Page 282 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
Soil Consistency and Engineering Classification 277

that air in the pores is trapped and compressed by water entering the capillaries,
causing the soil clod literally to explode and disintegrate. The same soil will not
slake when saturated. Slaking therefore can provide an immediate clue that a soil
has been compacted too dry, discussed in the next chapter.

12.15 SUMMARY

This chapter describes laboratory tests relating the plastic behavior to moisture
content, which form the basis for engineering classifications. Two classification
methods are presented, one that is more commonly used in highway soil
engineering and the other in foundation engineering. Soils may be classified by
either or both methods as part of a laboratory testing program. Classification is
useful for determining appropriate uses of soils for different applications, but is
not a substitute for engineering behavioral tests.

Results of classification tests can be influenced by air-drying, so soil samples
preferably are not air-dried prior to testing. If they are air-dried, considerable
mixing and aging are required to ensure complete hydration of the clay minerals
prior to testing. As soils used in classification tests are remolded, the results are
not directly applicable to most field situations, exceptions being soils that are
being remolded in the base of active landslides, in mudflows, and soils that have
been liquefied by vibrations such as earthquakes. Classification therefore is more
commonly a diagnostic than a performance tool.

Problems
12.1. Define liquid limit, plastic limit, plasticity index, and activity index.
12.2. Four trials in a liquid limit test give the following data. Plot a flow curve
and determine the liquid limit.

Number of blows Moisture content, %
45 29
31 35
21 41
14 48

12.3. If the plastic limit of the soil in Problem 12.2 is 13%, what is the plasticity
index?
12.4. If the soil in Problem 12.2 contains 30% 2 mm clay, what is the activity
index?
12.5. The liquid limit of a soil is 59%, the plastic limit is 23, and the natural
moisture content is 46%. What is the liquidity index? What is its
significance?

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.