2. GRADATION. In addition to their use in classification, grain-size
analyses may be applied to seepage and drainage problems, filter and grout
design, and evaluation of frost heave. See Table 1 for test standards.
a. Grain-Size Parameters. Coefficient of uniformity, C+u,, and
which are particle size diameter corresponding respectively to 60%, 30%, and
10% passing on the cumulative particle size distribution curves. C+u, and
C+z, indicate the relative broadness or narrowness of gradation. D+10, is
an approximate measure of the size of the void spaces in coarse-grained
soils. See Chapter 1.
b. Testing Program. Gradations of a large number of samples usually
are not required for identification. Samples should be grouped in principal
strata by visual classification before performing grain-size analyses on
specimens of major strata.
3. ATTERBERG LIMITS. For classification of the fine-grained soils by
Atterberg Limits, see Chapter 1. In addition to their use in soil
classification, Atterberg Limits also are indicators of structural
properties, as shown in the correlations in this chapter. Atterberg Limit
tests should be performed discriminately, and should be reserved for
representative samples selected after evaluating subsoil pattern. Determine
Atterberg Limits of each consolidation test sample and each set of samples
grouped for triaxial shear tests. For selected borings, determine Atterberg
Limits on samples at regular vertical intervals for a profile of Limits and
corresponding natural water content. See Table 1 for test standards.
1. APPLICATIONS. Permeability coefficient is used to compute the quantity
and rate of water flow through soils in drainage and seepage analysis.
Laboratory tests are appropriate for undisturbed samples of fine-grained
materials and compacted materials in dams, filters, or drainage structures.
See Table 2 for test standards and recommended procedures.
a. Fine-Grained Soils. Permeability of fine-grained soils (undisturbed
or compacted) generally is computed from consolidation test data or by
direct measurement on consolidation or triaxial shear specimens. For soils
with permeability less than 10. -6- cm/sec, a sealant must be used between
the specimen and the wall of the permeameter.
b. Sand Drain Design. Sand drain design may require complete
permeability data for soils to be stabilized, including determination of
permeabilities in both vertical and horizontal direction.
c. Field Permeability Tests. The secondary structure of in situ soils,
stratification, and cracks have a great influence on the permeability.
Results of laboratory tests should be interpreted with this in mind, and
field permeability tests (Chapter 2) should be performed where warranted.