#civil engineering(diploma) # civil(specialized in hydropower)

1. 3. Bridge survey
3.1 Introduction
Bridges are the structures that are constructed with the purpose of
connecting two places separated by deep valleys, gorges, rivers or
stream. In countries like Nepal, where the land is undulated and there
are plenty of rivers, bridges are the most economic and convenient way
to join the two places. Bridge construction is an important aspect in the
development of transportation network. For the construction of bridge,
surveying is required for topographical mapping, while the knowledge of
longitudinal section of river and cross-section at both upstream and
downstream is essential. The river flow level in different seasons should
also be taken into consideration before designing a bridge.
3.2 Objectives:
 To select the possible bridge site and bridge axis for bridge
construction.
 To collect preliminary data of the site i.e. water flow level, high flood
level etc.
 To study about the geological features of the ground.
 To carry out surveying for topographical mapping, longitudinal and
cross- sectioning at both the upstream and downstream side of the
river.

2. 3.3 Brief description of the site:
The bridge survey is done outside the premises of camp. It takes around
10-15 mins to reach the site from the camp. The bridge site had slopes
which are very stable. The river is fully dry and the elevation of river bank
is very high. The trees and shrubs has maintained the greenery of the
area. There are few cottages around the site. Very few locals are seen
around the site and the road to approach the site is in bad condition.
Fig: Some glimpse of bridge site

3. 3.4 Technical Specification (norms):
The main objective of the bridge site survey is to give the students the
preliminary knowledge on selection and planning of a possible bridge
site for future bridge. The purpose of the bridge site is not only to
prepare plan and layout of the bridge site but also, from the engineering
point of view, to collect the preliminary data about the site such as bed
level, normal water level, geological features of the ground for planning
and designing of the bridge.
 Carry out reconnaissance survey of the bridge site area. Establish
necessary triangulation stations to determine bridge axis length as
well as horizontal and vertical control of the area. Well conditions
traingles should be formed while selecting the triangulation stations.
 In triangulation, distance of base line must be measured in an
accuracy of 1:200.
 Observed two sets of horizontal circle reading by the theodolite to
measure the angle of two sets should be within a minute. Angular
misclosure for base triangle should be ±30”√N and other triangle
±1’√n.
 Compute the length of purposed bridge axis by triangulation survey
from two adjacent base triangles by using sine law and determine the
length of bridge axis by taking average length. Computed mean length
of bridge axis from two base triangles should be ≥1:2000
 Conduct fly levelling to transfer the RL from given BM to the nearest
triangulation station of the bridge axis and make circuit close for
checking error of closure.
 Carry reciprocal levelling to transfer level from one bank to other
bank of the river/stream within a precision of ±24√k mm. determine
the RL of the other triangulation stations by fly levelling from the end
point.
 Plot a topographic map indicating contour lines at suitable interval
(contour level = 1m). interpolate the index contour lines precisely by
arithmetic calculation method with the help of guide points and then
interpolate remaining contour lines either by graphical method or

4. estimation method. Do not erase RL of guide points in the original
sheet even after plotting of contours.
 Draw longitudinal section along the river bed up to 150m up-stream
and 50m downstream assuming 0.00 at center of bridge axis. Draw
cross section at 25m interval from topographic map and one at the
bridge axis.
Scale for plotting:
a. Topographic map:1:500
b. L-section: horizontal scale= 1:50 or 1:100 and vertical scale=1:50 or
1:100
c. Cross- section: horizontal scale= 1:50or 1:100 and vertical scale =1:50
or 1:100
 Carry out hydrological survey of river/stream. Collect hydraulic data
such as velocity of flow, high flood level, normal water level and low
water level. All these should be shown in map and cross section of
river.
3.5 Instrument used:
 Theodolite
 Ranging rods
 Auto level
 Staff
 Total station
 Prism
 Prism clamp
 Measuring tape
 Hammers
 Pegs
 Tripods
 Marker

5. 3.6 Methodology:
The methodology of the surveying is based on the principle of surveying,
which includes:
o Working from Whole to the Parts
o Independent Check
o Accuracy Required

6.  Reconnaissance:
Reconnaissance (reccee) means the exploration or scouting of an area.
In survey, it involves walking around the survey area and roughly
planning the number of stations and the position.. Reccee is primarily
done to get an overall idea of the site. This helps to make the necessary
observations regarding the total area, type of land, topography,
vegetation, climate, geology and indivisibility conditions that help in
detailed planning. The bridge site was observed and the overview of
the placement of axis was made.
 Station Fixation:
The station placement was chosen such that it should be at well-
defined and stable banks and must form well- conditioned triangles
and not affect the ecological balance of the flora and fauna of the site
area. The site should be on a straight reach of the stream. The bridge
axis should be so located that it should be fairly perpendicular to the
flow direction and at the same time, the river width should be narrow
from the economical point of view. A site which blends with the
topography and landscape will be aesthetically pleasing. Keeping in
minds the above factors, the station placement was done.
 Angle and distance measurement:
For the topographic survey of the bridge site, triangulation was done.
First the bridge axis was set and horizontal control stations were fixed
on either side for detailing. Distances between stations on the same
sides of river i.e. base line were measured with tape precisely. Then the
interconnecting triangles were formed and horizontal angles were
measured with theodolite.

7.  Well-condition triangle formation/bridge axis fixation:
 A triangle is stated to be well- conditioned triangle when all angle in
it is more than 30° or lesser than 120°. After horizontal angle
measurement for each case, they are adjusted to satisfy the
geometrical condition since the closing angular error is within the
permissible limit. While applying the correction, only unaffected
angles are taken into account and bridge axis is fixed. The bridge axis
length or span was calculated by solving the triangles using the sine
rule. Triangulation was performed for the determination of the
approximate span of the bridge axis.
 Detailing:
 Longitudinal section:
The longitudinal section of the river is required to give an idea about the
bed slope, nature of river bed, and variation of the elevation of the
different points along the length of the river. Keeping the instrument in
the control station on the river bank, the readings were taken at
different points along the center line of the river at an interval of around
5m up to 50m upstream and 30m downstream of the river. The
elevations of the control points being known previously, the RLs of
different points at the center lines was measured using total station.
 Cross- section:
For the cross-section of the river, the readings were taken.
.Approximately, at every 5m chain age the readings were taken for cross
sectioning. The spot heights were taken where the change in slope was
noticed or remarkable points were noticed such as water level low flood
level, high flood level, river bank etc.
Total station was used for detailing of the entire bridge site. The
detailing was done with respect to the skeleton formed by triangulation.
The details are booked up to 50m upstream and 30 m downstream. The
details includes natural as well as artificial features like trees, cottages,
canals etc.

8. Further plotting is done on Auto cad