Page 57 - Water Engineering Hydraulics, Distribution and Treatment

Riprap
Clay
Sand
core
Gravel
Clay
Pervious alluvium
cutoff
Bedrock
(a)
Axis
Roadway
El. 2533 Sand Gravel 2.10 Dams and Dikes 35
Max. normal res. W.S. El. 2513
2.0 1.80
1.0 1.0
Riprap
Transition zone Rockfill
Cofferdam Rockfill Transition zone
El. 2247 Clay core El. 2231
Backfill Random fill Random fill
Strip to rock under rockfill zone Strip to rock under rockfill zone
(b)
Figure 2.11 Zoned earth-fill and rock-fill dams: (a) earth-fill dam on pervious alluvium; (b) rock-fill dam on
bedrock.
are more or less horizontal, berms do slope inward to gut- Where rock outcrops on canyon walls can be blasted
ters; moreover, they are pitched lengthwise for the gutters into the streambed or where spillways or stream diversion
to conduct runoff to surface or subsurface main drains and tunnels are constructed in rock, rock embankment becomes
through them safely down the face or abutment of the dam, particularly economical. In modern construction, rock fills
eventually into the stream channel. are given internal clay cores or membranes in somewhat
Earth embankments are constructed either as rolled fills the same fashion as earth fills (Fig. 2.11). Concrete slabs
or hydraulic fills; rock embankments are built as uncom- or timber sheathing once much used on the upstream face
pacted (dumped) or compacted fills. In rolled earth fills, suc- can be dangerously stressed and fail as the fill itself, or its
cessive layers of earth 4–12 in. (100–300 mm) thick are foundation, settles. They are no longer in favor.
spread, rolled, and consolidated. Sheep’s foot rollers do the
compacting, but they are helped in their work by heavy earth-
moving vehicles bringing fill to the dam or bulldozing it into 2.10.2 Masonry Dams
place. Portions of embankment that cannot be rolled in this
In the construction of gravity dams, cyclopean masonry and
way are compacted by hand or power tampers. Strips adja-
mass concrete embedding great boulders have, in the course
cent to concrete core walls, the walls of outlet structures, and
of time, given way to poured concrete; in the case of arched
the wingwalls of spillway sections are examples.
dams rubble has also ceded the field to concrete. Gravity
In hydraulic fills water-carried soil is deposited differ-
dams are designed to be in compression under all conditions
entially to form an embankment graded from coarse at the
of loading. They will fit into almost any site with a suit-
two faces of the dam to fine in the central core.
able foundation. Some arched dams are designed to resist
Methods as well as materials of construction determine
water pressures and other forces by acting as vertical can-
the strength, tightness, and stability of embankment dams.
tilevers and horizontal arches simultaneously; for others, arch
Whether their axis should be straight or curved depends
action alone is assumed, thrust being transmitted laterally to
largely on topographic conditions. Whether upstream curves
both sides of the valley, which must be strong enough to
are in fact useful is open to question. The intention is to
serve as abutments. In constant-radius dams, the upstream
provide axial compression in the core and prevent cracks
face is vertical or, at most, slanted steeply near the bottom;
as the dam settles. Spillways are incorporated into some
the downstream face is projected as a series of concentric,
embankment dams and divorced from others in separate
circular contours in plan. Dams of this kind fit well into U-
constructions.
shaped valleys, where cantilever action is expected to respond