V. REMEDIAL ACTION: PERMANENT SEEPAGE MEASURES
V-16
DRAINAGE (Continued)
In several recent cases, drainage tunnels or adits have been excavated into the downstream
abutments of embankment dams to control seepage through the abutments. The seepage then
exits from the tunnel or adit rather than into the downstream embankment drains. Another use
has been to divert seepage away from poorly prepared contacts between fractured foundation
rock and embankment materials prone to erosion. In each of these cases, a number of
precisely drilled drain holes at various orientations have been installed from the tunnels to
increase the effectiveness of control.
Some of the limitations of various drainage methods are listed below.
• While toe (or trench) drains are most effective in drying up wet surface areas or
disposing of seepage collected by drainage blankets, it is seldom practical to trench
sufficiently deep to control deep seepage or seepage in a stratified foundation.
• Relief wells are effective in controlling deep seepage in stratified foundations but are
ineffective in mopping up shallow seepage.
• Inclined drains on slopes or abutments or inside dams or berms are susceptible to
contamination from construction activity or slope wash, which causes siltation and
reduces their capacity. They should always slope down from the high phreatic level to
the low level or exit.
• Horizontal blankets, because of the low gradients available, must be relatively pervious
to have acceptable flow capacity. As a consequence, they must often be multi-layered
to provide both the filter action and discharge capacity. Perforated pipes are frequently
added to provide capacity.
All drain systems are subject to deterioration. Monitor all drain systems and either repair or
replace them, as necessary, to maintain adequate efficiency.
COMBINED MEASURES
In many instances, more than one method of seepage control may be used. Often, it is
necessary to use multiple lines of defense to control seepage and to ensure a safe dam project
with regard to seepage