Selection Criteria For Bridge Site
- The choice of the right site is a crucial decision in the planning and designing of a bridge.
- It may not be possible always to have a wide choice of sites for a bridge.
- This is particularly so in case of bridges in urban areas and flyovers.
- For river bridges in rural areas, usually a wider choice may be available.
The characteristics of an ideal site for a bridge across a river are:
a. straight reach of the river.
b. Steady river flow without cross currents:
c. A narrow channel with firm banks
d. Suitable high banks above high flood level on each side.
e. Rock or other hard in erodible strata close to the river bed level.
f. Economical approaches danger of floods, the approaches should be free from obstacles such as hills, frequent drainage crossings, scared places, graveyards or built up areas or troublesome land acquisition
g. Absence of sharp curves in the approaches;
h. Absence of expensive river training works;
i. Avoidance of excessive underwater construction.
Selection Criteria For Bridge Site
- For selecting a suitable site for a major bridge, the investigating engineer should make a reconnaissance survey to get impression of the landscape and to decide on the type of the structure to the site.
- Care should be taken to investigate a number of probable alternative sites and then decide on the site which is likely to serve the needs of the bridge at the least cost.
- A brief description of the reasons for the selection of a particular site should be furnished in the investigation report along with salient details of alternative sites investigated and rejected
Selection Criteria For Bridge Site Preliminary Study(techno-economic Feasibility Survey) Different studies performs during PRELIMINARY SURVEY are:
- Catchment area
- Geo-technical data
- Construction resources
- Nearby bridges
- Traffic data
Details can be obtained from Survey of India Map.
In addition, one cross section each across the river at the selected sites should be taken.
This will also get from the same map (Survey of India)
Used mainly for the flood analysis.
Study about the low water level, highest flood level , slope of surface of water, flood velocity and discharge of river.
Data obtained from local enquiries or from the data available for the nearest gauging site from irrigation or flood control dept.
GEO-TECHNICAL AND SEISMIC DATA
Study perform to get the stability of the river, location of faults, their activity and their likely repercussions on a major structure to be put up and particulars of past earthquakes in the site vicinity.
Study perform also to get soil classification, grain size and depth at which hard strata is likely to be met with.
Mainly Augur boring test is used to collect the soil samples and further to study its engineering properties.
Some kind of navigation will exist on almost all major rivers.
Study investigate about the size, density and volume of traffic of vessels and boats so that it can be safely cross the river without any nuisance to both the vessels and bridge.
Investigation includes availability of quarry, skilled labor and need for special equipment such as crushers, batching plants, handling equipment etc.
DETAILS OF OTHER BRIDGES ACROSS THE RIVER
Study conducting mainly to correlate general criteria used for selection of sites and design of that bridge.
Also to get the behavior of the river at existing bridges on either reach. Will help considerably in determining the protection works, depth of foundation, type of foundation etc required at each site.
If the alternative locations can be separated by a considerable distance, the volume and type of traffic that will pass at each location may be different in some cases.
Economic Point of View.
Detailed traffic survey have to be conducted to get awareness of growth of traffic, density, volume and future possibilities of expansion of traffic lanes etc.