Concrete Dams and Three Gorges Dam U.S. Delegation to TGP
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ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams and Three Gorges Dam Kamran M. Nemati Visiting Professor Tokyo Institute of Technology Concrete Dams U.S. Delegation to TGP-Nov. 99 Professor Kamran M. Nemati Second Semester 2005 1 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams Three Gorges Dam Goals: g g g Location: g Flood Prevention Navigation improvement Power generation Yangtze River downstream from Three Gorges World’s Largest: g Height 181 meters Power 18 200 MW Reservoir volume 39.3 billion m3 g Concrete volume 27.94 million m3 g g Concrete Dams Timeline Professor Kamran M. Nemati Second Semester 2005 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 2 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams Stages of Construction Phase 1 (1993-1997) g g Phase 2 (1998-2003) g Water diversion channel Construction of transverse cofferdams Construction of the spillway, left powerhouse and navigation facilities Phase 3 (2004-2009) g construction of the right bank powerhouse Concrete Dams Structure of Gravity Dams Professor Kamran M. Nemati Second Semester 2005 Triangular shape Vertical Upstream face Uniformly sloped Downstream face Grout curtain 3 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams Forces acting on the dam Major Forces: Gravity of Dam Force of Reservoir Uplift Force Others: Thermal Stress Internal structural forces Sedimentation pressure Concrete Dams Calculation of Forces Force of Gravity of the Dam Concrete volume = 27.94*106 m3 Density of concrete = 2407.82 kg/ m3 F = mg=(6.727*1010 kg)(9.81 N/kg) Mass = 6.727*1010 kg Force = 9.599* 1011 N 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 Professor Kamran M. Nemati Second Semester 2005 Pressure = 58 kg/m2 Pressure = 28 kg/m2 4 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China 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: Prevention measures: Professor Kamran M. Nemati Second Semester 2005 Abrasion of spillway and structure Accelerated wear of turbine runners Increased pressure on dam structure Dikes to prevent sediment from settling Silt-flushing outlets in the water intakes Erosion prevention via tree planting Dredging to remove build up 5 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams Mass Concrete 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. Goal g g Prevent cracking of structure Must control heat fluctuations Concrete Dams Heat of Hydration 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. •Dry Cement •No hardening properties •Compounds in nonequilibrium, high-energy states Professor Kamran M. Nemati Second Semester 2005 •Hydrated Cement •Has hardening properties •Compounds move to stable, low energy states •Produce heat as a byproduct 6 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China 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. 3CaO ⋅ SiO 2 Limestone → CaO + CO2 2CaO ⋅ SiO2 → Clay → SiO2 + Al2 O3 + Fe2 + H 2 O 3CaO ⋅ Al2 O3 4CaO ⋅ Al2 O3 ⋅ Fe 2O3 Concrete Dams Heat Dissipation over time Professor Kamran M. Nemati Second Semester 2005 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. 7 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams Pozzolans Advantages: Example: Fly Ash 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 Flue dust from coal burning power plants Used for 35% of paste material in TGP Increases the workability of the cement Develops a strong cementlike nature upon reacting with hydrated cement PortlandCement : C 2S + H 2 O → C − H − S + CH fast slow → C − H − S PortlandPozzolanCement : Pozzolan + CH + H 2 O Concrete Dams Aggregate Professor Kamran M. Nemati Second Semester 2005 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 8 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams Concrete Properties Elastic Modulus (E) and Coefficient of Thermal Expansion (α) E = Va E a + Vp E p α = Va α a + Vp α p Dependent on the Volume ratio of aggregate to paste in the concrete Concrete Dams Thermal Stress Mechanics Strain: ∆L/L Stress: Force/Area Thermal Strain:ε=α(∆T) Elastic Modulus: Young’s Modulus: Stress Y= Strain Thermal E= Thermal Stress σ = ThermalStrain ε Solving for Thermal Stress (with additional restraint term R): σ = REα∆T Professor Kamran M. Nemati Second Semester 2005 9 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams Thermal Stresses in Concrete σ = Eε ⇒ ε = α∆T ⇒ σ = REα∆T Where: σ = Thermal stresses R = Restraint (0 < R < 1) E = Modulus of elasticity α = Coefficient of thermal expansion ∆T = Temperature drop E You have control on: α ∆T Very little you can do about E and α because they are function of aggregate available on site The only control you have is the amount of temperature drop, ∆T. Concrete Dams Computation of ∆T: Temperature change with time ∆T = Placement temperature of fresh concrete + Adiabatic temperature rise – Ambient temperature – Temperature drop due to heat losses. Professor Kamran M. Nemati Second Semester 2005 10 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China 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: Professor Kamran M. Nemati Second Semester 2005 Imbed thin-walled metal pipes in the structure Run cold water through pipes to lower maximum temperature 11 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams Power Generation Water Flow Reservoir Penstock (entrance tube) Turbine Draft Tube Concrete Dams Power Potential Determined by three factors: g g g Professor Kamran M. Nemati Second Semester 2005 Net head (H) Discharge available (Q) - dependent on the size of the turbine system, and flow rate of the water Efficiency of the turbine system (e) P(kw) = HQwe 737 ftkw−lb With two constants: g g Weight of water (w) Proportional constant to find Power in kW 12 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams Parts of a Francis Turbine Scroll Case Concrete Dams Parts of a Francis Turbine Professor Kamran M. Nemati Second Semester 2005 Scroll Case Stay vanes 13 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams Parts of a Francis Turbine Scroll Case Stay vanes Guide vanes Concrete Dams Parts of a Francis Turbine Professor Kamran M. Nemati Second Semester 2005 Scroll Case Stay vanes Guide vanes Runner 14 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China 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 g Professor Kamran M. Nemati Second Semester 2005 Rotor - driven by the turbine’s shaft Stator 15 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams Energy Transmission generated power Power from Generators: 700 MW g g Voltage: 20 kV Alternating Current: 35 kA 15 transmission lines: g g transformer AC to DC conversion transmission 500kV AC to central China and Chongqing DC to AC conversion ±500kV DC eastern China transformer power usage 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 Professor Kamran M. Nemati Second Semester 2005 16 ATCE --IIII ATCE ATCE-II Advanced Topics in Civil Engineering Concrete Dams: Three Gorges Dam in China Concrete Dams Rectifiers and Thyristors Conversion of power: AC - DC - AC Why DC? g smaller volume of conductor to transmit a given amount of power lack of continuous capacitance charging current g fewer insulation difficulties g Concrete Dams Impacts of the TGP Positive Flood control Power generation: 18,200 MW installed capacity Navigation improvement: sea-faring Negative Population relocation: 1.2 million people must move Loss of farmland Flooding of cultural relics: historical landmarks and remnants of ancient civilizations ships able to travel additional 630km upriver Professor Kamran M. Nemati Second Semester 2005 17
