Hydraulic Structures: Fourth Edition

A specification for field compaction of earthfill must ensure that a

dry density is attained which will ensure strength and other characteristics

adequate to fulfil stability, settlement or other criteria. The dry density

achieved in the field is usually less than (^) d maxobtained in the laboratory.
The ratio of field dry density to (^) d max, expressed as a percentage, is defined
as the relative compaction. This ratio may be used to specify the required
degree of site compaction, with limits placed on the water content of
the soil. For major fill projects, such as embankments, it is now increas-
ingly common to use a ‘method’-type specification, based on compre-
hensive field trials. In broad terms this type of specification defines the
layer thickness and water content limits and requires that a specified
minimum number of passes of a nominated roller be applied to each layer
of soil.
Compaction of a soil modifies the major engineering characteristics
as summarised below.

1. Shear strength.Although shear strength is increased by compaction,
   the shear strength obtained under a given compactive effort varies
   with the water content. The maximum shear strength generally
   occurs at a water content less than woptand so at a dry density less

than (^) d max. In practice, the greatest long-term shear strength is likely
to be achieved by compaction at a water content slightly greater than
wopt. In earthfills, particularly in the core of a dam, an increase in
plasticity is also an important consideration in relation to reducing
the risk of internal cracking or hydraulic fracturing (Section 2.7), and
may encourage compaction at a few percent above wopt.

2. Compressibility.A higher degree of compaction will reduce settle-
   ment due to subsequent compression and consolidation.


3. Volume changes (due to changes in water content).For a compress-
   ible soil compaction at a high water content generally tends to
   reduce subsequent swelling and increase potential shrinkage. Com-
   paction at a low water content has the reverse effect.


4. Permeability.Greater compaction results in reduced permeability.
   The effects are rather less apparent with cohesive soils, where the
   very low permeability values are dictated by, inter alia, pore nature
   and structure and the presence of the molecular adsorbed water
   layer.

2.3.6 Representative engineering properties for soils

Representative ranges of values for the principal engineering properties

of a number of generic soil types in their natural state are presented in

Table 2.3.

56 EMBANKMENT DAM ENGINEERING