Ch 2 Highway Alignment and Survey

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Chapter II

Highway Surveys

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Highway Surveys

4 Hours

2.1 Process of identifying best route location

2.2 Physical surveys: map study and reconnaissance, preliminary and detailed surveys

2.3 Highway alignment and controlling factors

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Flow Chart of Route Location Process

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Map study:

By careful study of topographical maps, it is possible tohave an idea of several alternate routes so that furtherdetails of these may be studied later at the site.

The probable alignment can be located on the mapfrom the following details available on the map.

– Alignment avoiding valleys, ponds or lakes.

– Possibility of crossing through a mountain pass.

– Avoiding bend of the river for crossing rivers.

– Keeping the permissible gradient.

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Reconnaissance:

• The second stage of surveys for highway location is thereconnaissance to examine the general character of thearea for deciding the most feasible routes for detailedstudies. Some of the details to be collected duringreconnaissance are given below.

• Valley, ponds, lakes, ridge, hills, permanent structuresand other obstructions along the route which are notavailable in the map.

• Approximate values of gradient, length of gradient andradius of curves of alternate alignment.

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Cont………..

• Number and types of cross drainage structure,maximum flood level and natural water level.

• Soil type and observation of geologicalfeatures.

• Sources of construction materials, water andlocation of stone quarries.

• Type of rocks, seepage flow, may observewhen the road passes through hilly ormountainous terrain.

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Preliminary surveys

The main objectives of the preliminary survey are;

• To survey the various alternate alignmentsproposed after the reconnaissance.

• To collect all the necessary physical informationand details of topography, drainage, and soil.

• To compare the different proposals in view of therequirements of a good alignment.

• To estimate quantity of earth work.

• To finalize the best alignment.

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Detailed survey;

• The alignment finalized at the design office is to

be located on the field by establishing center line.

• The center line stakes are driven at suitable

intervals (50m intervals in plain and rolling terrain

and 20 m intervals hilly terrain.)

• Bench marks are fixed at interval of about 250m

and all drainage structures.

• Levels along the final center line should be taken

at all staked points.

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Cont……..

• The cross section levels are taken up to the

desired width.

• All river crossing, valley etc. should be

surveyed in detail

• Detailed soil survey is carried out

• The data during the detailed survey should be

complete for preparing detailed plans, design

and estimate of the project

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Highway Alignment:

• The position or the layout of the centre line of

the highway on the ground is called

alignment.

• Horizontal alignment – (straight path, curves,

horizontal deviations)

• Vertical alignment – (Changes in gradient and

vertical curves)


Disadvantages of improper alignment

• Increase in construction cost

• Increase in maintenance cost

• Increase in vehicle operation cost

• Increase in accident rate.


Basic requirements of an ideal alignment

• Short –It is desirable to have a shortest

alignment between two terminal stations.

• Easy – The alignment should be such that it is

easy to construct and maintain easy with

minimum problems.


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Cont…• Safe – The alignment should be safe enough for

construction and maintenance from the view point of stability of natural hill slopes, embankment and cut slopes and foundation of embankments. It should be safe for the traffic operation with safe geometric features.

• Economical – If the total cost including initial cost, maintenance cost and vehicle operation cost is low the road alignment could be considered economical.


Factors controlling the highway

alignment

• Obligatory points

• Traffic

• Geometric design

• Economy

• Others


Obligatory point:

Points through which the alignment is to pass

• Bridge site

• Industrial area

• Hill pass

• Intermediate town

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Alignment along the hill pass


Connecting intermediate town


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Cont….

Points through which the alignment should not

pass:

• Waterlogged area

• Historical or archeological site

• Restricted zone for defense/national security

• Densely populated area

• Very costly structures

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Traffic

• The alignment should suit traffic

requirements. Origin and destination study

should be carried out in the area.


Geometric design

• The geometric design of a highway deals with

the dimensions and layout of visible features

of the highway such as gradients, radius of

curve, sight distance, and super elevation etc.


Economy

• The alignment should also be economical

Others

• Various other factors such as drainage,

hydrological factors, political considerations,

monotony, Sub-surface water level, seepage

flow and high flood level are to be keeping in

view


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Ch 2 Highway Alignment and Survey