Embankment dam details - Geotechnical Engineering of Dams - page 550

Table 13.6. Tolerances for embankment construction.

Towards axis Away from axis

of dam of dam

(A) Tolerances on zone boundaries
(a) Outside faces of dam embankment
Crest Zero 250 mm

Downstream slope Zero 500 mm

Upstream slope Zero 500 mm

(b) Division lines between zones upstream of dam axis
Zone 1 and Zone 2A Zero 1000 mm

Zones 3A and 3B 1500 mm 1500 mm

(c) Division lines between zones downstream of dam axis
Zone 1 and Zone 2A Zero 1000 mm

Zones 3A and 3B 1500 mm 1500 mm

(B) Tolerances on widths and thicknesses
(a) Width of filter zones and rip-rap
Zones 2A, 2B, 2C Plus 250 mm or 500 mm

Minus Zero

Zone 4 Plus 500 mm or 1000 mm

Minus Zero

(b) Thickness of horizontal filter drains
Zones 2A, 2B Plus 250 mm

Minus Zero

13.5 CONDUITS THROUGH EMBANKMENTS

The placement of conduits or outlet pipes through earth dam embankments is a common
cause of piping failure and accidents, particularly for small dams which do not have filters.
Foster et al. (1998, 2000a) record that historically about half of piping failures can be attrib-
uted to the presence of conduits.
Figure 13.12 shows a dam which has lost all the reservoir water in the large pipe which
has formed around the outlet conduit.
The traditional approach to overcome this problem has been to provide concrete cutoff
collars around the conduit. These lengthen the seepage path and penetrate into the fill,
lessening the likelihood of a stress induced crack penetrating outside the collar. Figures
13.13 and 13.14 show typical cutoff collars for an outlet works conduit.
USBR (1987) indicate the cutoff collars were generally 0.6 m to 0.9 m high, 0.3 m to
0.45 m wide and spaced from 7 to 10 times their height along the portion of the conduit
within the core of the dam. For a conduit founded on soil, the collars should completely
encircle the conduit. A bituminous or other joint filler is provided between the collar and
the conduit to minimize stress concentration effects on the conduit.
While many dams have been built and operated successfully using this approach others
have had problems.
Fell and Foster (1999) discuss the causes for this based on case studies, the literature
and their experience. It is necessary to consider the mode of the piping (piping into the
conduit, along and above the conduit or out of the conduit and the four phases of piping
(initiation, continuation, progression and breach).

13.5.1 Piping into the conduit

(a) Initiation - the conduit allows erosion into the conduit if it is cracked, corroded, or
joints have opened. This is most likely to occur if:
- Settlement of the conduit has occurred due to compressible (soil) foundations;