Unit:2 Highway Alignment and Engineering Survey ( Transportation Engineering-I for Third Year First Part)

2.1 Highway Alignment

The position or layout of centre line of road or highway on the ground is called highway alignment. To determine the precise position or layout of the road centre line from the design and drawing on ground during construction it is essential to determine three co- ordinates of all points the centre line. It can be broadly defined into two types:

1 Horizontal alignment

It is the projection of highway alignment on the horizontal plane and covers straight path, curves. It is defined by two co-ordinates (x, y)

2.Vertical alignment

It is the projection of highway alignment. During highway alignment following factors areconsidered:

i.Road should be straight to minimize the length.

ii.A road can be curve to consider the neighboring village, local market etc.

iii.Aligned road should cross the road canal, rivers etc. at right angle.

ivAligned road should be away from fertile land.

v.Aligned road should be located at division line/high point/ridge.

vi.Aligned road should not cross swamp area.

vii.Aligned road should be in balance position w. r. t. cutting & filling.

viii.There should be minimum crossing in aligned road.

Requirements of an ideal alignment:-

The basic requirements of an ideal highway alignment between two terminal stations are that it should be short, easy, safe and economical.

1.Short

There should be minimum travel distance between two terminals as far as practicable i.e. the alignment should be minimum between two terminal points.

2.Easy

The alignment should be such that, it should be easy to construct and maintain the road with minimum obstruction.

3.Safe

The alignment should be safe for construction and maintenance from view point of stability of natural slopes.

4.Economical

The alignment should be considered economical only if the total cost including initial cost, maintenance cost, and vehicles operation cost is lowest.

2.3 Factors controlling highway alignment

The various factors that controls the highway alignment are:

1.Obligatory points

These are the control points governing the alignment of highway and are of two categories:

I. Obligatory point through which the alignment should pass.

II. Obligatory point through which the alignment should not pass

I.Obligatory point through which the alignment should pass such as:

(a) suitable bridge site

Bridge should be located at place where river has straight and permanent path Curve near the bridge site and skew crossing is avoided. In figure straight alignment between stations A&B which possess as above is to be deviated along path shown at proper Location.

(b) Industrial areas, Tourist spots, Intermediate Towns etc:

Straight alignment AB may be shifted along ACB (Option-I) to connect important areas. Also by link road (Option-II) connection with C achieved avoiding the deviation of the straight alignment.

(c) Hill Pass

Various alternatives to cross hill range, mountainous or high ridge are to cut tunnel, to go round the hills or to deviate until a suitable hill pass is available. Figure shows the straight alignment AB is deviated along the hill pass slope; this avoids a tunnel or heavy cutting.

II. Obligatory point through which the alignment should not pass:

a.Marshy place, water logged area, Lakes etc

b.Historically and archeologically important property

c.Restricted zone for defense, national security

d.Costly structural elements requiring heavy compensation

e.Densely populated area

f. Foreign territory
2.Composition of traffic
For a highway with intense and heavy vehicles minimum length of steep slope is desirable. For highway towards tourist spots, picnic spots with more cars, bike etc steep slope can be selected.
3.Geometric design/ features

Permissible limits of ascending and descending gradients, radius of curve, width of roads, sight distance and other geometric features determines the highway alignment.

4.Economy

The alignment finalized based on the above factors should also be economical. In working out the economy, initial cost, maintenance cost and vehicles operation cost should be taken into account.

5.Other factos

Hydrological factors (sub -surface water level, seepage flow, high flood level) etc.

1.Political factors

2.Community perception

3.Monotony breaking

In spite of above, following special considerations are also taken while aligning road in hilly areas:

Stability of hill road

While aligning hill road special care should be taken to align, the road along the side of the hill which is stable. Unstable hill features and frequent landslide area should be avoided. Steep terrain and other inaccessible area also are avoided as far as possible.

Drainage

Numerous hill side drains should be provided for adequate drainage across the road.But cross-drainage works or structures being costly, attempt should be made to align the road in such a way where the numbers of cross- drainage structures is minimum.

Standard Geometrical

The route or alignment should be such that the ruling gradients are obtained most of the length. As far as possible the hair pin bend should be avoided in hill road alignment.

Resisting length

While aligning hill road at no point's road should be allowed to acquire down grade. The road alignment is considered the base which shows the least total distance while ascending and least total ascent while descending between the extreme points. In brief the resisting length of alignment should be kept as low as possible. Thus the ineffective rise and excessive fall should be maintained.

Engineering Survey or highway/Road survey:

Before any highway alignment is finalized, various engineering survey are required to be carried out the survey work may be carried out in four stages:

1.Map study

In this stage topographical map of the concerned area are studied. These maps are available from the survey departments. The map study give throughout idea about various possible alignment

2.Reconnaissance survey

In this type of survey fairly wide stretch of land along the proposed routes is studied using very simple instruments. Or simply walk over the whole areas to be surveyed and prepare free hand sketch and compare with map study.

3.Preliminary survey

It is a large scale investigation of the alternative routes marked during reconnaissance survey. In preliminary survey an accurate traverse line along the route already recommended during reconnaissance survey is drawn. It is completed in the following stages:

i.Preparation of base line traverse.

ii.Leveling along base line traverse.

iii.Collecting topographical and other details

iv.Preparation of preliminary maps

v.Paper location or preliminary design

4.Location survey

The construction of the project is based on the result of the final location survey.The purposed of the final location survey ids to fix the centre line of the selected alignment in the field and to collect additional data for the preparation of drawings. The location survey is carried out in two steps:

I. Location of centre line

The centre line of the road is finalized in the drawing is to be translated on the ground during the location survey.

II.Detailed survey

All topographical details are noted down and plotted using conventional signs.A detailed soil survey is carried out to enable drawing of the soil profile. It includes the following stages:

i.Pegging the centre line

ii.Centre line leveling

iii.Cross-sectioning

iv.Property line

v.Intersecting roads

vi.Materials site survey

vii.Special site survey.