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1. Curve Setting

2. LIST OF TOPICS
1
Introduction
Curve Setting
2
Simple Circular Curves
3
Transition curves
Transition curve and super elevation
4
Development Surveys
Setting a point of known coordinate, control of direction and gradient in drifts, tunnels,
raises and winzes; application of lasers; Problems of underground traversing.
Elements, laying of simple circular curves on surface and belowground.

3. Introduction
• In the geometric design of motorways,
railways, pipelines, etc., the design and
setting out of curves is an important
aspect of the engineer’s work.
• A curve is required to facilitate gradual
change of direction from one straight path
to another.
• In surface and underground mines the
design of haulage tracks, haul roads
frequently require setting out of curves so
as to overcome any obstacle intervening a
straight path or in order to avoid
derailment or skidding of haulage tubs etc.

4. Classification of curves
Curves can be listed under three main headings, as
follows:
(1) Circular curves of constant radius.
(2) Transition curves of varying radius (spirals).
(3) Vertical curves of parabolic form.
Curves
Horizontal curves
Simple curve
Compound curve
Reverse Curve
Transition curve
Lemniscate curve
Vertical curve
Summit curve
Valley curve
• Curves used in horizontal planes to connect two
straight tangent sections are called horizontal curves.
• Those curves that exist in vertical planes, are called
vertical curves.

5. Different types of circular curves
A simple curve
is a circular arc
connecting
two tangents.
It is the type
most often
used.
A compound
curve is
composed of
two or more
circular
arcs of different
radii tangent to
each other, with
their centers on
the same side of
the alignment.
The combination
of a short length
of tangent (less
than 30 m)
connecting two
circular arcs that
have centers on
the same side is
called a broken-
back curve.
A reverse curve
consists of two
circular arcs
tangent to each
other, with their
centers on
opposite sides of
the alignment.

6. Degree of a Circular Curve
The rate of curvature of circular curves can be designated either by their radius (e.g., a 1500-m curve), or by their degree
of curve. There are two different designations for degree of curve, the arc definition and the chord definition.
By the chord definition, degree of curve is the angle at
the center of a circular arc subtended by a chord of 30 m
(usually). This definition is convenient for very gentle
curves and hence is preferred for railroads.
By the arc definition, degree of curve is the
central angle subtended by a circular arc of
30 m (usually). This definition is preferred
for highway work.

7. Elements of a Circular Curve
• The point of intersection PI, of the two tangents is also called the
vertex, V. In stationing, the back tangent precedes the PI, the forward
tangent follows it.
• The beginning of the curve, or point of curvature PC, and the end of the
curve, or point of tangency PT, are also sometimes called BC and EC,
respectively.
• Other expressions for these points are tangent to curve, TC, and curve
to tangent, CT.
• The curve radius is R. Note that the radii at the PC and PT are
perpendicular to the back tangent and forward tangent, respectively.
• The distance from PC to PI and from PI to PT is called the tangent
distance, T.
• The line connecting the PC and PT is the long chord LC. The length of the curve, L, is the distance from PC to PT, measured along
the curve for the arc definition, or by 30 m (100 feet) chords for the chord definition.
• The external distance E is the length from the PI to the curve midpoint on a radial line.
• The middle ordinate M is the (radial) distance from the midpoint of the long chord to the curve’s midpoint.
• Any point on curve is POC; any point on tangent, POT.
• The degree of any curve is Da (arc definition) or Dc (chord definition).
• The change in direction of two tangents is the intersection angle I, which is also equal to the central angle subtended by the
curve.

8. Geometrics of a Circular Curve
Length of chord
Radius of Curve
Length of Arc
Tangent distance

9. Setting out Simple Circular Curve
Curves may be set out in various ways depending on
• The location of curve
• Its length
• The degree of accuracy required
• The instruments available, and
• The presence of obstacles
Depending on the instruments used the methods
of setting out simple circular curves may be
grouped in to two classes
• Linear methods: Used when high degree of
accuracy is not desired and the length of curve
is short.
• Angular methods: Usually a theodolite is used
with or without chain or tape. Nowadays,
advanced instruments like total station are
used.
Curve
setting
methods
Chords and offsets outside
the curve
Tangents and offsets
Chords and angles method
Rankine’s method or
tangential angles method
By two theodolites

10. Setting out by Offsets from the long chord
Before a curve is set out, it is essential to locate
• The tangents
• Point of intersection
• Point of curve, and
• Point of tangent

11. Setting out with Perpendicular Offsets from the
tangent

12. Setting out with Perpendicular Offsets from the
tangent

13. Setting out with Radial Offsets from the tangent

14. Setting out with Radial Offsets from the tangent

15. Setting out by Rankine’s Method
T

16. Setting out by Rankine’s Method
T

17. Setting out by Two Theodolites Method

18. Setting out by Two Theodolites Method

19. Chainages along Simple circular curves
Ans.
a) 346.41 m
b) 1710.03 m
c) 2338.35 m
d) 600

20. Superelevation
When the vehicle is running along a straight, the only force acting is the weight of
the vehicle W, acting vertically downwards and the weight is equally shared by the
two wheels.
As soon as the vehicle starts moving on a curve, there are two forces, P acting
horizontally outward and other is W acting downwards. The resultant R of these
two forces will be OA meeting the road surface at A.
Let AE and AB are its two components. The horizontal component AE resisted by
the friction between the wheel and ground. The vertical component AB is to be
shared unequally by the wheels C and D.
In such situation, as may be observed from the figure that load shared by wheel C
is more.
Now, the position of R depends upon the force P, which in turn depends on the
speed of the vehicle. Thus, if R will move nearer and nearer to wheel C, with
further increase in the force P, a time may come when due to increased speed the
position of R may pass over wheel C, in such case the whole load will be borne by
wheel C and none by wheel D. Thus the vehicle may topple.
Therefore, in order to equalize the pressure on two wheels the outer edge of the
road is raised by an amount h called superelevation, so that R should be
perpendicular to the surface of the road.

21. Let O be the center of the curve, R be the radius of the curve,
and v be the speed of the vehicle
t be the time required to travel an arc PP’, θ be the angle
subtended by the arc PP' at the center. As the vehicle moves
along the curve from P to P’, the direction of the speed after
time t becomes along MN, where angle POP' is small and equals
to θ. Resolving the speed v parallel and perpendicular to PO.
Component along PO is v sin θ. Component along PK is v cos θ.
Superelevation – Travelling on a curve

22. Superelevation - Estimate
Centrifugal ratio
The ratio of centrifugal force to the weight is called
centrifugal ratio.

23. Transition Curves
A transition curve or easement curve is a
curve of varying radius introduced
between a straight and a circular curve
for the purpose of giving easy changes of
direction on a route.
• It provides comfort to the passengers.
• It allows higher speed at the turnings.
• It eliminates the danger of derailment
and prevents the vehicle from toppling
over on curves.
• There will be less wear upon the running
gear.
Advantages

24. Transition Curves
To avoid these effects, a
small length of curve is
needed between the
straight and the circular
curve. The length of this
curve should be such
that its radius from
infinity at the straight
should decrease
gradually at a certain rate
so as to reach to a value
R of the circular curve,
when it joins the circular
curve. The same type of
curve will also be
required to join when a
vehicle moving along the
curve is required to join
the straight.

25. Length of transition curve

26. Length of transition curve
Example 17.1. Calculate the length of a transition curve to be introduced between a straight and a curve such that
15 cm superelevation may be introduced over the circular curve. Assume the rate of superelevation as 1 in 500.
(Ans. 75 m)
Example 17.2. Calculate the length of a transition curve to be inserted between a straight and a circular curve such
that a superelevation of 15 cm over a circular curve may be attained. Assume the rate of attaining superelevation as
2.5 cm per second and average speed of the vehicles as 60 km/hour. (Ans. 100 m)
Example 17.3. The maximum allowable speed on a curve is 80 km/hour and the rate of change of radial acceleration
is 30 cm/sec2. Calculate the length of the transition curve if the radius of the circular curve is 200 metres. (Ans. 182.9
m)

27. Length of transition curve
Ans. (a) 113.28 m (b) 136.04 m
Example:
A road deflects at an angle of 60° at a certain point to follow the path of another road. It is desired to connect the two
straights with a circular simple curve. If the maximum speed of the vehicle is 60 km/hour and the centrifugal ratio for
a road is 1/4, calculate
(a) The radius of the circular curve, and
(b) The length of the transition curve.
The rate of change of radial acceleration may be taken as 30 cm/sec2

28. Correlation Survey
Transferring the surface alignment through a vertical shaft is difficult
operation in view of the small size of the shaft. Generally, plumb wires
are used to transfer directions underground. Essentially, the plumb
wires produce a vertical reference plane, and on the surface the plane
can be placed in the line of sight; below ground, the line of sight can be
sighted into that plane. This is known as co-planing, and the line of
sight when established can be used to set up floor or roof stations
within the tunnel.
Accurate transfer of surface alignment down a vertical shaft using two
plumb wires can be achieved by Weisbach triangle method.
In Fig. 9.3, p and q are plan positions of the plumb wires P and Q on the
ground surface alignment above the tunnel, respectively. A theodolite,
reading directly one second, is set up at A’, approximately in line with p
and q. In triangle pA'q, the angle pA'q is measured by the method of
repetition, and the lengths of sides are also measured correct up to
millimeter. The angle pq A’ is also calculated by applying sine rule.
Now, the perpendicular distance d of A' from the line qp
produced, is calculated from the following expression.
The point p and q are joined by a fine thread, and a
perpendicular AA' equal to d in length is dropped from A'
on the thread. The foot of perpendicular A is the required
point on the line qp produced which may be occupied by
the theodolite for fixing the points on the floor or roof of
the tunnel.