Environmental Engineering Reference
In-Depth Information
13.3
EMBANKMENT CREST DETAILS
13.3.1
Camber
Embankment dams are subject to settlement after construction. In the case of earthfill this
can be related to consolidation of the earthfill as pore pressures reach equilibrium but
post construction settlement also occurs in rockfill, as with time high contact pressures
between particles of rock are crushed.
Data on observed post construction settlement of earthfill and earth and rockfill dams
are presented in Section 6.2.2, Table 6.8 and Figure 6.42.
From this it can be seen that most earth and earth and rockfill dams will experience less
than 0.5% settlement in the first 10 years and a further 0.25% from 10 years to 100
years, i.e. a total of 0.75%.
Figure 13.8 shows the long term crest settlement rates for some concrete face rockfill
dams, plotted as a function of dam height and the strength of the rock in the rockfill. The
data is for well compacted rockfill.
It can be seen that, for high strength rockfill, an 80 m high dam could be expected to
settle 0.3% in the 100 years after construction. This is consistent with Sherard and Cooke
(1987) who suggest that settlements will generally be 0.15% to 0.3% in 100 years. They
indicate that for dumped rockfill the crest settlements are 5 to 8 times that for compacted
rockfill and the time dependent rate 10 to 20 times.
To maintain freeboard it is common to build the crest of the dam higher than the design
level, i.e. to provide camber.
The camber relating to settlement of the embankment is best estimated by comparison with
performance of other dams. Hence a fairly conservative approach would be to provide
camber of about 0.5% to 1% for earthfill and earth and rockfill dams and about 0.2% to
0.4% for CFRD, depending on the height of the dam and strength of the rock in the rockfill.
This is usually provided by oversteepening the upper slopes of the embankment, possibly
by up to 0.25H:1V. The camber changes as a smooth curve or series of straight lines along
the dam crest and is provided proportional to the height of the dam above the foundation.
The camber relating to settlements of the dam foundation is best calculated using con-
ventional soil and rock mechanics principles.
0.30
Medium
strength rock
Gravels
Very high strength rockfills
High strength rockfills
Medium strength rockfills
Kotmale
0.25
Mangrove Creek
0.20
R
2
coefficient of regression
High strength rock
R
2
0.79
0.15
0.10
Very high strength
rock and gravels
R
2
0.51
0.05
0.00
0 0 0 0 0 0120
140
160
180
Embankment Height (m)
Figure 13.8.
Long-term crest settlement rates vs embankment height for compacted rockfills (Hunter
2003; Hunter & Fell 2003c).
