Case study on the Hoover Dam

Case study on the Hoover Dam

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Hoover Dam

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By- Balaji.T.K, CE02B011
CONTENTS
No     Description     Page no
1.     Hoover dam –an Introduction     1
2.     Requirements posed by structural design     2
3.     Requirements posed by other details     6
4.     Type of Concrete     7
5.     Guidelines for Mix design     9
6.     Fabrication and Installation     10
7.     Formwork     11
8.     Cooling of concrete     12
9.     Temperature control of Mass Concrete     12
10.     Quality Assurance     13
11.     Bibliography     14










          


Hoover Dam- an Introduction!

It still stands tall as an engineering marvel high above the Colorado River between Arizona and Nevada. Hoover Dam attracts over 7 million visitors from around the new world every year feeding vast tourism into the Las Vegas Nevada and Arizona economy.
The building of Hoover Dam took the brilliance of over 200 engineers to pull-off what many deemed as almost impossible. And it was the fortitude of over 7,000 dam workers that endured amazingly harsh conditions and extreme dangers to complete Hoover Dam almost two years ahead of schedule

The Mission of the Dam:
1.     Flooding along the Colorado River as it made its way to the Gulf of California had to be controlled.
2.     The water-flow had to be harnessed to provide much needed water to the fertile, yet arid agricultural areas of California and Arizona.
3.     Hydroelectric energy was to satisfy the requirements of millions and millions of people in adjacent regions.
Some Statistics About the dimensions of the dam:
Hoover Dam is 726 feet tall and 1,244 feet long. At its base, Hoover Dam is 660 feet thick which is 60 feet longer than two football fields laid end-to-end. Combined with its top thickness of 45 feet, there is enough concrete (4.5 million cubic yards) in Hoover Dam to build a two-lane highway from Seattle Washington to Miami Florida. Or imagine a four-foot wide sidewalk around Earth at its equator.
A scenic by-product of Hoover Dam is the gigantic reservoir of Lake Mead, a stunningly beautiful water recreation wonderland. This boating, sailing, fishing and house-boating paradise attracts over 10 million visitors a year. Lake Mead covers 550 miles of majestic shoreline and 247 square miles of area which is twice the size of Rhode Island. Its capacity of 1 1/4 trillion cubic feet of water would cover the entire state of Pennsylvania one foot deep.
Requirements for concrete posed by Structural design:
The Hoover Dam is an arch dam. Arch dams transmit most of the horizontal thrust of the water stored behind them to the abutments by arch action and hence thinner cross sections are sufficient (compared to the massive cross-sections of the gravity dams). Narrow V-shaped canyons(just like the Black canyon where the Hoover dam was constructed) will be suited for locating arch dams since they can withstand the thrust produced by the arch section.

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Fig 1 Free body Diagram of an arch dam

Fig. 2 Constant Radius Arch Dam



Fig 3 .Constant Angle Arch Dam







Fig 4 Variable-Radius Arch Dam
          




Fig 5 Double curvature - Cupola Dam


Vertical curvature is introduced the weight of the dam will offset vertical tensions due to
water load. Cupola dams are ideal for narrow valleys and are similar to the thin arch dams in regard to foundation requirements.

In other words, the design considers action of segments of vertical cantilevers and horizontal arches working simultaneously. The object should gain the same deflection at their point of intersection with a water load on the face and with varying thicknesses of the dam.

As Hoover Dam is one of the largest in the world, it bears a tremendous amount of load due to the hydrostatic pressure of the water stored in its reservoir.

Therefore, some of the possible structural requirements are listed below.

1.     The concrete to be used should be able to bear large loads.



Tunnels to divert the flow of the river to construct the dam.
Before actual Hoover Dam construction could begin, the Colorado River had to be temporarily diverted around the dam construction site. To divert the river's flow around the construction site, four tunnels were driven through the canyon walls, two on the Nevada side and two on the Arizona side. These tunnels were to be 56 feet in diameter. Their combined length was nearly 16,000 feet (more than three miles).
Tunneling began at the lower portals of the Nevada tunnels in May 1931. Shortly after, work began on two similar tunnels in the Arizona canyon wall.
To construct such tunnels the requirements for the concrete would be:
1.     It should withstand the pressure due to the rushing water in the tunnel. But, the it does not require the same strength as the concrete used for building the dam itself.
2.     The concrete should be highly fluid so that it could be easily poured into whatever shape that is chosen for form works.
3. The concrete should be abrasion resistant.











Requirements for concrete imposed by mode of construction and other details:
1.     Obviously, the concrete should be completely impermeable.
2. Since the dam was located in a totally isolated area transportation of aggregates would have been a huge problem so they might have had to use the aggregates which were locally available.
3.     The concrete should not harden quickly as it had to be transported for a considerable amount of time before it could be poured into the forms.
4. The concrete should not react with any components present in the reservoir for example salts.
5. The structure is expected to have a long life.
1.     The concrete should not crack by shrinking as the whole purpose of the dam will be spoilt if water enters through the cracks.


An approximate salt content in rivers is tabulated below. Though the actual salt content will depend on various other factors, given below is an average estimate.
Chemical Constituent     Percentage of total salt content in rivers
Silica (SiO2)     15.24
Calcium(Ca)     16.62
Magnesium(Mg)     4.54
Sodium(Na)     6.98
Potassium(K)     2.55
Chloride(Cl)     8.64
Nitrate(NO3)     1.11
Sulphate(SO4)     21.41
Bicarbonate(HCO3)     31.90

Type of concrete that could be used and the important components:

With the given requirements, the ideal type of concrete to be used would be High Performance Concrete (HPC).But, considering that the structure was built in the 1930’s there was no other way than to use the usual concrete with a thicker base.

Mass Concrete could be used for the construction of the dam. Mass concrete could be defined as “any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of cement.”

A control plan, and mock-up for the high strength mass concrete elements, is
needed for the large concrete elements because heat can potentially compromise
the durability of concrete due to temperature induced stresses and possible
chemical alteration of cement paste. Induced stresses may cause thermal cracking,
including micro cracking, as a result of uncontrolled volume change. Chemical
alteration from too high of a peak curing temperature can allow for the potential
for delayed or secondary ettringite formations that may occur years later. A high
peak temperature can also restrict the hydration process due to self-desiccation. In
general, ultimate strengths are lower as curing temperature rises.

The concrete will have to be reinforced with steel on the upstream side.

For the tunnels the initial lining could be done using Shotcrete (Sprayed Concrete).This will ease the procedure for further construction of the tunnels by a huge margin.

Components that are to be used in the concrete

The main concept of High Performance Concrete is to reduce the pores in it so that its more compact and therefore can withstand more pressure. For the HPC the following components could be used.

1.     Cement
a.     The cement should be chloride and sulphate resistant as the reservoir will contain theses salts.
b.     The cement should be of high strength

2.     Aggregates
a.     The aggregate must be clean and free of clays, salts and organic matter. A source of aggregate near the Dam was used so that it would not have to be transported too far.

3.      Super plasticizers could be used to decrease the water to cement ratio in the concrete there by reducing the pores

4.      The concrete could be steel fiber reinforced to prevent from shrinkage cracking.

5.     Fly ash could be added to the concrete mix.

For the Shotcrete the following components are to be used:

1.     Aggregates
2.     Cement
3.     Silica fume (for cohesion)
4.     Viscosity enhancing agent to prevent from rebounding when sprayed.
5.     Accelerator.
6.     Super plasticizer could be used if wet process has to be adopted.














Guidelines for Mix Design:

•     Water: Cement ratio : Since there are other substances other than Portland cement in the mix, w/c ratio is not very clearly defined.But since it’s a high strength concrete w/c ratio should in no case be more than 0.35.

•     Water : Binder Ratio :This is the ratio water is to all the materials that are smaller than Portland cement. It is important from a technical point of view. The water is to binder ration in no case shall be more than 0.40

•     Cement: The cement should be of high strength therefore we use 450kg/m3 dosage. The cement should also not react with chlorine or sulphates as the concrete is always exposed to such salts.

•     Workability: Since the concrete has to be moulded into different forms the concrete should have high workability. The concrete should preferably have a slump >17cm.
The mix design measures included keeping the cementitious content under 6 sacks per cubic yard, replacing up to 35% of the portland cement with fly ash, disallowing the use of high performance pozzolans such as silica fume and metakaolin, and using inch and a half aggregate. The initial placement temperature has been kept to either a maximum of 18.5°C (65° F), or 21 °C (70 °F) if all mix water was added as ice. Though these have been the
practices used for mass concrete designed to be 4 ksi or less, these measures have
also been adequate for concrete achieving actual strengths near 5 ksi.











Fabrication and Installation:
The initial concrete required for the dam was mixed in a river-level mixing plant which was located approximately 3/4 of a mile upriver from the dam site This plant provided the concrete for the linings in the diversion tunnels and for the lower levels of the dam. The concrete was loaded into buckets which were transported to the site initially by truck. Eventually, the concrete buckets were transported by electric trains.Initially, nearly all of the concrete produced at this plant., went into the linings of the diversion tunnels.
Placing the 12-foot long sections, some weighing 185 tons and others 135 tons, into the plugged diversion tunnels along with smaller penstocks in their lined tunnels was another major challenge. A 250-ton-capacity cableway was built which swung the pipe section over the canyon and lowered it with 10 cables coming from the cablecar. The largest sections were locked together with 3-inch-diameter pins; smaller sections were joined with hot-rivets.
As the dam rose in height, a new concrete mixing plant was constructed on the canyon rim. Completely automated, the hi-mix plant measured ingredients, mixed and dispensed the concrete. It was capable of producing 24 cu .yd. of concrete every three and a half minutes. The hi-mix plant was used to produce all of the concrete placed in the dam above the 992 foot level.
The first concrete was placed into the dam on only after one year after the beginning of he construction. The concrete was placed in the dam using dump buckets. These buckets were lifted from the cars and lowered into place by overhead cable ways. There were a total of nine of these cable ways used to place the concrete. Five of the cable ways were connected to moveable towers, which allowed them to be repositioned to work on different parts of the dam when necessary.

To build the dam, a series of uneven separate columns were erected and keyed to each other. The columns on the outside (upstream face) of the arc were 60 feet square, while those on the inside were 25 feet square to round the contour of the dam.

Formwork:
Probably the most widely used lift is 1.5m, however, on large dams a height of 2.3-3.0m is frequently used. With the larger lifts there are fewer movements of forms and fewer horizontal lift surfaces to be cleaned. The high-lift formwork is unique and expensive with less prospect for re-use, heavier equipment is required for lifting the forms and the heat problems and risks of cracking in the concrete are accentuated. Modern steel formwork is of cantilever design, see figure. Where possible the use of slip forms will expedite the work and lower the costs. At some locations it may be expedient to use precast concrete slabs for formwork with set-retarding agent on the inner surface.










Cooling of Concrete –
The method of cooling concrete during the first few days after placing can be of the utmost importance if cracking is to avoided. It is essential to give attention to both internal and external factors that may induce cracking;
•     Temperature rise, which will depend upon the heat of hydration of the cement, the quantity of cement per cubic metre, the concrete placing temperature and the rate of construction;
•     Heat dissipation, which will depend upon the conditions of exposure - including the temperature of the underlying concrete and the thermal diffusivity of the concrete. If it is considered necessary to heat the underlying concrete the rate of rise of its temperature should not exceed 2° Celsius per day;
•     The effects of restraint from a cold surface, i.e. rock or concrete say 14 days old, it will depend upon the temperature gradient which can be reduced by placing concrete in half lifts for a predetermined height, say 3m above the cold surface;
•     The arrangement of cooling pipes - at 0.25 and 0.75 of the height of the lift may be more efficient than on the top of the old lift and at mid-height of the new lift. The horizontal spacing will depend upon the rate of heat removal required and the temperature of the cooling water (i.e. river water of varying temperature or refrigerated water);
•     The local weather conditions - humidity, temperature and wind.
Temperature control of Mass Concrete:
Temperature control of mass concrete is necessary to prevent cracking caused by excessive tensile strains that result from differential cooling of the concrete. The concrete
is heated by reaction of cement with water and can gain additional heat from exposure to the ambient conditions. Cracking can be controlled by methods that limit the peak temperature to a safe level, so the tensile strains developed as the concrete cools to equilibrium are less
than the tensile strain capacity.

The temperature control methods available for consideration all have the basic objective of reducing increases in temperature due to heat of hydration, reducing thermal differentials within the structure, and reducing exposure to cold air at the concrete surfaces that would create cracking. The most common techniques are the control of lift thickness, time interval between lifts, maximum allowable placing temperature of the concrete, and surface insulation. Post-cooling may be economical for large structures. Analysis should be made to determine the most economic method to restrict temperature increases and subsequent temperature drops to levels just safely below values that could cause undesirable cracking.

Quality Assurance For High performance Concrete:

The organization should have a materials engineer (or technician) reporting directly to the resident engineer and being responsible for all phases of quality assurance from
aggregate production through curing and protection of the concrete. The organization should include a shift supervisor (quality assurance representative) for each shift. Under each
shift supervisor, the organization should include one placing quality assurance representative for each location at which concrete is being placed, one mixing plant quality assurance
representative, and one quality assurance representative assigned to verification of cleanup, curing, protection, and finishing. The organization should include a laboratory
technician and assistants as required to handle acceptance testing of aggregates and concrete, to consolidate reports, prepare summary reports, and keep records. The laboratory
technician should report directly to the materials engineer. On a very large project, several contracts on the same project may be supervised by a single area engineer. Under
such circumstances, the engineer reports to the resident engineer and supervises all concrete activity on the project. The quality assurance force required for aggregate quality
verification depends upon job conditions. Where the Contractor’s quality control has been proven satisfactory, sampling should be required only at the point of acceptance
(mixing plant) and the quality verification should consist only of routine observation of plant operations; this work can be handled jointly by the shift supervisors and the
engineers or by special assignment. Usually quality verification of aggregate processing operations does not require continuous assignment of personnel.

Some of the Salient features to be taken care of are:
•     Aggregate grading : . In order to determine compliance with the specification requirement that aggregates must be within certain grading limits as delivered to the mixers, samples must be obtained as delivered to the mixer.
•     Aggregate Quality: To confirm that the quality of aggregate has not changed since it was tested during design stage.
•     Free moisture on aggregates. Adjustments of batch weights to compensate for variation in aggregate moisture are a basic contractor responsibility.
•     Slump and air content: Slump test is made to check the uniformity of the concrete being sent to the forms and to check if the concrete is placable.
•     Concrete temperature. The temperature of cooled concrete should not be measured until 20 minutes after mixing.
•     Compressive Strength: Compressive strength of the concrete being sent to the formwork has to be verified if it is strong enough to be placed.

Bibliography:

1.     Design of gravity dams -http://www.usace.army.mil
2.     http://www.romanconcrete.com
3.     Arizona online travel guide
4.     http://www.usbr.gov












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