Ancillary works constructionembraces the construction of spillway
and stilling basins, culverts or tunnels for outlet works etc., valve towers
and similar control works. It also includes completion of crest details, e.g.
roadway, drainage works, wavewall etc. and, where climatic conditions
allow, grassing of the downstream face slope (Section 4.4).
Embankment construction practice, including planning and control,
is further discussed in Thomas (1976), Wilson and Marsal (1979),
USBR (1987), Jansen (1988) and Fell, MacGregor and Stapledon
(1992). Two recent projects in the UK are described in greater depth
in Coats and Rocke (1982) (Fig. 2.10a); for geotechnical details see Mill-
more and McNicol (1983)) and Bridle, Vaughan and Jones (1985). Recent
construction practice is also described in Banyard, Coxon and Johnston
(1992.)
2.6 Seepage analysis
2.6.1 Seepage
The phreatic surface of the seepage régime, i.e. the free surface, must
be kept well clear of the downstream face to avoid high porewater
pressures which may promote slope instability (Section 2.7). In the
extreme case of the seepage line emerging on the face, local softening and
erosion will occur and may initiate sloughing as a prelude to instability.
Seepage pressures and velocities must also be controlled to prevent
internal erosion and particle migration, particularly from the core.
Seepage control is effected by the incorporation of vertical chimney drains
and horizontal drainage layers, protected by suitable filters and transition
layers.
In this section a basic knowledge of seepage theory and flownet
construction is assumed, including entry and exit conditions, as provided
in the soil mechanics texts referred to in Section 2.1. A more exhaustive
general discussion of flownets and seepage analysis is provided in
Cedergren (1977). Embankment flownets in particular are considered in
depth, with numerous illustrations in Cedergren (1973). Embankment
seepage control is also discussed in Volpe and Kelly (1985).
The fundamental relationships applicable to flownets for two-
dimensional seepage are summarized below.
For anisotropic soils, with the coefficient of horizontal permeability
khkv, the coefficient of vertical permeability, the horizontal scale
transform factor, , and the effective permeability, k, are respectively
given by
(kv/kh)1/2 (2.13)
78 EMBANKMENT DAM ENGINEERING
