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Lesson 24. Earthen Dam Drainage and Failure

24.1 Drainage System Drainage in earthen dams is primarily provided to bring the phreatic line (upper surface of zone of saturation) in the embankment well within the downstream face so that water does not seeps through the body of the dam. It is achieved using the filter material at the downstream end of the earthen embankment (Fig.24.1). It drains the excess water inside the confining structure and thus reduces the pore pressure and thus internal erosion

The code of practice recommended by Indian standards (IS 9429:1999) lists different type of filters based of their relative placement in the embankment. These are presented in Fig. 24.2.

24.2 Criteria for Selection of Drainage Features The drainage features primarily address the problem of seepage force in form of pore water pressure and piping. 24.2.1 Impervious Fill Material The seepage control in the dam body is achieved either by providing a barrier or control in drainage. The impervious fill material can act asa barrier to the seepage flow through the body of embankment dam. 24.2.2 Inclined/Vertical Filter with Horizontal Filter

The inclined or vertical filters abutting downstream face of the impervious core are provided to collect seepage water to keep the downstream shell relatively dry. The top level of the vertical filter should be kept as equal to the top of the impervious core level at least. The minimum thickness should be kept as one meter. At the base, this filter should be connected to the horizontal filter that will ultimately carry the seepage water to the toe drain (see Fig. 24.3).

24.2.3 Horizontal Filter Horizontal filter collects seepage water from body of the dam through inclined or vertical filter and transmits it to the toe

drain and thus minimize the risk of piping from the dam base. Horizontal filter is usually provided in the downstream side of the homogenous dam. A minimum slope of 1 in 100 towards toe filter has to be maintained for quick disposal of seepage water to the toe drain. A minimum thickness of one meter is required for the horizontal drainage. 24.2.4 Intermediate Filter Horizontal layer of filter in both upstream and downstream at intermediate level is required in the cases where pore pressure exerted during construction and sudden draw down in expected. Italso helps in reducing pore pressure rise after prolonged rainfall. The thickness and vertical interval are decided on the basis of height, permeability of earth of dam body and filter. However, provision of 0.6 m thickness at 6.0 m interval is considered as adequate for small earthen dam. This filter can be extended upto outer slope of the embankment but care should be taken to avoid the connection with vertical/inclined filters. 24.2.5 Rock Toe The major function of the rock toe is to protect the lower part of downstream embankment. 24.2.6 Toe Drain Toe drain (Fig. 24.2) is provided at the downstream side of the earthen dam to collect seepage from the horizontal filter or inner cross drains, through the foundation as well as the rain water falling on the face of the dam.

24.3 Design Criteria for Filter Material The filter material used for drainage system should satisfy the following criteria. 1.

2.

Filter material should be more pervious than base material The particle of base material should not clog the voids of filter material

Based on percentage of particles finer than 75 micron, the base soil are graded (Table 24.1). Table 24.1. Categories of base soil Sl. No. i

Catagory 1

% finer than 75 micron >85

ii

2

40-85

iii

3

15-39

iv

4