Bridges are the structures that are constructed with the purpose of connecting two places separated by deep valleys or gorges or rivers and streams. Bridges are usually the cross drainage and hence a part of roads making them shorter and hence economical. For places, where the ground is uneven and undulated and where the number of rivers is large, bridges are the most economic and efficient way. It is a very convenient way.
- To select the possible bridge site and axis for the construction of bridge.
- To carry out surveying for topographical mapping, longitudinal and cross sections at both the upstream and downstream side of the river.
Brief description of the site
The bridge site was surrounded by bushes. The ground was damp and swampy. The soil was soft. The hill slopes on both sides were very steep.
Technical Specifications (Norms)
The following norms were followed while performing the bridge site survey in the field:
- Control point fixing as well as determining the length of the bridge axis had to be done by the method of triangulation. While forming triangles, proper care had to be taken such that the triangles were well conditioned, i.e., none of the angles of the triangle were greater than 120° or less than 30°.
- In triangulation, distance of Base Line must be measured in an accuracy of 1:2000.
- The triangulation angle had to be measured on two sets of readings by Theodolite and the difference between the mean angles of two sets of readings had to be within a minute.
- Carry out reciprocal levelling to transfer level from one bank to other bank of the river within a precision of ±25*sqrt(k) mm. Determine the RL of the other triangulation stations by fly levelling from the end point of bridge axis.
- Plot a topographic map indicating contour lines at suitable interval (contour interval = 1m).
- The scale for plotting the topographical map was given to be 1:500
Equipment & Accessories
The equipment used in the survey during the preparation of topographic map in bridge site are as follows:
- Levelling Machine
- Ranging Rods
- Measuring Tapes
- Leveling Staffs
- Pegs & Arrows
- Marker Pen
The various methods performed during the bridge site survey were site selection, triangulation, leveling (fly levelling and reciprocal levelling), detailing by theodolite, cross section, and L-section. The brief descriptions of these methodologies are given below:
Site selection is the first and foremost step for the construction of bridge. Several governing factors are there for the site selection of the bridge. Geological condition, socio-economic and ecological aspect etc. guides the way of selection of bridge site. Therefore, the site was chosen such that it is laid on the very stable rocks at the bed of river as far as possible and not affect the ecological balance of the flora and fauna of the site area. The location of the bridge was selected in such a way that the heights of the roads joined by the proposed bridge were almost the same. This prevented a lot of cutting and filling to maintain a gentle gradient. The bridge site was chosen in such a way that the bridge axis was perpendicular to the flow direction and was also shorter in span so as to make the construction economical.
For the topographic survey of bridge site, triangulation was done. Triangulation is the process of measuring the angles of a chain or a network. The main purpose of the triangulation was to determine the length of the bridge axis. The triangulation also serves the control points for detailing. The bridge axis was set and horizontal control stations were fixed on either side. Distances between stations on the same sides of river i.e. base lines were measured with tape precisely. Then the interconnecting triangles were formed and angles were measured with the theodolite. The bridge axis length or span was calculated by solving the triangles using the sine rule. For vertical control, the level was transferred from the given arbitrary benchmark and RL was transferred to the stations on the next bank by reciprocal leveling while direct level transfer method was used or the same bank.
L-Section & Cross Section
For gaining an idea about bed slope, nature of the riverbed, and the variation in the elevations of the different points along the length of the river, L-section is carried out. Keeping the instrument at the control (traverse) stations on the river banks, the staff readings were taken at different points along the center line of the river up to a 150 meters upstream and 50 m downstream. Then the L-Section of the riverbed was plotted on a graph paper on scale for vertical and horizontal.
Cross-section of a river at a particular point is the profile of the lateral sides from the centerline of the river cut transverse to the L-Section at that point. The cross section can be used to calculate the volume and discharge of water at the particular section if the velocity at the cross section is known.
a. Transferring R.L. from B.M. to control points:
The benchmark was on the rock. R.L. was transferred to the triangular station from the B.M. by fly leveling by taking the back sight-reading to the bench mark which should be within the given accuracy. The R.L. was transferred to the opposite bank of the river by reciprocal leveling.
b. Reciprocal Leveling:
This method is applicable when taping is obstructed but not the vision. For transferring the RL across the bridge reciprocal leveling was performed. This method eliminates the error due to focusing, collimation, earth’s curvature and refraction of atmosphere etc.
Taking the average of the two differences we get the difference in elevation between A and B.
Theodolite was used for detailing of the entire bridge site. The reading was taken from the different station set up. The detailing was done with respect to the skeleton formed by triangulation. The vertices of triangles serve as a control point. The details were booked, up to 150m upstream and 50m downstream. The data and the calculations have been tabulated in a systematic way.
Comments and Conclusion
Economy and durability determine the way how a bridge is designed. The bridge axis should be designed such that the span length should be minimum and the location is safe. The bridge axis should not be below the highest flood level. The bridge span was found out to be …… meters. The cross-section was taken at the banks of river and at the middle of the river to get the profile of the flowing river.