Bridge Site Selection & Guidelines
• Social Feasibility Survey and
• Technical Survey
Both surveys are of equal importance. The social feasibility survey establishes community ownership and responsibility, and the technical survey ensures that bridge construction is sound and safe.
SOCIAL FEASIBILITY SURVEY
- Level of local participation
• size of area of influence
• size of traffic flow
• socio-economic benefits produced by the proposed bridge
The technical survey includes:
- Bridge site selection and Topographic Survey of the selected bridge site
GENERAL DATA COLLECTION
-Location of bridge site
– Transportation distance
– Nature of crossing and affordability
-Availability of local materials –
– Availability of local bridge builders
– Local participation
The bridge site should fulfill a number of general conditions:- traditional crossing point ::the bridge site should be selected at or near to the traditional crossing point.
- Favorable:selected site is at or nearby the traditional crossing point
- unfavorable:selected site is far from the traditional crossing point
minimum free board:: the freeboard between the lowest point of the bridge and the highest
- flood level should not be less than 5 m. For this, sufficient clearance
- between the lower foundation saddle and HFL should be maintained.
maximum bridge span ::
The bridge span in this standard is limited to 120 m span.
- Favorable:span, lis equal or shorter than 120 m
- unfavorable: span, lis longer than 120 m
space for the bridge foundations::
Level Difference between two banks
the level difference h between the two foundation blocks should
not be more than l/25.
- Favorable:h is equal or less than l/25
- unfavorable:h is bigger than l/25
Space for Foundation
Foundation should be placed at least 3 m behind the soil slope and 1.5 m behind the rock slope from the front edge of the riverbank.
The bridge foundation should be placed behind the line of the angle of internal friction. (Angle of internal friction is the angle of slope of soil or rock at which it is still stable and does not slide).
In order to protect the bridge from sudden over-flooding and strong erosion, the bridge site should not be located near the confluence area of two rivers.
- Favorable:Bridge site far from river confluences
- Unfavorable: Bridge site near river confluences