2.4 Principles of embankment dam design
2.4.1 Types and key elements
In its simplest and oldest form the embankment dam was constructed with
low-permeability soils to a nominally homogeneous profile. The section
featured neither internal drainage nor a cut-off. Dams of this type proved
vulnerable to problems associated with uncontrolled seepage, but there
was little progress in design prior to the 19th century. It was then increas-
ingly recognized that, in principle, larger embankment dams required two
component elements (Section 1.3 should also be referred to):
- an impervious water-retaining element or core of very low per-
meability soil, e.g. soft clay or a heavily remoulded ‘puddle’ clay
and
- supporting shoulders of coarser earthfill (or of rockfill), to provide
structural stability.
As a further design principle, from c.1860 the shoulders were frequently
subject to a degree of simple ‘zoning’, with finer more cohesive soils
placed adjacent to the core element and coarser fill material towards either
face.
Present embankment dam design practice retains both principles.
Compacted fine-grained silty or clayey earthfills, or in some instances
manufactured materials, e.g. asphalt or concrete, are employed for the
impervious core element. Subject to their availability, coarser fills of dif-
ferent types ranging up to coarse rockfill are compacted into designated
zones within either shoulder, where the characteristics of each can best be
deployed within an effective and stable profile.
The principal advantages of the embankment dam which explain its
continuing predominance were outlined in Section 1.3. Figures 1.2 and 1.3
illustrated the more important variants of embankment dam, and brief
supplementary notes on each are given below.
Homogeneous embankments (Figs 1.2(a) and (b)) are now generally
confined to smaller, less important dams and to dykes in river engineering
(see Section 2.10 and Chapter 8). They require ready availability of suffi-
cient low permeability soil, and careful design and internal detailing is
necessary to control seepage and porewater pressures.
The central core earthfill profile, illustrated in Figs 1.2(c)–1.2(e), is
the most common for larger embankment dams. Narrow cores of soft com-
pressible ‘puddle’ clay or of concrete, as in the profiles of Figs 1.2(c) and
1.2(d), have been displaced since 1940–1950 by the technically superior
wide rolled clay core profile of Fig. 1.2(e). The characteristics of soft
puddle clays, as used in the obsolete profile of Fig 1.2(c), are reviewed in
60 EMBANKMENT DAM ENGINEERING
