(e) Outlet works: tunnels and culverts
Outlet works (see also Section 4.8) should where practicable be con-
structed as a tunnel driven through the natural ground of the dam abut-
ment. Where this is difficult or uneconomic a concrete culvert founded on
rock is a satisfactory alternative, provided that care is taken to ensure that,
if not founded in a shallow rock excavation (Fig. 2.17), a relatively rigid
projecting culvert does not promote shear cracking of the embankment fill
as the latter settles. The culvert cross-section should therefore be ogival,
with transverse joints at intervals of 10–15 m and an external slip coating
to assist settlement of the fill relative to the culvert. It is potentially dan-
gerous to construct an outlet culvert on a compressible foundation within
the fill itself owing to the effects of differential deformations and possible
cracking.
Concrete culverts sometimes incorporate a number of external trans-
verse seepage collars or plates at intervals along their length to inhibit
preferential seepage and possible erosion at the culvert-fill interface
(USBR, 1987). The effectiveness of such features can be questioned.
(f ) Upstream face protection
Methods of wave prediction and the issue of possible wave overtopping,
with its potential to degrade downstream slope stability, are discussed in
Yarde, Banyard and Allsop (1996) (see also Section 4.4).
Several options are available for protection of the upstream face
against wave erosion, ranging from traditional stone pitching with grouted
joints through concrete facing slabs to the use of concrete blockwork and
rock armouring. A heavy and thick protective layer is necessary between
crest level and the minimum operating or drawdown level, with reduced
protection typically in the form of beaching, i.e. smaller uniform rocks, pro-
vided down to the reservoir bed. Pitching is expensive and, while durable, it
is not an efficient dissipator of wave energy. Concrete slabs have similar
limitations, allowing considerable wave run-up which has to be included
within the design freeboard. Open-jointed heavy concrete blockwork
bedded on gravel and a granular filter is preferable, and is now widely
employed. Where durable rock is available in large, angular sizes, rock
armouring and dumped rock riprap provide the most effective protection.
The wave protection facing plus under-layer interacts with the imme-
diately underlying embankment fill to perform as a composite system. The
performance of the overall protection is particularly dependent upon the
under-layer fulfilling the following functions:
- filtration, to resist movement of embankment fill by wave-induced
under-layer water flows;
- drainage, to relieve wave-induced uplift forces under the facing;
70 EMBANKMENT DAM ENGINEERING
