Transcript
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 1
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
and
Three Gorges Dam
Kamran M. Nemati
Visiting Professor
Tokyo Institute of Technology
Concrete Dams
U.S. Delegation to TGP-Nov. 99
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 2
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Three Gorges Dam
� Goals:g Flood Prevention
g Navigation improvement
g Power generation
� Location:g Yangtze River downstream from
Three Gorges
� World’s Largest:g Height 181 meters
g Power 18 200 MW
g Reservoir volume 39.3 billion m3
g Concrete volume 27.94 million m3
Concrete Dams
Timeline
� 1919 - Sun Yat-sen proposed project
� 1931 and 1935 - Floods killed over 200,000 people
� 1944 - J. L. Savage, the chief designer of both the
Grand Coulee and Hoover dams, sent by United
States Bureau of Reclamation to survey area and
consult with Chinese engineers
� 1970 - Construction began on Gezhouba dam
� 1992 - Chinese Government adopted official plan for
the dam project
� 2009 - Expected completion of the TGP
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 3
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Stages of Construction
� Phase 1 (1993-1997)
g Water diversion channel
g Construction of transverse cofferdams
� Phase 2 (1998-2003)
g Construction of the spillway, left powerhouse and navigation
facilities
� Phase 3 (2004-2009)
g construction of the right bank powerhouse
Concrete Dams
� Triangular shape
� Vertical Upstream face
� Uniformly sloped Downstream face
� Grout curtain
Structure of Gravity Dams
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 4
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Major Forces:� Gravity of Dam
� Force of Reservoir
� Uplift Force
Others:� Thermal Stress
� Internal structural forces
� Sedimentation pressure
Forces acting on the dam
Concrete Dams
Calculation of Forces
� Force of Gravity of the DamConcrete volume = 27.94*106 m3
Density of concrete = 2407.82 kg/ m3
F = mg=(6.727*1010 kg)(9.81 N/kg)
� Horizontal Pressure of water
Upstream:
Depth = (1/3)*175 m = 58 m
Density of water = 1 kg/ m3
Downstream:
Depth = (1/3) * 83 m = 28 m
Mass = 6.727*1010 kg
Force = 9.599* 1011 N
Pressure = 58 kg/m2
Pressure = 28 kg/m2
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 5
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Uplift Force
� Newton’s 3rd Law - Action/Reaction
� Due to choice of foundation most force of
gravity dam dissipated to surrounding area
� Uplift force, compared with gravity is
minimal
Concrete Dams
Sedimentation
� Major concern for engineers
� Potential cause of:
� Abrasion of spillway and structure
� Accelerated wear of turbine runners
� Increased pressure on dam structure
� Prevention measures:
� Dikes to prevent sediment from settling
� Silt-flushing outlets in the water intakes
� Erosion prevention via tree planting
� Dredging to remove build up
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 6
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Mass Concrete
� Goal
g Prevent cracking of structure
g Must control heat fluctuations
Mass concrete is “…any large volume of cast-in-place concrete with dimensions large
enough to require that measures be taken to cope with the generation of heat and
attendant volume change to minimize cracking”
Higginson, Elmo C. Mass Concrete for Dams and Other Massive Structures.
Concrete Dams
Heat of Hydration
•Dry Cement•No hardening properties
•Compounds in non-
equilibrium, high-energy
states
Conservation of Energy:
The reaction creating the cement is reversed by the hydration process. This causes all of
the energy added to the compounds to produce the cement to be released again in the
reaction.
•Hydrated Cement•Has hardening properties
•Compounds move to
stable, low energy states
•Produce heat as a by-
product
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 7
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Cement Production
� Made from a homogenous mixture of
calcium silicates, heated in a kiln.
� Process requires fossil-fuel energy
input, at about 800 kCal per kilogram of
clinker.
Limestone → CaO + CO2Clay→ SiO2 + Al2O3 + Fe2 +H2O
→
3CaO ⋅ SiO2
2CaO ⋅ SiO23CaO ⋅ Al2O3
4CaO ⋅ Al2O3 ⋅Fe2O3
Concrete Dams
Heat Dissipation over time� Heat from hydration is produced non-uniformly with
time and thus the cooling-off process is very long
� Mass concrete takes many years to cool due to large
volumes of material and relatively small surface area.
� Long hydration process and large temperature drop
can cause fracturing within the structure.
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 8
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Pozzolans
Advantages:� Pozzolans react with a
product of the hydration
reaction of portland cement
� Pozzolans’ reactions do not
produce extremely high
temperatures
� Reduce the amount of
cement needed
Example: Fly Ash� Flue dust from coal burning
power plants
� Used for 35% of paste
material in TGP
� Increases the workability of
the cement
� Develops a strong cement-
like nature upon reacting
with hydrated cement
PortlandCement : C 2S + H 2Ofast
→ C − H − S + CH
PortlandPozzolanCement : Pozzolan + CH + H2Oslow → C − H − S
Concrete Dams
Aggregate
� Varies from fine sand to coarse gravel and crushed rock
� TGP aggregate: maximum size of 150 millimeters in length
� Type depends on what properties the concrete design calls for and what types of aggregate is available
� Drastically decreases the amount of cement paste needed - 80% of TGP concrete is aggregate
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 9
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Concrete Properties
� Elastic Modulus (E) and Coefficient of Thermal
Expansion (α)
� Dependent on the Volume ratio of aggregate
to paste in the concrete
E = VaEa +VpE p
α = Vaαa +Vpαp
Concrete Dams
Thermal Stress
� Mechanics
Young’s Modulus:
� Thermal
Elastic Modulus:
Solving for Thermal Stress (with additional restraint term R):
σ = REα∆T
E =ThermalStress
ThermalStrain=σ
εY =Stress
Strain
Strain: ∆L/L Thermal Strain:ε=α(∆T)Stress: Force/Area
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 10
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Thermal Stresses in Concrete
� Where: σ = Thermal stresses
R = Restraint (0 < R < 1)
E = Modulus of elasticity
α = Coefficient of thermal expansion
∆T = Temperature drop
� You have control on:
εσ E=
∆T
E
α
Very little you can do about Eand α because they are function of aggregate available on site
T∆=⇒ αε TRE ∆=⇒ ασ
� The only control you have is the amount of
temperature drop, ∆T.
Concrete Dams
Computation of ∆∆∆∆T:
∆T = Placement temperature of fresh concrete +Adiabatic temperature rise – Ambient temperature
– Temperature drop due to heat losses.
Temperature
change with time
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 11
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Cooling Techniques: Pre-Cooling
� Cool components before
batching
� Mix concrete at night or early
morning
� Use ice chips instead of water
Result: Placing temperatures of 7° Celsius
Much lower maximum temperature and less time to cool
Concrete Dams
Cooling Techniques: Post-Cooling
Purpose:� Control temperature change in
structure
� Regulate thermal shrinking to
maintain stable volume
Process: � Imbed thin-walled metal pipes
in the structure
� Run cold water through pipes
to lower maximum temperature
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 12
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Power Generation
Water Flow
Reservoir
Penstock
(entrance tube)
Turbine
Draft Tube
Concrete Dams
Power Potential
� Determined by three
factors:
g Net head (H)
g Discharge available
(Q) - dependent on the
size of the turbine
system, and flow rate
of the water
g Efficiency of the
turbine system (e)
� With two constants:g Weight of water (w)
g Proportional constant -
to find Power in kW
P(kw) =HQwe
737 ft −lb
kw
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 13
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Parts of a Francis Turbine
� Scroll Case
Concrete Dams
Parts of a Francis Turbine
� Scroll Case
� Stay vanes
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 14
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Parts of a Francis Turbine
� Scroll Case
� Stay vanes
� Guide vanes
Concrete Dams
Parts of a Francis Turbine
� Scroll Case
� Stay vanes
� Guide vanes
� Runner
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 15
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Parts of a Francis Turbine
� Scroll Case
� Stay vanes
� Guide vanes
� Runner
� Tailrace
Concrete Dams
Generators
� Uses Faraday’s law coil of conducting material
traveling through a magnetic field
causes an electromagnetic force
to be induced in the coil
� Parts
g Rotor - driven by the
turbine’s shaft
g Stator
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 16
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Energy Transmission
� Power from Generators:
700 MWg Voltage: 20 kV
g Alternating Current: 35 kA
� 15 transmission lines:g 500kV AC to central China and
Chongqing
g ±500kV DC eastern China
generated power
transformer
AC to DC conversion
transmission
power usage
DC to AC conversion
transformer
Concrete Dams
Transformers
� Lenz's law: induced voltage and current occur in
direction opposite to the
change that produces it
� 20kV to 500kV
uses step-up
transformer N1< N2
� 500kV to 220V uses
step-down transformer
N1> N2
Concrete Dams: Three Gorges Dam in China
Professor Kamran M. Nemati
Second Semester 2005 17
Advanced Topics in Civil EngineeringATCE-II ATCEATCEATCE---II II II
Concrete Dams
Rectifiers and Thyristors
� Why DC? g smaller volume of conductor to transmit a given
amount of power
g lack of continuous capacitance charging current
g fewer insulation difficulties
Conversion of power: AC - DC - AC
Concrete Dams
Impacts of the TGP
Positive
� Flood control
� Power generation:
18,200 MW installed
capacity
� Navigation
improvement: sea-faring ships able to travel additional
630km upriver
Negative
� Population relocation: 1.2 million people must move
� Loss of farmland
� Flooding of cultural
relics: historical landmarks and remnants of ancient
civilizations
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