2.4.3 Design features and practice
The considerations summarized in Section 2.4.2 have major implications
with regard to certain design features and good construction practice.
Some of the more important points are outlined below, and representative
examples of embankment dams illustrating modern practice are shown in
Figs 2.10(a) and (b) and Fig. 2.17.
(a) Zoning of shoulder fills
The careful and correct zoning of the available materials is an important
aspect of embankment design. The principles are as follows.
- The core width should be as great as is economically viable.
- The downstream shoulder should be underlain by a drainage
blanket, or base drain, of free-draining material.
- Finer shoulder material should be zoned closest to the core, with an
intervening vertical drainage zone connecting to the base drain (see
Fig. 2.10(b)).
- Shoulder zones should be of progressively coarser material as the
face slopes are approached.
- Where a major change in the characteristics of material in adjacent
zones is unavoidable, interface effects should be eased by the inser-
tion of an intermediate or transition zone.
The permeability of successive zones should increase toward the
outer slopes, materials with a high degree of inherent stability being used
to enclose and support the less stable impervious core and filter. Pervious
materials, if available, are generally placed in upstream sections to permit
rapid porewater pressure dissipation on rapid drawdown (Section 2.7.1(a)
and Fig. 2.10(b)).
The stability of an embankment and its foundation is determined by
their collective ability to resist shear stresses. Embankments constructed
with cohesive materials of low permeability generally have slopes flatter
than those used for zoned embankments, which have free-draining outer
zones supporting inner zones of less pervious fill material.
Lower quality random fill materials may be satisfactorily employed
in areas within the dam profile where neither permeability nor shear
strength is critical and bulk and weight are the primary requirements.
Examples include the placing of stabilizing fill at the toes of embankments
on low-strength foundations or so-called ‘random zones’ within the heart
of either shoulder.
General points regarding zoning and core profile include the
following.
66 EMBANKMENT DAM ENGINEERING
