5 dams
TRANSCRIPT
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ZM UTP DAMS 1
HYDRAULIC STRUCTURES
DAMSby:
Dr. Zahiraniza Mustaffa
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General Content:
• Introduction
– Introduction to Dams
– Dams Classification
• Material classifications
• Concrete Gravity Dam
– Forces (Loads) on the Dam
– Load Combination
– Stability Analysis
• Ancillary Structures
– Spillways etc. (will be covered later)
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Introduction
– What is a dam?
• A dam is a barrier structure placed across a watercourse to store water.
– Why do we need dams?
• To fulfill many functions like water supply (domestic, irrigation & industrial), flood mitigation, hydropower development and irrigation.
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Dam
Energy Dissipator
Structures
Hydraulic jump
Reservoir
Q
Spillway
Typical Layout of a Dam
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Dams Classification
• Dams can be classified in many ways: • Size:
Dams vary in size from a few meters in height to massive structures of over 100 m in height.
– Large Dam (H >15 m or Reservoir Volume > 3 x 106 m3)
– Small Dam
• Purpose:
- Water Supply (domestic, irrigation & industrial), Flood Mitigation, Hydropower and Irrigation Dams.
• Material:
- Earthfill, Rockfill, Gravity (Concrete), Arch, Buttress etc
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Kenyir Dam, Terengganu
(10-11 April, 2004)
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Kenyir Dam, Terengganu
(10-11 April, 2004)
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Dams Classification – Material
• Earthfill (Embankment) Dam
• Rockfill Dam
• Concrete Gravity Dam
• Buttress Dam
• Arch Dam
• Roller Compacted Concrete (RCC) Dam
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Fine material
Coarse material
Filter material
EARTHFILL DAM
• An embankment that uses earth soil (natural materials excavated nearby the area) to provide stability.
• The materials are compacted.
• Impermeable materials at the centre – to prevent seepage
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ROCKFILL DAM
Impervious face
Rock
• An embankment that uses variable sizes of rocks to provide stability.
• A thin membrane (impervious) on its upstream face for water tightness.
• More stable than an earthfill dam. Cheaper than concrete dams.
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CONCRETE GRAVITY DAM
Concrete
• A dam that applies its weight (gravitational forces) for stability.
• Normally in triangular shape (side view).
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ARCH DAM
Concrete
• Narrow in size, in which the abutments are of massive rock of the canyon.
• Is designed to transfer the imposed loads to the adjacent rock walls on either side of the canyon.
• Hard to construct. Cheaper than concrete gravity dams.
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BUTTRESS DAM
Concrete
Buttress
• A hollow gravity dam.
• Buttresses of reinforced concrete rest on the
rock foundation and support a watertight
sloping face of the dam.
• Cheaper than concrete gravity dams.
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Concrete Gravity Dam
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• Concrete gravity dams are designed so that the weight of the dam itself (gravity force) is sufficient to resist overturning by the applied forces.
• The forces that must be considered in the design of a dam are:
1. Weight of the dam
2. Hydrostatic forces (u/s and d/s of the dam)
3. Hydrostatic uplift force
4. Earthquake force
5. Silt force
6. Ancillary forces (roadway etc)
7. Others (ice, waves, wind forces etc)
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ICE JAMS ALONG A RIVER
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ICE JAMS NEAR A BRIDGE
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ICE JAMS
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ZM UTP DAMS 22FU
WFp1
Fp2
Ww
1
2
Forces Acting on a Dam
HW
TW
HW = headwater
TW = tailwater
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RFy
Fx
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1. Weight of Dam (W)
• Necessary to include:
– Weight of the dam, W• The weight of dam per unit (1 m) length,
– Weight of other ancillary structures like gates, bridges, roadways etc.
• The resultant weight acts at the centroid of the dam
i.e. at 1/3 of the dam width, b (from the heel).
Forces on Dam
where, Ac is the area of the dam (side view) and, c is the
specific weight of concrete (24 kN/m3 or 2400 kg/ m3).
(kN/m) cc AW
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b/3
b
W
Heel
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2. Hydrostatic Forces (Fp)
• Sometimes referred to as external hydrostatic
pressure.
• Hydrostatic forces are forces acting at the
upstream and downstream faces of the dam.
• The hydrostatic force, Fp per unit (1 m) length is
given by:
2
2hF w
p
where, w is the specific weight of
water (9.81 kN/m3) and h
is the vertical depth of
water.
(kN/m)
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b’/3
Fp1
Fp2
Ww
1
2
h1 /3
h2 /3
b’
h2
h1
Toe
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• For a vertical surface, Fp is acting
horizontally at 1/3 of the water depth,
measured from the base of the dam.
• For an inclined surface, there are 2 forces
acting on the surface, namely Fp (acts
horizontally) and weight of water,Ww (acts
vertically).
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• Ww is described as follows:
• Its magnitude is equal to the weight of
volume of water per unit (1 m) length
directly above the inclined face of the
dam.
• It is acting through the centroid of the
volume of water, i.e. at 1/3 of b’,
measured from the toe.
www AW
where, Aw is the area of the
water (side view)(kN/m)
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3. Hydrostatic Uplift Force (FU)
• Sometimes is referred to as internal
hydrostatic pressure.
• Hydrostatic uplift force is a force produced by
water (under pressure) in the pores of the
concrete dam and foundation.
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After the reservoir is filled, water will tend to
move/seep from u/s to d/s/.
It will seep into the pores of the concrete
(despite the low permeability of the concrete) and
its foundation.
When the seepage water is stable (resulting
in a saturated condition), a pressure head
gradient will develop along the base of the
dam.
This will give extra pressure force to the
dam!
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For a dam without tailwater (TW) effect:
• FU drops linearly from u/s to d/s; resulting
in a triangular pressure distribution
diagram, decreasing from wh1 to 0.
For a dam with tailwater (TW) effect:
• FU drops linearly from u/s to d/s; resulting
in a trapezoidal pressure distribution
diagram, decreasing from wh1 to wh2 .
How does a pressure head gradient look like?
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FU
b/3
b
h1
w h1
A dam without tailwater (TW)
at downstream section
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FU
b
h1
h2
w h1
w h2
A dam with tailwater (TW)
at downstream section
TW
x
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• The uplift force, FU per unit (1 m) length is
determined by:
• FU is measured at the centroid of the uplift
pressure distribution diagram, measured from
the toe of the dam.
uwu AF
where, w is the specific weight of water (9.81
kN/m3), Au is the area of uplift pressure
distribution diagram.
(kN/m)
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• Is FU good for the stability of the dam?
Why?
• How can we control FU ?
– Constructing cut-offs:
• Grout curtain
• Drainage curtain
– Creating a more impervious zone at the
foundation
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• Grout Curtain
– A line constructed at
the foundation to
block water
seepage from u/s to
d/s of the dam.
– A hole of 4-6cm are
drilled at the heel.
Cement grout is
pumped into the
holes (to seal the
cracks in the rocks).
• Drainage Curtain
– A row of holes
drilled just d/s from
the grout curtain.
– To intercept any
seepage which may
escape past the
grout curtain. The
seepage is collected
in the drain and
flows away by
gravity or pump.
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Grout curtain
Holes
Grout Curtain
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Holes
Drain curtain
Drain Curtain
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Impervious Zone
Impervious
zone
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4. Earthquake Force (Fe)
• When an earthquake occurs, the earth
shakes (vibrates) at an acceleration, a.
• The dam will be accelerated due to the
earthquake with an initial force, Fe but at
opposite direction to a.
• Fe is acting at the centroid of the dam.
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• Fe is given by,
Fe = Ma
• a can be in the range of 0.05g to 0.5g, with
g stands for acceleration due to gravity.
where, M is the mass of the dam and a is the
earthquake acceleration.
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Load Combination
• Not all loads mentioned earlier are considered when designing a dam. Why?
• The load selections are based on below conditions:
– Normal Load Combination (NLC)
– Unusual Load Combination (ULC)
– Extreme Load Combination (ELC)
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Load Combinations
Load Source Qualifications NLC ULC ELC
Primary
Secondary (if applicable)
Headwater
Tailwater
Self-weight
Uplift
Silt
Ice
Exceptional
Earthquake
At DFLAt NFL
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Stability Analysis
• The stability of a dam can be checked by
using the Simple Gravity Method.
• The stability analysis checks:
1. Safety against stresses
2. Safety against sliding
3. Safety against overturning
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Safety Against Stresses
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Let’s talk about stress…
• Stress, .
• Unit of stress = N/mm2
• Two common stresses:
– Tensile stress leads to tension
– Compressive stress leads to compression
Stress =
Pressure?
compression
tension
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Toe
Heel
Tensile stress Compressive stress
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CrushingCracking
Heel Toe
Why are stresses not desired in a dam?
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• There are many stresses acting on a
dam but the focus will be given on
vertical normal stresses, acting on a
horizontal plane.
• Uplift load, Fu is excluded in the stress
determination.
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d/su/s
Stress Diagram at
Dam Foundation
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• At the base of a dam, the normal stresses can
be either tensile or compressive.
• BUT, it is not desired to have any tensile stress
at the heel, so only the compressive stresses
are allowed at BOTH heel and toe, given by:
b
e
b
Fy
heel
61
'
b
e
b
Fy
toe
61
'
concrete
foundation
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where,
Fy’ is the resultant vertical forces above
the plane considered (exclusive uplift),
b is the base width of the dam and e
is eccentricity of the resultant load R (the
horizontal distance from the centre of
the base to the point where R acts) .
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• e is obtained from the equation,
• e MUST be,
if not, u/s will be negative, i.e tensile stress, which
leads to tension at the heel. This will cause
cracking. Not good!
• A good dam design is when the dam is free from
tensile stress at the heel. How to strengthen the
heel from developing tensile stresses?
'yF
Me
6
be
where, is the summation of
moments at toe and is the
summation of all vertical forces
(exclusive uplift).
xM'yF
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b/2b
Lxe
+M
Fx
Fy
R
Fx
Fy
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• Allowable concrete stress, con(allw) :
2000 kPa < con(allw) < 4000 kPa
• Allowable foundation stress, found(allw) :
Foundation Materials Allowable stress, found(all)
(kPa)
Granite
Limestone
Sandstone
Gravel
Sand
Stiff Clay
Soft Clay
4000 – 6000
3000 – 4000
2500 – 3500
300 – 600
200 – 400
200 – 400
50 – 100
Note: Pa = N/m2
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Safety Against Sliding
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• Sliding?
• How would you hold yourself from
sliding if somebody pushed you?
• A dam can resist sliding if the ratio of the
horizontal force, Fx to the vertical force, Fy is
smaller than a safety factor, f . Or,
fF
F
y
x
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Sliding
Worst scenarios that could
happen to a dam!
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• f can be obtained from laboratory analyses
as summarized below:
Materials f
Sound rock, clean and irregular surface
Rock, some jointing and laminations
Gravel and coarse sand
Sand
Shale
0.8
0.7
0.4
0.3
0.3
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Safety Against Overturning
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• Overturning?
• Overturning would occur if the resultant
force, R fell outside the toe.
• But sometimes as R is moving closer to the
toe, the dam already experiences many
failures like crushing, cracking and sliding.
This is explained in the next slide:
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Overturning
Worst scenarios that could
happen to a dam!
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Will cause
overturning
Safe from
overturning
RR
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As R moves closer to the toe (e is closer to
toe), pressure at heel decreases and
pressure at toe increases.
Tension occurs at heel, resulting in a further increase in
uplift pressure, and excessive compressive stresses at
toe result in crushing.
Eventually, before a dam overturns, it experience crushing
(toe), cracking (heel) and increasing in uplift and sliding.
Therefore, a dam is safe from overturning if the criteria
of no tension on the upstream face, the resistance
against sliding, and the quantity of concrete/foundation
is good.
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• A dam can resist overturning if the ratio of the summation of all restoring (+ve) moments to the summation of all overturning (-ve) moments is within the allowable safety factor, fo. Or,
with,
fo 1.5 is desirable, and
fo 1.25 is generally regarded as acceptable.
o
ve
ve fM
M
+ve
M