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dams on non-rock foundations
Q: Could RCC dams be founded on non-rock foundation?
 A:
Although most concrete dams including RCC dams are founded on sound
rock, for small dams, a rock foundation is not always necessary.
However, before placing RCC dams on other than high-modulus rock
foundation, design engineers need to carefully evaluate several
factors including differential settlement, seepage, uplift pressure
distribution, piping potential, and hydraulic erosion of the foundation
due to spillway or conduit flows.
History has shown that the potential for failure of concrete dams
is extremely remote. However, nearly all failures related to concrete
dams have occurred through foundation materials rather than through
the concrete in the dam. Because each potential dam site is unique,
engineers experienced in the evaluation of foundations for dams
should investigate the site and determine what foundation improvements
are needed.
 Sound
rock foundations are considered most suitable for RCC dams because
they provide high bearing capacity, prevent excessive settlement,
and have high degree of erosion and seepage resistance. Nevertheless,
there are at least seven small RCC dams in the United States successfully
placed on non-rock foundations. The tallest amongst these dams is
the 88-foot (29-m) high Big Haynes Dam near Conyers, Ga. A 500-foot
(152-m) section of the dam is founded on variable strength foundation
varying from soil to hard rock. This portion of the dam was built
in three monoliths; one on hard rock, the middle on soil, and the
other on partially weathered rock.
Another example is the 30-foot (9-m) high Cedar Falls Dam southeast
of Seattle, Wash. This RCC structure was built on glacial deposits
up to 600-feet (183-m) deep. The design required replacing the upper
most 16 feet (5 m) of existing low-density sand with compacted fill
to reduce the potential for earthquake-induced liquefaction, reduce
seepage, and reduce settlement. Additionally, the design included
a 20-foot (6-m) deep cutoff wall and an upstream concrete blanket
to further reduce seepage as well as lengthening the seepage path.
Detailed information on small RCC dams is included in the Portland
Cement Association publication Design
Manual for Small RCC Dams (EB225). RCC design
professionals are also encouraged to refer to the article titled
“Design Considerations for Small RCC Dams,” by K. D.
Hansen, published in the International Journal on Hydropower
& Dams, Issue 3, 2008. View
a PDF copy of the article.
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