Engineering Surveys and the location of highway alignment

Hi,

Location of the highway alignment is done after carrying out survey of the area, these surveys are called Engineering Surveys. 

We have to locate an alignment which fulfill the basic requirements like the path must be short, safe, economic, easy and useful. To check all these basic requirements we can carry out the

Engineering Surveys in the following phases:

1.  Map Study
2.  Reconnaissance Surveys
3.  Preliminary Surveys
4. Location Surveys

•  Map Study: 

This is the first step of the Engineering survey, using a topographic map of the area under consideration, which can be availed from the Survey of India, we can propose different alternatives of the road alignment. 
This topographic map in general have a contour interval of around 30 m to 40 m. 

We can get the details of the natural and artificial features of the area using the topographic map, and accordingly we can suggest a numbers of alternatives for the road alignment. These routes are further studied in the Reconnaissance survey.

•  Reconnaissance Survey: So in the second phase/step a survey team is headed to the area under study with the minor surveying instruments like Abney level, Tangent Clinometer etc. to do a rough survey of the area under study. 

The rough survey is done along the alternatives proposed in the map study and feasibility of the road alignment is checked along the different routes. 

Some of the routes may be cancelled out or they may be changed if they appear to impossible in this study. So finally they will have a set of routes which are to be further studies in the next step.

• Preliminary Survey: In this step the alternative routes which are proposed after a rough survey in the second step are surveyed in details using some advanced instruments like levels, chain and theodolite. Aerial Photogrammetry is best suited for this type of survey.                                                                
    All the necessary details to carry out the comparative study of the different routes are collected and then finally we have to decide one alignment best suited for the alignment of the road. 

Here various details are found out along the stretches of the routes, which can also be found using the aerial techniques by taking photographs along the routes and then further processed to find out the final details of the area.                         

Different kind of surveys like, Soil investigations, cross sectioning and profiling, marine surveying, hydrology data collection, obligatory points, industries and  population surveys are necessary along the routes and only then it is possible to have a fair comparison of the different routes. 

So finally one among all of them is chose and drawings are prepared on the sheet which will show its alignment to be shifted on to the ground.

• Location Survey: 

In this fourth phase of the Engineering Survey for the highway location, we have a drawing of the alignment and we have to go through the further two processes:

   (a) Location      (b) Detailed Survey

•  Location:- Location of the center line of the road is done with very much precise instrument like Theodolite and Chain using the drawing prepared or the details gathered in the third step(i.e. Preliminary survey). This is done by staking the ground with the stakes inserted at the intervals of 50 m to 100 m in the plain area, 50 m to 75 m in the rolling terrain and 30 m to 50 m in the hills and steep terrain.                                                                                  

Pegs may driven at all the control points. At the curves control points, starting of the transition curve, starting of the circular curve and terminal of the circular curve and the terminal of the transition curve the pegs/stakes are driven into the ground to firmly locate these control points. Bench marks are located at and interval of 250 m and they are necessarily located at the sites of the cross drainage works.                                                        

•   Detailed Survey: In this part we have carry out the detailed study of the final route using some very precise instruments like Theodolite and Chain to gather all the necessary data for the final estimation, design and preparing drawings using which the construction can be started. A detailed project report is to be prepared and all the necessary data is collected to prepare that report.

 So, the profiling, cross-sectioning and soil investigation are carried out very precisely. CBR values are also found to find out the design thickness of the pavement. After collecting the data a final drawing a report is prepared which concludes the highway planning part. 

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Relevant books:

Basic Requirements of a Highway alignment on plain and hill roads

Hi

There are some basic requirements of the highway alignment in the plain and hill roads which must be fulfilled. In general the basic requirements are:

(1) Short: The alignment must be the shortest of the various alternatives available. Of course the shortest path between any two points is a straight line but the topography of the area or other factors may necessitate it do divert and take some other route, but as far as possible it should be kept minimum.
(2) Easy: Alignment should be such that the road must be easy to construct and easy to maintain or repair. If curves are of large radius and the gradient is gentle it would be easy to construct the road, rather than opposite.

(3) Safety: Safety is again the basic requirement of the highway alignment and special care must be taken to align the road in such a way that it must have the safe or minimum Sight distances and Radius of the curves, means the geometrical design features like Sight Distance, Radius of the curves and the gradient of the road must be given special attention.

(4) Economical: Road alignment must be designed to have the initial cost of construction, maintenance cost and the vehicle operation cost to a minimum. Also the locally available materials should be checked  before and it may decrease the over all cost. There must be a balance in the cutting and filling on the alignment of the road.

Some other basic requirements specially on the hill roads:
Hill roads have some other basic requirements also which govern the alignment of the hill roads:

(1) Drainage: Drainage of the road must be kept in mind and it must be insured that enough drainage structures can be built on the route. As far as possible alignment must avoid the drainage works means it must have the minimum numbers of the drainage works.

(2) Economy: Economy is governed by the numbers of the drainage works, cutting filling and the gradient.

(3) Safety: Safety is governed by the sight distance, superelevation and the design radius of the curves. It must be kept in mind that gradient must be kept below the ruling gradient. In hill roads special attention must be given to the side slopes, and thorough geological surveys must be carried out to ensure the safety while construction as well as while traffic movement.

(4) Minimum resisting length: The unnecessary rise and fall of the gradient must be minimized to reduce the cost and length of road.

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Sight Distance - Stopping Sight Distance and Overtaking Sight Distance

Hi,

Sight Distance:
It is the length of the road ahead visible to a driver at any instance. IRC (Indian Roads congress) has standardized the definition by assuming the eye level of driver as 1.2 m above the road surface and height of obstruction 0.15 m from the road surface.

These are the three main types of sight distances, that are considered while the design of the roads:
(a) Stopping Sight Distance
(b) Overtaking Sight Distance
(c) Sight Distance at the Intersections

(a) Stopping Sight Distance: Stopping Sight Distance is the sight distance required for a driver to effectively apply the brakes and stop the vehicle without collision with the obstruction on the road.
  Stopping Sight Distance = Lag Distance + Braking Distance = v.t + v^2/2gf   where, v is in m/sec

(b) Overtaking Sight Distance: Overtaking Sight Distance is the sight distance which is required by the vehicle running at the design speed to overtake a vehicle running in the same lane at relatively smaller speed.
OSD = d1+ d2+ d3 = vb.t + vb.T + 2S + v.T      

(c) Sight Distance at the Intersection: Sight distance on the uncontrolled intersections should be sufficient enough  for the safety of the traffic, it should be at least equal to the stopping sight distance. 

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Length of Transition Curves

Hi, how've you been?

Length of Transition Curves:

Transition curves are provided in between a straight road and the Curve of a design radius. 

The radius of a transition curve varies from infinity to the design radius or vice verse.  The length of the transition curve must fulfill some requirements. It is designed to fulfill the following three conditions:

(a) Rate of change of centri-fugal Acceleration(C):

 C = (v^2/R)/t =  (v^2/R)/ (Ls/v) = v^3/( LsR)  m/sec^3

As per IRC recommendations, C= 80/(75+v)   m/sec^3

Here, C= allowable rate of change of centrifugal acceleration ( m/sec^3)

Ls= Length of the transition curve.

(b) Rate of introduction of Designed super-elevation:

If pavement is rotated about centre line,  then

1/N = (E/2)/Ls

=> Ls= EN/2 = e.B.N/2 = e.(W+We).N/2

If pavement is rotated about inner edge, then

I/N = E/Ls

=> Ls= EN = e.B.N= e.(W+We).N

 where, Ls= Length of transition curve

              B= width of the pavement

(c) By Empirical Formula given by IRC(Indian Roads congress):

It should not be less than
(i) For plain and ruling terrain:   Ls = 2.7 V^2/R

(ii) For mountainous and steep terrain: Ls = V^2/R

Find out the greatest length of the transition curve by the above three criteria and use to construct the transition curve.

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Super-elevation at horizontal curves

Hi,

Super-elevation: It is the inward transverse slope provided throughout the length of the horizontal curves to counteract the centrifugal force and therefore to check the tendency of the vehicle to over turn or skid.

Outer edge of the pavement is raised with respect to the inner edge of the pavement, the ratio of the height raised to the width of the pavement is called super-elevation.

Superelevation on the horizontal curves

e = E/B = tan(angle w.r.t. horizontal)

Or E = e.B

e+f = v^2/ g.R
Here, e = rate of super-elevation
          f= design value of the lateral friction co-efficient = 0.15
        v = speed of vehicle in m/sec.
        R= Radius of the horizontal curve, m
  If velocity is in Kmph, then
  e+f = V^2/(127.R)

Methods of Providing Super-elevation:

Superelevation is provided at a gradual rate along the length of the transition curve. It  is done by changing crowned camber  a single cross slope before the start of the circular curve.  Full superelevation is attained at the end of transition curve or at the start of the circular curve.

Attainment of the Superelevation may be completed in following steps:

(i) Elimination of the crown of the cambered section.

(ii) Rotation of the pavement to attain a full super-elevation.

Second step can be completed in three ways,  (a) by rotating the pavement with respect to inner edge,  (b) by rotating the pavement with respect to the center of the pavement or  (c) by rotating the pavement with respect to the outer edge of the pavement.

Superelevation is introduced by rise in the outer edge of the pavement at a rate not exceeding 1 in 150 in plain and rolling terrain and 1 in 60 on mountainous and steep terrain as per the recommendations of the IRC(Indian Roads Congress).

Designing Superelevation:

The design procedure for providing the superelevation follows the following Four steps:

(1) Friction is neglected and super-elevation is designed for 75% of the design speed value,

     e = (0.75. v)^2/ (gR)   where v is in m/sec

 or e = (0.75. V)^2/ (127R)   where v is in Kmph/sec

(2) If e < 0.07 then value so obtained is provided

If e>0.07, then provide the maximum super-elevation equal to 0.07 and proceed with steps 3 and 4.

(3) Check the co-efficient of friction developed for the maximum value of e equal to 0.07 for the full design speed v.
   f= v^2/(g.R) - 0.07
if  f< 0.15, then e = 0.07 is safe for the design speed. If not, calculate the restricted speed as follows

(4) 0.07 + 0.15 = Va^2/ (g.R)
         Where, Va = allowable safe speed.

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Widening of Pavement on Horizontal Curve

Hi, 

If you have observed the pavement width on the horizontal curves, you will find that to be somewhat larger than the width on the straight roads. The pavement is extra widened on the horizontal curves due to the following reasons:

1. The wheel base of the vehicles is rigid and therefore while taking the turn, only front wheel are able to change direction. Path traveled by the front will be different and will be at certain distance outwards from the path traced by the inner wheel. This can be understood with the help of the diagram and image shown below.

   

   Extra widening on horizontal curves

2. There is a tendency of the driver to take the outer path at the curves to have more sight distance visible ahead.
3. While overtaking operations on horizontal curves driver will need more spacing from the other vehicles to feel safer.

Widening of the Pavement on the Horizontal Curves:

Horizontal curve on a hilly road (Gocind Sagar view point-Himachal Pradesh)

Widening of the pavement on the horizontal curves is governed by the following factors:

(a) Length of the wheel base

(b) Radius of the curve negotiated, R

(c) Psychological factor which depends upon the velocity of the vehicle and the Radius of the curve.

 In general extra width is provided on the horizontal curves when the radius is less than 300 m.

Now, 

Extra widening = mechanical Widening + Psychological Widening

IRC recmmended values for the Extra widening of pavement on Horizontal curves

           We = Wm + Wps

           We =  nl^2/ 2R + V/ [9.5R^(1/2)]

Here, n = number of traffic lanes

           l = Length of the wheel base

           V= Design speed in kmph

            R = Radius of the horizontal curve in m

The figure given above shows the IRC recommended values of the extra width for the curves of different radius.

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Highway Curves

Hi,
Here are the images which explains the Highway curves, be it horizontal or vertical curves. Horizontal curves are simple circular curves, compound curves, reverse curves etc.
There are transition curves, and at end vertical curves are explained.

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