Your Dam Your Responsibility (PDF~1.2MB)

5 Construction of a Dam
is inviting trouble! Remember that tracked plant has tracks in order to keep the
contact pressure low (for example, to cross soft ground), whereas what is required
for good compaction is a combination of much higher contact pressures and ability
to penetrate layers and knead them to ensure good bonding and eliminate leakage
paths. Proper compaction plant is designed to do these things.
Remember bulldozers, scrapers and tractors may be large machines but they are not
designed for soil compaction. In fact, they are designed to create minimum pressure.
Good compaction is achieved using appropriate compaction rollers on soil at just the
right amount of moisture content.
5.5 Outlet Pipes or Structures in
Embankment
Outlet pipes under the dam wall are used for the
controlled release of water from the dam. If not
properly installed, they can readily provide a path
for leakage from the dam, resulting in failure of the
embankment and uncontrolled release of water.
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Proper installation requires the backfilling and
compaction to be carried out with extreme care.
Using cut-off ‘collars’ at intervals along pipes Refer Compensation Flow Section 4.5.2.
passing under a bank can reduce the possibility of
leakage. However, special care is required to ensure
adequate compaction in these areas, and this may require (among other things) the use
of special
small equipment.
Outlet pipes or other works should be placed in original ground rather than in the
embankment. As far as possible, the location of such structures in the embankment
itself should be avoided. It is likely that they will be damaged during construction of
the embankment and/or when any settlement occurs, but such damage will be unseen.
Because of difficulties with compacting backfill properly around an outlet pipe it is better
to backfill with concrete. Appendix C outlines the procedure. Never use sand or other
granular material as pipe backfill or bedding.
Warning: Even if these suggested practices are used, great care is required, especially if
such items are incorporated in embankments or foundations of dispersive clays. It is then
highly likely that failure will result, unless special care, overseen by an expert, is applied.
Ideally, steel pipe with cement mortar lining should be used and backfilled with concrete.
Unlined steel pipe can corrode and perforate within 20 years and lead to internal erosion
of the embankment or abutment and result in dam failure. The use of concrete cutoff
collars and an upstream guard valve is considered to be highly desirable, but such
features themselves create an additional need for special care and attention to detail in
both design and implementation.
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