Seepage control measures can be broadly categorized as either seepage barriers or exit and
drainage controls. The intent of seepage barriers is to reduce seepage and reduce the
hydraulic gradient causing seepage, while the intent of exit and drainage controls is to provide
safe discharge of seepage. Barriers are seldom perfect, and exit or drain controls may increase
the flow of seepage. In most cases, water barriers and filters/drains are used together. As an
example, a grout curtain under a concrete gravity dam is nearly always followed by a curtain of
drain holes and drain gallery located immediately downstream of the grout curtain.
Seepage control measures are usually conservatively designed because seepage analysis is
seldom accurate. The estimates of flow volume may be off by an order of magnitude or more.
Designs often incorporate a safety factor of 2 orders of magnitude for flow quantities.