Basic Design Provision For
Existing Dams
C.S.Mathur Manoj Kumar Ankit Kumar,
Dam Design Engineer, Dy. Director, Design Engineer,
EGIS DSR dtc. EGIS
(Ex. Chief Engineer, Central Water
Central Water Commission
Commission)
Aspects Looked into While Reviewing Safety of
an Existing Dam
1. Design flood
2. Flood routing studies for arriving at revised MWL
3. Checking the adequacy of free board available above
the revised MWL
4. Examination of need or otherwise for an additional
spillway
5. 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 equipment
8. 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 codes
IS 10635 – Freeboard requirements for
embankment dams
IS 6512 – Criteria for design of solid gravity
dams.
As per the above codes
Not withstanding the requirements regarding the wave
height and wind set up , 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 – Guidelines for fixation of spillway capacity
For gated spillways 10% of gates with a minimum of one
gate being inoperative may be considered as an
emergency condition (like earthquake) and a reduced
freeboard may be accepted under this condition.
‐ This is followed for new dams.
2. Issue of Consideration of 10% Gates as
Inoperative
Proposals for Existing dams:‐
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 – Code of practice for stability analysis of
Earth dams
Methods of Analysis :‐
Swedish slip circle (Fellenius) Method
Sliding wedge Method
Fellenius Method considers only the moment
equilibrium and not the force equilibrium
It provides a factor of safety on lower side (results in a
conservative design)
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 method
o Janbu method
o Spencer method
o 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 are
a. Morgenstern - Price Method
b. 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 –Criteria for design of concrete/ masonry solid gravity
dams, cracking is not permitted in dams.
No tensile stress is permitted in normal loading condition (Reservoir at
FRL with drains operative )
However some nominal tensile stresses are permitted in other conditions
as under
Permissible tensile stress
Load Combination
Concrete Masonry
C (Reservoir at MWL with drains 0.01fc 0.005 fc
operative)
E (Reservoir at FRL with drains 0.02 fc 0.01 fc
operative and Earthquake)
F ((Reservoir at MWL with drains 0.02 fc 0.01 fc
inoperative)
G (Reservoir at FRL with drains 0.04 fc 0.02 fc
inoperative and Earthquake)
fc = Compressive strength of concrete/mortar for masonry
4. Stability Analysis for an Existing Concrete/Masonry Gravity
Dam with Reference to Permitting Cracking in the Dam
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
The approach given by USBR in its publication Design
of Small Dams could be adopted.
4. Stability Analysis for an Existing Concrete/Masonry Gravity Dam
with Reference to Permitting Cracking in the Dam
Assumptions involved (for static condition)
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
4. Stability Analysis for an Existing Concrete/Masonry Gravity
Dam with Reference to Permitting Cracking in the Dam
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.
4. Stability Analysis for an Existing Concrete/Masonry Gravity
Dam with Reference to Permitting Cracking in 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 )
4. Stability Analysis for an Existing Concrete/Masonry Gravity
Dam with Reference to Permitting Cracking in the Dam
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.
