Investigation for bridge construction

1. 1
Investigation for
Bridge Construction

2. 2
Aim of investigation
To select the most suitable location, type of
structures satisfying,
• Optimum cost
• Road alignment
• demands of traffic, the stream, safety and
aesthetics.

3. 3
Investigation for Bridges
Selecting the following parameters should be carried
out under bridge investigation.
• Service life
• Location of the bridge
• Deck level
• Bridge type
• Span
• Piers
• Abutments
Bad decisions regarding location and the structure can
lead to several problems. A high investment is needed
to construct a bridge, so it’s important to make the
right decisions first.

4. 4
Preliminary Engineering – Desk Study
• Collect topographical maps – contour
surveys available from the Survey
Department.
• Remote sensing images if available.
• Aerial photographs – stability of stream by
viewing photographs of multiple years.

5. 5
Preliminary Data to be Collected
1. Name of the river / road, probable bridge
location, any close by bridges on the same
river
2. Location of the nearest available
benchmark
3. Present and anticipated future volume and
nature of traffic
4. Hydraulic data pertaining to river

6. 6
Preliminary Data to be Collected
5. Soil profiles
6. Navigational requirements of the stream (if
any)
7. Material availability to locations (source
locations and transportation)
8. Utility and service supply during
construction

7. 7
Design Service Life
• Permanent structure (Service Life > 40 years) ; to
carry the specified loads
• Temporary structure ; when an anticipated future
development after some time will increase the
desired capacity (low cost temporary bridge until
the development occurs – permanent abutments
and a light deck may be a wise alternative)

8. 8
Location of the Bridge
Following features are ideal conditions,
• Straight reach of river

9. 9
Meandering River

10. 10
Meandering River

11. 11
Location of the Bridge (contd.)
• Steady stream flow - no serious whirls/
cross currents
• Narrow channel with firm banks
• Suitable high banks (above high flood level)
• Rock bed (or any other hard inerodible
strata) close to river bed level
• Economical approaches

12. 12
Location of the Bridge (contd.)
• Appropriate horizontal alignment of the
road to be connected (no sharp curves in
approach)
• Appropriate vertical alignment
• Absence of expensive river training work
• Absence of excessive underwater
construction
• Traditional crossing point

13. 13
Location of the Bridge
• 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
• However, it is not always possible to
have a wide choice of sites for a bridge

14. 14
Deck Level
• Determination of the deck level should be done based
on a hydraulic study
• When a bridge structure and its associated
embankments encroach upon the flow of a river in
flood, there is a risk to the structure, the embankments
and the surrounding land
• It is not economical, however, to build a bridge to clear
a wide floodplain because bridge works cost more than
earth embankments
• Often bridges are designed to accommodate annual
high flows without excessively restricting the flow of
the river

15. 15
River Survey
• Obtain the following data:
o Ordinary Flood Level (OFL)
o Lowest Water Level (LWL)
o Highest Flood Level (HFL) - highest known
flood is termed the high flood (HF)
o Design Flood Level (DFL) - The annual high
flow is termed the design flood

16. 16
River Flow
Maximum discharge to be estimated by;
• Empirical formulae
• Rational method
• Area-velocity method
• Records of flood discharge
It is preferred to estimate the flood discharge by at
least two of the above methods

17. 17
River Flow
• Restricting the flow of the design flood can cause either excessive
backwater, resulting in flooding upstream, or scour that may
damage the structure.
• The bridge parts should also be designed to withstand the high
flood
• On important roads the bridges may be designed for a design flood
occurring no more than once every ten years and a high flood once
every hundred years.
• A minor road may be served by bridges or low water crossings
(causeways) allowing water to flow over for a few days every year.
• The bridge designer is required to select a design flood level (DFL),
a design discharge (ie. flow volume), and a design velocity, on
which to base calculations of waterway geometry, foundation
depth, scour protection and vertical clearance.

18. 18
Vented Causeway

19. 19
River Flow
• The design flood is the maximum flow that can
pass through the bridge without: • causing
unacceptable disruption to traffic; • endangering
the pier and abutment foundations with scour; •
damaging approach embankments; • causing
flood damage on the upstream side of
embankments.
• The high flood is the rarely occurring flow that it
is uneconomical to include in the design flood,
but which may be considered when designing the
superstructure and piers of the bridge.

20. 20
Hydraulic Design

21. 21
Hydraulic Design

22. 22
Hydraulic Design

23. 23
Vertical Clearance at DFL
Vertical Clearance is required to,
• Allow for errors in estimation of High Flood Level
(HFL)
• Allow for floating debris
Discharge
(m3/s)
Minimum Vertical
Clearance
(mm)
< 0.3 150
0.3 to 3.0 450
3.0 to 30.0 600
30 to 300 900
> 300 1200

24. 24
Scouring
• Scour is the erosive effect of water flow on the
river bed or banks.
• Scouring occur when the velocity of the stream
exceeds the limiting velocity of the particles in the
stream bed could withstand
• Bridge works may alter the existing scour pattern
by restricting the free flow of the stream.
• About 50% of river bridge failures are due to
scour

25. 25
Scour Protection
• River and scour protection is usually required
where a bridge is built across a meandering
stream or when some restriction to the flow of
the design flood occurs at a bridge
• Protection measures can take the form of:
o Rip rap on slopes or river bed
o Gabions
o Groynes
o Piled walls
o Vegetation.

26. 26
Scour Protection

27. 27
Scour Protection

28. 28
Scour Protection

29. 29
Scour Protection

30. 30
Gabion Basket

31. 31
Scour Protection

32. 32
Bridge Type
• The designer should consider all the preliminary
data made available to him from the detailed
investigation before arriving at a solution
• The entire complete structure should be the most
suitable to carry the desired traffic, adequately
strong to support incident loads, economical and
aesthetically pleasing

33. 33
Bridge Type
Factors influencing the choice
• Economics / available funds
• Approaches
• Level of permissible interference to the flow
below
• Climate and environmental conditions
• Scenery from the bridge
• Topographic conditions
• Subsoil conditions
• Type of traffic
• Navigational clearance

34. 34
Bridge Width
• Single lane (less than 200 vehicles per day; 3.7m
for traffic and 1.5m each for shoulders)
• One and a half lanes (two lanes of light traffic;
4.6m)
• Two lanes or more

35. 35
Span
• When the span is high, superstructure cost is high
and substructure cost is low
• Experience has shown that at the most
economical span length, superstructure cost =
substructure cost.

36. 36
Span

37. 37
Piers
Piers are required to reduce the span of the deck;
• Carries the vertical and horizontal loads of the
deck
• Generally, piers are under water
• Should be designed to withstand hydraulic
pressures and impact loads

38. 38
Shapes of Bridge Piers

39. 39
Piers

40. 40
Abutments
Abutments serve the following purposes:
• Supports the dead loads of the deck
(superstructure)
• Resists the vertical and horizontal live loads
• Retain the approach embankments
• Provide a smooth transition from the road surface
to the bridge deck

41. 41
Abutments

42. 42
Abutments

43. 43
Abutments

44. 44
Abutments

45. 45
Location of Piers and Abutments
Factors to be considered
• Foundation conditions
• Economical span
• Navigational requirements
• Aesthetic requirements
• River flow
It is preferred to align the piers and abutments
parallel to the main direction of flow in the stream.

46. 46
Location of Piers and Abutments

47. 47
Traffic
• Estimate the type and volume of traffic likely to
use the bridge over the service life
• Determine the current traffic and use a
reasonable growth factor

48. 48
Design Loading
• Most countries have some form of design loading
standards for bridges
• Available standards
o British Standards
o AASHTO
o Euro Code

49. 49
Subsoil Exploration
• The aim of the preliminary exploration is to get a
general idea
• The detailed investigation would call for careful
sampling and thorough study of the engineering
properties of soil and rock
• Following methods can be used:
o Test pits
o Hand auger boring
o Cable percussion boring
o Rotary drilling
o Geophysical surveying

50. 50
Information from Soil Investigation
• Nature of soil deposits
• Depth and thickness of soil strata
• Groundwater table
• Depth to bed rock
• Engineering properties of soils and rock

51. 51
Borehole Log

52. 52
References
• Bridge Engineering Handbook, Ed: Wai-Fah Chen
and Lian Duan, CRC Press LLC, Chapter 4: Planning
of Major Fixed Links