Page 344 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

8.6 THE WORK

Curves may also be defined according to the length of the radius, that is, the distance from any
point on the curve to the center. The radius of a 1° curve is 5,730 feet. A 2° curve will have half
that radius, or 2,865 feet. A short radius means a sharp curve.
Either measurement may be converted into the other by using one of the formulas:
5,730
Radius
degree of curve
5,730
Degree of curve
radius
If someone were in a hurry, or trying to work the problem out in his or her head, the person could
change the 5,730 to 6,000 and be less than 5 percent off.
A highway curve may be compound. A compound curve includes two or more arcs having dif-
ferent degrees of curve, and may include some short straight lines also.

Tangents. In highway work and in many other surveys, straight lines are called tangents. This
is because they are tangent to the connecting curves. This one word makes clear that a straight line
forms a smooth continuation of an adjoining curve.
Tangents vary in length from a few inches up to many miles.
Whenever two tangents are joined by a curve, lines are drawn on the plan continuing them until
they cross each other outside the curve. The point where they meet is called the point of intersection,
or PI for short. The tangent distance is measured between the point where the tangent meets the
curve (called PC at the beginning of the curve, or PT at its end) to the PI. The side or deflection
(def.) angle formed by the intersection of the extended tangents equals the central angle of the
simple curve between the tangents.

Baseline. In making a road survey, the engineers first lay out a baseline that follows the general
route of the road, but that may be partly or wholly outside of the right of way. This line is often
made before the exact location of the highway has been decided.
The baseline is very carefully surveyed and marked. Some points on it, called hubs, are more
important to the engineers than any of the road line stakes. They may be conspicuously marked,
or concealed with leaves or rocks.
No contractor or employee should ever destroy, move, or otherwise interfere with any stake or
marker on the job, regardless of whether it seems to fit in with the markers he or she recognizes.
Centerline. The centerline is the basic location reference for the highway itself. It is the center
of the pavement in a single road, or the center of the median division of a dual highway whose
two roadways are a fixed distance from each other.
The engineers set the centerline according to angles and distances from baseline points.
Measurements are made along it with a steel tape (an operation often called chaining), and stakes are
set at 100-foot intervals.
All distances are measured along the centerline, and structures and stakes are located in refer-
ence to it. It is also the basis for grade calculations for single roadways.
There are a number of construction lines that run parallel or almost parallel to the centerline.
These include pavement, shoulder, gutter, and slope edges. They are usually measured off from
the centerline, at right angles to it on straight stretches, and along radial lines on curves.
Profile. The profile of a road is the vertical alignment of the centerline or of a theoretical grade
line. It is a representation of its rise and fall, without indication of whether its route is straight or
curved.
Two profiles are prepared, one of the existing ground surface, the other of the proposed pave-
ment surface. Both of these are drawn on one sheet or roll of cross-section paper marked off in
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squares of ⁄ 10 inch, with inch squares indicated by heavier lines.