TECHNICAL REPORT ON SURVEYING (Setting out a simple curve)
Chapter one
INTRODUCTION
Curves are provided whenever a
road changes its direction from right to S (vice versa) or change sits
alignment from up to down (vice versa). Curves are a critical element in the pavement
design. They are provided with a maximum speed limit that should lie
followed very strictly. Following the speed limit becomes essential as the
exceed in speed may lead to the chances of the vehicle becoming out of control
while negotiating a turn and thus increase the odds of fatal accidents. Also,
it is very necessary that appropriate safety measures be adopted at all
horizontal and vertical curves to make the infrastructure road user friendly
and decrease the risks of hazardous circumstances. The center lines of highways
and railroads consist of a series of straight lines connected
by curves. The curves of fast traffic are normally circular, although
spiral curves may be used to provide gradual transitions to or from the
circular curves. There are different kinds of curves, the simple curve,
compound curve, the reverse curve, and the spiral curve. The simple curve is
consists of a single arc. The compound curve consists of two or more arcs
with different radii. The reverse curve consists of two arcs that curve in
different directions. And the spiral curve (or transition curve) has a varying
radius, so that the curve starts out very flat and increases in sharpness as we
move into the curve.
In this field work, simple curve is the one that was focus. A
curve is initially laid out with two straight lines or tangents. These lines
are extended until they intersect and the point of intersection is called P.I. The first tangent encountered
is called the back tangent and the second one is
called the forward tangent
SURVEY: Is define as taking a general view of, by observation and
measurement determining the boundaries, size, position, quantity, condition,
value etc. of the land , estate, buildings, farms mines etc. and finally
presenting the survey data in a suitable form.
This covered the
work of the valuation surveyor and quantity surveyor, the building surveyor,
the mining surveyor, and so forth, as well as land surveyor.
The process of surveying is therefore in three stages namely
1
taking a general view
2
observation and measurement
3
presentation of datas
TYPES OF SURVEY
On
the bas
Definition of Curves:
Curves
are regular bends provided in the lines of communication like roads, railways
etc. and also in canals to bring about the gradual change of direction. They
are also used in the vertical plane at all changes of grade to avoid the abrupt
change of grade at the apex.
Curves provided
in the horizontal plane to have the gradual change in direction are known as
Horizontal curves, whereas those provided in the vertical plane to obtain the
gradual change in grade are known as vertical curves. Curves are laid out on
the ground along the Centre line of the work. They may be circular or
parabolic.
Types of curves
(i)
Simple curve,
(ii)
Compound curve
(iii)
Reverse curve
(iv)
Spiral Deviation curve
(i) Simple Curve:
A simple curve consists of a single arc
of a circle connecting two straights. It has radius of the same magnitude
throughout. In fig. 11.1 T1 D T2 is the simple curve with T1O as its radius.
Chapter two
AIMS OF CARRYING OUT SIMPLE CURVES
1 The goal of this field work is to be
able to lay a simple curve by deflection angle.
2 To
set out the simple curves in between two center line of highways which are meet
at one point by deflection angle method.
In this
method, curves are staked out by use of deflection angles turned at the point
of curvature from the tangent to points along the curve. The curve is set
out by driving pegs at regular interval equal to the length of the
normal chord. Usually, the sub-chords are provided at the beginning and end of
the curve to adjust the actual length of the curve. The method is based on the
assumption that there is no difference between length of the arcs and their
corresponding chords of normal length or less. The underlying principle of this
method is that the deflection angle to any point on the circular curve is
measured by the one-half the angle subtended at the center of the circle
by the arc from the P.C. to that point.
INSTRUMENTS
1 Range Poles- is a surveying instrument used for marking the
position of stations and for sightings of those stations as well as for
ranging the straight lines.
2 Chalk Used to put marking on pavements
3 50 Meter Tape: These are designed for most conventional
measurements in surveying and in engineering work.
4. Theodolite -a surveying instrument
with a rotating telescope for measuring horizontal and vertical angles
5 peg : to mark out important
points in the field.
6 hammer : to drive the pegs in every
points.
PROCEDURES
1. The obtained length of long chord is
first set out on the field by proper ranging and midpoint is established.
2. The length of offsets at mid length
is to be set out . for this, a person holds the tape at required point and
aligns the slit with the end station ranging poles. At this instant another
looks through the normal slit and guides a person with a poles to come into its
view thus along this line normal to long chord, the calculated offset is setout
3. The tape is shifted to the next point distance ‘X’ as specified
and above steps is repeated the offset corresponding to that distance is setout
from the point
4. Pegs are marked at the end of the
offsets, the joining of which completes the setting
OBJECTIVES
To be able to lay a simple curve by deflection angle.
To master the skill in leveling, orienting and using the transit
effectively.
RESULT
The simple curve is set out by the method of deflection angle in the field to change direction.
RESEARCH AND DISCUSSIONS
The degree of curvature is defined as the
central angle to the ends of an arc or chord of agreed length. Various lengths are
commonly used in different areas of practice. This angle is also the
change in forward direction as that portion of the curve is traveled. Curvature
is usually measured in radius of curvature. A small circle can be easily laid
out by just using radius of curvature, but if
the radius is large as a km or a mile,
degree of curvature is more convenient for calculating and laying out
the curve of large scale works like roads and
rail roads. By this method curve setting can be easily done with the
help of a transit or theodolite and a chain, tape or rope of a prescribed
length. One of the methods in getting a simple curve is by incremental
chords and deflection angle method. In this method, curves are staked out by
use of deflection angles turned at the point of curvature from the tangent to
points along the curve. The curve is set out by driving pegs at regular interval
equal to the length of the normal chord. In the incremental method, the
instrument is setup on the PC, deflection angles are measured from the back
tangent, and chords are measured from the previous set station. Usually, the
sub-chords are provided at the beginning and end of the curve to adjust the
actual length of the curve. The method is based on the assumption that there is
no difference between length of the arcs and their corresponding chords of
normal length or less. The underlying principle of this method is that the
deflection angle to any point on the circular curve is measured by the one-half
the angle subtended at the center of the circle by the arc from the P.C. to
that point. The deflection method is the premier curve location method for use
when running grade lines. It allows you to check special situations, such as
sharp, v-shaped draws to see if a particular curve will fit to the location
without undue cuts or fills. It can also be used to check grade line (the
variation or deflection from one tangent to the next, or from one ribbon
location to the next) to see if the deflection-angle exceeds a critical value,
meaning that the minimum radius rule (e.g. 60 ft) may not be met.
APPLICATION
Curves are used on highways and railways where it is
necessary to change the direction of motion
CONCLUSION
There are too many process how surveyors
does a curve on a railroad, highways, expressways, and etc. It is not easy for
it to just set a curve without any aid of surveying. If we will notice
some of the roads, some of it has its mistakes, which led to accidents,
particularly in the sharp curves , it has the most critical part on the roads.
In civil engineering, we do not only consider on the structure itself but
also the safety of each individual who will benefit to it.
As
mentioned, the objectives of the activities were to be able compute the simple
curve by deflection angle method and to be able to lay out a simple curve
through a location of point on the curve.
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