V. REMEDIAL ACTION: PERMANENT SEEPAGE MEASURES
V-13
CUTOFF WALLS (Continued)
FIGURE V-2. CONCRETE CUTOFF WALL
For dams with dispersive clay cores on fractured rock foundations, a thin layer of lime-treated
clay placed directly on the rock should aid in preventing the clay from washing into small cracks
in the rock. The lime treatment makes the clay non-dispersive. For new dams, rigorous
treatment of all fractures and cracks with slush grout and dental concrete is imperative and
should be combined with downstream filters as a second means of defense. Recent research
has shown that sand filters with an average D15 size of 0.5 mm will also safely control and seal
concentrated leaks through the great majority of dispersive clays. (In a mechanical gradation
analysis, the D15 size of a material is the sieve size, which 15 percent of the material would
pass.)
Where the original design and construction did not include such defensive measures, seepage in
dispersive clays can be very serious. While surface erosion problems can be fixed, if inadequate
filters or foundation treatment were not incorporated in the structure, ensuring the safety of the
dam against seepage problems can be difficult and expensive. Adding an inclined drain to an
existing embankment is expensive but may be the only viable alternative for this problem