Page 275 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
270 Geotechnical Engineering

Figure 12.12
Chart for
classifying
fine-grained soils
by the AASHTO
system.

The ranges of the liquid limit and the plasticity index for fine-grained soils in
groups A-4, A-5, A-6, and A-7 are shown in Fig. 12.12, which has been arranged
to be comparable to the Unified chart.

Example 12.2
Classify a soil containing 65% of material passing a No. 200 sieve and having a liquid limit
of 48 and a plasticity index of 17.

Answer: Since more than 35% of the soil material passes the No. 200 sieve, it is a
silt-clay material and the process of determining its classification can begin by examining
the specified limits for group A-4, where the maximum is 40. Since the liquid limit of
the soil being classified is 48%, it cannot be an A-4 soil so we proceed to the columns to
the right, where it will be seen that it meets the liquid limit requirement of A-6 and A-7,
but meets the plasticity index requirements of A-7. The soil therefore is A-7. This procedure
is simplified by reference to Fig. 12.12, which also is used to separate A-7 soils into two
subgroups, A-7-5 and A-7-6.

12.12.3 Size Grade Definitions

AASHTO definitions of gravel, sand, and silt-cIay are as follows:

Gravel
Material passing a sieve with 75 mm (3 in.) square openings and retained on a
No. 10 (2 mm) sieve.

Coarse Sand
Material passing the No. 10 sieve and retained on the No. 40 (425 mm) sieve.

Fine Sand
Material passing the No. 40 sieve and retained on the No. 200 (75 mm) sieve.

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