Page 297 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
292 Geotechnical Engineering

A maximum density,
max , is more difficult to define because it depends on
the compaction mechanics and confinement. The determination has been
standardized and involves putting loose soil into a cylindrical mold that is set
on a vibrating table (Fig. 13.9). A surcharge weight is placed on top to confine the
soil, and the table is actuated with electromagnetic vibrations at 50 or 60 Hz
depending on availability. The vibration time is adjusted to give a uniform
number of cycles.

Tests have shown that
max also depends on the geometry of the equipment, the
applied load, and the acceleration ratio, which is the ratio of acceleration from the
vibrations to that of gravity, so the conditions under which the test is performed
must be carefully controlled (ASTM Designation D-4253).

Minimum and maximum densities often are determined with the soil wet and after
oven-drying, and the higher sets of values used for determining relative density.
While capillary effects reduce the efficiency of compaction, they also tend to hold
grains together after densification instead of letting them fly apart from the
continuing vibrations.

13.6.5 Applications of Relative Density
The relative density determination is limited to soils with 15 percent
or less noncohesive fines passing the No. 200 sieve. Soils containing 5 to
15 percent noncohesive fines also may be impact-compacted for comparative
purposes.

Example 13.1
3
3
A field density determination gives
¼ 110 lb/ft (17.3 kN/m ). Laboratory tests give
3
3
3

max ¼ 127 lb/ft and
min ¼ 102 lb/ft (19.95 and 16.0 kN/m ). What is the relative density
of the field soil?
Answer:
127 110 102Þ
ð
D d ¼ 100 ¼ 37%
110 127 102Þ
ð
13.7 COMPACTION SPECIFICATIONS FOR FINE-GRAINED SOILS

13.7.1 The Proctor Relationship
Most compaction specifications are written around an optimum moisture content
and maximum density from a Proctor density test (Fig. 13.2). The moisture
content of a soil in the field is adjusted by adding water or aerating and drying
until it is within a specified range of the optimum moisture content.

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