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Basic Design Provision For Existing Dams

C.S.MathurDam Design Engineer,EGIS(Ex. Chief Engineer, Central Water Commission)

Manoj KumarDy. Director,DSR dtc.Central Water Commission

Ankit Kumar,Design Engineer,EGIS

Aspects Looked into While Reviewing Safety of an Existing Dam

1. Design flood2. Flood routing studies for arriving at revised MWL3. Checking the adequacy of free board available above

the revised MWL4. Examination of need or otherwise for an additional

spillway5. Examination of the causes of distress viz. Excessive seepage through dam Cracking in dam Damages to concrete in Spillway/Energy Dissipation

Arrangements Indications of any boiling/piping etc.

Aspects Looked into While Reviewing Safety of an Existing Dam

6. Checking the stability of dam and spillway due to :- Revised MWL (due to revision in design flood) Changes in seismic parameters/zone

7. Issues related to Hydro-Mechanical equipment8. Issues related to Outlet works.

Design Provisions for Dams

In general our BIS codes/standards provide guidelines for designing new dams .

The same standards are used for existing dams also. However while applying our standards for safety

evaluation of existing dams, some changes may be necessary.

In this paper the changes proposed have been discussed in respect of some basic aspects.

1. Freeboard Aspects

Relevant BIS codesIS 10635 – Freeboard requirements for

embankment dams IS 6512– Criteria for design of solid gravity

dams.As per the above codes Notwithstandingtherequirementsregardingthewaveheightandwindsetup,1m high solid parapet wall is required to be provided above the top of dam.

But it is not to be considered as part of freeboard

1. Freeboard Aspects

However for existing dams the following is proposed:-I. For an existing embankment dam, the u/s solid

parapet wall may be taken as part of the free board, in case the parapet wall is otherwise structurally safe.

II. The maximum water level for the revised flood should however be lower than the top of impervious core of the embankment dam.

III. For existing gravity dams the revised maximum water level could be allowed to reach upto the dam top, where 1 m high solid parapet wall is provided.

2. Issue of Consideration of 10% Gates as Inoperative

IS 11223– Guidelinesforfixationofspillwaycapacity

Forgatedspillways10%ofgateswithaminimumofonegatebeinginoperativemaybeconsideredasanemergencycondition(likeearthquake)andareducedfreeboardmaybeacceptedunderthiscondition.

‐ Thisisfollowedfornewdams.

2.Issue of Consideration of 10% Gates as Inoperative

ProposalsforExistingdams:‐

In general , it is felt there may not be any need to check an existing dam for this condition.

However it needs to be ensured that all gates are in working condition and that O&M guidelines are being followed meticulously.

3.Stability Analysis for an Existing Embankment Dam – Method to be used for analysis

IS 7894– CodeofpracticeforstabilityanalysisofEarthdams

MethodsofAnalysis:‐ Swedishslipcircle(Fellenius)Method SlidingwedgeMethod

FelleniusMethodconsidersonlythemomentequilibriumandnottheforceequilibrium

Itprovidesafactorofsafetyonlowerside(resultsinaconservativedesign)

3. Stability Analysis for an Existing Embankment Dam – Method to be used for analysis

There are various other methods of Stability analysis like :o Bishop methodo Janbu methodo Spencer methodo Morgenstern - Price method etc.

3. Stability Analysis for an Existing Embankment Dam – Method to be used for analysis

It may be preferable to adopt a method which :- Satisfies both moment equilibrium and force

equilibrium Takes into account the inter slice forces (normal

and shear ) and results in a closed polygon Methods recommended for stability analysis of existing

dams area. Morgenstern - Price Methodb. Spencer Method

Can be analyzed using software's like SLOPE/W of GEO-SLOPE International.

4. Stability Analysis for an Existing Concrete/Masonry Gravity Dam with Reference to Permitting Cracking in the Dam

As per IS 6512–Criteriafordesignofconcrete/masonrysolidgravitydams,crackingisnotpermittedindams.

Notensilestressispermittedinnormalloadingcondition(ReservoiratFRLwithdrainsoperative)

Howeversomenominaltensilestressesarepermittedinotherconditionsasunder

Load Combination Permissible tensile stressConcrete Masonry

C (Reservoir at MWL with drainsoperative)

0.01fc 0.005 fc

E (Reservoir at FRL with drainsoperative and Earthquake)

0.02 fc 0.01 fc

F ((Reservoir at MWL with drainsinoperative)

0.02 fc 0.01 fc

G (Reservoir at FRL with drainsinoperative and Earthquake)

0.04 fc 0.02 fc

fc = Compressive strength of concrete/mortar for masonry


For Existing dams it is proposed to allow cracking in load combination F (Reservoir at MWL with drain inoperative) and in load combination G ( Reservoir at FRL with earthquake and drains inoperative) when the tensile stress exceeds the permissible tensile stress prescribed in IS 6512

TheapproachgivenbyUSBRinitspublicationDesignofSmallDamscouldbeadopted.


Assumptionsinvolved(forstaticcondition)

Vertical stress distribution along a horizontal plane without uplift is linear between the u/s & d/s faces of dam.

The uplift pressure in the entire cracked width portion is equal to the reservoir pressure corresponding to the reservoir level. Thereafter the uplift varies linearly from crack tip to tail water pressure at d/s face.

Crack extends up to the point of no stress. The uncracked length is entirely in comparison


Stability Criteria

o Net vertical compressive stress should not exceed the allowable compressive stress

o Sliding stability to be checked using the uncracked base width of the dam.


Additional assumption involved in a pseudo static (Earthquake) condition

In the cracked length , the uplift pressures are assumed to be zero. ( During Earthquake event the opening /closing of the crack is so rapid that it does not allow the reservoir water, and associated pressure, to penetrate )


If the sliding and stress levels are satisfactory for the cracked section during earthquake event, post earthquake static condition should also be checked.

Post earthquake analysis should include full uplift pressure throughout the crack.

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