Bridge site survey includes the work to carry a survey for the construction of the bridge to join two banks of theriver and allowcontinuation of the roadandprovide easyaccess for the people and the vehicles. Bridge construction is an important aspect which plays vital role in the development of a nation and its economy basically in the context of our country. Surveying is required for topographic mapping, while longitudinal section of the river and the cross-section of the upstream and downstream sides of the river are essential. Also the river flow level in the different sections as well as in different seasons should be taken into consideration before the design process of the bridge.
The mainobjective ofthe bridge site survey isto obtainthe preliminary knowledge on selection and planning of possible bridge site and axis for the future construction of the bridge. The purpose of the bridge sitesurveyisnotonlytoprepare planandlayout ofthebridge sitebutalso from the engineering point of view, the purpose is to collect the preliminary data about the site suchas normalwaterlevel, highfloodlevel, geologicalfeaturesofthe ground,catchmentsareas for planning and designing of the bridge from the details taken from the surveying. Moreover, bridge construction is an important aspect which plays major role in the development of transportation network. Thus, surveying is required for topographic mapping, knowledge of longitudinalsectionsoftheriverandcross-sectionsatboththeupstreamaswellasindownstream sides of the river for the construction of the bridge. The objectives of the bridge site survey can be summed as follows:
To find the best location for a bridge over the river considering the factors like convenience,economy and geologicalstability.
To determine the physical properties of the river such as discharge, velocity of water, bed slope etc.
To exercisethe reciprocal levelingfor RL transfer in bridge axis.
To perform triangulation method for measuring the bridge axis and detailing 120m upstreamand 60 m downstream.
Topreparetopographicmapofthebridgesiteandhencepreparelongitudinalandcross- sections of the river atthe required upstream anddownstream of the river.
To accumulate the past information about the high flood level, normal flow level, river cut direction, river shifting pattern etc.
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3.3 BRIEF DESCRIPTION OF THE AREA
The area for Bridge site is about 500 m from the KITC .Due to the season of the year(winter season) there was no water level.The area is totally covered with vegetation .
3.4NORMS (TECHNICAL SPECIFICATIONS)
1.Control point fixing as well as determining the length of the bridge axis had to be accomplished by the method of triangulation. While forming the triangles, proper care had to be taken such that the triangles were well conditioned i.e. none of angles of triangles were greater than 120° or less than 30°.
2.The triangulation angles were to be measured by two sets of reading and the difference between the meanangles of two sets was to be within 1’.
3.RL was transferred from the BM to stations by the method of fly leveling and then to the stations onanother bank of the river by the method of reciprocal leveling.
4.The scale of the plotting topographic map was 1:500. The contours were drawn in an interval of 1m.
5.In order to plot the longitudinal section of the river, detailing had to be done 150m upstreamand75mdownstream.Theplotforthe longitudinalsectionalongtheflowline had to bein a scale of 1:50 for vertical and 1:500for horizontal.
6.For the cross section profile data was taken at 20m intervals both upstream and downstream, and one at the bridge axis. Observations had to cover minimally 20m across the bank of the river onthe either side. Thescale of the cross sections was 1:100 for both vertical and horizontal.
The equipment’s used during bridge site survey are summed up asfollows:
1.Total Station with tripod
3.Prisms and poles
5.Auto level with tripod
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10.Enamel paint or marker
3.6.1 Site Selection
Site selection is the major or the vital aspect of the bridge site survey. The selection of a bridge site is governed by both tactical and technical considerations. Tactical requirements fixed the generalareaforthebridgesite.Technicalrequirementsfixtheexactlocationandmaysometimes eliminate the site that has been tactically acceptable. For permanent constructions, technical considerations govern the bridge location.
Therearevariousfactorswhichdirectlyorindirectlyaffectsthesiteselectionofthebridge.They are geological conditions, socio-economic and ecological aspects, etc. The foundation of the proposed bridge shouldbe laidabove verystable hard rocksatthebedofriver asfaras possible for the stability and longlife of the bridge. The proposed bridge site should not affect or disturb the ecological balance of the flora and fauna of the locality in the site. The axis of the bridge should be laid such that the width of the river channel should be least from economical point of view. The startingpointof the axisof the bridge or the startingof thebridge itself shouldnot lie or touch the curved portion of the road. In other words, it can be said that the axis of the bridge shouldnotbefollowedbythecurvewhileselectingthebridge.Forthepurposeofhavingshorter span of the bridge, the stations should be selected such that the direction of the bridge axis is perpendiculartotheflowdirectionoftheriver.Thereshouldnotbecuttingorerosionoftheriver banks by the river waterat the proposed bridge site.
As thefirststep, reconnaissancewasdonefortheentireareawhere the bridgewasto belocated. The site for the bridge was selected at a location that was a continuation of the straight portion of the existing road.
The following things were considered while selecting a bridge site:
1.The rocky outcrops were studied and as they were strong enough to support the bridge and the dead as wellas the live loads the site wasconcludedto be suitable.
2.Theslopeoftheriverbedwastobegentleattheproposedsitebecausesteepslopemeans thegreatermagnitudeofthewatercurrent,alotoferosionandwearandteartakesplace on the bridge foundation. Hence to avoid that condition, location of the bridge was chosenat the place where there was lesscurrent.
3.The high flood level of the river was measured accurately and hence the height of the river above the river wasdesigned taking intoconsideration the occasionalflooding.
4.The bridge was chosen at such a place that it is very convenient to approach. Hence its use ispredictedto be huge thus providing service to a large number of thelocalpeople.
Besides, the stones and the coarse aggregates could also be used from local sources nearby reducing the transportation cost.
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Triangulation is the process of forming number of connecting triangles so as to provide the controlpoints.Triangulationinvolvestwoprocesses,measurementofhorizontaldistancesofany two side of the triangle formed on the opposite side of the frame between two control points by tape and the measurement of all the interior angles with the help of angle measuring instrument i.e. Theodolite. All the remaining lengths of the sides of the triangle are computed with the help of any one measuredlength andall the measuredangles of the trianglesformed. Theaxis length of the proposed bridge was computedas mentioned earlier using sine lawin the triangles.
3.6.3 Profile Levelling (Longitudinal Sectioning)
Profile levelling is the process of determining the elevations of the points at the short-measuredintervals along a fix line or alignment such as the centre line of the railway, highway, canal or sewer. The fixed line may be a single straight line or it may be composed of a succession of straight lines orofa series ofstraight linesconnected bycurves. Itisalso knownaslongitudinal levelling. By means of such sections the engineer is able to study the relationship between the existingground surfaceand the levels oftheproposedconstruction inthe directionof itslength. The profile is usually plotted on specially prepared profile paper, on which the vertical scale is much larger thanthe horizontal.
Profile leveling, like differential leveling, requires the establishment of turning points on which both back and fore sights is taken. In addition, any number of intermediate sights may be taken on the points along the line from each set up of the instrument. It is generally best to set up the instrument to one side of the profile line to avoid too short sights on the points near the instrument. For each set up, intermediate sights should be taken after the fore sight on the next turning point has been taken. The position of the intermediate points on the profile is simultaneouslylocatedbychainingalongthe profileandnotingtheirdistancesfrom the point of commencement.
3.6.4 Cross Sectioning
Cross-sections are run at right angles to the longitudinal profile and on the either side of it for the purpose of lateral outline of the ground surface. They provide the data for estimating quantities of earth work and for other purposes. The cross-sections are numbered consecutively from the commencement of the centre line and are set out at right angles to the main line of section with thechain and tape. Cross-sectionsmaybe takenateachchain. Thelength of cross- sectiondependsuponthenatureofthework.Thelongitudinalandcross-sectionsmaybeworked together or separately asper the requirement.
The spot heights were taken where the change of the slope of the ground was abrupt. Cross- sections must be taken at normal flood level, high flood level, and river banks etc. Total Station was usedfor this purpose as atachometer.
Transferring R.L. from B.M. tocontrol points
For the determination of the reduced level of all the control points fly leveling must be carried outtotransferreducedlevelofthenearestpossiblebenchmarktoallthecontrolpoints.TheR.L.
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of the bench mark was given to be 1420.000m which was transferred to all the triangulation stations within given accuracy. For transferring the reduced level to opposite bank of the river Reciprocal Leveling wasused.
When it is necessary to carry out leveling across a river, ravine or any obstacle requiring a long sight betweentwo pointsso situated that no place for level can be foundfrom which the lengths of foresight and back sight will be even approximately equal, special method, i.e. reciprocal leveling must be used to obtain accuracy and to eliminate the error in instrument adjustment, combinedeffectofearth’scurvatureandtherefractionoftheatmosphereandthevariationinthe average refraction.
LetAandBbethepointsandobservationsbemadewithalevel.Forthefirstsetupofinstrument, three wire readings on both the staff held at the point A and B are observed and the level difference between themis calculated.
figure 7 reciprocallevelling
The level difference is calculatedas:
Staff reading on A = ha Staff reading on B = hb -e
Similarly, for second setup of instrument
Staff reading on A = ha׳ – e Staff reading onB = hb׳
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Taking average of two true difference ofelevation, we get
True difference of elevation isequalto mean of the two apparent differences inelevation
The detailing or detailedsurvey wascarriedout with the help of Total Station. The important or theharddetailswhichwereomittedornottakeninthelongitudinalsectioningaswellasincross sectioning were taken. Trigonometric leveling was also performed in the case of determination of reduced level of inaccessible points. All the detailed points were noted for the topographic view ofthe bridge site.
3.6.7 Computation and Plotting
Inthecaseoftriangulation,the horizontalandthe verticalcontrolsoftriangulationstationswere determinedandthe horizontalas wellas verticalcontrols of theotherstations weremanipulated later. The elevations were calculatedandthe usualchecks were applied. Then plottingwasdone in appropriate scale.
3.7 COMMENTS AND CONCLUSION
The bridge axis was selected keeping in mind all the norms and specifications. The required drawings were prepared in the respective scales. During the selection of the site all the considerations like geological, socio-economical and topographical were made and the best site was selected. The flow in the river was normal and showed no danger of changing its direction of flowfor the design period of the bridge.
The work of clearing the bushes and hedges during the survey works was very tedious and time consuming. It is suggested that the site should be chosen for the future references in order to minimize theclearing works