Transcript Document
SIVA
Soil Mechanics-II Earthen Dames
Dr. Attaullah Shah
ground
1
Components of a Dam
2
Types of Earthen Dams
3
DAMS
Classifications based on type and materials of construction Criteria for selection of best dam type: 1. Feasibility -topography, geology, and climate (& its effect on materials) 2. Cost -availability of construction materials near the site; accessibility of
Types
transportation facilities
Materials of Construction
A. Gravity B. Arch Concrete, rubble masonry Concrete C. Buttress D. Embankment Concrete, also timber & steel Earth or rock
Foundation Requirements
Strong foundation
Minimal differential settlement
No bearing capacity shear failure
Sand/Gravel or rock
Low hydraulic conductivity
Silt and/or Clay, non-fractured rock
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DAMS
GRAVITY DAMS Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight. They use their weight to hold back the water in the reservoir. Can be made of earth or rock fill or concrete.
DAMS Gravity
-Depends on its own weight for stability -Usually straight in plan although slightly curved
Forces on Gravity Dam 1. Gravity (weight of dam) W = V
x
γ
= (volume)(specific weight of material) (lb) = (ft 3 )(lb/ft 3 )
2. Hydrostatic pressure H h = γ h 2 / 2
(lb/ft) = (lb/ft 3 ) (ft) 2 (horizontal component) /2 where, h = depth of water at that section
γ
= specific weight of water Where,
H v = γ V / h
(vertical component) (lb/ft) = (lb/ft 3 ) (ft 3 ) / ft V = volume of the dam at that point
DAMS Gravity
Forces on Gravity Dam 3. Uplift
the water under pressure that comes b/t dam and foundation and results in upward (uplift) forces against the dam h 1 = depth of water @ upstream face, aka “heel” (higher) h 2 = depth of water@ downstream face, aka “toe” (lower)
γ
= specific weight of water t = base thickness of dam.
4. Ice pressure
pressure created by thermal expansion exerts thrust against upstream face of the dam
5. Earthquake forces
results in inertial forces that include vertical motion, oscillatory increase, or decrease in hydrostatic pressure (all put force against dam)
DAMS
GRAVITY DAMS Causes of Failure: 1. Sliding along horizontal plane (shear failure) net force > shear resistance at that level 2. Rotation about the toe 3. Failure of material
Gravity Dams
DAMS
Friat Dam
Gravity Dams
DAMS
Tygart Dam, West Virginia
DAMS
ARCH DAMS Curved dam which is dependent upon arch action for its strength.
Transmits most of horizontal water thrust behind them to the abutments by the arch action. Thinner and requires less material than any other type of dam.
Used only in narrow canyons.
DAMS
ARCH DAMS Arch dams includes: *series of horizontal arches *series of vertical cantilevers Load distribution Near bottom of dam Near top of dam Most of load carried by cantilevers (Known as Trial Load arches Method) I. Constant-center (Constant radius) best for U shaped canyons II. Variable-center (Variable radius, constant-angle) best for V shaped canyons
ARCH DAMS
DAMS
Boundary Dam, Seattle
ARCH DAMS
DAMS
Salmon Creek Dam, Alaska
DAMS
BUTTRESS DAMS Buttress dams are dams in which the face is held up by a series of supports. Buttress dams can take many forms -- the face may be flat or curved. Usually, buttress dams are made of concrete and may be reinforced with steel bars.
DAMS
BUTTRESS DAMS sloping membrane that transmits the water load to a series of buttresses @ right angles to axis of dam -Increased formwork & reinforced steel compared w/gravity dam -Less massive than gravity dam (requires 1/3 to 1/2 as much concrete) -Use on weaker foundation -Same forces as gravity and arch dams, however, ice pressures not as prevalent; gaps b/t buttresses relieve majority of uplift forces Types 1. Flat-slab 2. Multiple-arch Water Supporting Membrane flat, concrete-reinforced slabs series of arches
BUTTRESS DAMS
DAMS
Daniel-Johnson Dam, Quebec
BUTTRESS DAMS
DAMS
Bartlett Dam, Colorado
DAMS
EMBANKMENT DAMS
Embankment dams are massive dams made of earth or rock. They rely on their weight to resist the flow of water, just like concrete gravity dams.
DAMS
EMBANKMENT DAMS Types: 1. Simple Embankment (homogeneous throughout)
(upstream less permeable material)
2. Impervious Foundation 3. Impervious Core (Zoned embankments)
DAMS
EMBANKMENT DAMS
Generally have some sort of water proof insides (called the core), which is covered with earth or rock fill. Water will seep in through the earth or rock fill, but should not seep into the core. The water will stop at the seepage line. seep into the core material and should Forces on Embankment Dams: 1. Force of the water (main force) 2. Uplift force
EMBANKMENT DAMS
DAMS
Wolf Creek Dam, Nashville
DAMS
COFFER DAMS temporary structures (sheet-pile, water-tight) that allow construction operations diverts flow from construction areas until work completed
Hoover Dam
• 726.4 feet high ..
• 660 feet thick at the base .. • weighs 6.6 millions ton ... • 1,244 feet across at the top ..
• 45 feet thick at the top ..
• 17 generators giving it the capacity to produce over 2,000 megawatts • can store up 2 years 'average' flow from the Colorado River ...
• total storage capacity can be measured in 30,500,000 acre feet ...