Engineering Survey and its Stages

The engineering surveys are to be carried out before a Road alignment is finalized in Road project. The Surveys are completed in four stage, first three stages consider all possible alternate alignments keeping in view of the various requirement of road alignment. The fourth stage is meant for the detailed survey of the selected alignment.

The four stages of the engineering surveys are:

       i.          Map Study

When the topographical map of the area is available, it will be easier to find out alternate path of the highway. In Nepal, topographic maps are available from the department of survey, Government of Nepal. Then it is possible to suggest the likely routes of the road. The new maps are available in 1:25,000 to 1:50,000 scales. The main features like rivers, hills, valley is carefully shown in color in these maps. By the careful study of such maps it is possible to have an idea of the several possible alternate routes so that the further details of these may be studied later at the site. The probable alignment can be located on the map from the following details map available on the map,

1.     Alignment avoiding valleys, ponds or lakes.

2.     When road has to cross a row of hills, possibility of crossing through a mountain pass.

3.     Approximate location of bridge site for crossing rivers, avoiding bend of the river if any.

Hence from the map study the alternate routes can also be suggested. It may also be possible from the map study a drop a certain route in view of any unavoidable obstructions or undesirable ground enroute. Thus, map study gives a rough guidance of the routes necessary for the survey.

     ii.          Reconnaissance

It is the second stage of surveying deciding the road location. The field survey parties inclusive of an advisor inspect a broad stretch of land along the proposed alternative routes of map in the field. All relevant details not available in the map were collected and noted down. Some of the details collected during reconnaissance are mentioned as follows:

Ø  Ponds, marshy land, ridge, hills, permanent structures and other obstruction along the route, which are not available in the map study.

Ø  Number and types of cross drainage structures, maximum flood level and natural ground water level along the probable routes.

Ø  Soil type along the routes from field identification tests and observation of geological features.

Ø  Sources of construction materials, water and location of stone quarries.

For the vast area or when terrain is difficult, reconnaissance may be one by aerial survey. From the details collected during the reconnaissance, the alignment proposed after map study may be altered or even changed completely for better design of road.

    iii.          Preliminary Survey

The main objective of the preliminary survey are given as below:

Ø  The Survey of the various alternate alignments proposed after the reconnaissance and to collect all necessary details of topography, drainage and soil.

Ø  Comparison of different proposals in view of the requirements of a good alignment.

Ø  To finalize the best alignment from all considerations.

 

    iv.          Final Location and Detailed Survey

The alignment finalized after the preliminary surveys is first located on the field by establishing the centerline. Next detailed survey is carried out collecting the data necessary for the preparation of plans and construction details for the road Project. The key terms used in the course of detailed survey are as stated hereunder:

a)     Plan

It is top view of project in a map. It consists of

                        i.             Northing

                      ii.             Location of IPs and BMs with reference

                    iii.             The road centerline, formation width, side drain and right of way

b)     Benchmark

It is a permanent point of reference whose elevations w.r.t. some assumed datum is known.

c)     Leveling

It is the branch of surveying. The objective of leveling is to find the elevations of given points w.r.t. a given elevations or at a different elevations w.r.t. a given or assumed datum. Leveling deals with measurements in a vertical plane. For the purpose of road construction, the profile leveling is carried out to determine the R.L. of the centerline located with driven pegs. The leveling determines the alignment of the road. The leveling is taken at suitable intervals according to the site.

d)     Composition of Traffic

The alignment of road passing from the shortest route is deviated due to the volume and composition of traffic. For road with intensive heavy vehicles and high volume of traffic alignment yielding minimum length of steep ascend/descend is much desirable than the shortest route distance.

Similarly, a road leading to a recreation spot, or tourist spot that might have predominant by light passenger car and a few buses, alignment may be chosen with higher slope. In addition, the origin and destination study should be carried out in the area and the desire lines be drawn showing the trend of traffic flow. The alignment should be chosen based on origin and destination study, traffic desire lines, flow pattern, future trends etc.

e)     Geometric Features

Geometric design factors such as permissible limit of descending or ascending slopes, sight distance requirements, degree of curvature and bends, slope of camber, super elevation, and width of the road, extra widening and many other dimensional features of the road may also govern the final alignment of the highway.

f)      Economy

The appropriate alignment should also be economical. The economic analysis is done  taking into account all the possible costs which includes construction cost, maintenance cost, vehicle operation cost, accident cost, travel time cost etc. The analysis should also include cost of environmental and social mitigation measures, land acquisition for construction and development of road. For road alignment to be economical, the sum of all road components should be as less as possible. The initial construction cost seem to be more initially but in reality  the vehicle operation cost and road user travel time cost is more. However, due to budget constraint sometimes – initial construction cost might be the governing factor and alignment selected accordingly even if the road yields highest maintenance cost and vehicle operation cost. The Initial cost of construction can be decreased if high embankments and deep cuttings are avoided and the alignment is chosen in a manner to balance the cutting and filling.