chapter 2 handout

JIGJIGA UNIVERSITYRoad & Bridge Construction

Chapter 2: ROUTE SELECTION AND SURVEYS

IMPORTANCE OF ROUTE SELECTION

Once the necessity of the highway is assessed, the next process is deciding the alignment and route location. Improper route selection may lead to capital loss in construction as well as loss in costs of maintenance and vehicle operation. Once the route is selected and constructed, it is not easy to change the alignment due to increase in cost of adjoining land and construction of costly structures by the road side. Hence careful considerations should be taken while finalising the selection of a route in construction of new road.

STEPS IN ROUTE LOCATION:

Determine the terminal points. From the study of a map of the area, identify and locate:

National parks Any ancient relics, castles and the likes Existence of monasteries Mining sites Existing transport facilities Other public facilities (electricity, water) Location of construction materials

Conduct preliminary and reconnaissance surveys and collect information on pertinent details of topography, climate, soil, vegetation, and any other factors.

Based on the information collected in the previous two steps select a corridor. Make a preliminary design for the possible alternative alignments and plot on a base

map. Examine each of the alternative alignment with respect to grades, volume of

earthwork to select the best alternative route. Make final design and location of the selected best alternative route.

Instructor : Shivaraj B S 1


GUIDELINES FOR ALIGNMENT AND ROUTE LOCATION

There are certain guidelines that must be borne in mind in selecting the alignment and locating the route.

The route of the highway should be so selected that it can handle the traffic most efficiently and serve the inhabited localities.

A direct alignment usually results in overall economy. The location should minimize the use of agricultural land. The location should involve the least impact on the environment. Obstructions such as places of worship, archaeological and historical monuments

should be passed through. Interference with utility services like electric overhead transmission lines, water

supply mains, sewers, pipelines should be avoided as far as possible. Frequent crossing of railway lines should be avoided. Locate the highway close to sources of embankment materials and pavement

materials. Avoid areas liable to flooding. Steep terrain should be avoided as much as possible. When the alignment has to cross major rivers, the crossing point should be fixed

carefully.

SURVEYS

In order to select the best route location, the following engineering surveys are usually carried out:

Reconnaissance Surveys Preliminary Surveys Detailed (Location) Surveys



RECONNAISSANCE SURVEYSThe purpose of the reconnaissance survey is to evaluate the feasibility of one or more routes for a road construction between specific points. Good reconnaissance survey can be the greatest money-saving phase in the construction of a new road. Hence the engineer should make ample provision in both time and finance for this stage of highway location study. The following is a useful checklist of the general information required in the first phase of the reconnaissance study for a major highway, irrespective of whether it is in a rural or in urban area.

1. General Land Surveya) Location of site on published maps and chartsb) Aerial survey, where appropriatec) Site boundaries, outlines of structures, and building linesd) Ground contours and natural drainage featurese) Above ground obstructions to view and flying, e.g. transmission linesf) Meteorological information

2. Permitted use and restrictionsa) Planning and statutory restrictions applying to the particular areasb) Tunnels, mine-works (abandoned, active and proposed)c) Ancient monuments, burial grounds, etc.

3. Approaches and access (including temporary access for construction purpose)a) Roadb) Railwayc) By waterd) By air

4. Ground conditionsa) Geological mapsb) Flooding, erosion, landslide and subsidence historyc) Construction and investigation records of adjacent sitesd) Seismicity

5. Sources of material for the constructiona) Naturalsb) Tips and waste materialsc) Imported materials

6. Drainage and seweragea) Names of the sewerage and land drainageb) Location and levels of the existing systemsc) Existing flow quantitiesd) Flood risk to the proposed works

7. Water supply8. Electric and gas supply9. Telephone and others

After completion of the reconnaissance survey, Engineer should design the more detailed geotechnical investigations which should be followed during road construction. Design should include all social, ecological, traffic, economic, and political aspects. The results of these studies are presented in a



reconnaissance report. Once the reconnaissance survey is clear and perfect, the best route can be determined.

PRELIMINARY LOCATION SURVEY

The preliminary survey is a large-scale study of one or more feasible routes. It results in a location and alignment that defines the line for the subsequent final location survey. The preliminary survey is made for the purpose of collecting the additional physical information that may affect the location of the highway. Information like, the shape of the ground, the positions of trees, banks and hedges, bridges, culverts, existing roads, power lines and pipe lines, houses and monuments are determined and noted. These are then translated into maps, and engineers will use it for cost estimations. Normally we have 2 types of preliminary survey, Aerial Surveys and Ground Surveys.

Steps involved in preliminary survey are listed below, The first step is the carrying out of a baseline traverse – a series of connecting

straight lines and tangents stationed continuously from the beginning to the end of the survey.

Angles between connecting lines should be measured in accordance. Levels should be taken at all marked stations, as well as at all important breaks in

the ground. After the baseline has been pegged and levels run over it, the topography

elevations should be taken. At the same time locations of all trees, fences, building and important elements are

noted so that they can be shown on the preliminary map.

FINAL LOCATION SURVEY

Most of the surveys in this final location survey are carried in preliminary surveys. Data's collected in preliminary surveys are examined thoroughly in final location survey. It involves preparing detailed layout of selected route and establishing final horizontal & vertical alignments as well as establishing final positions of structures & drainage. Finally a plan is prepared with the help of collected data’s, which will be useful for the construction process.



SOIL CLASSIFICATION

INTRODUCTIONSoil is the most important material used in foundation and construction of pavement

structures. For the foundation, the soil present on the site without disturbance may be used directly. If the soil is disturbed or the quality is not standard, soil may be transported from the other place. For construction of pavement structure, soil in its natural form like sand or gravel may be directly used based on the quality. If the quality is not matching the standards, soil treated for the stabilization can be used for the construction. Soil classification is thus, an integral part of the location, design and construction of highways.Soil classification for highway purposes involve the exploration of the soils along the highway routes and the identification of suitable soils for use as subbase and fill materials. The uses of soil classification are listed below:

i) Selection of roadway alignment; ii) Decision on the need for subgrade or embankment foundation treatment; iii) Selection and design of the roadway pavement; iv) Location and evaluation of suitable borrow and construction materials; andv) Design of foundations for bridges and other structures.

SOIL CLASSIFICATIONAs a means of obtaining general behavior, soils are systematically categorized on the

basis of some common characteristics obtained from visual description, during soil boring, and laboratory tests. As a result very many soil classification systems are in use throughout the world in different areas of study. In highway engineering, soils are classified by conducting relatively simple tests on disturbed samples to serve as a means of identifying suitable materials and predicting the probable behavior when used as subgrade or subbase material.The most commonly used classification systems for highway purposes are,

1. American Association of State Highway and Transportation Officials (AASHTO)2. Unified Soil Classification System (USCS).

These classification systems only help engineers to predict how the soil will behave if used as a subgrade or subbase material, however, the information obtained should not be regarded as a substitute for the detailed investigation of the soil properties.



(AASHTO)The AASHTO Classification System is based on the Public Roads Classification System

that was developed from the results of extensive research conducted by the Bureau of Public Roads, now known as the Federal Highway Administration of the United States. Several revisions have been made to the system since it was first published. The system has been described by AASHTO as a means for determining the relative quality of soils for use in embankments, subgrades, subbases, and bases.

Table 1: AASHTO soil classification system

In this system of classification, soils are categorized into seven groups, A-1 through A-7, with several subgroups, as shown in Table 1. The classification of a given soil is based on its particle size distribution, Liquid Limit, and Plasticity Index.



In general, according to the AASHTO system of classification, the suitability of a soil deposit for use in highway construction can be summarized as follows.

1. Soils classified as A-1-a, A-1-b, A-2-4, A-2-5, and A-3 can be used satisfactorily as

subgrade or subbase material if properly drained. In addition, such soils must he properly

compacted and covered with an adequate thickness of pavement for the surface load to be

carried.

2. Materials classified as A-2-6, A-2-7, A-4, A-5, A-6, A-7-5, and A-7-6 will require a layer

of subbase material if used as subgrade. If these are to be used as embankment materials,

special attention must be given to the design of the embankment.

(USCS)Originally developed by Casagrande during World War II for use in airfield construction,

USCS has been modified several times to obtain the current version. The fundamental premise used in the USCS system is that, the engineering properties of any coarse-grained soil depend on its particle size distribution, whereas those for a fine-grained soil depend on its plasticity. Thus, the system classifies coarse-grained soils on the basis of grain size characteristics and fine-grained soils according to plasticity characteristics.

The particle distribution by sieve analysis is shown below



Thus, the system classifies coarse-grained soils on the basis of grain size characteristics and fine-grained soils according to plasticity characteristics.

Coarse-grained soils. Soils with more than 50 percent of their particles being retained on the No. 200 sieve are classified as coarse-grained soils. The coarse-grained soils are subdivided into,

Gravels (G) soils having more than 50 percent of their particles larger than4.75 mm (i.e., retained on No. 4 sieve)

Sands (S) those with more than 50 percent of their particles smaller than 4.75 mm (i.e., passed through No. 4 sieve).

Fine-grained soils. Soils with less than 50 percent of their particles retained on the No. 200 sieve are classified as fine-grained soils. The fine-grained soils are subdivided into clays (C) or silt (M) based on a plasticity chart plotted between Liquid Limit of the soils and Plasticity Index of the soil from which a dividing line known as the "A" line separates the more clayey materials from the silty materials. Soils that fall below the “A” line are silty soils, whereas those with plots above the "A” line are clayey soils.

The plasticity chart used in classification of the USCS is shown below.


chapter 2 handout

Documents

best route location

route locationthere

road construction

improper route selection

construction of new

reconnaissance study

direct alignment

good reconnaissance