Long-term post-construction settlement, (^) r2, is governed by a rela-
tionship of the form
(^) r2(logt 2 logt 1 ). (2.43)
The constant in equation (2.40) normally lies in the range 0.2–0.5, and
timest 1 andt 2 (years) are determined relative to the completion of con-
struction and/or first impounding.
The controlling parameters in optimizing the field compaction of
rockfill, which should be near saturated to minimize later settlement, are
those which apply also to earthfill compaction, i.e. layer thickness, plant
characteristics and number of passes. Layer thickness may range up to
1.0–2.0 m dependent upon maximum rock size and shape. Excessively
heavy plant and/or overcompaction can result in undesirable crushing and
degradation of softer or weathered rockfills, with the generation of exces-
sive fines to the detriment of drainage capability. The optimum number of
passes is a compromise between further incremental gain in compacted
density and cost per unit volume of compacted rockfill, and is generally
determined from construction of a controlled trial fill.
Rockfill embankment analysis and design are discussed further in
Hirschfeld and Poulos (1973), Jansen (1988) and Leps (1988). The use of
low grade rockfill is discussed in Wilson and Evans (1990). Asphaltic con-
crete decks and diaphragms are discussed in Creegan and Monismith
(1996) with reference to USA practice.
2.10 Small embankment dams, farm dams and flood banks
2.10.1 Small embankment damsThe principles of good practice in embankment design and construction
are unrelated to the size of dam. There are nevertheless a number of
points of difference which may emerge in the engineering of smaller dams,
defined for the purposes of this Section as dams up to 10 m in height. The
differences are essentially technical in the context of design and construc-
tion standards applied, but many of these can be linked to differences in
ownership pattern and other non-technical factors. As a generalization,
therefore, it is realistic to anticipate that the small dam maybe engineered
to different standards. The reasons for such differences are numerous, but
on the basis of UK experience they include the following:
- small dams are more likely to be in singular private ownership;
- small dams are frequently ‘low-cost’ designs;
- surveillance and maintenance will be a low priority;