Highway Alignment and Engineering Survey

Highway Alignment and Engineering Survey

 

Highway Alignment: 

A highway alignment may be defined as the position occupied by the centerline of the road on the ground. To determine the precise position or the layout of the road centerline from the design drawing on the ground during construction. It is essential to determine three coordinates of all points of the centerline.

a. Horizontal alignment: - 

it is the projection of highway alignment on a horizontal plane & covers a straight path, the horizontal deviations & curves. It has two coordinates x & Y.

b. Vertical alignment: - 

it is the projection of highway alignment on a vertical plane & covers changes in slope & vertical curves. It has two coordinates x & z.

When the horizontal alignment showed along with the road width & other cross-section elements in the plan the drawing thus formed is known as the plan of the road. A longitudinal section of the road cut along its center is usually referred to as the longitudinal profile of the road; a section cut perpendicular to the axis of the road is called a cross-section. 

A new road should be aligned very carefully, as improper alignment would result in one or more of the following disadvantages: - Increase in construction cost 

• - Increase in maintenance cost 
• - Increase in vehicle operation cost 
• - Increase in accident rate 

Best Route Location/ Ideal Alignment / Requirements of highway Alignments 

The basic requirement of an ideal alignment between two terminal stations is that it should be:- 

(a) Short

(b) Easy

(c) Safe &

(d) Economical 

Short: - the alignment should be short as far as practicable. However, sometimes various factors cause to deviate from these criteria. Easy: - easy to construct, maintain & vehicle operation. 

Safe: - safe for construction, maintenance from viewpoint of stability & also safe for traffic operation, 

Economical: - total cost including initial cost, maintenance cost & vehicle operation cost is lowest. 

 

Factors controlling highway alignment 

If there have been no prominent (famous, well-known) factors that dictate engineers on selecting the alignment one will easily accept the shortest route ( air route) between the two given terminals. Other site conditions remaining identical a shortest route will be the cheapest & most economical. The shortest route however is not always technically feasible & economical too. The shortest route may have a very steep grade, demanding high cost for operating vehicles. Similarly, there may be a lot of construction & maintenance problems along the shortest route. The various factors which control the highway alignment, in general, may be listed as: - 

a)    Obligatory points 

b)    Composition of traffic  

c)    Geometric design / features 

d)    Economy 

e)    Other considerations 

In hill roads additional care has to be given for: - Stability, -drainage -a geometric standard of hill roads &

 

Obligatory points: -

these are the control points governing the alignments of highways & areas of two categories: -

1. Obligatory points through which a highway is to pass such as; 

- An industrial area or mine zone to which a highway is to serve additionally 

- Tourist’s spot - bridge site (suitable) - tourist spot -hill pass 

- Link with the intermediate town -health post, -VDC, - DDC 

- School areas, -College areas 

2. Obligatory points through which highway should not pass 

• - Marshy place, Waterlogged area, etc 
• - Historically & archeologically important property 
• - Restricted zone for defense, national security 
• - Costly structural elements requiring heavy compensation 
• - Densely populated area/ densely forest due to eye of environment 

Composition of traffic:

for a highway with intensive heavy vehicles & a high volume of traffic alignment yielding minimum length of steep ascend/ descend is much more desirable than the shortest route. Similarly, a highway leading to a recreation spot, picnic spot, or tourist spot which might have been predominant by light passenger car had a few buses alignment may be chosen with a higher slope. Also, the alignment should be chosen based on origin/ destination study, traffic desire lines, flow patterns, etc. 

Geometric features:

the permissible limit of descending or ascending slopes, sight distance requirements, degree of curvatures, bends, the width of road & so many other dimensional features of the road also may dictate the alignment. 

Economy:

the alignment finalized based on the above requirements should also be economical. As noted down in the criteria for ideal highway alignment, the sum of the total cost of all road components should be minimum. However, due to budget constraints, sometimes initial construction cost might be the governing factor & alignment selected accordingly even if the road yields the highest maintenance cost & vehicle operation cost. 

Others: 

-Necessity to break monotony -political pressure-social pressure 

-Defense purpose -foreign territory -hydrological factors 

In hill road additional care has to be given for: - 

(a) Stability: stable & against sliding 

(b) Drainage: the cross-drainage structure being costly attempts should be made to align the road in such a way where the number of cross-drainage structures is minimum. 

(c) Geometric standard of hill road: the route should enable the ruling gradient to be attained in most of the length, minimizing steep gradient, hairpin bends & needless rise & fall. 

(d) Resisting length: the resisting length of a road may be calculated from total work to be done to move the loads along the route taking horizontal length, the actual difference in levels between the two stations & the sum of ineffective rise & fall above floating gradient. The resisting length should be kept as low as possible.