Engineering Surveys for Highway Alignment

Before a highway alignment is finalized in a new highway project, engineering surveys are to be carried out. These engineering surveys may be completed in the following four stages.

i) Map Study

ii) Reconnaissance Survey

iii) Preliminary Surveys

iv) Final Location and Detailed Surveys

1) Map Study

If the topographic map of the area is available, it is possible to suggest the likely routes of the road. In India, topographic maps are available from the Survey of India with 15 or 30 meter contour intervals. The main features like rivers, hills valleys, etc. are also shown on these maps. By careful study of such maps, it is possible to have an idea of several possible alternate routes so that further details of these may be studied later at the site. The probable alignment can be located on the map from the following details available on the map.

1. Alignment avoiding valleys, ponds or lakes
2. When the road has to cross a row of hills or mountains, possibility of crossing through a mountain pass
3. Approximate location of bridge site for crossing rivers, avoiding bend of the river, if any
4. When a road is to be connected between two stations, one of the top and the other on the foot of the hill, then alternate routes can be suggested by keeping in view that the design or ruling gradient and the maximum permissible gradient.

2) Reconnaissance

The second stage of engineering surveys for highway alignment is the reconnaissance survey. During the reconnaissance, the engineer visits the site and examines the general characteristics of the area before deciding the most feasible routes for detailed studies. A field survey party may inspect a fairly broad stretch of land along the proposed alternative routes of the map in the field. Only very simple survey instruments are used by the reconnaissance party to collect additional details rapidly, but not accurately. All relevant details which are not available in the map are collected and noted down. Some of the details to be collected during reconnaissance are given below.

1. Valleys, ponds, lakes, marshy land, ridge, hills, permanent structures and other obstructions along the route which are not available in the map.
2. Approximate values of gradient, length of gradients and radius of curves of alternate alignments.
3. Number and type of cross drainage structures, maximum flood level and natural ground water level along the probable routes.
4. Soil type along the routes from field identification tests and observation of geological features
5. Sources of construction materials, water and location of stone quarries
6. When the road passes through hilly or mountainous terrain, additional data regarding the geological formation, type of rocks, dip of strata, seepage flow etc. may be observed so as to decide the stable and unstable sides of the hill for highway alignment.

3) Preliminary Survey

The main objectives of the preliminary survey are:

1. To survey the various alternate alignments proposed after the reconnaissance and to collect all the necessary physical information and details of topography, drainage and soil.
2. To compare the different proposals in view of the requirements of a good alignment
3. To estimate quantity of earth work materials and other construction aspects and to work out the cost of alternate proposals
4. To finalize the best alignment from all considerations the preliminary survey is carried out to collect all the physical information which are necessary in connection with the proposed highway alignment.

The preliminary survey may be carried out by any one of the following methods.

i) Soil Survey

Soil survey is an essential part of the preliminary survey as the suitability of the proposed location is to be finally decided based on the soil survey data. The soil survey conducted at this stage also helps in working out details of earth work, slopes, suitability of materials, subsoil and surface drainage requirements and pavement type and the approximate thickness requirements. All these details are required to make a comparative study of alternate proposals.

ii) Material Survey

The survey for naturally occurring materials likes stone aggregates, soft aggregates, etc. and identification of suitable quarries should be made. Also, availability of manufactured materials like cement, lime, brick, etc. and their locations may be ascertained.

iii) Traffic Survey

Traffic surveys conducted in the region form the basis for deciding the number of traffic lanes and roadway width, pavement design and economic analysis of the highway project. Traffic volume counts of the classified vehicles are to be carried out on all the existing roads in the region, preferably for 24 hours per day for seven days. Origin and destination surveys are very useful for deciding the alignment of the roads. This study may be carried out on a suitable sample of vehicle users or drivers. In addition, the required traffic data may also be collected so that the traffic forecast could be made for 10 to 20 year periods.

iv) Determination of Final Centre Line

After completing the preliminary surveys and conducting the comparative studies of alternative alignments, the final centre line of the road is to be decided in the office before the final location survey. For this, the preliminary survey maps consisting of contour plans, longitudinal profile and cross sections of the alternate alignments should be prepared and carefully studied to decide the best alignment satisfying engineering, aesthetic and economical requirements. After selecting the final alignment, the grade lines are drawn and the geometric elements of the horizontal and vertical alignments of the road are designed.

v) Rapid Method Using Aerial Survey and Modern Technique Using GPS

Aerial photographic surveys and photogrammetric methods are very much suited for preliminary surveys, especially when the distance and area to be covered are vast. The survey may be divided into the following steps.

Taking aerial photographs of the strips of land to be surveyed with the required longitudinal and lateral overlaps. Vertical photographs are necessary for the preparation of mosaics.

1. The photographs are examined under stereoscopes and control points are selected for establishing the traverses of the alternate proposals. The control points are located on the maps.
2. Using stereo-pair observations, the spot levels and subsequently contour details may be noted down on the maps
3. Photo-interpretation methods are used to assess the geological features, soil conditions, drainage requirements etc.

4) Final Location and Detailed Survey

The alignment finalized at the design office after the preliminary survey is to be first located on the field by establishing the centre line. Next detailed survey should be carried out for collecting the information necessary for the preparation of plans and construction details for the highway project.

i) Location

The centre line of the road finalized in drawings is to be transferred on the ground during the location survey. This is done using a transit theodolite and by staking of the centre line. The location of the centre line should follow, as closely as practicable, the alignment finalized after the preliminary surveys. Major and minor control points are established on the ground and centre pegs are driven, checking the geometric design requirements. However, modifications in the final location may be made in the field, if found essential. The centre line stakes are driven at suitable intervals, say at 50 metre intervals in plain and rolling terrains and at 20 metre in hilly terrain.

ii) Detailed Survey

Temporary bench marks are fixed at intervals of about 250 m and at all drainage and under pass structures. Levels along the final centre line should be taken at all staked points. Levelling work is of great importance as the vertical alignment, earth work calculations and drainage details are to be worked out from the level notes. The cross-section levels are taken up to the desired width, at intervals of 50 to 100 m in plain terrain, 50 to 75 m in rolling terrain, 50 m in built up areas and 20 m in hilly terrain. The cross sections may be taken at closer intervals at horizontal curves and where there is abrupt change in cross slopes. All river crossing, valleys etc. should be surveyed in detail up to considerable distances on either side. All topographical details are noted down and also plotted using conventional signs. Adequate hydrological detail is also collected and recorded.

Drawings and Report

Drawings

The following drawings are usually prepared in a highway project.

i) Key Map

Key map should show the proposed and existing roads, and important places to be connected. The size of the plan generally should not exceed 22 x 20 cm. The scale of the map is chosen suitably depending upon the length of road.

ii) Index Map

Index map should show the general topography of the area. The details are symbolically represented. The index map should also be of suitable scale, the size being 32 x 20 cm.

iii) Preliminary Survey Plans

Preliminary survey plans showing details of the various alternate alignments and all information collected should be normally drawn to scale of 10 cm = 1 km to 25 cm =1 km.

iv) Detailed Plan and Longitudinal Section

Detailed plans show the ground plan with alignment and the boundaries, contours at intervals of 1 to 2 m in plain terrain and 3 to 6 m in hills, showing all details including existing structures. A scale of 1/2400 in close country and a scale of 1/1200 may be adopted for detailed plans. The size of the drawing may be A2 size or 60 x 42 cm approximately.

Longitudinal sections should be drawn to the same horizontal scale of the ground as in detailed plan. Vertical scale may be enlarged 10 times of the longitudinal scale. The longitudinal section should show the details such as datum line, existing ground surface, vertical profile of the proposed road and position of drainage crossings.

v) Detailed Cross Section

Detailed cross sections are generally drawn to natural scale of 1 cm = 2.0 to 2.5 m. Cross section should be drawn every 100 m or where there are abrupt changes in level. In hill roads the cross sections should be drawn at closer intervals. The cross-section drawings should extend at least up to the proposed right of way. The cross-section number, the reduced distances and the area of filling and/or cutting should be shown on cross section drawings.

vi) Land Acquisition Plans

Land acquisition plans and schedules are usually prepared from the survey drawings for land acquisition details. These plans show all general details such as buildings, wells, nature of gradients and other details required for assessing the values. The scale adopted may be 1 cm = 40m or less.

vii) Drawings of Cross Drainage and Other Retaining Structures

Detailed design for cross drainage and masonry structures are usually drawn to scale of 1 cm = 1 m. For details of any complicated portion of the structure enlarged scales up to 8 cm = 1 m or up to half full size may be employed. However, the size of drawing should not exceed the standard size. Cross sections of streams should be to a scale of not less than 1 cm = 10 m.

viii) Drawings of Road Intersections

Drawings of road intersections should be prepared showing all details of pavement, shoulders, islands etc. to scale.

ix) Land Plans Showing Quarries, etc.

Where quarries for construction materials are to be acquired for new projects, separate land plans should be prepared. The size of these maps and scales may be similar to those suggested under land acquisition.

Estimates

The project estimates should consist of general abstract of cost and detailed estimates for each major head. If the project work is proposed to be executed in stages, the estimate should be prepared for each stage separately.

Project Report

The first phase of project report soon after completing the preliminary surveys, feasibility and EIA studies is to prepare a 'Feasibility Report'. The Detailed Project Report (DPR) should be prepared after completing all the detailed studies including final location survey, preparation of longitudinal and cross sections, soil and material surveys, drainage studies, etc. The design details of the pavements and all Cross Drainage structures including major bridges should be carried out and the relevant drawings prepared as specified in the terms of reference for the project preparation.

Highway Projects

In a new highway project, the engineer has to plan, design and construct either a network of new roads or a road link. There are also projects requiring redesign and realignment of existing roads of upgrading the geometric design standards. Once a highway is constructed, development takes place along the adjoining land and subsequent changes in alignment or improvements in geometric standards become very difficult. A badly aligned highway is not only a source of potential traffic hazard, but also causes a considerable increase in transportation cost and strain on the drivers and the passengers. Therefore, proper investigation and planning are most important in a road project, keeping in view the present day needs as well as the future developments of the region.

New Highway Project

The new highway project work may be divided into the following stages.

1. Selection of route, finalization of highway alignment and geometric design details.
2. Collection of materials and testing of sub grade soil and other construction materials, mix design of pavement materials and design details of pavement layers.
3. Construction stages including quality control.

i) Route Selection

The selection of route is made keeping in view the requirements of alignment and geological, topographical and other features of the locality. However special care should be taken as regards the geometric design standards of the road for possible upgrading of speed standards in future, without being necessary to realign the road. After the alignment if finalized, the plans and working drawings are prepared.

ii) Materials and Design

The soil samples collected from the selected route during the soil surveys are tested in the laboratory in order to design the required pavement thickness and the design of embankment and cut slopes. The basic construction materials such as selected soil, aggregates etc. are collected from the nearest borrow pits and quarries and stacked along the road alignment after subjecting these materials to the specified laboratory tests. In order to design the mixes for the pavement component layers and to specify quality control test values during road construction, mix design tests are carried out in the laboratory.

iii) Construction

The construction of the road may be divided into two stages,

i) Earth work

ii) Pavement construction

The earth work consists of excavation and construction of the embankments. During the excavation for highway cuts, the earth slopes, their protection and construction of drainage network are taken care of. Highway embankments may be best constructed by rolled fill method by compacting the soil in layers under controlled moisture and density using suitable rollers. In the case of high embankments, the stability of the embankment foundation and slopes and the possible settlement of the embankment with time are to be investigated. The pavement construction is subsequently taken up starting with the preparation of sub grade and the construction of sub base, base and surface courses of the pavement.