CLASSIFICATION OF EARTHEN DAMS
Earthen dams may be classified in two ways:
- Classification based upon the method of construction.
- Classification based upon the section of the dam.
1. Classification Based Upon the Method of Construction.
Under this classification earthen dams can be
(i) rolled fill earth dams and
(ii) hydraulic fill earth dams.
(i) Rolled-fill earth dams.
- In the construction of such dams successive layers of moistened earth are laid one after the other. Each layer is laid after the layer of earth.
- Sheeps foot roller is mostly used for carrying out the process of compaction.
- Bulldozers, scrapers, draglines, like heavy earth moving machinery is use for the purpose of earth work.
- Moisture content during compaction is maintain at the level of optimum moisture.
- Proper slope of the dam is maintained during construction.
(ii) Hydraulic fill dams.
- Outer edges of the embankment are maintained slightly higher than the middle part of the dam.
- The mixture of excavated materials with water, is pumped and discharged at the outer edges of the dam.
- The mixture of excavated material and water consists of coarse.
- When this mixture is discharg near the outer edges, the coarser material of the mixture would settle at the edges while finer material settles at the centr.
- No compaction is required in this const.
- process is continued till dam of required height is constructed.
- In this dam coarser material lies near outer edges which is not very impervious.
- The finer material deposited in the central part of the dam consists of clay mostly and thus provides imperviousness to the dam.
2. Classification Based Upon the Section of the Dam.
According to this classification the dams can be of the following three types:
(i) Homogeneous earth dams,
(ii) Zoned earth dams, and
(iii) Diaphragm type earth dams.
(i) Homogeneous earth dams.
- This dam is made from a single material.
- It is suitable for low heights only.
- Purely homogeneous section can be modified a little by constructing rock toe at the down stream lower end of the dam and also by putting horizontal filter drains.
- Both these measures control the seepage and thus enable to construct much steeper slopes of the dam.
- These measures also keep the phreatic line of seeping water, well within the body of the dam.
- The homogeneous dams are made from impervious or semi-impervious soils.
- This is essential to put a barrier against the flow of seeping water.
- Upstream slope of the dam is generally kept flat so as to reduce the path of seeping water and to counter act the effect of sudden draw downs.
(ii) Zoned earth dam.
- This dam is made by using more than one material. In this case, the central part of the dam, which is known as core is made from impervious material.
- Considerably more pervious material is used on both the sides of core.
- The dam also consists of a rock toe, a system of horizontal drains, and sometimes even system of inclined filters to carry out proper drainage of seeping water from the dam.
- If, at certain place, a variety of soils is available the dam should always be of zoned type. Impervious material should be used for core, while pervious soils at flanks of core.
- U/S pervious soil provides stability against rapid drawdown, while that on D/S side acts as a drain to control the seepage line.
- Central core checks the seepage.
(iii) Diaphragm type earth dam.
- This is a sort of combination of both the previously, mentioned earth dams.
- The bulk of the embankment is made from pervious soil, but a thin diaphragm of impermeable material is provided at the central part of the dam to check the seepage.
- The impervious diaphragm may be made of impervious clayey soil, cement concrete, masonry or of any other impervious material.
- It can be located somewhere in the central part of the dam or it may be put on the upstream face of the dam also.
- The difference in zoned and diaphragm type earth dams is due to thickness of impervious core or diaphragm only.
- If thickness of impervious core is less than 10 m or less than the height of the dam above that level the dam is known as diaphragm type, otherwise it would be zoned earth dam.
SEEPAGE LINE OR PHREATIC LINE
- Seepage line and phreatic line means same thing.
- This is such a line in the body of the dam below which there are positive hydrostatic pressures.
- On the line itself the hydrostatic pressure is zero. Above the line, there is a zone of capillary saturation.
- The effect of capillary is however neglected in dams. Analysis of seepage has been given ahead.