ENGINEERING SURVEYS FOR LOCATION OF HIGHWAY ALIGNMENT
Before a highway alignment is finalised, the following engineering surveys are to be carried out.
1) Map Study
3) Preliminary survey
4) Detailed survey (Location Survey)
1) Map Study :
In india topographic maps are available from the survey of India, with 15 or 30 m contour intervals.
The main features like rivers, hills, valleys, etc. are shown on these maps.
By cares study of such maps, it is possible to have an idea of several possible alternative routes to the further details of these may be studied later at the site.
2) Reconnaissance :
The object of reconnaissance survey are :
i) To examine the general character of the area.
ii) To collect the information about the important features of the area.
The details to be collected during reconnaissance are :
a) Obstructions along the route like, valleys, ponds, lakes, marshy land, ridge, hill
b) Approximate values of gradient, length of gradients and radius of curves of alternate alignment.
c) Number and types of cross drainage structures, maximum flood level, natural gro water level etc.
d) Soil types along the routes from field identification tests.
e) Sources of construction materials, water and location of stone quarries.
3) Preliminary Survey :
The main objects of Preliminary surveys are:
i) To survey the various alternative alignments proposed after the reconnaissance and collect all the necessary physical information and details of topography, drainage an soil.
ii) To compare the different proposals in view of the requirements of a good alignment.
iii) To estimate quantities of earthwork and other construction aspects and to workout the cost of alternate proposals.
iv) To finalise the best alignment from all considerations.
Procedure of Preliminary Survey :
The various steps in preliminary survey are :
a) Primary traverse :
The first step in the preliminary survey is to establish the primary traverse.
As these traverse are open traverses no adjustment of error is possible later, so the angles should be measured accurately by the theodolite.
The length of centre line should be measured accurately by accurate chaining methods, or by tacheometry or by modern instruments.
b) Topographical features :
After establishing the centre lines of preliminary survey the topographical features are recorded.
All geographical features and man made features along the traverse and for a centain width on either side are surveyed and plotted.
c) Levelling Work :
Levelling work is required to give centre line profiles and typical cross sections.
The levelling work in the preliminary survey is kept to a minimum just sufficient to obtain the approximate earth work in the alternate alignments.
d) Drainage Studies :
Drainage investigations and hydrological data are collected so as to estimate the type, and approximate size of cross drainage structures.
For vertical alignment of the highway hydrological and drainage data like.
- Depth of water table
- Amount of surface runoff
- Ponded water level, etc.
are also collected.
e) Soil Survey :
The proposed location of highway is finally decided based on soil survey data, thus survey is very important.
The soil survey data is useful for :
- To decide suitability of materials to decide amont of earthwork
- Details of slopes
- Subsoil and surface drainage requirements
- To decide type of pavement and its thickness.
At this stage a detailed soil survey is not necessary.
Soil samples may be collected to depth of 1 to 3 m by augers.
The electrical resistivity method or geophysical methods may be adopted for soil investigation.
Soil profile is obtained by drawing longitudinal section along the proposed road alignment.
The types of soils encountered along the route up to the depth of exploration are marked on the soil profile by suitable colour coding.
f) Material Survey :
The survey for naturally occuring materials like stone aggregates, soft aggregates and iden tification of suitable quarries should be made.
Also availability of materials like cement, bricks, lime, fly ash etc. and their locations may be ascertained.
j) Traffic Survey :
Traffic surveys are helpful for deciding the number of traffic lanes, width of roadway, Traffic volume study are to be carried out on all existing roads in the region, preferably pavement design etc. for 24 hours.
Origin and destination study is useful for deciding the alignment of the roads.
h) Determination of final centre line :
After completing the preliminary surveys and conducting the comparative study of alternate alignments the final centre line of the road is to be decided in the office before the final location survey.
For this, contour plans, longitudinal profile and cross sections of alternate alignments should be prepared and carefully studied to decide the best alignment satisfying engineering asthetic and economical requirements.