backhoe trenches are used
to evaluate near-surface and geologic conditions (i.e., up to 15 ft deep), with borings being
used to investigate deeper subsurface conditions. Backhoe trenches are especially useful
for performing fault studies. For example, Figs. 5.8 and 5.9 show two views of the
FIGURE 5.5
Down-hole logging. Note that the arrow points to the top of the steel cage used for the down-
hole logging.
