Moffat (2002), as there are many such dams in service. The slender core
can prove vulnerable to fracturing and internal erosion (Section 2.7.2), the
wide core offering lower internal hydraulic gradients. The change to the
wide core was coincident with the development of soil mechanics theory
and with the introduction of high-capacity earthmoving and compaction
plant. Core base width is now generally 20–40% of the height of the
embankment (refer also to Figs 2.10(a) and (b)). Central and inclined core
dams with shoulders of graded compacted rockfill are shown in Figs 1.2(f ),
1.3(a) and 1.3(b).
The inclined core profile of Fig. 1.3(b) is sometimes considered
advantageous in moderating the risk of core cracking as a result of load
transfer between compressible core and stiffer rockfill shoulder (Section
2.7).
The decked rockfill embankment is illustrated in Fig. 1.3(c) and
depicts an asphaltic or concrete impermeable upstream membrane. Thin
asphaltic membranes (0.15–0.30 m thick) are now widely employed where
soil suitable for core construction is either not available or uneconomic.
An asphaltic membrane can accept a degree of deformation without
rupture. Thicker (0.6–1.2 m) asphaltic membranes are also widely
employed in the less vulnerable central position indicated in Fig. 1.3(d)
(and Fig. 2.17).
Selection of the optimum type of embankment for a specific location
is determined largely by the nature and availability of different fill mater-
ials in sufficient quantity. The much steeper face slopes possible with com-
pacted rockfill shoulders (Figs 1.2(f ) and 1.3(a)–(d)) can reduce the
quantities of fill required for a given height of dam by 30–50%.
The primary loads acting on an embankment do not differ in prin-
ciple from those applicable to gravity dams and outlined in Section 1.7.
There are, however, the conceptual differences there referred to with
regard to the water load which, in the case of all but decked embank-
ments, is exerted inside the upstream shoulder fill. Self-weight load,
similarly a distributed internal body load, is significant with respect to
stability and internal stress for the embankment and for a compressible
soil foundation. Because of such differences, embankment dam analysis
is less formalized and is carried out quite differently from concrete
dam analysis (Chapter 3). This will be developed further following
consideration of the defects and failure modes which may affect embank-
ment dams.
Charles (1998) presents a thoughtful discourse on the embankment
dam.
amelia
(Amelia)
#1