Environmental Engineering Reference
In-Depth Information
It is suggested in Gomez et al. (1999) that these cracks could have developed due to
tensile stresses generated as a result of differences in modulus between the compacted
alluvium (upstream half of embankment) and rockfill (downstream half of embank-
ment). Remedial works to reduce the leakage rate have included the placement of silty
sediments over the cracks. Recent dam performance shows leakage rates respond to
reservoir level and rainfall events, with an overall long-term trend (1995-2000) of
increasing leakage rate based on data presented by Gomez et al. (1999).
(c)
Golillas Dam
- Amaya et al. (1985) reported significant leakage during first-filling of
Golillas Dam with distinct relationship between reservoir level and leakage rate.
Maximum reported leakage rate during first filling was in excess of 1000 l/s, prima-
rily associated with the perimetric joint and opening of joints within the rock in the
left abutment. Initial remedial works, involving removal of the PVC and mastic water-
stops in some areas and replacement with burlap, lead fibre and mastic, did not result
in any real reduction in leakage. Drawdown of the reservoir enabled cleaning and re
sealing of joints in the abutment and abutment support using shotcrete. This remedial
work, along with the washing of fines into the perimetric joint, resulted in reduction
in leakage rate to long-term values of the order of 270-500 l/s.
15.7.2
Early CFRD and other dams which experienced large leakage
Early designs of CFRD (1920s-1960s) were typically characterised by a main body of
dumped rockfill and a thick upstream zone of derrick-placed rockfill supporting the con-
crete face. Over the period from 1920 to 1960 embankment construction involved dump-
ing in high lifts (up to 20-50 m) followed by sluicing. The minimal control on placement
of the rockfill resulted in large deformations within the embankment as illustrated in
Table 15.8
, for dams such as Dix River, Salt Springs, Courtright and Wishon. Typical post-
construction vertical strain for these cases was in excess of 1% (compared to modern CFRD
cases which typically show less than 0.2% vertical strain).
As a result of these large settlements, many early CFRD developed significant face
cracking and opening of joints (particularly perimetric joints), resulting in high leakage
rates.
Table 15.13
presents data on leakage rates observed in early CFRD during first-fill-
ing and during operating.
Other dams which have experienced large leakage or flows through rockfill include
Scofield Dam (Sherard, 1953) which experienced leakage on a piping incident of
1400-5600 l/s, and Hell Hole, which began unravelling in a flood during construction at
a flow of 540 m
3
/sec (Leps, 1973).
15.8
FRAMEWORK FOR ASSESSING THE LIKELIHOOD OF FAILURE OF CFRD
15.8.1
Overview of approach
CFRD are inherently very unlikely to fail, because (as discussed in Sections 15.6.4 and
15.7.1) provided they are designed and constructed with adequate permeable zones (e.g.
free draining rockfill, or screened and washed gravels in “gravel” dams) the rockfill can
accommodate large leakage flows. The concrete of the face slab, and surrounding the peri-
metric and other joints or cracks which may form in the face slab, is essentially non erodi-
ble, so leakage should not develop rapidly.
However the rockfill, and even the Zone 2D and 2E materials are often poorly graded,
and potentially subject to internal instability, with the finer particles potentially being
washed out. This can lead to settlement, and further opening of joints or cracks; with
increasing leakage flows. Left unchecked, the process may not be self-limiting, and very
