5 (2.21b)
and
25 (2.21c)
whereD 15 etc. refer to the 15% passing size etc. as determined from particle
size analysis. Expressions (2.21a) and (2.21b) set out piping and permeabil-
ity criteria respectively; expression (2.21c) further defines permeability ratio.
A recent development is the suggestion that, in view of the potential
problem of hydraulic fracturing and cracking etc., with risk of progressive
erosion (Section 2.7.2), rational filter and transition design should be
based on considerations of relative permeability (Vaughan and Soares,
1982). This approach introduces the concept of basing specification of the
filter to protect a clay core on considerations of seepage water chemistry
and clay floc or particle size in relation to filter void size and effectiveness.
It is considered to define the way ahead in filter design for the protection
of very fine soils such as clays, the empirical approach based on particle
size ratios having proved unsafe in a number of instances (Vaughan et al.,
1970).
2.7 Stability and stress
2.7.1 Stability analyses
Embankment dam stability must be assessed in relation to the changing
conditions of loading and seepage régime which develop from construc-
tion through first impounding into operational service, including reservoir
drawdown. The slope stability analyses generally employed are detailed
in the soil mechanics texts referred to in Section 2.1. In this section a
basic understanding of established limit-equilibrium methods of two-
dimensional stability analysis is assumed, and only a brief appreciation of
certain fundamental points is given below. (Three-dimensional limit-
equilibrium techniques have been developed, but they have not won wide
acceptance for a number of reasons and are not considered further.)
Two-dimensional limit-equilibrium analysis is based on considera-
tion of the static equilibrium of the potentially unstable and ‘active’ mass
of soil overlying a conjectural failure surface. The factor of safety, F, is
defined by
F∑f/∑ (2.22)
D 50 (filter)
D 50 (soil)
D 15 (filter)
D 15 (soil)
82 EMBANKMENT DAM ENGINEERING
