Investigation and Survey for Construction of Bridges - Chapter 3

Investigation and Survey for Construction of Bridges

Minor Bridges

• Provided on local drainage crossings, field channels, and at canal crossings
• Pipes are normally proposed where sufficient cushion below the sleeper is available
• Skew crossings should be restricted to 30°
• Investigation should cover catchment area, soil characteristics, anticipated flood level, and other relevant hydraulic particulars

Major Bridges

• Investigation is confined to one particular site and should cover:
  • Topographical details
  • Catchment area characteristics from Survey of India maps
  • Hydrological particulars like low water level, high flood level, discharge, flood velocity, and surface slope
  • Geotechnical investigations for soil particulars needed for foundation design
  • Seismology of the area, wherever necessary
  • Navigational requirements to determine clearance
  • Construction resources like labor, materials, accommodation, water, power, transport, and communication infrastructure
  • Details of nearby bridges across the same river or stream

Survey of Rivers

• River should be surveyed for 10 km upstream and 5 km downstream of the bridge location
• Cross-sections of the river bed should be taken at suitable points
• High flood levels should be reliably obtained and noted
• Diversion of rivers should be examined, necessary surveys and sections made, and the diversion shown on the survey plan
• Protection works required to prevent encroachment or mitigate the effect of bursting of tanks or scouring should be carefully considered

Hydrological Investigations

• Extent depends on the type and importance of the bridge
• Data to be collected includes:
  • Area of the catchment
  • Shape of catchment
  • Details of the main stream and tributaries
  • Longitudinal slope of the main stream and average land slope of the catchment
  • Nature of soil in the catchment
  • Extent of vegetation
  • Probable changes in catchment characteristics
  • Rainfall records
  • Other climatic conditions
  • Changes in the course of the channel
  • Nature of material through which the channel flows
  • Bank erosion and bed scour
  • Maximum observed scour depth
  • Description of existing bridges

Plan and Cross-sections of Rivers

• Plan should be drawn to a scale of 1:5000 and cover the river and tributaries within 10 km of the bridge site
• Three cross-sections of the river bed should be plotted to a natural scale of 1:100
• Levels of highest known flood, ordinary flood, and ordinary low water should be indicated

Factors Governing Site Choice

• Straight reach of the river, free of whirls, eddies, and excess currents
• Firm, fairly inerodibl e high banks
• Located near a nodal point in a meandering river
• Suitable strata at reasonable depth for foundations
• Minimum width and normal alignment to the river
• Avoid locations with frequent changes in river course or tendency for aggradation/degradation
• Secure approach banks not liable to flash floods or major spills

Design Discharge (Q)

• Based on actual hydrometeorological observations or computed flood with 100-year recurrence interval
• For foundation design, a higher flood discharge (Qf) is used, increased by 10-30% depending on catchment size

Design of Waterways

• Equal to the width of water spread between stable banks for discharge Q
• May be increased beyond bank-to-bank width if river spills over banks appreciably
• Minimum clear span of 18 m and minimum headroom of 6 m should be provided

Vertical Clearance

• Varies from 3 to 12 m depending on design discharge
• For arch bridges, minimum clearance measured to the crown of the intrados is 6-12 m depending on arch span
• May be relaxed by up to 20-30% if adoption of prescribed values results in heavy expenditure or construction difficulties

Freeboard (F)

• Minimum 0.6 m from water level of design discharge Q to formation level of railway embankment or top of guide bund

Investigation and Survey for Construction of Bridges

• Minor bridges are generally provided on local drainage crossings, field channels, and at canal crossings
• Pipes are normally proposed where sufficient cushion below the sleeper is available
• Where a skew crossing is unavoidable, it is advisable to restrict the skew to 30°
• The investigation should cover the particulars of catchment area, the soil characteristics, the anticipated flood level, and other relevant hydraulic parameters

Investigation for Major Bridges

• The investigation is mostly confined to one particular site and should cover:
  • Topographical details
  • Catchment area characteristics from Survey of India maps
  • Hydrological particulars like low water level, high flood level, discharge data, flood velocity, and surface slope
  • Geotechnical investigations to get the soil particulars for foundation design
  • Seismology of the area wherever necessary
  • Navigational requirements to determine the clearance
  • Construction resources like labor, materials, accommodation, water, power, transport, and communication infrastructure
  • Details of nearby bridges across the same river or stream

Investigation for Important Bridges

• Investigation for important bridges may be carried out in three stages:
  • Technical feasibility study (reconnaissance survey)
  • Techno-economic feasibility study (preliminary engineering survey)
  • Detailed survey and project report stage (final location survey)
• In case of meandering courses in alluvial and quasi-alluvial rivers, multiple alternative sites may be available for locating a bridge, and investigations may be carried out for each site.

Techno-economic Feasibility Study

• This report should bring out the comparative merits and demerits of the various alternative sites, the salient features of the bridge, its estimated cost, and cost-benefit ratio.
• The approximate waterway may be based on discharge calculated using the "Regional flood frequency approach" developed by RDSO.

Hydrological Investigations

• Hydrological investigations should collect data on the area of the catchment, shape of catchment, details of the main stream and tributaries, longitudinal slope, soil nature, vegetation extent, rainfall records, and other climatic conditions.
• Changes in the course of the channel and nature of the material through which the channel flows should be described based on actual borehole particulars.
• Full description of existing bridges upstream and downstream, including relief and overflow structures, should be provided.

Factors Governing the Choice of Site

• The reach of the river, especially the upstream, should be straight
• The river in the reach should have a regime flow free of whirls, eddies, and excess currents
• The site should have firm, high banks that are fairly inerodible
• For a meandering river, the site should be located near a nodal point
• The site should have suitable strata at a reasonable and workable depth for founding piers and abutments
• The bridge should be aligned normal to the river as far as possible
• Wide Khadirs for bridge location should be avoided, and the location with respect to Khadir width should be carefully decided.

Design Discharge (Q)

• The design discharge should be based on actual hydrometeorological observations or computed flood with a probable recurrence interval of 100 years.
• The design discharge for foundations (Qf) should be higher than Q, with the percentage increase based on the catchment area.

Design of Waterways

• The waterway shall be designed to be equal to the width of the water spread between the stable banks for the design discharge Q.
• If the river spills over its banks, the waterway shall be suitably increased to carry the spill discharge as well.
• Minimum clear span of 18 m should be provided in new bridges, and minimum headroom of 6 m should be provided for proper inspection and maintenance.

Vertical Clearance

• The minimum vertical clearance from the water level of design discharge Q, including afflux, shall be based on the discharge range.
• For arch bridges, the minimum clearance measured to the crown of the intrados of the arch shall be based on the arch span.
• Relaxation in clearance provisions can be made by the Principal Chief Engineer/Chief Bridge Engineer in specific cases.

Freeboard (F)

• The freeboard from the water level of the design discharge Q to the formation level of the Railway embankment or the top of the guide bund, including afflux, shall not be less than 1.5 m.

This quiz covers the investigation and survey methods for constructing minor bridges, including site selection and feasibility studies. It also discusses the importance of adequate foundation and site surveys.