One of the basic requirements for the design of an earth or rockfill dam is to ensure safety against internal erosion, piping and excessive pore pressure in the dam. A suitably designed drainage system is therefore essential to satisfy these requirements. The seepage of reservoir water through the body of the dam or at the interfaces of the dam with the foundation or abutment creates two main problems, apart from causing excessive water loss and thereby reducing usable storage of reservoir:

1. Seepage force causing excessive water loss
2. Piping

In spite of taking all measures in design as well as construction to minimize seepage, it does take place either through the body of the dam or at interfaces. Whatever may be quantum of seepage, if it is not safely drained away from the toe of embankment dam into nearby drainage, valley, etc, it may lead to failure or heavy damage to the embankment, by way of slips of slopes and/or development of internal erosion leading to formation of sink holes, boiling, settlement, etc, besides creating unfriendly environment on downstream faces and areas of embankment dams.

The design is mostly governed by type and permeability of base materials as well as filter materials, water depth in reservoir, topographical features of dam site, etc. The conventional types of seepage control and drainage features generally adopted for the embankment dam are:

a) Impervious core,

b) Inclined/vertical filter with horizontal filter,

c) Network of inner longitudinal drain and cross drains,

d) Horizontal filter,

e) Transition zones/transition filters,

f) Intermediate filters,

g) Rock toe, and

The drainage system may comprise of either one or a combination of more than one of these drainage features, and typical sections are shown for homogeneous dams, in Figure 85 and for zoned dams, in Figure 86.

Inclined/Vertical Filter:

Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of core/transition zone and thereby keeping the downstream shell relatively dry. In the eventuality of hydraulic fracturing of the impervious core, it prevents the failure of dam by piping.

Horizontal Filter:

It collects the seepage from the inclined/vertical filter or from the body of the dam, in the absence of inclined/vertical filter, and carries it to toe drain. It also collects seepage from the foundation and minimizes possibility of piping along the dam seat.

Inner Longitudinal and Inner Cross Drains

When the filter material is not available in the required quantity at reasonable cost, a network of inner longitudinal and inner cross drains is preferred to inclined/vertical filters and horizontal filters. This type of drainage feature is generally adopted for small dams, where the quantity of seepage to be drained away is comparatively small.

Transition Zones and Transition Filters

Transition zones/filters in earth and rockfill dams in the upstream and downstream shells are necessary, when the specified gradation criterion is not satisfied between two adjacent zones. When such zones/filters are placed on either side of the impervious core, they help to minimize failure by internal piping, cracking, etc, that may develop in the core or by migration of fines from the core material.

The filter material used for drainage system shall satisfy the following criteria:

a) Filter materials shall be more pervious than the base materials;

b) Filter materials shall be of such gradation that particles of base material do not totally migrate through to clog the voids in filter material; and

c) Filter material should help in formation of natural graded layers in the zone of base soil adjacent to the filter by readjustment of particles.

Horizontal Filters at Intermediate Levels

Horizontal filter layers at intermediate levels are sometimes provided in upstream and downstream shells, to reduce pore pressures during construction and sudden drawdown condition and also after prolonged rainfall (see Figure 87).

The filter layers should be extended upto the outer slopes of the embankment so as to drain out the collected water. These filter layers should not be connected with inclined or vertical filters. A minimum space of 2.0 m or more, should be kept between the face of inclined/vertical filter and downstream intermediate filter.

Rock Toe

The principal function of the rock toe is to provide drainage. It also protects the lower part of the downstream slope of an earth dam from tail water erosion. Rock available from compulsory excavation may be used in construction of the rock toe. Where this is not possible and transportation of rock is prohibitively costly, conventional pitching should be used for protecting the downstream toe of the dam. The top level of the rock toe/pitching should be kept above the maximum tail water level (TWL). In the reach where the ground level at the dam toe is above the maximum tail water level, only conventional pitching should be adopted. The top of such pitching should be kept 1.0 m above the top of horizontal filter, or stripped level, whichever is higher. A zone of coarse filter should be introduced between the rockfill/ pitching and the fine filter. A combination of partial rock toe and pitching may also be considered to effect economy.