Geology Reference
In-Depth Information
Figure 2-5
Core Box Containing
Rock Core Samples.
sandstone), its texture (coarse-grained or fine-grained, or some mixture
of the two), degree of stratification (such as laminations), orientation of
rock formation (bedding planes vertical, horizontal, or in between), and
the presence of weathering, fractures, fissures, faults, or seams can be
observed. Also, compression tests can be performed on core samples to
determine the rock's compressive strength, and permeability tests can
be done to see how underground water flow might be affected. All of the
foregoing information can be invaluable in the design process and to
prevent costly “surprises” that may be encountered during excavations.
Core recovery is the length of core obtained divided by the distance
drilled. For example, a laminated shale stratum with a number of clay
seams would likely exhibit a relatively small percentage of core recov-
ery because the clayey soil originally located between laminations may
have been washed or blown away by the water or air, respectively, dur-
ing the drilling process. On the other hand, a larger percentage of core
recovery would be expected in the case of granite.
Preceding paragraphs have discussed some of the more common
types of borings. Once a means of boring has been decided upon, the ques-
tion arises as to how many borings should be made. Obviously, the more
borings made, the better the analysis of subsurface conditions should be.
Borings are expensive, however, and a balance must be made between the
cost of additional borings and the value of information gained from them.
As a rough guide for initial spacing of borings, the following are
offered: for multistory buildings, 50 to 100 ft (15 to 30 m); for one-story
buildings, earthen dams, and borrow pits, 100 to 200 ft (30 to 60 m); and
for highways (subgrade), 500 to 1000 ft (150 to 300 m). These spacings
may be increased if soil conditions are found to be relatively uniform
and must be decreased if found to be nonuniform.
Once the means of boring and the spacing have been decided upon,
a final question arises as to how deep the borings should be. In general,
borings should extend through any unsuitable foundation strata
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