(g) Embankment crest
The crest should have a width of not less than 5 m, and should carry a sur-
faced and well-drained access road. (With old dams the latter provides
valuable resistance to the erosive effect of occasional limited and short-
term overtopping.)
2.5 Materials and construction
2.5.1 Earthfill materialsThree principal categories of fill material are necessary for earthfill
embankment dams to fulfil the requirements for core, shoulder and
drainage blankets, filters, etc.
Core fillshould have low permeability and ideally be of intermediate
to high plasticity to accommodate a limited degree of deformation without
risk of cracking (Section 2.7.5). It is not necessary, and possibly disadvan-
tageous, to have high shear strength. The most suitable soils have clay
contents in excess of 25–30%, e.g. glacial tills etc., although clayey sands
and silts can also be utilized. The core is the key element in an embank-
ment and the most demanding in terms of material characteristics and
uniformity, the properties of the compacted clay core being critical to
long-term watertight integrity. The principal performance characteristics
of the more important groups of soils suitable for rolled cores are sum-
marized in Table 2.5.
Representative values for the more important engineering para-
meters of compacted earthfills, to be utilized in core or shoulders as appro-
priate, are summarized in Table 2.6. Note that the comparable parameters
for compacted rockfill are also tabulated.
Shoulder fillrequires sufficiently high shear strength to permit the
economic construction of stable slopes of the steepest possible slope angle.
It is preferable that the fill has relatively high permeability to assist in dis-
sipating porewater pressures. Suitable materials range across the spectrum
from coarse granular material to fills which may differ little from the core
materials. The shoulder need not be homogeneous; it is customary to
utilize different fills which are available within predetermined zones within
the shoulders (rockfill for shoulders is discussed briefly in Section 2.9).
Drain/filter materialmust be clean, free draining and not liable to
chemical degradation. Processed fine natural gravels, crushed rock and
coarse to medium sands are suitable, and are used in sequences and
gradings determined by the nature of the adjacent core and/or shoulder
fills. The cost of processed filter materials is relatively high, and the
requirement is therefore kept to a minimum. The properties of core
materials are further discussed in McKenna (1989).