concrete dams and three gorges dam

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

� 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

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

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

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

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

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

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

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

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

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

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

Parts of a Francis Turbine

� Scroll Case

Concrete Dams


� Scroll Case

� Stay vanes





Concrete Dams


� Scroll Case

� Stay vanes

� Guide vanes

Concrete Dams


� Scroll Case

� Stay vanes

� Guide vanes

� Runner





Concrete Dams


� 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

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

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

concrete dams and three gorges dam

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