dam maintenace

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DAM MAINTENANCE


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A good maintenance program will protect a damagainst deterioration and prolong its life. Apoorly maintained dam will deteriorate and canfail. Nearly all the components of a dam and the

materials used for dam construction aresusceptible to damaging deterioration if notproperly maintained. The cost of a propermaintenance program is small compared to thecost of major repairs or the loss of life and

property.

INTRODUCTION


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1.Critical Conditions

2.Periodic Maintenance

3. Continued maintenance

4.Embankment Maintenance

5.Outlet Maintenance

TYPES OF MAINTENANCE


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Erosion, slope failure or other conditions which areendangering the integrity of the dam.

Piping or internal erosion as evidenced by

increasingly cloudy seepage or other symptoms. Spillway blockage or restriction.

Excessive or rapidly increasing seepage appearinganywhere near the dam site.

1.CRITICAL CONDITIONS


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Remove bushes and trees from the embankment and

abutments.

Repair erosion gullies. Repair defective gates or valves.

Repair deteriorated concrete or metal components.

Maintain riprap or other erosion protection.

2.PERIODIC MAINTENANCE


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Test, clean and lubricate gates and valves.

Inspect and maintain instrumentation and gaugingequipment.

Remove debris from dam area and emergencyspillway approach and exit channel.

Remove debris from embankment face and from areasaround the intake structures.

Clean and remove debris from seepage weirs and

small drainage ditches.

3. CONTINUED MAINTENANCE


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1. Fill erosion gullies with properly compacted cohesive soilmaterial. Seed or riprap repaired area to stabilize from futureerosion.

2. Fill rodent burrows with slurry of soil, cement and water.

Remove the rodents.3. Maintain grass cover by spraying weeds, fertilizing andwatering as needed.

4. Remove brush, bushes and trees from embankment andfrom within 25 feet of the groins and 50 feet of the toe of

embankment. Remove tree roots, fill with compacted soil andre-seed area.

4.EMBANKMENT MAINTENANCE


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5. Add or repair riprap where displacement or other damageoccurs.

6. Maintain grading of the embankment crests to prevent

potholes, rutting or other potential for standing water toaccumulate.

7. Maintain fences to provide site security and to excludelivestock from the embankments. Repair and revegetatedamaged embankment surfaces.

8. Perform regular inspections of the embankments andabutments to identif otential maintenance items.


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1. Test gates and valves semi-annually.

2. Lubricate gates and valves annually or as recommendedby the manufacturer.

3. Repair defective gates and valves to ensure smoothoperation and prevent leakage.

4. Repair deteriorated concrete or metalwork.

5. Remove debris from the outlet channels annually, inspectand repair erosion protection.

6. Repair and verify calibration of water measurementequipment.

5.Outlet Maintenance


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Leveling of Embankment Tops & Slopes:

Existing embankments have to be repaired or reconditioned inadvance,

so that these can safely withstand pressure of the likely

highest waterlevel against them, in the ensuing monsoon. The mostimportant point,

therefore, is to bring the embankment to the proper grade or

levels andsection. To achieve this the re uired free-board for desi ned

GENERAL REPAIR &

MAINTENANCE


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Item of Earthen and Annual Repairs

All hollows and depressions in the embankment section,where ever existing must be made-up with rammed earth after

clearing the site of all loose and vegetable material. Where the

top of embankment is dusty or sandy, 30 cm of soil containing

10 to 15 percent of clay should be put on and well rammed orrolled.


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Opening up and Refilling Leaks

A register of leaks should be maintained showing the exactlocation and

the action taken during the monsoon period (that is, whether

it has been

fully opened out and refilled with good soil duly watered andrammed, or

just plugged at its upstream face). During the monsoon

period deep nicks

on the slope and sides are also left at the site of each leak as


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Repairs to Masonry Works

It is also important to inspect very carefully masonry worksso that there

is no danger of seepage of water along the plane of contact

between the

earth and masonry. The earth adjacent to the masonry workshould be

brought to designed sections laying in very thin layers and

then

consolidating by reamers.


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Plantation

The plantation of trees on embankments are not allowablebecause their roots tend to loosen the structure of theembankment when shaken by wind storms and these encouragecracks and the development of leaks, which are difficult to close.When trees are removed, it is necessary to remove the rootsthoroughly because, if the root is left in, when the organic matterdies and decays and crumbles, it leaves dangerous hollowswhich cause settlement and lead to further trouble. On the otherhand short grass growing on the embankments (tufts of sod) isgood protection against erosion and wave wash. The side-slopesof all embankments and land beyond the toes up to 6 m on the

countryside and 3 m on the waterside should be kept clear of alltrees and other rowths exce t short rass and lai.


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Filling Up of Borrow Pits on Land Side

Any borrow pits on the land side made in emergency duringflood season near the rear toe may cause excessive seepage

and form a surface of saturation along which the

superincumbent material may slide or slip,

endangering the stability the embankment. These should beproperly

filled up.


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POSSIBLE DAM FAILURES


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TYPES OF FAILURE & THEIR

CONTROL

1.Seepage

2. Cracking

3. Erosion

4.Instability


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SEEPAGE : Seepage all embankment dams pass water through theembankment materials. the passage of water through the embankment

materials is called Seepage .


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Controlling methods

Most modern embankment dams have internal drains tocontrol seepage. Internal drains are designed to intercept

seepage and to discharge it safely. Many different types ofdrains can be used to control seepage. Three common typesof drains are the Toe Drain, the Horizontal Blanket Drain, andthe Chimney Drain with Blanket.


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Relief wells may be installed in the downstream toe area to reduce

potentially

damaging uplift pressure from foundation seepage through pervious

materials

that were not cut off. Uplift pressure from excessive seepage can

cause

internal erosion of foundation material or embankment instability.


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CRACKING

Cracks are splits that appear in the crest or slopes of thedam. Cracking in an embankment dam falls into the

following three major categories.

1. Desiccation Cracking

2. Transverse Cracking3. Longitudinal Cracking


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Desiccation cracking

Desiccation cracking is caused by the drying out and

shrinking of certain types of embankment soils.

Remedial action: If the depth of the cracking extends

below the reservoir level, the place shall be excavated

in the form of trench up to the end of the crack andshall be filled with well compacted soil, of course, not

with a plastic soil.


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Transverse Cracking

The indication of a differential settlement with in the embankmentor

underlying foundation appears in the form of TRANSVERSE

CRACKING in a direction perpendicular to the length of the dam.

Remedial action:

The place shall be excavated in the form of a trench up to the endof the crack and filled with well compacted soil, if the cracks are upto 1.50 m and above the water level of the reservoir. If the cracksare above 1.50 m and extends below the water level of the reservoira trench of 1.50 to 2.0 m depth along the crack shall be cut. Theremaining portion of the crack, i.e. from the excavated trench is tobe filled with well compacted clayey soil, of course not plastic innature.


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Longitudinal Cracking

Uneven settlement between adjacent embankment zone of

differing

compressibility and the beginning of an unstable slope appears

as a

longitudinal cracking in a direction parallel to the length of the

dam

Remedial action:The material up to the depth of the crack shall be removed whenthe reservoir level is at low, the material shall be spread in thinlayers and rolled after deciding on the designed density, thethickness of the layers, the amount of rolling and the weight of

rollers.


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EROSION

Erosion of slopes, abutments, and spillway discharge channels isone of the most common maintenance problems with

embankment structures. Erosion is a natural process, and its

continuous forces will eventually wear down almost any surface

or structure. Erosion can be caused or aggravated by improper

drainage, settlement, pedestrian traffic, inadequate vegetation,

animal burrows, or other factors. The cause of the erosion will

have a direct bearing on the type of repair needed. Erosion in and

around dams can lead to failure of a dam if left untreated.

Periodic and timely maintenance is essential in preventing

continuous deterioration and possible failure.


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Remedial action

Prompt repair of vegetated areas that develop erosion is required to

prevent more serious damage to the embankment. Rills and gullies

should be filled with suitable soil (the upper 4 inches should be

topsoil, if available), lightly compacted, and then seeded. Erosion from

runoff, either rills, gullies or bare areas? Improve grass cover; reshape

embankment to improve drainage pattern. Runoff from rainfall

concentrates in these gutter areas and can reach erosive velocities

because of the steep slopes. Berms on the downstream face thatcollect surface water and empty into these gutters add to the runoff

volume. Sod gutters may not adequately prevent erosion in these

areas. Small animals often construct burrows underneath concrete

gutters, possibly because burrowing is easier due to existing

undermining. A well graded mixture of rock with stones 9 to 12 inchesin diameter or lar er laced on a sand filter enerall rovides the best


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A serious erosion problem which can develop on theupstream slope is "beaching. Waves caused by high winds or

high-speed power boats can erode the exposed face of theembankment. Waves repeatedly strike the surface just abovethe pool elevation, rush up the slope, then tumble downwardinto the pool. This action erodes material from the face of theembankment and displaces it farther down the slope, creating

a "beach". Erosion of unprotected soil can be rapid andduring a severe storm could lead to complete failure of a dam.The upstream face of a dam is commonly protected againstwave erosion and the resultant beaching by placement of alayer of rock riprap over a layer of filter material. In some

cases other materials such as steel, bituminous or concretefacing, bricks or concrete blocks are used.


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INSTABILITY

SLIDE, SLUMP, OR SLIP

Foundation movement or a toosteep

slope can cause earth or rocks

to move along a slip plane

which can lead to a slump of the

embankment.Remedial action:

If there is a slip, the reservoir

shall

immediately be lowered or

drained to safe limit to preventth ibl

dam maintenace

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