Page 124 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
The Soil Profile 119

detail in the last chapter of this book. However, it is important to emphasize that
a brief review of the site geology and soils is presented as introductory material.
Geological and pedological information helps to define and optimize the approach
used in the investigation. For example, if the site is on an upland having a uniform
geology, the boring and testing program may be reduced if results are consistent
with expectations. On the other hand, if the site is on a floodplain of a meandering
stream, borings must be close enough that they will not miss the clay-filled oxbow
that may be hidden away underneath the ground surface.

In some cases, for example in a limestone sink or undermined area, no reasonable
amount of drilling will provide absolute assurance, and a geophysical investiga-
tion may be warranted. The geotechnical report must emphasize that it can
describe only what has been encountered in the borings and not what lies between.
The more detailed the investigation, the more reliable the results but the higher the
cost. It is important to maintain communications between all parties so that there
are no misunderstandings. A credible discussion of site geology and soils is basic
to any site investigation.

Problems
5.1. Weathering to form clay minerals is most intense in the A horizon, so where
does the clay go? Give evidences that support your answer.
5.2. Strength tests show that a dry, clayey B 2t behaves like a gravel. (a) With
reference to par. 5.2.6 what is the meaning of the subscript ‘‘t’’? (b) Explain
the peculiar behavior of this clay. (c) How might clay coatings on the peds
affect this behavior?
5.3. A soil is described as having A over C g horizons. (a) What can you infer
regarding groundwater conditions? (b) What are the order and suborder?
5.4. A C k horizon occurring in the southwestern United States is crushed and
used on gravel roads. (a) What is the cementing component in this material?
(b) What is another name for this horizon? (c) What is the order?
5.5. The iron-cemented or laterite layer of tropical soils is frequently used as
road metal. (a) What is the horizon designation? (b) What is the soil order?
5.6. The next chapter describes the clay mineral that dominates in Vertisols.
What is it?
5.7. Identify hue, value, and chroma for (a) 10YR 5/4; (b) 10YR 2/2; (c) N 9/0.
5.8. Borings indicate the following sequences of horizons from the top down.
Give one or more possible explanations for each sequence: (a) A, C; (b) A,
B, C, B, A; (c) A, B, C, A, B, C; (d) B, C.
5.9. Compare the suitability for foundation bearing: B 2t ,B x ,B ss .
5.10. (a) Examine soil horizons exposed in a roadcut in your area and identify A,
B, and C horizons. (b) Do the same from soils obtained from a boring made
with a hand auger.

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