Earthen Dams and Rock Fill Dams
Earthen dams and earthen levees are the most ancient type of embankments, as they can be built with the natural materials with a minimum of processing and primitive equipment. But in ancient days, the cost of carriage and dumping of the dam materials was quite high. However, the modern developments in earth moving equipments have considerably reduced the cost of carriage and laying of the dam materials. The cost of gravity dams on the other hand, has gone up because of an increase in the cost of concrete, mesonry, etc. Earthen dams are still cheaper as they can utilize the locally available materials, and less skilled labour is required for them.
Gravity dams and arch dams require sound rock foundations, but earthen dams can be easily constructed on earth foundations. However, earth dams are more susceptible to failure as compare to rigid gravity dams or arch dams. Before the development of the subject of Soil-Mechanics, these dams were being designed and constructed on the basis of experience, as no rational basis for their design was available. This led to the failure of various such earthen embankments. However, in these days, these dams can be designed with a fair degree of theoretical accuracy, provided the properties of the soil placed in the dam, are properly controlled. This condition makes the design and construction of such dams, thoroughly interdependent. Continuous field observations of deformations and pore water pressures have to be made during the construction of such dams. Suitable modifications in the design, are then made during construction, depending upon these field observations.
Type of Earthen Dams
The earthen dam can be of the following three types:
1. Homogeneous Embankment type
2. Zone embankment type
3. Diaphragm type.
These three types of dams are described below:
(1) Homogeneous Embankment Type. The simplest type of an earthen embankment consists of a single material and is homogeneous throughout. Sometimes, a blanket of relatively impervious material may be placed on the upstream face. A purely homogeneous section is used, when only one type of material is economically or locally available. Such a section is used to moderately high dams for levees. Large dams are elected designed as homogenous bankments.
(a) Homogenous type embankment
Figure: Zoned type embankment
The central core checks the seepage. The transition zone prevents piping through cracks which may develop in the core. The outer zone gives stability to the central impervious fill and also distribute the load over a large area of foundations.
This type of embankment are widely constructed and the materials of the zones are selected depending upon their availabilities. Caly, inspite of it being highly impervious, may not make the best core, if it shrinks and swells too much. Due to this reason, clay is sometimes mixed with fine sand or fine gavel, so as to use it as the most suitable material for the central impervious core. Silts or silty clays may be used as the satisfactory central core materials. Freely draining materials, such as coarse sands and gravels, are used in the outer shell. Transition filters are provided between the inner zone and the outer zone, as shown in fig. This type of transition filters are always provided, whenever there is an abrupt change of permeability from one zone to the other.
(3) Diaphragm Type Embankments. Diaphragm type embankments have a thin
impervious core, which is surrounded by earth or rock fill. The
impervious core, called diaphragm, is made of impervious soils, concrete,
steel, timber or any other material. It acts as a water barrier to prevent seepage
through the dam. The diaphragm may be placed either at the centre as a central
vertical core or at the upstream face as a blanket. The diaphragm must also be
tied to the bed rock or to a very impervious foundation material, if excessive
under-seepage through the existing previous foundations has to be avoided
Figure: Diaphragm type embankment
The diaphragm type of embankments are differentiated from zoned embankments, depending upon the thickness of the core. If the thickness of the diaphragm at any elevation is less than 10 elevation is less than the height of the embankment above the corresponding
elevation, the dam embankment is considered to b equals or exceeds or exceeds these limits, it is considered to be of zoned embankment type.
Methods of Construction
There are two methods of constructing earthen dams:
(1) Hydraulic-fill Method; and
(2) Rolled-fill Method.
(1) Hydraulic-fill Method. In this method of construction, the dam body is constructed by excavating and transporting soils by using water. Pipes called flumes, are laid along the outer edge of the embankment. The soil materials are mixed with water and pumped into these flumes. The slush is discharged through the outlets in the flumes at suitable intervals along their lengths.
The slush, flowing towards the centre of the bank, tends to settle down. The coarser particles get deposited soon after the discharge near the outer edge, while the fines get carried and settle at the centre, forming a zoned embankment having a relatively impervious central core.
Since the fill is saturated when placed, high pore pressures develop in the core material, and the stability of the dam must be checked for these pressures. This type of embankment is susceptible to settlement over long periods, because of slow drainage from the core.
Hydraulic-fill method is, therefore, seldom adopted these days. Rolled-fill method for constructing earthen dams is, therefore, generally and universally adopted in these modern days.
(2) Rolled-fill Method. The embankment is constructed by placing suitable soil materials in thin layers (15 to 30 cm) and compacting them with rollers. The soil is brought to the site from burrow pits and spread by bulldozers, ect. In layers. These layers are thoroughly compacted by rollers of designed weights. Ordinary road rollers can be used fro low embankments (such as for levees or bunds); while power-operated rollers are to be used for dams. The moisture content of the soil fill must be properly controlled. The best compaction can be obtained at a moisture content somewhere near the optimum moisture content. (The optimum moisture content is the moisture required for obtaining optimum density in the fill). Compaction of coarse gravels cannot be properly done by rolling and is best done by vibrating equipment. Detail of rolling and
compacting different types of soils are available i by the same author.
Shearing Strength of Soils
Before we describe the causes of failure of earthen dams and the criteria for their safe design; we shall review a few important conceptions of soil-mechanics and their importance in the design of earth dams.
Irrigation projects, are classified into two Main categories. They are
1. Diversion works
2. Storage works
The purpose of a diversion work is mainly to raise the water level and divert the river flows into canals to feed the ayacut. A diversion work consists of an anicut or a weir across the stream or river, by which the water level could be raised to divert it into canals on either side of the river or stream.
There is no question of any storage here.
The main component works in this case are:
1. A weir or Anicut across the river or stream in question
2. Head sluices or Head Regulators at either end to feed the canals on either side.
3. Scour sluices on either side to prevent silting of the approaches to the Heal Regulator.
A detailed design of a River weir is given i
In big irrigation projects, where the river, the main source of water, has enough flows in the year and which go waste un-utilized, storage works are resorted to, to conserve the water by creating a reservoir, conserve the flows as far as possible without detriment to the riparian rights of the areas below the reservoir on the same river. These projects involving construction of huge reservoirs are also designed to serve as multipurpose projects. A multipurpose project means that it should serve the following useful purposes:
1. Flood control
3. Power generation.
5. Improve fish culture, and
6. Supply of drinking water.
A storage work, consists of the following components
1. A dam across the river to create a reservoir of adequate capacity to irrigate the command area, with provision for letting down the surplus water down stream.
2. Head Regulators on either side to feed the main canals.
3. Main canals and distributory system to feed the commanded area.