Environmental Engineering Reference
In-Depth Information
15
Concrete face rockfill dams
15.1 GENERAL ARRANGEMENT AND REASONS FOR SELECTING
THIS TYPE OF DAM
15.1.1
Historic development of concrete face rockfill dams
The development of concrete face rockfill dams (CFRD) has been described by Galloway
(1939), ICOLD (1989a), Cooke (1984, 1993, 1999, 2000) and Regan (1997) and is sum-
marized in
Table 15.1
.
The first rockfill dam to have a concrete face was constructed in California in 1895.
This followed on construction of timber faced dumped rockfill dams beginning in the
1850s.
These early CFRDs often had steep (0.54 H:1 V to 0.75 H:1 V) slopes, with a skin of hand
placed rocks to stop the face from raveling. Then dams of greater height, up to 100 m high
(Salt Springs Dam, California), were constructed with concrete faces and rockfill dumped in
thick layers, often greater than 20 m or 35 m, and placed without compaction other than
sluicing.
The design of faced rockfill dams was (and still is) mainly empirical, based on experi-
ence and judgement. The typical features of designs of CFRD up until the late 1950s are
shown in
Figure 15.1
.
Many of these dams performed well. However several higher dams leaked excessively due
to deformation of the concrete face, with resulting opening of joints and cracking. This could
be attributed to the low modulus of the dumped rockfill, and to the detailing of joints, which
allowed compression of horizontal and vertical joint fillers in the central part of the dam face
(which is under compression) and resulted in increased opening of other joints, including the
perimetral joint. The leakage did not endanger the dam stability but in some cases was unac-
ceptably high for operating reasons.
Over the period 1955 to 1965 there was a general adoption of compaction of rockfill.
This change was brought about by a realisation that dumping and sluicing of rockfill led
to signifi-cant segregation, with the accumulation of larger rock at the base of the layer,
leaving large voids and being particularly compressible. There was also a realisation that
weaker rocks tended to lose strength on saturation leading to settlement if placed as
dumped rockfill. Cooke (1984) and ICOLD (1989b) attribute the major change in
approach to Terzaghi (1960a). Terzaghi (1960b) also introduced the change from a deep
cutoff trench in rock as shown in Figure 15.1, to the adoption of the use of a plinth, or toe
slab, on the grounds that excavation for the trench could loosen and fracture the rock,
making it more permeable, and that the plinth could be an adequate cutoff if founded on
suitable rock, grouted and anchored to the rock with steel bars.
15.1.2
General arrangement - modern practice
Figure 15.2
shows the modern practice for zoning of CFRD constructed of sound, free
draining rockfill, on a strong rock foundation.
