1.5.4 Cut-offs
Seepage under and round the flank of a dam must be controlled. This is
achieved by the construction of a cut-off below the structure, continued as
necessary on either flank. Modern embankment cut-offs are generally
formed from wide trenches backfilled with rolled clay, if impervious strata
lie at moderate depths, and/or by drilling and grouting to form a cut-off
screen or barrier to greater depths. Grout screen cut-offs are also custom-
arily formed in the rock foundation under a concrete dam.
1.5.5 Internal drainage
Seepage is always present within the body of any dam. Seepage flows and
their resultant internal pressures must be directed and controlled. Internal
drainage systems for this purpose are therefore an essential and critical
feature of all modern dams. In embankments drainage is effected by suit-
ably located pervious zones leading to horizontal blanket drains or outlets
at base level. In concrete dams vertical drains are formed inside the
upstream face, and seepage pressure is relieved into an internal gallery or
outlet drain. In the case of arch dams, seepage pressures in the rock abut-
ments are frequently relieved by systems of bored drains and/or drainage
adits or tunnels.
1.5.6 Internal galleries and shafts
In addition to their function alongside drains in effecting local control of
seepage, galleries and shafts are provided as a means of allowing internal
inspection, particularly in concrete dams. The galleries, shafts and any
associated chambers to accommodate discharge valves or gates can also be
used to accommodate instrumentation for structural monitoring and sur-
veillance purposes (Chapter 7).
The ancillary structures and design features referred to are further
described in subsequent chapters. Additional illustrations of these and
other ancillary works are also contained in Thomas (1976), USBR (1987),
Fell, MacGregor and Stapledon (1992) and Kennard, Owens and Reader
(1996).
22 ELEMENTS OF DAM ENGINEERING
