Profession : An occupation, such as law, medicine, or engineering, that requires specialized education at the university level.

Special Terms

Engineering: The practical application of the findings of theoretical science so that they can be put to work for the benefit of mankind. An engineer is a member of the engineering profession, although this term is also used to refer to someone who operates or maintains certain kinds of equipment-а railroad locomotive engineer, for example. In the latter context, the person referred to is a technician rather than a professional engineer.

Profession: An occupation, such as law, medicine, or engineering, that requires specialized education at the university level.

Empirical Information: Information that is based on observation and experience rather than theoretical knowledge.

Civil Engineering: The branch of engineering that deals with the design and construction of structures that are intended to be stationary, such as buildings, dams, and bridges. Among its subdivisions are structural engineering, dealing with permanent structures; hydraulic engineering, dealing with the flow of water and other fluids; and environmental/sanitary engineering, dealing with water supply, water purification, and sewer systems, as well as urban planning and design.

Mechanical Engineering: The branch of engineering that deals with machines and their uses.

Mining and Metallurgy: The branch of engineering that deals with extracting metal ores from the earth and refining them.

Chemical Engineering: The branch of engineering that deals with processes involving reactions among the elements, the basic natural substances. Petroleum engineering is a subdivision which deals specifically with processes involving petroleum.

Electrical and Electronic Engineering: The branch of engineering that deals with the effects and processes that result from the behavior of tiny particles of matter called electrons.

Nuclear Engineering: A modem branch of engineering that deals with the processes that result from breaking up some particles of matter.

Aqueduct: A structure that is used for transporting water over long distances.

Stress: Physical pressure or other forces exerted on an object. The force of gravity, the natural pull of the earth, for example, is one of the stresses that acts on an object. Silt: Sand or earth transported from one location by water and deposited as sediment at a second location.

Environmental Impact Study: A study that shows the effect a proposed structure will have on its surroundings: the air, water, human, animal, and plant life, for example. Such studies are now required for most major construction projects in the United States.

Vocabulary Practice

What does engineering mean?

What is a profession? Give some examples.

How does a railroad locomotive engineer differ from a professional engineer?

What is empirical information?

What does a civil engineer deal with?

What are some of the subdivisions of civil engineering? With what is each of them concerned?

What does a mechanical engineer deal with?

What does a mining and metallurgical engineer deal with?

What does a chemical engineer deal with? Name a subdivision of chemical engineering.

What do electrical and electronic engineers deal with?

What do nuclear engineers deal with?

What is an aqueduct?

What is stress?

Define silt.

What is an environmental impact study concerned with?

What does quantification mean?

The Engineering Profession

Engineering is one of the oldest occupations in the history of mankind. Indeed, without the skills that are included in the field of engineering, our present-day civilization could never have evolved. The first toolmakers who chipped arrows and spears from rock were the forerunners of modern mechanical engineers. His craftsmen who discovered metals in the earth and found ways to process and refine them were the ancestors of mining and metallurgical engineers. And the skilled technicians who devised irrigation systems and erected the great buildings of the ancient world were the civil engineers of their time. One of the earliest names that have come down to us in history is that of Imhotep, the designer of the stepped pyramid at Sakkara in Egypt about 3,000 B.C.

Engineering is often defined as the practical application of theoretical sciences, such as physics or chemistry, for the benefit of mankind. Many of the early branches of engineering, however, were based not on science but on empirical information, that is, information that depended on observation; and experience rather than theoretical knowledge. Many of the structures that have survived from ancient times, such as the aqueducts of Rome, exist because they were built with greater strength than modem standards require. But at least the Roman engineers were sure that buildings would last for a long time. Probably die oldest text in engineering is the work of a Roman architect and engineer named Vitruvius Pollio, who wrote a book in the first century B.C. about the engineering practices of his day. Many of the problems encountered by Vitruvius Pollio were similar to those that modern engineers still must confront.

The term civil engineering originally came into use to distinguish it from military engineering. Civil engineering dealt with permanent structures for civilian use, whereas military engineering dealt with temporary structures for military use. An example of the latter is the bridge built across the Rhine in 55 B.C. that is described in Julius Caesar's Commentaries on the Gallic War. A more appropriate definition of civil engineering is that it deals with the design and construction of objects that are intended to be stationary. In practice, this definition includes buildings and houses, dams, tunnels, bridges, canals, sanitation systems, and the stationary parts of transportation systems-highways, airports, port facilities, and roadbeds for railroads.

Civil engineering offers a particular challenge because almost every structure or system that is designed and built by civil engineers is unique. One structure rarely duplicates another exactly. Even when structures seem to be identical, site requirements or other factors generally result in modifications. Large structures like dams, bridges, or tunnels may differ substantially from previous structures. The civil engineer must therefore always be ready and willing to meet new challenges.

Since the beginning of the modem age in the sixteenth and seventeenth centuries, there has been an explosion of knowledge in every scientific field: physics and chemistry, astronomy and physiology, as well as recently evolved disciplines like nuclear and solid-state physics. One reason for this rapid increase in scientific knowledge was the development of the experimental method to verify theories. At least of equal importance has been the use of quantification, that is, putting the data from the results of experimentation into precise mathematical terms. It cannot be emphasized too strongly that mathematics is the basic tool of modem engineering.

As scientific knowledge increased, so did the practical applications. The eighteenth century witnessed the beginning of what is usually called the Industrial Revolution, in which machines began to do more and more of the work that previously had been done by human beings or animals. In the nineteenth century and in our own day, both scientific research and the practical applications of its results have progressed rapidly. They have given the civil engineer new and stronger materials; the mathematical formulas which he can use to calculate the stresses that will be encountered in a structure; and machines that make possible the construction of skyscrapers, dams, tunnels, and bridges that could never have been built before.

Another result of the explosion of knowledge was an increase in the number of scientific and engineering specialties. By the end of the nineteenth century, not only were civil, mechanical, and mining and metallurgical engineering recognized, but courses were also being offered in the newer specialties of electrical engineering and chemical engineering. This expansion has continued to the present day. We now have, for example, nuclear, petroleum, aerospace, and electronic engineering. Of course, many of these disciplines are subdivisions of earlier specialties-electronic engineering from electrical engineering, for example, or petroleum engineering from chemical engineering.

Within the field of civil engineering itself, there are subdivisions: structural engineering, which deals with permanent structures; hydraulic engineering, which is concerned with systems involving the flow and control of water or other fluids; and sanitary or environmental engineering, which involves the study of water supply, purification, and sewer systems. Obviously, many of these specialties overlap. A water supply system, for example, may involve dams and other structures as well as the flow and storage of water.

Many different kinds of engineers often work on large projects, such as space exploration or nuclear-power development. In the space program, for example, the launching pads and the rocket assembly and storage building at Cape Canaveral, Florida - the largest such structure in the world - are primarily the work of civil engineers. In a nuclear power plant, civil engineers are responsible for the design and construction of the plant itself, as well as the protective shielding around the nuclear reactor. In both these cases, however, the civil engineers work with specialists in aerospace, nuclear, and electrical engineering. In projects of this kind, the engineer is a member of a team that is often headed by a systems engineer who coordinates the contributions of all members of the team. Because teamwork is necessary in so many engineering projects nowadays, an important qualification for engineers is the ability to work successfully with other people.

Still another result of the increase in scientific knowledge is that engineering has grown into a profession. A profession is an occupation like law, medicine, or engineering that requires specialized, advanced education; indeed, they are often called the "learned professions." Until the nineteenth century, engineers generally were craftsmen or project organizers who learned their skills through apprenticeship, on-the-job training, or trial and error. Nowadays, many engineers spend years studying at universities for advanced degrees. Yet even those engineers who do not study for advanced degrees must be aware of changes in their field and those related to it. A civil engineer who does not know about new materials that have become available cannot compete successfully with one who does.

The word engineer is used in two ways in English. One usage refers to the professional engineer who has a university degree and an education in mathematics, science, and one of the engineering specialties. Engineer, however, is also used to refer to a person who operates or maintains an engine or machine. An excellent example is the railroad locomotive engineer who operates a train. Engineers in this sense are essentially technicians rather than professional engineers.

Engineers must be willing to undergo a continual process of education and be able to work in other disciplines. They must also adapt themselves to two requirements of all engineering projects. First, the systems that engineers produce must be workable not only from a technical but also from an economic point of view. This means that engineers must cooperate with management and government officials who are very cost-conscious. Therefore, engineers must accommodate their ideas to the financial realities of a project.

Second, the public in general has become much more aware, especially in the last ten years or so, of the social and environmental consequences of engineering projects. For much of the nineteenth and twentieth centuries, the attitude of the public could be summed up by the phrase, "Science is good." The most visible part of science was the engineering work. No one can avoid seeing the great dams, the bridges, the skyscrapers, and the highways that have created an impressive, engineered environment around us.

Nowadays, however, the public is more conscious of the hidden or delayed hazards in new products, processes, and many other aspects of civil engineering systems. For instance, new highways in the United States are no longer approved routinely; instead, highways and other similar projects must now undergo environmental impact studies to assess the project's effect on air pollution and other environmental concerns.

A recent news story which reported that the Egyptian government now permits public criticism of the Aswan High Dam underlines this concern. The Aswan Dam is one of the engineering wonders of modern times, but several undesirable effects have been noted. The dam has, for instance, blocked the flow of silt down the Nile, so that the fertility of the land below the dam has decreased. Nutrients that were once carried down the river have been held back by the dam, and consequently schools of fish that once thrived around the Nile Delta have gone elsewhere. Still another reported effect of the dam has been the increase of the salinity of the soil which is irrigated by the water behind the dam. These and other problems might have been prevented by more thorough studies before construction was undertaken.

In other words, engineers do not work in a scientific vacuum. They must consider the social consequences of their work. We have, after all, described engineering as a profession that makes practical application of the findings of theoretical science. Successful engineers must include in their definition of practical the idea that the work is also desirable and safe for society

Discussion

Who was Imhotep?

Review

B. below are some of the projects on which a present-day engineer might work. Indicate which branch of engineering (civil, mechanical, chemical, and so on) would be involved. Some of the projects may involve more than one kind of engineering; if so, indicate all of those that you think should be included.

Special Terms

Surveying: Measuring the earth's surface. Plane surveying measures surfaces as though they were flat, without taking into account the earth's curvature, whereas geodetic surveying includes calculations for the curvature. A surveyor is the person who makes a survey.

Chain: A device 66 feet long (surveyor's chain) or 100 feet long (engineer's chain) for measuring distance. Today, a steel tape has replaced the chain, but the men who hold the tape are usually still called chainmen.

Plane-table Alidade: An alidade is another kind of telescope used to measure both distances and vertical angles. It is set on a plane-table (a flat table), which a surveyor can use as a drawing board to make maps in the field.

Surveyor's Level: Another kind of telescope with a bubble level, a tube of fluid with an air bubble in it. The surveyor can sight a rule called a level rod through the telescope in order to measure elevation.

Vocabulary Practice

What is surveying?

What are contour lines?

What is a bench mark?

What does boring mean?

Surveying

Before any civil engineering project can be designed, a survey of the site must be made. Surveying means measuring-and recording by means of maps-the earth's surface with the greatest degree of accuracy possible. Some engineering projects- highways, dams, or tunnels, for example-may require extensive surveying in order to determine the best and most economical location or route.

There are two kinds of surveying: plane and geodetic. Plane surveying is the measurement of the earth's surface as though it were a plane (or flat) surface without curvature. Within areas of about 20 kilometers square-meaning a square, each side of which is 20 kilometers long-the earth's curvature does not produce any significant errors in a plane survey. For larger areas, however, a geodetic survey, which takes into account the curvature of the earth, must be made.

The different kinds of measurements in a survey include distances, elevations (heights of features within the area), boundaries (both man-made and natural), and other physical characteristics of the site. Some of these measurements will be in a horizontal plane', that is, perpendicular to the force of gravity. Others will be in a vertical plane, in line with the direction of gravity. The measurement of angles in either the horizontal or vertical plane is an important aspect of surveying in order to determine precise boundaries or precise elevations. In plane surveying, the principal measuring device for distance is the steel tape. In English-speaking countries, it has replaced a rule called a chain, which was either 66 or 100 feet long. The 66-foot-long chain gave speakers of English the acre, measuring ten square chains or 43,560 square feet as a measure of land area. The men who hold the steel tape during a survey are still usually called chainmen. They generally level the tape by means of plumb bobs, which are lead weights attached to a line that give the direction of gravity. When especially accurate results are required, other means of support, such as a tripod - a stand with three legs - can be used. The indicated length of a steel tape is in fact exactly accurate only at a temperature of 20° centigrade, so temperature readings are often taken during a survey to correct distances by allowing for expansion or contraction of the~tape.

Distances between elevations are measured in a horizontal plane. When distances are being measured on a slope, a procedure called breaking chain is followed. This means that measurements are taken with less than the full length of the tape.

Lining up the tape in a straight line of sight is the responsibility of the transitman, who is equipped with a telescopic instrument called a transit. The transit has plates that can indicate both vertical and horizontal angles, as well as leveling devices that keep it in a horizontal plane. Cross hairs within the telescope permit the transitman to line up the ends of the tape when he has them in focus.

Angles are measured in degrees of arc. Two different systems are in use. One is the sexagesimal system that employs 360°, each degree consisting of 60 minutes and each minute of 60 seconds. The other is the centesimal system that employs 400 grads, each grad consisting of 100 minutes and each minute of 100 seconds. A special telescopic instrument that gives more accurate readings of angles than the transit is called a theodolite.

In addition to cross hairs, transits and theodolites have markings called stadia hairs (stadia is the plural of the Greek word stadion, a measure of distance). The stadia hairs are parallel to the horizontal cross hair. The transitman sights a rod, contour lines, the lines on a map that enclose areas of equal elevation.

 Contour maps can be made in the field by means of a plane-table alidade. The alidade is a telescope with a vertical circle and stadia hairs. It is mounted on a straight-edged metal plate that can be kept parallel to the line of sight. The surveyor can mark his readings of distances and elevations on a plane (or flat) table that serves as a drawing board. When the marks epresenting equal elevations are connected, the surveyor has made a contour map.

Heights or elevations are determined by means of a surveyor’s level, another kind of telescope with a bubble-leveling device parallel to the telescope. A bubble level, which is similar to a carpenter's level, is a tube containing a fluid that has an air bubble in it. When the bubble is centered in the middle of the tube, the device is level. The surveyor sights a rule called a level rod through the telescope. The rod is marked off to show units of measure in large, clear numbers. The spaces between the marks usually are alternately black and white in order to increase visibility. The number that the surveyor reads on the level rod, less the height of his or her instrument, is the vertical elevation.

Heights are given in relation to other heights. On maps, for example, the usual procedure is to give the elevation above sea level. Sea level, incidentally, can be determined only after averaging the tides in a given area over a definite period. A survey carried out by level and rod often gives the elevation in relation to a previously measured point that is called a bench mark. Approximate elevations can also be measured with an altimeter, which is a device that takes advantage of changes in atmospheric pressure. Readings taken with an altimeter are usually made at two, and sometimes three, different points and then averaged. The readings must be corrected for humidity and temperature, as well as the weight of the air itself.

Modern technology has been used for surveying in instruments that measure distance by means of light or sound waves. These devices direct the waves toward a target that reflects them back to a receiver at the point of origin. The length of time it takes the waves to go to the target and return can then be computed into distance. This surveying method is particularly useful when taking measurements over bodies of water.

Aerial photography is another modern method of surveying. A photograph distorts scale at its edges in proportion to the distance the subject is from being in a direct vertical line with the lens of the camera. For this reason, the photographs for an aerial survey are arranged to overlap so that the scale of one part joins the scale of the next. This arrangement is called a mosaic, after the pictures that are made from hundreds of bits of colored stone or glass.

Geodetic surveying is much more complex than plane surveying. It involves measuring a network of triangles that are based on points on the earth’s surface. The triangulation is then reconciled by mathematical calculations with the shape of the earth. This shape, incidentally, is not a perfect sphere but an imaginary surface, slightly flattened at the poles, that represents mean sea level as though it were continued even under the continental land masses.

In addition to measuring surfaces for civil engineering projects, it is often necessary to make a geological survey. This involves determining the composition of the soil and rock that underlie the surface at the construction site. The nature of the soil, the depth at which bedrock is located, and the existence of faults or underground streams are subsurface factors that help civil engineers determine the type and size of the structural foundations or the weight of the structure that can rest on them. In some areas, these can be critical factors. For example, Mexico City rests on a lakebed with no bedrock near the surface; it is also located in an earthquake zone. The height and weight of buildings must therefore be carefully calculated so that they will not exceed the limits that are imposed by the site. Geological samples are most often obtained by borings, in which hollow drills bring up cores consisting of the different layers of underground materials. Other devices that are used in geological surveys are gravimeters and magnetometers. The gravimeter measures the earth's gravitational pull; heavier rocks like granite exert a stronger pull than lighter ones like limestone. The magnetometer measures the strength of the earth's magnetic field. Again, the denser the rock, the more magnetic force it exerts. A third instrument is the seismograph, which measures vibrations, or seismic waves, within the earth. It is the same instrument that is used to detect and record earthquakes. In a geological survey, it is used by setting off small, man-made earthquakes. The waves created by a blast of dynamite buried in the ground reflect the different kinds of rock under the surface; hard or dense rocks reflect the waves more strongly than soft or porous rocks.

Discussion

What can be used to make contour maps in the field? What does the surveyor use to make his sightings? Where does he mark his readings? What has he made when he connects the marks representing equal elevations?

Discussion

1. Whose principles of road-building are still applied to the building of modem highways?

2. What elements were included in the roads that these men designed?

3. How did the roads designed by McAdam differ from those designed by Tresaguet and Telford?

4. Why were Me Adam's roads adequate in the nineteenth century but inadequate in the twentieth?

5. How was the surface of such roads first improved to handle automobile traffic?

6. In what two ways was road-building improved in the twentieth century?

7. What kind of roads has traffic engineering produced?

8. What is the first step in the construction of a new road?

9. Why should the civil engineer carefully calculate the amount of earth to be moved?

10. What kind of machine is probably used to prepare the footing for the road? What does it do?

11. How can the soil be stabilized? On what do soil stabilization techniques depend?

12. What are the base course and the wearing surface of modern highways usually made of?

13. Why must a concrete surface be laid in segments?

14. What two methods are used to lay a reinforced concrete surface?

15. What machines are used for laying the concrete slabs of a highway? How is the reinforcing mesh laid with the second machine? How quickly do they work?

16. How do airport runways differ from highways? What special factors must be considered in constructing runways?

17. When was the steam locomotive invented? What rail lines were in existence before that time?

18. Why did the great age of railroad building generally seem to be over by 1920?

19. What is one example of the renewed interest in railroad building?

20. In what other kind of transportation system is there considerable interest?

21. What must be done before construction of a railroad line can begin?

22. At what grade do steam locomotives stop operating efficiently? What has this resulted in?

23. What special considerations were necessary in designing curves on railroad lines?

24. What steps are followed in conventional railroad building methods?

25. What sort of base is laid down when a railroad line passes over swampy or unstable ground?

i

26. What technological advances have taken place in railroad construction in the last few years? What do these permit?

27. What are some experimental or proposed forms of rail transportation systems?

28. What disadvantages does the automobile have that have caused an increased interest in new mass transportation systems?

4.3. Review

A. Match each expression on the left with one of the statements on the right.

1. Footing	a. The paved strip on which aircraft land or take off at an
2. Base course	airport.
3. Wearing surface	b. A place where earth is deposited or dumped during
4. Crown	construction.
5. Compacting	c. A machine used to compact soils.
6. Cut	d. The surface on which a foundation rests.
7. Fill	e. A wood or prestressed concrete beam to which the rails
8. Soil mechanics	for a railway line are fastened.
9. Soil stabilization	f. A new science dealing with the classification of different
10. Vibrating roller	types of soils.
11. Concrete train	g. A layer of crushed rock that rests on a footing for a road.
12. Slipform paver	h. Crushed rock laid on top of a footing for a railroad

 

13. Runway 14. Ballast	roadbed. i. A single machine that lays a concrete wearing surface for
15. Sleeper (crosstie)	a highway. j. The top level of a road that directly receives wear from traffic.
	k. A convex curve in the surface of a road that permits drainage. l. Techniques for making soils denser and firmer so they can bear a heavier load. m. A place from which earth is removed during construction. n. A group of machines that work together to lay a concrete wearing surface for a highway. o. Pressing down soil to make it denser and firmer.

 

B. Recently, the world has been facing an energy crisis because of the increased consumption, short supply, and high prices of fuels, particularly petroleum products like gasoline. Discuss the possibilities for engineering new transportation systems or improving present ones so that they would consume less fuel and still provide adequate service.

 

UNIT FIVE BRIDGES

Special Terms

Span: The distance between two supports of a bridge. Span is also used as a verb, as in: The bridge spans a distance of 200 meters.

Masonry Arch Bridge: A bridge of masonry with arches between piers. Today the term usually refers to bridges made of stone.

Pontoon: A hollow drum that can float. Pontoons are used as supports for a pontoon bridge. Beam Bridge: The simplest kind of bridge. It consists of a rigid beam between two supports.

Suspension Bridge: A bridge supported by cables that are usually hung from towers.

Truss: A framework strengthened by diagonal beams that form triangles with horizontal and vertical beams. Trusses are used to strengthen beam bridges.

Cantilever: A type of structure in which a horizontal beam extends beyond its support. A cantilever bridge is a type of beam bridge.

Steel Arch Bridge: A bridge with an arch made with steel beams.

Deck: The roadway or trafTic-bearing surface of a bridge.

Stay: A cable that runs at a diagonal from the main supporting cable to the deck of a suspension bridge.

Concrete Arch Bridge: A bridge with an arch made of reinforced concrete. It is really a kind of masonry arch bridge, but it can span a much greater distance than a stonearch.

Lift Bridge: A movable bridge with a span that is raised by elevators.

Swing Bridge: A movable bridge with a span that swings open parallel to the channel. It is also known as a pivot bridge.

Bascule Bridge: A movable bridge with a span that is raised at an angle by means of a counterweight. There are two kinds: single-leaf with one section, and double-leaf with two sections.

Cofferdam: A watertight enclosure made of piles or steel sheets sunk into a water bed. It can be pumped dry so that construction work can be done inside it.

Pneumatic Caisson: A cylinder with a cutting edge that can be sunk into the water bed. Water is forced out by compressed air.

The Bends: A crippling or fatal condition, caused by excess nitrogen in the blood, that can result from working in compressed air. It is also known as caisson disease. Keying: Extending the piers of a bridge into bedrock instead of simply resting them on top of the rock.

Falsework: The temporary scaffolding used to support an arch while it is being constructed.

Crane: A device that moves and lifts heavy weights. It is also called a jack. Anchorage: The terminal support, usually a huge block of concrete, for the main cables of a suspension bridge.

5.1. Vocabulary Practice

1. What is a span? Use the word as a verb.

2. What does the term masonry arch bridge usually refer to today?

3. What is a pontoon? How is it used in connection with bridges?

4. What is a beam bridge?

5. What kind of bridge is a suspension bridge?

6. What is a truss? How are trusses used in bridges?

7. What is a cantilever?

8. What is a steel arch bridge?

9. What does deck refer to in connection with a bridge?

10. What is a stay?

11. How does a concrete arch bridge differ from other masonry arch bridges?

12. What is a lift bridge?

13. What is a silting bridge? What is another term for a swing bridge?

14. What is a bascule bridge? What two different kinds are there?

15. What is a cofferdam? What does it make possible?

16. What is a pneumatic caisson?

17. What are the bends? What is another name for this condition?

18. What does keying mean?

20. What is a crane?

21. What is the anchorage of a suspension bridge?

Bridges

Bridges are among the most important, and often the most spectacular, of all civil engineering works. The imposing bridges that have survived from ancient times are arched structures of heavy masonry, usually stone or brick. Herodotus, the Greek historian of the fifth century B.C., however, mentions a wooden bridge across the Euphrates River at Babylon. In Rome, the bridge of Fabricius, built in 62 B.C. and named for its engineer, still carries traffic across the Tiber River, as does the Sant'Angelo Bridge, built in about 136 A.D. Both of these bridges, and many other Roman bridges, have a series of arche supported by heavy piers that extend down to bedrock. Ancient sources also mention pontoon bridges, usually in connection with military operations. A pontoon is a hollow drum that can float; a series of pontoons anchored to a riverbed can support a roadway. The Incas of pre-Columbian Peru built remarkable suspension bridges, supported by cables of natural fibers, that crossed many of the deep gorges in their mountainous country.

The sudden expansion in transportation systems that began in the eighteenth century, and still continues in our own day, has enormously increased the need for bridge^ as a part of highways and railroads. Better understanding of the forces that are exerted on structures and the improved materials that became available in the nineteenth century have made it possible to build increasingly longer and stronger bridges. With the ability to span greater distances, the damlike effect of masonry arch bridges with several heavy piers that block the flow of a stream can be largely eliminated.

The simplest type of span is a beam bridge, consisting of a rigid beam between two supports. Today most simple beam bridges are strengthened by a truss, which is based on the triangle. Diagonal beams that extend between the horizontal and vertical beams give support against both compression and tension. Many early truss bridges were built of wood; one that was erected across the Susque-hanna River in Pennsylvania in 1815 had a span of 110 meters. Iron and then steel were later used in the construction of truss bridges, still further increasing their strength. Trusses are not only strong but also light, because all unnecessary material has been eliminated in their design.

Another type of beam bridge is the cantilever, in which a horizontal beam extends beyond its support. Cantilever bridges, like trusses, had also been built before iron and steel became available. Most cantilever bridges have two arms of truss structure that meet or support a section between them. Cantilevers enabled bridge builders to span longer distances than truss bridges. During the nineteenth century, cantilevers were frequently used to build railroad bridges. The Quebec Bridge, which crosses the St. Lawrence River in Canada, is the longest cantilever bridge in the world, with a span of 540 meters. It was completed in 1917, and until 1929 it was the longest bridge of any type in the world.

A third type of modern bridge is the steel arch bridge, which can carry a roadway either above or below its arch of steel beams. An arch exerts strong downward and diagonal thrusts, so the piers that support it must be especially strong. Probably the most famous steel arch bridge is the Sydney Harbor Bridge in Australia, with a span of 495 meters. The Bayonne Bridge between New Jersey and Staten Island in New York has a span one meter longer.

Suspension bridges span even longer distances than other types of bridges. The longest bridge of any type is the Verra-zano-Narrows Bridge in New York, with a span of 1,280 meters. The deck or roadway of a suspension bridge is suspended from steel cables that are supported by massive towers. The first modem suspension bridges used linked chains made of wrought iron. Some of them survived for many years, like one across the Danube River in Budapest, Hungary. It was completed in 1849 and destroyed during World War II, nearly a hundred years later.

When steel became available, cables of steel wires replaced chains of wrought iron. Several suspension bridges built in this manner collapsed, however, as a result of storms or the movement created by the rhythm of the loads moving across them. It was later discovered that these failures were caused by the lack of truss supports for the deck. The first major cable-type suspension bridge to overcome these faults was designed by John A. Roebling at Niagara Falls. Its span of 250 meters was strengthened by trusses between the two decks. Roebling also used stays, inclined cables that ran from the main supporting cables to the deck, to stabilize the bridge. Roebling went on to design the Brooklyn Bridge in New York, which was completed in 1883 by his son, George Washington Roebling. The Brooklyn Bridge, with a span of 486 meters, is one of the most important - and one of the most esthetically satisfying - bridges ever built. The method devised by the Roeblings for laying the component wires that make up the cables for the Brooklyn Bridge is essentially the same technique used today.

The development of reinforced and prestressed concrete has given engineers other important materials for bridge building. Concrete has been used particularly for relatively short-span bridges that are a part of freeway systems. These bridges often use precast concrete beams. Many arch bridges have also been constructed of concrete. Currently, the longest concrete arch bridge is the Gladesville Bridge in Sydney, Australia. It has a span of 305 meters, and its deck is above the arch. This is another example of an esthetically pleasing bridge.

Many bridges that pass over rivers or canals must be movable so that shipping can pass under them. One type is the lift bridge, with towers that can raise the entire span between them by means of counterbalances and electric motors. Another type is the swing or pivot bridge, which pivots the span on a pier so that the bridge can swing open parallel to the river or canal. A third type is the bascule bridge, which has one or two arms that can open upward at an angle by means of counterweights. A bascule with one arm is a singleleaf bridge, and with two arms it is a double-leaf bridge.

Bridge construction can present extraordinary difficulties. Usually the foundations for the piers must rest on bedrock, and often under water. One technique for working in these conditions is by means of a cofferdam. Piles usually made of interlocking steel plates are driven into the water bed. The water is then pumped out from within the area that has been enclosed.

Another technique is the use of the pneumatic caisson. The caisson is a huge cylinder with a bottom edge that can cut into the water bed. When compressed air is pumped into it, the water is forced out. Caissons must be used with extreme care. For one thing, workers can only stay in the compression chamber for short periods of time. For another, if they come up to normal atmospheric pressure too rapidly, they are subject to the bends, or caisson disease as it is also called, which is a crippling or even fatal condition caused by excess nitrogen in the blood. When the Eads Bridge across the Mississippi River at St. Louis was under construction between 1867 and 1874, at a time when the danger of working in compressed air was not fully understood, fourteen deaths were caused by the bends.

When extra strength is necessary in the piers, they are sometimes keyed into the bedrock - that is, they are extended down into the bedrock. This method was used to build the piers for the Golden Gate Bridge in San Francisco, which is subject to strong tides and high winds, and is located in an earthquake zone. The drilling was carried out under water by deep-sea divers.

Where bedrock cannot be reached, piles are driven into the water bed. Today, the piles in construction are usually made of prestressed concrete beams. One ingenious technique, used for the Tappan Zee Bridge across the Hudson River in New York, is to rest a hollow concrete box on top of a layer of piles. When the box is pumped dry, it becomes buoyant enough to support a large proportion of the weight of the bridge.

Each type of bridge, indeed each individual bridge, presents special construction problems. With some truss bridges, the span is floated into position after the piers have been erected and then raised into place by means of jacks or cranes. Arch bridge can be constructed over a falsework, or temporary scaffolding. This method is usually employed with reinforced concrete arch bridges. With steel arches, however, a technique has been developed whereby the finished sections are held in place by wires that supply a cantilever support. Cranes move along the top of the arch to place new sections of steel while the tension in the cables increases.

With suspension bridges, the foundations and the towers are built first. Then a cable is run from the anchorage - -a concrete block in which the cable is fastened-up to the tower and across to the opposite tower and anchorage. A wheel that unwinds wire from a reel runs along this cable. When the reel reaches the other side, another wire is placed on it, and the wheel returns to its original position. When all the wires have been put in place, another machine moves along the cable to compact and to bind them. Construction begins on the deck when the cables are in place, with work progressing toward the middle from each end of the structure.

5.2. Discussion

1. How were the bridges that have survived from ancient times constructed?

2. What other kinds of bridges do ancient sources mention?

3. What kind of bridges did the Incas of pre-Columbian Peru build?

4. What has enormously increased the need for bridges?

5. What has made it possible to build longer and stronger bridges?

What advantage does a longer span give?

6. What is the simplest type of span?

7. How are most beam bridges strengthened today? What advantage in addition to strength does this construction give to a bridge?

8. What is another type of beam bridge? Are all bridges of this type built from iron or steel?

9. What is the longest cantilever bridge in the world? When was it built?

10. What is another type of modem bridge? Why must the piers that support this type of bridge be especially strong? What are two examples of this type of bridge?

11. What kind of bridge can span longer distances than other types of bridges? What is the longest bridge in the world?

12. How is the deck of a suspension bridge supported? What other system was once used? Give an example.

13. What happened to some of the earlier suspension bridges that had steel cables? What caused these failures?

14. How did John A. Roebling overcome the difficulties in steel cable suspension bridges? What two bridges did he design?

15. What two types of bridges are often built today with reinforced concrete? What is the longest concrete arch bridge?

16. Why must some bridges be movable?

17. What are three types of movable bridges? How do they differ from each other?

18. Where do the foundations for a bridge usually rest?

19. What is one technique used in the construction of the foundations for the piers‘of a bridge?

20. What is another technique that is used for working on foundations below water?

21. Why must great care be taken when caissons are used? What happened during the construction of the Eads Bridge in St. Louis?

22. What is sometimes done when extra strength is necessary in the piers?

23. What technique is used when bedrock cannot be reached?

24. What special technique was used for the Tappan Zee Bridge in New York?

25. How is the span sometimes raised into place with truss bridges?

26. What is the usual construction method for concrete arch bridges?

27. What method has been developed for the construction of steel arch bridges?

28. What is built first for a suspension bridge?

29. How are the cables put in place on a suspension bridge?

30. What part of the bridge is constructed after the cables are in place?

5.3. Review A. Match each expression on the left with one of the statements on the right. 1. Span a. A stone or brick bridge with arches spanning the 2. Pontoon bridge distance between piers. 3. Beam bridge b. A movable bridge that opens by swinging on a pivot. 4. Suspension bridge c. A cylinder with a cutting edge that can be sunk into a 5. Truss water bed. Water is forced out by compressed air. 6. Cantilever d. The traffic-bearing surface c a bridge. 7.Masonry arch bridge e. The distance between two supports of a bridge. 8.Concrete arch bridge f. A bridge with an arch of steel. 9. Steel arch bridge g. A bridge consisting of a rigid beam between two 10. Deck supports. 11. Lift bridge h. Temporary scaffolding use-to support an arch under 12. Swing bridge construction. 13. Bascule bridge i. A movable bridge that swings upward at an angle by 14. Cofferdam means of a counterweight. 15. Pneumatic caisson j. An enclosure of piles sunk into a water bed. Water can 16. Keying be pumped out of the enclosed area.

 

	17. Falsework		k. A framework strengthened by diagonal beams between horizontal and vertical beams. l. A movable bridge with counterweights and electric motion to raise the span. m. Extending the piers of a bridge into the bedrock. n. A bridge supported by cables usually hung from towers. o. A structure in which a horizontal beam extends beyond its support. p. A bridge supported by hollow drums that can float. q. A bridge with an arch of reinforced concrete.

 

 

UNIT SIX TUNNELS

Special Terms

Tunneling Shield: A large cylinder with a cutting edge that can be moved forward by jacks. It is used when tunneling through clay or soft rock.

Tail: The back part of a shield. The lining for a tunnel is usually assembled in the tail. Pneumatic Drill: A compressed air machine that is used to make holes in rock. Heading: The point from which work progresses on a tunnel. Most tunnels are bored from two headings.

Pilot Tunnel: A small exploratory tunnel bored in advance of a tunnel project along the same route. It provides geological information, as well as ventilation.

Immersed Tube: A tunnel made of prefabricated sections that are sunk into position. Cut-and-cover: A tunneling technique. A trench is excavated and the tunnel lining assembled in it. The trench is then filled in.

Dredging: Pumping silt or sand usually from the bottom of a body of water such as a river or a harbor.

Duct: A tube or channel that carries something, as an aqueduct carries water. A system of ducts is used for ventilation in most tunnels.

Mole: A tunneling machine that can bore through hard rock.

6.1. Vocabulary Practice

1. What is a tunneling shield? When is it used?

2. What is usually assembled in the tail of a tunneling shield?

3. What is a pneumatic drill?

4. What is the heading of a tunnel? How many headings are tunnels usually bored from?

5. What is a pilot tunnel? What are its purposes?

6. What is an immersed tube?

7. Describe the cut-and-cover tunneling technique.

8. What does dredging mean?

9. What is a duct? What are ducts used for in tunnels?

10. What does the term mole refer to in tunneling?

Tunnels

Most of the tunnels before the great age of railroad construction were built in connection with mines, water supply systems, or canals. As we noted Unit 4, the maximum grade at which a steam locomotive could operate efficiently was 2 percent. This fact encouraged the construction of a great many tunnels. As more tunnels were built, many significant technical advances were made in boring both through underwater clay and through rock.

One of the most remarkable tunnel building feats was the construction of the first tunnel under the Thames River in London. This tunnel is still used by the London Underground Railway System. It was dug out of clay beneath the riverbed between 1825 and 1842 under the direction of Sir Marc Brunei, who had designed a tunneling shield that made the work possible. The shield offered the workers protection while they dug out clay and mud in the face of the shield. It was then moved forward by jacks so that the process could be repeated.

One of the first major tunnels built through rock was the Mon Cenis Tunnel in the Mps between France and Italy. It is fourteen kilometers long and was built between 1857 and 1871. When construction began on it, progress was only twenty- two centimeters a day. Fortunately, the pneumatic drill, which uses compressed air to bore holes in rock, was invented a few years after construction began.Thereafter, the tunneling speed was increased to two meters a day. Like most tunnels, the Mont Cenis was bored from two different headings, one in France and one in Italy, which met in the middle.

Another tunnel under the Alps, the Simplon, remains a major engineering accomplishment. It runs for a distance of nineteen kilometers, and at one point, it is more than 2,100 meters under the crest of a mountain. It was built between 1898 and 1906, and extraordinary difficulties had to be overcome: extremely high temperatures, rock that burst off the walls because of the pressure, springs of both cold and hot water, and layers of soft stone that required special supports. It remains the deepest tunnel ever constructed.

The usual technique for tunneling through hard rock is to drill holes in the face of the heading. The holes are filled with an explosive that is detonated after the workers and equipment are withdrawn to a safe distance. After fumes and rock dust have settled, the rock brought down by the explosion is removed, often on conveyor belts. In many projects, like the Simplon, for example, a small pilot tunnel is driven before the full diameter of the tunnel is excavated. This technique helps engineers to determine the geological features of the rock through which the tunnel is passing so that many difficulties can be anticipated. In some cases, work on the main tunr.el progresses a few meters behind the pilot tunnel so that the latter provides ventilation, always a major problem in tunneling operations.

Shields that are modifications of the one that Brunei developed for boring the Thames Tunnel are used in excavating through clay or soft rock. A shield has a sharp edge that is driven into the tunnel face by hydraulic jacks. The top edge of the shield projects for a short distance in order to protect the workers. Behind the cutting edge is the tail, which has a smaller diameter. The permanent lining of the tunnel is assembled in this area. The space between the lining and the larger diameter that has been excavated by the forward part of the shield is filled with a grout that is pumped in under pressure. Modern shields are highly mechanized so that many phases of the tunneling process can be performed almost completely by machine.

Silt, the soft mud that is typical of riverbeds and other underwater tunneling sites, presents difficult problems. In Brunei's Thames Tunnel, water broke through on several occasions, and the tunnel had to be pumped dry. In addition, Brunei had loads of clay dumped into the riverbed to make it more impervious to water. Modem engineers, of course, have developed and now use more sophisticated soil stabilization techniques when they tunnel through silt, sand, or loose material such as gravel. Most underwater tunnel construction through these materials utilizes shields with compressed air, similar to the caissons we described in the previous unit. The same precaution must be taken to protect workers from being exposed to the dangers of extreme pressure.

Another technique that has come into wide use in recent years is the immersed- tube system. In this technique a channel is dredged along the line of the tunnel; in other words, silt is pumped out of the water bed. Piles are then driven along the channel, and prefabricated sections of the tunnel are lowered into place onto the piles. The tunnel sections are closed by temporary faces that are removed after all the sections have been assembled. The joints between the sections are also made watertight at this stage of construction. Finally, the channel is filled in to give the tunnel greater stability. The Maas River Tunnel at Rotterdam is an immersed tube. Another is the recently opened tube between San Francisco and Oakland, California, for the Bay Area Rapid Transit System.

A technique that is often used for subway construction is the cut-and-co\ er method. Workers excavate a trench, inside of which the lining for the tunnel is built. It is then covered over with the earth or other fill that was originally removed. The cut-and-cover method obviously can only be used when the tunnel is immediately beneath the surface. One difficulty is that normal street traffic must often be allowed to continue during construction. In such a case, wooden beams or steel plates are laid down to cover the excavation.

Ventilation is a major problem in all tunnels, but particularly in those to be used by automobile traffic. The exhaust fumes of automobiles contain carbon monoxide, a deadly gas. Most automobile tunnels therefore have two systems of ducts. Huge fans pump in fresh air through one of them, while polluted air is sucked out through the other.

A good example of the complexity of engineering a modern highway system can be illustrated by the Chesapeake Bay Bridge-tunnel. This crossing between Maryland and Virginia over one of the principal waterways in the United States is twenty-eight kilometers long. Most of the highway is supported on concrete piles with short spans between them. It was also necessary, however, to leave four ship channels. Two of these are spanned by bridges, but the other two are provided by tunnels. The entrances to the two tunnel sections are from man-made islands in the bay. The tunnels themselves are steel tubes which were placed in a channel that had been dredged in the bay bottom.

Another great engineering project that is still in the process of construction is the Seikan Railroad Tunnel in Japan. This tunnel, thirty-six kilometers long, will connect the islands of Honshu and Hokkaido, passing 100 meters under the surface of the strait between them; the strait itself is 140 meters at its deepest point. The route is now being explored by a pilot tunnel in order to determine the geological formations and types of rock through which the tunnel must be bored. The Japanese are also experimenting with new types of machines called moles, which can bore through h^rd rock by mechanical means.

6.2. Discussion

1. Before the great age of railroad construction, in what connection were most tunnels built?

2. What fact encouraged the construction of many tunnels? What other result did increased tunnel construction bring about?

3. When was the first tunnel built under the Thames River in London? Under whose direction?

4. What made the construction of Brunei's Thames Tunnel possible?

5. What was one of the first major tunnels built through rock? How long is it? When was it built?

6. How rapid was progress at the beginning? What increased the speed of construction?

7. From how many headings was the Mont Cenis Tunnel bored? Is this a usual practice?

8. How long is the Simplon Tunnel? How far is it beneath the surface at one point?

9. What difficulties had to be overcome in excavating the Simplon Tunnel?

10. What is the usual technique for tunneling through hard rock?

11. What is sometimes excavated in advance of the main tunnel? What purposes does this ^c:erve?

12. Describe a tunneling shield.

13. Why is there a space between the excavated space and the tunnel lining? How is it filled?

14. What is silt and where is it often encountered?

15. How did Brunei deal with the problem of water in his tunnel?

16. How is tunneling through silt accomplished today?

17. How is a tunnel constructed by the immersed-tube technique? What are some examples of tunnels built by this method?

18. What technique is often used for subway construction?

19. What condition is necessary when employing the cut-and-cover method? What is one difficulty with this method, and how is it overcome?

20. Why is ventilation a major problem in automobile tunnels?

21. How is it dealt with in most automobile tunnels?

22. Describe the Chesapeake Bay Bridge-tunnel.

23. What major tunnel is now under construction in Japan? How long will this tunnel be? How far under sea level will it pass?

24. What are the Japanese experimenting with in the construction of this tunnel?

6.3. Review

A. Fill in each of the blanks in the following sentences with the appropriate word or phrase.

1. A tunneling shield is moved forward by....

2. The face of a tunneling shield has a larger... than its tail.

3. The lining of a tunnel excavated by means of a shield is somewhat smaller than the area excavated; the space is filled in by a... pumped in 'under pressure.

4. When a tunnel is being excavated through..., a shield with compressed air, a kind of caisson, is ordinarily used.

5. Workers under compressed air conditions must be protected against an illness called... or....

6.... drills, which operate with compressed air, are used to bore holes in hard rock.

7. When holes have been bored in hard rock, they are filled with an... which is then detonated.

8. A pilot tunnel is often bored in advance of the main tunnel in order to determine the... features that will be encountered.

9. It is common practice today to use soil... techniques for tunneling through silt, sand, or gravel.

10. When a tunnel is being built immediately beneath the surface, the... method is often employed.

11. Where traffic has to continue above a shallow trench, wooden... or... plates are often laid down to provide a surface.

12.... is a major problem in automobile tunnels because exhaust fumes contain carbon monoxide, a deadly gas.

13. Fresh air is pumped in through one while polluted air is sucked out through another.

14. A... is a new type of machine that can bore through hard rock.

B. Describe briefly the tunneling technique that would be used to bore through each of the different kinds of material given below. If more than one technique could be used, describe both.

1. Clay or soft rock.

2. Silt, sand, or gravel.

3. Hard rock.

4. Earth or loose rock immediately below the surface.

 

 

UNIT SEVEN

HYDRAULIC ENGINEERING DAMS AND CANALS

Special Terms

Hydraulics: The science that deals with the flow and control of water and other fluids. Hydraulic engineering is chiefly concerned with projects that involve water, such as dams and canals.

Dam: A barrier built to stop or control the flow of water.

Topography: A detailed survey or study of the physical features of a given area. A topographical map shows these details.

Masonry Dam: A dam built of some kind of masonry such as cut stone, concrete, and so on.

Embankment Dam: A dam built with a fill of compacted earth, crushed rock, and so on. Seepage: Slow leaking of water or another fluid through porous material like earth or some types of rock, such as limestone.

Gravity Dam: A masonry dam in which the water is held back by the weight of the structure. The vertical profile is triangular.

Gravity-arch Dam: A gravity dam curved in an arch or semicircle.

Buttress Dam: A masonry dam strengthened by buttresses.

Earth-fill Dam: An embankment dam in which the principal material is compacted earth.

Rock-fill Dam: An embankment dam in which the principal material is crushed rock that has been compacted.