The document discusses three main failure modes for gravity dams: sliding failures, overturning failures, and structural failures. Sliding failures occur when horizontal forces exceed the frictional resistance between the dam and foundation, causing the dam to slide. Overturning failures happen when horizontal and vertical forces create overturning moments that exceed the dam's weight, potentially causing the dam to rotate or overturn. Structural failures result from tensile or compressive stresses exceeding the strength of the dam's materials, which can cause cracking and eventual failure. Proper design and construction aim to balance forces and stresses to maintain stability and prevent these failure modes.

1.  Three basic problems are generally found in previous
foundations:
(1) the amount of under seepage,
(2) the pressures exerted by the seepage, and
(3) the potential for internal erosion.
 Under seepage
(1) To estimate the volume of under seepage that may be expected, it
is necessary to determine the coefficient of permeability of the
previous foundation.
Seepage Reduction and Control
Various methods of seepage reduction and/or control can be used,
Cutoff trenches back filled with compacted soil, cement-bentonite
cutoff walls, soil-bentonite cutoff walls, soil-cement cutoff walls,
concrete cutoff walls, upstream impervious blankets, or combinations
of these methods have been used to reduce seepage flow and to help
control related water pressures.
 Filters and Drains
Typical drainage features include drainage blankets, toe drains, and
drainage trenches.
 Relief Wells Pressure
relief wells are devices used to relieve water pressures deeper in
foundations by intercepting seepage at a level below the ground
surface that cannot economically be cut off or reached by drainage
trenches. Relief wells are generally used to prevent uplift or
blowout of impervious zones
 Cutoffs
When used primarily for seepage reduction, the cutoff trench is
preferably located beneath or upstream from the centerline of the
crest of the dam.
GRAVITY DAM FAILURE MODES
I. Gravity dam failures can be grouped into three general categories:
sliding failures.
II. overturning failures.
III. structural failures.
 Sliding Gravity Dam Failure
When the dam slides over its foundation or one part of the dam slides
over a part of itself, it is called a sliding failure. It occurs when
the net horizontal forces acting on the gravity dam exceed the
frictional resistance produced between the body of the dam and the
foundation. The resistance may be due to friction alone, or it may be

2. due to a combination of friction and shear strength, depending on how
the dam was constructed. The ability of the dam to resist a sliding
failure is commonly calculated by dividing the resistance to movement
by the horizontal forces acting on the dam. Its value varies by
loading condition, but generally, the result should vary between 1.3
and 1.5.
Overturning Gravity Dam Failure
Horizontal and vertical forces such as water pressure, wave pressure,
silt pressure, ice pressure, and uplift pressure can act against a
gravity dam, creating overturning force or rotation of the structure.
The structure resists this rotation by having adequate weight. If the
sum of all the forces acting on a dam acts, either through or outside
of the downstream toe of the dam, the dam will rotate and overturn.
While a structure can rotate about the toe, it is more likely that
the overturning forces would increase stresses to such a level at the
toe of the dam that structural failure would occur.
 Structural Failure of Gravity Dams
Structural failure occurs when the tensile or compressive stresses
in the dam exceed the strength of the materials that compose the dam.
Masonry and concrete are weak in tension, so masonry and concrete
gravity dams are designed to minimize tension in the structure. Steel
bars, known as re-bar, are incorporated within the concrete during
construction to resist anticipated tensile forces. As the structure
ages, re-bar can corrode or deteriorate, and tensile forces can
develop where re-bar is not present or weakened. This, coupled with
overturning forces, can redistribute stresses in the dam that could
cause cracking. In severe cases of the redistribution of stresses,
the dam can lose contact with the bottom foundation, increasing
uplift forces on the dam. This can significantly increase loading at
the toe of the dam leading to a compression failure. If the
compressive stresses exceed the allowable stresses of the masonry or
concrete, the materials may be crushed, leading to dam failure
A) main purposes of studying uplift pressure?
 To made proper design of dam
 To balance the the vertical force acting on the dam and protect
from failure.
 To reduce sliding,structural failure of dam.
 To maintain stability of the dam.
B) remedial measures to minimize this uplift pressure.
 Toe in the foundation in to the ground.
 Increase self weight of structure.
 Ground anchor.
 Pilling method.

3. DAMS Construction
material
Foundation
requirement
volume of structure Topographical requirement
Earth dam Soile and
rocks
Earth
foundation
Wider and
shorter.it is
smolle dam.
Suitable for almost
any foundation.
Concrete
dam
Concrete,stone
brick.
Concrete
foundat
Shalow and
tall
dam.larger
than earth
dam
Suitable only when
the foundation no
fissures,cracks,join
ts.