Selection of the type of dam 1. Environment and public opinion- a dam must be constructed

without disturbing the surrounding environment, at least to the minimum extent as is possible.

2. Availability of construction material- cost of transportation of construction material -

mimimum3. Unavailability of skilled workers-

skilled workers are an absolute necessity4. Seismicity- The behavior of the dam under earth quake vibrations- to check if a particular section is suitable or not.dynamic structural analysis computer program -A description of the program is available at the following

web-site: http://nisee.berkeley.edu/elibrary/getdoc?id=141382

5. Geology and foundation strength-The existence of joint patterns in the abutments (their orientation, inclination and infilling) may indicate the possibility of instability under loading from an arch dam and reservoir water.

the stresses expected at the base of a dam may have to be checked with the bearing capacity of the foundation material to propose the suitability of a particular section.

6. Hydrology- If, during the construction season, there are possibilities of the partially constructed dam being overtopped by the floods of the river water, then a concrete dam section would be preferred then an embankment dam section.

7. Valley shape and overburden-

gravity or buttress dams

embankment, gravity or buttress dams

arch or cupola dams

combination of two types of dams

Appurtenant structure and ancillary works

Each dam has to have certain appurtenant structures to enable them to discharge their operational functions safely and effectively.

Spillway

Spillways or passages for letting out flood waters when the reservoir, is over flowing has three major components:

Entry to the spillway, which may or may not be controlled using gates.

A channel for conveying the water from the reservoir side to the down stream of the dam.

And energy dissipating arrangement for the water flowing down the spillway channel as it reaches a lower elevation near the outlet of the channel.

The capacity of the water conveyance of the spillway should be such that it must safely pass the maximum design flood.

Outlets These include outlets for irrigation canals, power channels ortunnels, water conditions for domestic and industrial use etc.

The following types of sluices are used for following purposes: For used during river diversion, at the time of construction of the

concrete dam. If these sluices are left out in the body of the dam at a lower level, the construction of the dam can safely go on at higher levels.

To control the rate of filling of the reservoir. This would be necessary during the first time reservoir filling.

As part or the whole of the permanent spillway discharge, as it has been done for some arch dams.

To release the bottom water from a stratified lake. This action may be desirable to remove foul water from the bottom of the reservoir after initial filling

Cut off The seepage under and round the

flank of a dam must be controlled, or else the foundation of the dam may be weakened.

This is achieved by the construction of a cut off or a barrier below the dam penetrating the foundation.

Internal drainage arrangement embankment dam - a thin vertical wall of very

pervious material acts like a drain, collecting the seepage water and passes it downstream with the help of a connected horizontal layer of pervious material that extends up to the down stream face of the embankment dam.

concrete dams- vertical drains are formed near the upstream face of the dam which collects the seeping water - pass it on to a gallery near the base of the dam that extends from one abutment to the other. - From gallery, the collected water is drained off to the downstream by drains located at suitable points.

Instrumentation in Dams instruments are installed in a concrete

gravity dam to measure the various parameters that indicate the structural health of the dam and the state of the foundation

IS 7436(part2)-1997 Guide for types of measurements for structure in river valley projects and criteria for choice and location of measuring instruments.

Obligatory Measurements The following measurements are obligatory

for all dams: a)Uplift pressure at the base of the dam at

a sufficient number of transverse sections b) Seepage into the dam and appearing

downstream there-from; c) Temperature of the interior of the dam;

and

d) Displacement measurements - Except for very small structures (of height 20 m and below not involving any foundation complications). Displacement measurements should include one or more of the following types of -measurements:

1) Those determined by suspended plumb lines; 2) Those determined by geodetic measurements

where warranted by the importance of the structure;

3) Those determined by embedded resistance jointmeters at contraction joints where grouting is required to be done.

Optional Measurementsa) Stress b) Strain c) Pore pressure (as distinct from uplift

pressure), and d) Seismicity of the area and dynamic

characteristic of the structure.

Measurement of Uplift Pressure

It is important to determine the magnitude of any hydraulic pressure at the base of a dam. The effect of uplift on a dam is to reduce its effective weight on account of resulting buoyancy.

Measurement of Seepage the best indicator of the overall performance of a

dam because this reflects the performance of entire dam and not just the condition at discrete instrumented points

Any sudden change in the quantity of seepage without apparent cause, such as a corresponding change in the reservoir level or a heavy rainfall, could indicate a seepage problem.

when the seepage water becomes cloudy or discoloured, contains increased quantities of sediment, or changes radically in chemical content, a likely serious seepage problem is indicated

Grout curtain It is customary to provide grout

curtain near the upstream face of the dam. Besides, a drainage curtain in the foundation and porous drain in to intercept any seepage that passes through the grout curtain and through the body of the dam respectively. the body of the dam are provided

Measurement of Temperature during

Construction For concrete gravity dams it is very important to know the thermic variations in the dam during its construction which enables to determine whether the concrete setting process is normal

Temperature measuring devices are embedded within the dam body and also mounted on the surface according to a predetermined plan for useful observations.

Measurement of Temperature of the Dam

interior To measure temperature in the body of

concrete and masonry dams in order to ascertain the nature and extent of thermal stresses and the consequent structural behaviour of the dam

To ascertain when to undertake grouting of contraction joints that may have been provided for the structure

Measurement of Temperature of Reservoir Water and Air Measurement of temperature of

reservoir water and air is essential for distinguishing the effects of ambient and water temperatures on such measurements as deflection, stresses, strains, joint movements and settlements

Measurement of Displacement Measurement of displacement of

points either between two monoliths, or between foundation and body of the dam or the displacement of any joint of the dam with respect to the surrounding area will immediately reveal any distress conditions developing in the dam.

Internal Joint Movement Concrete and masonry dams are

generally built in blocks separated by transverse jointsrelative movement between two blocks.

movemnets - due to differential foundation behaviour

Surface Joint Movement Measurement of joint movement at the

surface of the locations accessible from galleries is made by detachable gauges

To assess the amount of joint opening of two blocks of the dam.

Used for observation of opening or of closing of surface cracks at any location.

Foundation displacement Measurement of vertical or horizontal

displacement of foundation provides information for taking preventive measures for inclination, distortion etc. of structures.

used for studying the elastic and inelastic properties of dam and foundation.

Involves vertical and horizontal displacement of part of foundation with respect to dam.

Displacement of One Part of the Dam Relative to other Parts of the Dam

It is a direct indication of structural behaviour of the dam.

These observations are made by regular and inverted plumblines.

The plumbline data together with other supporting data may be used to study the elastic behaviour of the dam.

Displacement of Dam with Reference to Surrounding Area

It is a direct indication of structural behaviour of the dam.

the deflections should be consistent with changes in reservoir water surface level and in temperature and should not change appreciably from year to year.

can be done by theodolite from fixed bases using either line-of-sight over the top of the dam or by turning angles to targets on the downstream face and at the crest

Measurement of Tilt

Tilt is measurement of rotation in vertical plane

It is measured with the help of tiltmeter system consisting of tiltmeter sensor, tilt plates and indicator.

Tilt plates are bonded to the surface of mass of structure under observation. The sensor is oriented on three pegs of tilt plate and senses change in tilt of tilt plate. The portable indicator gives the degree of rotation.

Measurement of Stress Direct measurement of stress developed inside

the mass of concrete or masonry helps in watching the structural behaviour of dams and their foundations.

adverse change in stress will indicate distress conditions - remedial measures can be taken.

Helps in studying the assumed stresses and actual stresses in dams -used in improving upon the design procedure

Measurement of Strain Factors like temperature, chemical

action, moisture change and stress result in volume changes which case strain in the structure

It is a common practice to measure the strain and to calculate from it the developed stress.

Measurement of Pore Pressure large concrete and masonry dams are

provided with internal formed drains located near the upstream face, a record of pore pressure development and its variations would indicate the effectiveness and adequacy of these drains

Increase in the pore pressures - choking up of the internal drains

any unusual reduction from the normal would indicate possibility of formation of cracks or establishment of flow channels in the body of the dam

Seismicity of the Area and Dynamic Characteristic of the Structure to know about the seismicity of the

region before taking up construction To know the change in the seismicity

pattern, due to creation of large reservoir

Measurement of Water Level on Upstream and Downstream Side This measurement is useful for

calculating the water pressure on the upstream face and downstream face of the dam.

A typical set of piezometer installations for an embankment dam is shown in Figure below

Causes of failure of earth dams

Various modes of failures of earth dams may be grouped under three categories:

1. Hydraulic failures 2. Seepage failures, and 3. Structural failures

Hydraulic failures type of failure occurs by the surface erosion of the dam by

water. 1. Overtopping of the dam which might have been caused

by a flood that exceeded the design flood for the spillway. Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the spillway. Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided. Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the dams downstream face, either complete or partial failure is inevitable (Figure 22).

2. Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23).

3. Erosion of downstream slope by rain wash. Though the downstream face of an embankment is not affected by the reservoir water, it may get eroded by heavy rain water flowing down the face, which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24).

4. Erosion of downstream toe of dam by tail water. This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base. (Figure 25).

2.Seepage failures

1. Piping through dam and its foundation: This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)

2. Conduit leakage: This is caused due to seepage taking place by the surface of a conduit enclosed within an embankment dam (Figure 27). The seepage of water may be from the reservoir to the downstream or due to the water leaking out of the conduit through cracks that might have developed due to unequal settlement of dam or by overloading from the dam. Further, the cracking of a conduit may also be caused when the soil mass lying below it settles and the conduit is not sufficiently strong to support the soil mass lying above.

3. Sloughing of downstream face: This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam. This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)

Structural failures 1. Sliding due to weak foundation: Due to the presence of faults

and seams of weathered rocks, shales, soft clay strata, the foundation may not be able to withstand the pressure of the embankment dam. The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe.

2. Sliding of upstream face due to sudden drawdown: An embankment dam, under filled up condition develops pore water pressure within the body of the dam. If the reservoir water is suddenly depleted, say due to the need of emptying the reservoir in expectation of an incoming flood, then the pore pressure cannot get released, which causes the upstream face of the dam to slump (Figure 30).

3. Sliding of the downstream face due to slopes being too steep: Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and / or too steep in relation to the shear strength of the shoulder material. This causes a sliding failure of the downstream face of the dam (Figure 31).

4. Flow slides due to liquefaction: Triggered by a shock or a movement, as during an earthquake, some portion of the dam or foundation may destabilize due to the phenomena called liquefaction. Here, even cohesionless soil cannot drain quickly enough as the movements are so sudden that the rate of extra loading on the soil becomes greater than the rate of drainage of the seepage water out of the soil. This causes excess pore water pressure to develop, where both the effective stress and the strength decrease. Under circumstances when the effective stress

5. Damage caused by burrowing animals or water soluble materials: some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly, carrying fine material along with. This phenomena consequently leads to piping failure within the body of the dam, finally leading to a complete collapse. Similarly, water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure.

6. Embankment and foundation settlement: Excess settlement of the embankment and/or the foundation causes loss of free board (Figure 32). The settlement may be more in the deeper portion of the valley, where the embankment height is more.

Design of earth dams

1. Safety against hydraulic failures due to overtopping , rain cuts, wave action or tail water.

2. Safety against seepage failures due to internal erosion and development of pore pressure due to insufficient drainage.

3. Safety against structural instability. 4. Special design requirements

Selection of the type of dam Slide 2Slide 3Slide 4Appurtenant structure and ancillary works Slide 6Slide 7Slide 8Slide 9Instrumentation in Dams Slide 11Slide 12Optional MeasurementsSlide 14Measurement of Seepage Grout curtainMeasurement of Temperature during Construction Measurement of Temperature of the Dam interior Measurement of Temperature of Reservoir Water and Air Measurement of Displacement Slide 21Surface Joint Movement Foundation displacement Displacement of One Part of the Dam Relative to other Parts of the Dam Displacement of Dam with Reference to Surrounding Area Measurement of Tilt Measurement of Stress Measurement of Strain Measurement of Pore Pressure Seismicity of the Area and Dynamic Characteristic of the Structure Measurement of Water Level on Upstream and Downstream Side A typical set of piezometer installations for an embankment dam is shown in Figure below Causes of failure of earth dams Hydraulic failures Slide 35Slide 36Slide 37Slide 38Slide 392.Seepage failures Slide 41Slide 42Slide 43Slide 44Structural failures Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52 Design of earth dams