03 Highway Alignment.pdf

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1. Highway Alignment

Portion of centerline of the highway on ground

31 January 2017 Highway Engineering

2. Alignment

o Horizontal: Straight path, horizontal deviations,
curves

o Vertical: Changes in the gradient, vertical curves

Construction cost, maintenance cost, operational cost,

accident rate

Once fixed, it is difficult and costly to change in future

Enough care is necessary as error may lead to losses

in Initial, Maintenance or Operation costs

31 January 2017 Highway Engineering

3. Basic Requirements

o Short: Straight is shortest and hence cheapest, may
not be possible always
Easy: w.r.t. Construction, Maintenance, Vehicle
Operation
Safe: w.r.t. Construction. Maintenance, Operation,
Slopes, land slides
Economical: Total cost of construction as well as
maintenance and vehicle operation should be
minimum

o Should have max. utility w.r.t service to Public
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4. Factors Affecting Alignment

All the requirements are rarely fulfilled by any single

alignment. Depends on...
Obligatory Points
Traffic
Geometric Design
Economics

Other Considerations

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5. Obligatory Points

o Controlling points which govern the alignment. Two
types of points... .
Thru which alignment should pass Ex. Bridge site,
intermediate town, quarry
Thru which should not pass Ex. Religious place, costly

land/structure, pond, valley, problematic area

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6. Traffic

Alignment should satisfy the traffic needs

O-D surveys are undertaken and desire lines are
established

The proposed alignment should be in tune with the

present as well as future trends and lines

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7. Geometric Design

Geometric Design factors like
Slope (Gradient) : Ruling or Design Gradient.
Radius of curvature: Minimum Radius of Curvature
and Transition Curve.
Sight distance: Min. SD
govern the alignment
o To accommodate the standards the alignment may

have to be changed

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8. Economy

o Initial cost depends on the earthwork, so, heavy
cutting and filling avoided
Besides initial cost the maintenance cost and vehicle

operation cost also to be considered before finalization

Other Consideration

o Slopes are governed by rainfall, seepage flow, GW level,
HEFL, and drainage requirements

o Monotony

o Foreign Teritory

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9. Special Requirements for Hill Roads

Stability:

o Landslides is common problem. Depends on cutting,
filling and side slopes. Care is necessary

Drainage:

o Large number of cross drainages required. For
minimum cost the no. should be min. hence alignment

with minimum c. d. works preferred.

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10. Special Requirements for Hill Roads

Geometric Standards:

o Relaxed standards for sight dist, curve radius and
other factors. Still affect..

Resisting Length:

o Amount of work for moving one unit of load along the
selected alignment
Should be as low as possible, i.e. ineffective rise and

excess falls avoided

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11. Surveys for Location of Alignment

Before finalising the alignment four stages of engineering
surveys are undertaken

Map study

Reconnaissance

Preliminary Survey

Final location and detailed survey

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12. Map Study

o Topographic maps, prepared by Survey of India are
studied first
Many alternatives are marked avoiding ponds, lakes
and valleys
Approximate location for bridges, hill passes etc. are
considered

These alternatives are further studied in next step

31 January 2017 Highway Engineering

13. Map Study

14. Reconnaissance

Field party visits the marked routes and checks for
obstructions if any, not shown on the map.

Simple instruments like clinometers, abney level,
barometer are used

Valleys, Ponds, Structures, hills and obstructions.
Possibility of geometric std. checked

Data for no and type of cd works, HFL, GW level, soil

strata, materials for construction, quarry noted down

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15. Preliminary Survey

Main objectives of the preliminary survey:
Survey of various alternative alignments
Compare different proposals
Estimate quantity of earthwork and other construction
aspects

To finalise the best alignment

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16. Preliminary Survey

Primary traverse

Topographical features

Levelling work

Drainage studies and hydrographical data

Soil survey: earth work slopes, suitability of materials,
subsoil and surface drainage

Material Survey

Traffic Survey: traffic lanes and roadway width,
pavement design

Determination of final centre line
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17. Final location and Detailed Survey

o The alignment finalised at the design office after the
preliminary survey is to be first located on the field by
establishing the centre line.

Final selection marked on ground by pegs using steel
tape and theodolite

TBM established at 300 m. Precise leveling carried
out. L sect and c/s sect, at closer interval are plotted
Adequate hydrological and soil data is collected
Detailed estimates and plans are prepared

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18. Drawings and Reports

Key map: proposed and existing roads, and
important places to be connected

Index map: general topography of the area
Preliminary survey plans and report

Detailed plan, L-sect, c/s sect.

Land acquisition plans

Masonry works, structures and details

Material location plans

Estimates

31 January 2017 Highway Engineering