giáo_trình_tiếng_anh_chuyên_ngành_cầu_-_Đường_(unit_5)

8/11/2019 Gio_trnh_ting_Anh_chuyn_ngnh_Cu_-_ng_(Unit_5)

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Authors: Do Viet Hai Phan Hoang Nam

1 English for Road and Bridge Engineering

Unit 5: ROAD DESIGN

Road selection and design depend on the nature of the subgrade; the traffic and

drainage conditions; the construction time available; the supply of local and

imported materials; and the engineer equipment, personnel, and expertise available.The completed design must then meet the requirements for the given load class and

allow safe and efficient traffic movement.

The load-carrying capacity of a road surface depends on continuous, stable support

furnished by the subgrade. Subgrade stability requires adequate drainage and proper

load distribution by the surface and base courses. Surface and base courses of

sufficient thickness and quality to spread the wheel loads over the subgrade are

necessary so that the applied stress is less than the unit load capacity of the

subgrade. In areas where seasonal freezing and thawing occur, the load-carrying

capacity of inadequately designed or improperly constructed roads can be

dramatically decreased to the extent that failure may occur.

For safe and speedy traffic movement, the geometric design requirement for given

road classes must be met. In a combat zone, military urgency dictates rough, hasty

work designed to meet pressing needs. An improved network of well-surfaced,

high-quality roads may be required in rear areas and near major airfields, ports, and

supply installations.

Figure 1 Road nomenclature


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Figure 2 Road cross section and nomenclature

Road design uses stage construction for the progressive improvement of the road to

meet increased traffic demands. Road design also uses many technical terms.

Figures 1 and 2, show terms used to designate road features and components.

GEOM ETRIC DESIGN

The geometric design process begins with good-quality topographic surveys. In

most cases, a minimum 5-foot contour interval is required to clearly describe the

terrain. The design process can be described in the following steps:

1.

Draw the proposed centerline on the topographic survey.

2.

Plot the centerline on plan-and-profile paper.

3. Calculate grades, the degree of curvature of horizontal curves, and curve

lengths of vertical curves.

4.

Compare the values of step 3 with the military road specifications.

5. Adjust the centerline, if possible, to reduce any calculated grades and limit

horizontal and vertical curves that exceed the specifications.

6.

Plot new tangents (straight sections of road) on the plan and profile in those

locations where horizontal and vertical curves exceed the military road

specifications.

7.

Design horizontal and vertical curves for all tangent intersections.

8.

Plot newly designed curves on the plan and profile.


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9.

Develop a mass diagram for the project. Balance the cuts and fills and

optimize ruling grade and earthwork volumes.

10.

Design superelevations (curve banking) and widening for all horizontal

curves.

11.

Draw typical cross sections.

12.Design the required drainage structures and bridges.

GRADE AND ALIGNMENT

Before building a road or an airfield, the engineer must determine the best vertical

and horizontal alignment of the facility concerned. Design both horizontal and

vertical alignment to keep sight distance restrictions to a minimum. Define the route

by a series of straight lines and curves to meet the stated mission and capacity. This

provides the shortest, most efficient route that requires the least construction effort.

Define the route vertically in a series of grades and curves that fall within

acceptable specifications and requirements. Horizontal and vertical alignment are

interrelated and must be considered concurrently. However, the principles on each

are best studied separately.


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Vocabulary

Word Pronounced Meaning

subgrade

traffic

drainage

expertise

surface course

base course

freezing

thawing

combat zone

through cutside-hill cut

culvert

traveled way

road bed

road way

traffic lane

interceptor ditch

cut slope

ditch slope

fill slope

crown

topographic survey

tangent

superelevation

alignment

terrain condition

simple

reverse

compound

spiral


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Further reading

HORIZONTAL ALIGNM ENT AND HORIZON TAL CURVES

The principles of horizontal alignment are summarized as follows:

Tangents (straight sections of road) should be as long as possible, because the

shortest distance between two points is the connecting straight line. Terrain

conditions, however, seldom permit the construction of a route between two points

in one tangent line. Therefore, the engineer should make each tangent as long as

possible, limit the number of curves, and provide long straight stretches, thereby

improving the route capacity.

Make curves as gentle as possible. Long, gentle curves increase the capacity of the

roadway by permitting higher speeds. They also provide a safer path of travel forthe vehicle. Making gentle, horizontal curves will increase the curve length, thereby

decreasing the tangent length. However, this reduction in tangent length is minor

compared to the benefits gained by reducing the total number of curves.

Tangents should intersect other roads and railroads at right angles. Military roads

normally supplement existing roadnets and have intersections at one or both ends of

the military road. Operating efficiency usually is improved when these intersections

approach right angles.

Frequently used horizontal curves are shown in Figure 3. The most common are the

simple curve, the reverse curve, the compound curve, and the spiral curve.

A simple curve uses the are of a circle to provide a smooth transition

between two tangents. This curve is used frequently in the TO because it fills

the needs of the low-speed design roads normally used and is easy to

construct. A reverse or compound curve can be designed using the same

basic equations.

A reverse curve uses two simple curves tangent to a common line at a

common point. Their centers are on opposite sides of the common line. The

radii of the curves may or may not be equal in length.


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A compound curve has two simple curves tangent to a common line at a

common point. The centers of these curves are on the same side of the

common line, and the curves have radii of different lengths.

A spiral curve is a simple curve in the center with parts of a spiral on each

end to smooth transition to the tangent. The spiral is used only on high-speed

roads (classes A and B). Low design speeds of class-C and -D roads do not

require spiral transition sections.

Figure 3 Types of horizontal curves

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