Unit 5 ROAD DESIGN study

Authors: Do Viet Hai – Phan Hoang Nam Unit 5: ROAD DESIGN Road selection and design depend on the nature of the subgrade; the traffic and drainage conditions; the construction time avai

Authors: Do Viet Hai – Phan Hoang Nam

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

Authors: Do Viet Hai – Phan Hoang Nam

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

GEOMETRIC 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

Authors: Do Viet Hai – Phan Hoang Nam

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

Authors: Do Viet Hai – Phan Hoang Nam

Vocabulary

subgrade

traffic

drainage

expertise

surface course

base course

freezing

thawing

combat zone

through cut

side-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

Authors: Do Viet Hai – Phan Hoang Nam

Further reading

HORIZONTAL ALIGNMENT AND HORIZONTAL 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 for the 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

Authors: Do Viet Hai – Phan Hoang Nam

 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