Highway Alignment and Engineering Survey

The alignment of a highway is the position of its centre line on the ground. An ideal alignment is one that is short, easy, safe and economical (the four requirements: short, easy, safe, economical).

Requirements

1. Short: the straight line is the shortest route; a short alignment saves construction and vehicle operating cost.
2. Easy: gentle gradients and flat curves so that vehicles can run smoothly at the design speed. The gradient should not exceed the ruling gradient.
3. Safe: adequate sight distance; avoid sharp curves, steep grades and unstable slopes; safe at intersections and bridges; geometric design standards (NRS 2070) followed.
4. Economical: low initial cost (earthwork, structures) as well as low maintenance and vehicle operation cost; balance cut and fill.
5. Serve the needs: pass through or near the important towns and traffic generation points.
6. Stable ground: avoid landslide-prone, flood-prone and poor soil areas; avoid marshy land.
7. Minimum cross-drainage works: cross the streams at right angles and at narrow, stable points; fewer bridges and culverts.
8. Construction materials: near the alignment.
9. Minimum land acquisition, and avoid religious, historical sites and forests as far as possible.
10. Future expansion: possibility of widening and upgrading.
11. Hill roads: keep to the contour, use hair-pin bends where necessary, avoid steep north-facing wet slopes.

The alignment of a highway is the route (centre line) selected on the ground. The selection depends on many factors, which can be grouped as follows.

1. Obligatory points (control points)

• Points the road must pass: bridge sites, mountain passes (saddles), important towns, or existing roads.
• Points the road must avoid: religious and historic places, graveyards, costly buildings, reserved forests, airports, lakes and unstable land.

2. Traffic

• Alignment should serve the origin-destination pattern, linking the main traffic generators.
• Traffic volume and composition decide the design standard.

3. Geometric design

• Design speed, gradient, curves, sight distance and widths from NRS 2070 (or IRC) limit what is possible.
• Avoid steep grades and sharp curves; keep sight distance adequate.

4. Economy

• Total cost = construction + maintenance + vehicle operation. The cheapest alignment is not always the shortest. Balance cut and fill and keep earthwork, structures and land acquisition low.

5. Topography

• Avoid very rough, steep or hummocky ground. In hills, follow the contour with ruling gradient; in plains, take nearly straight routes.

6. Hydrology and drainage

• Number and size of rivers and streams; cross the streams at right angles at narrow, stable places; avoid flood-prone, marshy and high-water-table land.

7. Geology and soil

• Stable rock or good soil; avoid landslide, unstable, swelling clay, and highly erodible zones.

8. Materials

• Availability of construction materials (aggregate, sand, water) near the road.

9. Climate and environment

• Rainfall, snow, wind, fog; avoid shady north-facing wet slopes in hills; avoid forests and wildlife areas; EIA requirements.

10. Political and social factors

• Land acquisition, resettlement, administrative boundaries, defence and security.

11. Future expansion

• Possibility of widening and upgrading later.

A good alignment is the one that satisfies the most of these factors at the least total cost.

To fix the best alignment, the engineering survey is done in stages, moving from broad study to detailed work. Each stage narrows the options.

1. Map study

• Using topographic maps (1:25,000 or 1:50,000), satellite images or aerial photographs, the possible routes are marked between the terminals through the obligatory points. Ridge lines, valleys, river crossings, passes and obstructions are studied.
• Gives alternative alignments at low cost.

2. Reconnaissance survey

• A quick field examination of the routes picked on the map, on foot or by vehicle, with simple instruments (abney level, compass, tape, altimeter).
• Collects: topography, geology, drainage and stream crossing, soil, materials, land use, climate/rainfall, settlements, forests, cost of land.
• Rough estimates of cost; best routes (one to three) are selected.

3. Preliminary survey

• A detailed survey of the selected route(s) to compare and fix the final one: a traverse with a theodolite/total station, levelling along it, contouring of a strip (a 100-200 m belt), cross-sections, stream data, soil and material survey, drainage and traffic data.
• Output: contour plan, preliminary estimate and comparison of alternatives.

4. Final location and detailed survey

• The chosen alignment is fixed on the ground using the plan from the preliminary survey: centre line pegging, curves set out, bench marks, profile and cross-sections, soil and material tests, hydrological data.
• Detailed drawings, bill of quantities and cost estimate for tender.

Map study -> Reconnaissance -> Preliminary -> Final location
 (paper)       (field, quick)     (detail)      (set-out, design)
 many routes      2-3 routes     1-2 routes      1 alignment

Route location is done in a funnel-like sequence, moving from a large area to a single centre line. Each step narrows the area and increases the detail of the study.

 Region  (whole area between terminals)
   |  map study, aerial photos
 Band    (feasible strips, obstacles removed)
   |  reconnaissance
 Corridor (probable narrow routes)
   |  preliminary survey
 Alignment (single centre line, final location)

1. Region: the whole area of interest between the two terminals, including the controls. Identified from small-scale maps.
2. Band: the part of the region left after eliminating zones with obstacles (mountains, big lakes, forests). Studied by maps and air photos.
3. Corridor: within the band, the narrower strips containing the likely alignments - width a few hundred metres to a few kilometres. Selected after reconnaissance.
4. Alignment: within the chosen corridor the exact centre line is fixed after preliminary and detailed surveys.

Process steps: (i) identify the terminals and obligatory points, (ii) map study, (iii) reconnaissance survey, (iv) preliminary survey, (v) final location/detailed survey, followed by design and cost estimate.

Highway alignment is the position or layout of the centre line of the highway on the ground. It has two parts: the horizontal alignment (straights, curves, as seen in plan) and the vertical alignment (gradients and vertical curves, as seen in profile). A good alignment is short, easy, safe and economical.

The preliminary survey is a detailed instrumental survey of the routes chosen after the reconnaissance, to compare them and fix the best alignment. The objectives are to collect the topographic and other data, estimate cost and select the final alignment.

Conventional method - procedure

1. Primary traverse: a closed or open traverse is run along each route using a theodolite or total station; chainage pegs at 20-30 m (or 50 m) intervals; traverse stations kept fixed with the main control points.
2. Levelling: longitudinal levelling along the traverse and cross-sections (about 100-150 m each side) at regular intervals; bench marks established at about every 500 m.
3. Topographic detail: details such as streams, buildings, roads, fields and trees are plotted in a strip about 100-200 m wide (contour interval 1-2 m) to prepare the contour plan.
4. Drainage and hydrology data: stream crossings, catchments, high flood level, existing structures.
5. Soil and geology survey: soil classification, CBR/subgrade tests at intervals; location of quarries and construction materials.
6. Traffic data and land use where needed.
7. Cost estimate of each alternative route, including earthwork, pavement, cross-drainage works and land.
8. Comparison of alternatives and selection of the final route.

Result

The contour plan, longitudinal section and estimates; the alignment is then marked in the field for the final location survey.

Modern practice uses total station, GPS, drone/aerial photogrammetry, and digital terrain model software (the aerial method), which saves time.

The final location and detailed survey transfers the alignment chosen from the preliminary survey to the ground and collects all data needed for design, estimate and construction.

Process

1. Location of centre line: the centre line from the plan is set out with a theodolite/total station and fixed by pegs at 20 m (30 m in plains) intervals.
2. Setting out of curves: tangents are fixed at the points of intersection; circular and transition curves are set out with deflection angles/coordinates; curve data recorded.
3. Levelling: the final longitudinal profile, cross-sections at 20-50 m intervals (and at changes of slope), permanent bench marks every 250-500 m.
4. Reference pillars are fixed outside the construction limits to restore the centre line.

Works to be carried out

• Detailed topographic survey at bridges, intersections, hairpin bends, and difficult sections (scale 1:500 to 1:1000 near structures).
• Soil survey: soil testing for subgrade and for borrow areas; CBR values.
• Hydrological survey: catchment area, flood levels, discharge, waterway needs for culverts and bridges.
• Geological/geotechnical investigation for slopes, cut and fill, foundations of structures.
• Material survey: quarries, sand, aggregates, water and their lead.
• Land and property survey: land boundaries, structures to be removed, utility lines.
• Traffic survey if required.

Data collected and outputs

Plan and profile drawings, cross-sections, culvert/bridge details, soil data, material sources, quantities, cost estimate, specification and tender documents.

The survey stages build on each other: the data from one stage is reused and refined in the next so that the work is not repeated, and the choice becomes firmer.

How the updating is done

1. Map-study routes are checked and corrected in the field during reconnaissance (e.g. a route marked over a ridge may be found to pass landslide area).
2. The reconnaissance observations on soil, streams and materials are measured accurately in the preliminary survey.
3. The preliminary contour plan is used to fix the centre line, and minor changes (smoother curves, balance of earthwork, better stream crossing) are made at the final location.
4. Estimates are updated: rough -> preliminary -> detailed.
5. Any new obstacle found in a later stage can send the study back to the previous stage.

Thus the information is continuously verified, refined and extended from broad to detailed.

Reconnaissance is a rapid field study of the alternative routes that were picked in the map study. The following information is collected.

1. Topography: general terrain, ridges, valleys, cross-slopes, possible gradients, mountain passes.
2. Streams and rivers: number and size of stream crossings, high flood level, river course, bed material, suitable bridge sites.
3. Soil: type of soil (clay, silt, sand, rock), suitability as subgrade, swamp or black cotton soil.
4. Geology: rock type, landslide zones, faults, unstable slopes.
5. Drainage conditions: water table, flooding, seasonal streams.
6. Land use: forests, farms, settlements, graveyards, religious sites, protected areas.
7. Construction materials: quarries, sand and aggregate sources, water and the lead distance.
8. Climate: rainfall, snow, fog, wind, temperature.
9. Existing roads and bridges, their conditions and possible use.
10. Cost of land and property, utilities (electric, telephone, pipes) to be shifted.
11. Labour and equipment availability, and accessibility to the site.
12. Social and environmental aspects, e.g. effect on local people.

Instruments used: Abney level, compass, tape, pocket altimeter, GPS and camera. The final output is a comparison of the routes with rough lengths and cost, from which 1-3 are selected for the preliminary survey.

Reconnaissance survey is done to check, in the field, whether the routes chosen by the map study are really feasible.

• To verify the map study: maps may be old and do not show new buildings, landslides, rivers, soil conditions.
• To collect data not available on maps: soil, geology, drainage, land use, materials, local conditions.
• To eliminate bad routes and to keep one to three promising routes for the costly preliminary survey, so time and money are saved.
• To find the obligatory points and stream crossing positions.
• To make approximate estimates of construction cost and to compare routes.
• To get a general idea of the problems (hydrology, slope stability, land acquisition) before the detailed work.

Basic principle of route location

The aim is to find the route which gives the minimum total cost to the nation - the sum of construction cost, maintenance cost, vehicle operating cost (including time cost) and accident cost over the life of the road - while serving the traffic and meeting the safety and environmental requirements. In short, the route should be short, easy, safe and economical, passing through the obligatory points and avoiding the points to be avoided.

The route is located in a systematic sequence: region -> band -> corridor -> alignment, using map study, reconnaissance, preliminary and detailed surveys.

Complexities of route location

1. Many conflicting objectives: the shortest route may be steep or costly; the cheapest may be long or unsafe, so a compromise is needed.
2. Large number of variables: terrain, geology, hydrology, land use, traffic, environment all affect the choice.
3. Uncertain and incomplete data: maps may be old and field data limited; future traffic is only a forecast.
4. Obligatory points may be far from each other or impose very difficult geometry (e.g. a pass).
5. Cost and benefit that cannot be easily valued: environmental harm, social impact, resettlement, loss of heritage.
6. Land acquisition and public opposition, political pressure and administrative boundaries.
7. Environmental restrictions (national parks, forests, wetlands, EIA).
8. Hydrological and geological risks in Nepal: floods, landslides, young unstable hills.
9. Interaction with other infrastructure (canals, power lines, existing roads).
10. Time and funding limits and need for staged construction.

Therefore, the location team should compare alternatives using a multi-criteria or economic analysis, and select the route that gives the best overall result.

Map study is the first stage of the survey in which possible alignments are studied on topographic maps before going to the field.

Importance

1. Quick and cheap: gives alternative routes without costly field work.
2. Identifies the obligatory points such as passes, bridge sites, towns.
3. Shows the topography: contours show ridges, valleys, steep slopes and the possible gradients.
4. Shows features like rivers, lakes, forest, settlement, existing roads and railways which must be avoided or crossed.
5. Helps to reject unsuitable routes early and limits the fieldwork to the promising ones.
6. Gives approximate length and earthwork and helps to estimate cost.
7. Guides reconnaissance: the field team knows what to check.
8. Can use modern tools: satellite images, DEM and GIS for better and faster results.

Limitation: maps may be old and give no soil or geology data, so the field survey must verify them.

In route location the study narrows step by step from the largest to the smallest area.

• Region (2 marks): the whole area, usually a large one, between the two terminals and including all the controls, within which the route can be placed. It is picked from small-scale maps, e.g. the zone between Kathmandu and Pokhara.
• Band (2 marks): a part of the region left after the large barriers (high mountains, big lakes, reserved forests) are removed. It contains the feasible routes and is found by studying maps and aerial photos. Its width is typically several kilometres.
• Corridor (2 marks): a narrower strip within the band which contains the probable alignments, found after more detailed map study and reconnaissance. Its width is a few hundred metres to a kilometre.
• Alignment (2 marks): the exact centre line of the road within the corridor, fixed after the preliminary and final location surveys, with all the curves and gradients.

 Region  >  Band  >  Corridor  >  Alignment
 (largest)                         (one line)

Economic appraisal checks whether a highway project is worthwhile for the nation by comparing all its costs and benefits over its life.

Components

A. Costs

1. Initial (capital) cost: land acquisition, earthwork, pavement, structures, drainage, engineering and supervision.
2. Maintenance costs (routine, periodic) and rehabilitation.
3. Operation costs of the highway agency (traffic control, lighting).
4. Environmental and social costs (resettlement, pollution).

B. Benefits (road user and others)

1. Savings in vehicle operating cost (fuel, tyre, repair, depreciation).
2. Savings in travel time of passengers and goods.
3. Savings from reduced accidents.
4. Benefits from generated and diverted traffic.
5. Regional and development benefits (agriculture, industry, tourism, land value).
6. Reduced maintenance on the old road, and salvage value at the end of the analysis period.

C. Analysis items

• Analysis period and discount rate, traffic forecast.
• Economic indicators: Benefit-Cost ratio (BCR), Net Present Value (NPV), Internal Rate of Return (IRR), First Year Rate of Return.
• Sensitivity and risk analysis (cost increase, traffic decrease).
• Conversion of financial prices into economic prices (shadow prices, removal of taxes and subsidies).