V. REMEDIAL ACTION: PERMANENT SEEPAGE MEASURES
V-14
UPSTREAM IMPERVIOUS BLANKETS
An impervious blanket immediately upstream of a dam can be used to seal the reservoir bottom
and sides and thereby reduce seepage quantities and pressures beneath a dam. If the dam
contains a permeable shell, the impervious blanket must be extended up the embankment slope
to effectively control the seepage problem.
Mechanical or manmade liners may be used, but have some disadvantages. Besides being
expensive, they require a relatively smooth surface for placement, and must be protected from
puncture and sometimes from UV rays from sunlight. Also, careful and proper seaming of the
sections is necessary during placement; otherwise, appreciable seepage may still occur.
If blankets will be exposed by pool fluctuation, they must be protected against erosion from
wave action and runoff, from desiccation or drying and cracking, from mechanical damage, and
from piping into coarse granular or fractured rock subgrades.
DOWNSTREAM BERM
A downstream berm can be used as a remedial treatment against seepage forces and uplift
pressures on the downstream face of the dam. A berm may prevent blowout by increasing the
overlying weight sufficiently to resist the uplift pressures. If the berm is of low permeability,
the seepage will be forced to exit further downstream.
The design of a pervious berm should ensure there is no upward migration of fine particles from
the foundation. This implies filter compatibility between the berm and the foundation. An
added advantage of a downstream berm is to increase slope stability because of the additional
resistance to sliding provided by the berm.
In some cases, it may be necessary to add on to the downstream side of the dam using
impervious elements, filter and drainage chimneys and blankets, and an outer stability shell.
These cases include:
• Where the downstream slope is steep and unstable because of seepage through the
dam.
• Where the dam has serious transverse cracking.
• Where dispersive soils were used in fill construction.
• Where the embankment was not properly zoned.
While these measures may be expensive, they are generally less expensive than building a new
dam.