Agricultural Structures and Environmental Control 3(2+1)

LESSON 14. Types of Rural Roads and Their Construction

1.2 TYPE  OF FARM ROADS:

From the service point of view the farm roads shall be grouped into the following two classes.

1. The road serving farm stead and

2. The road serving fields

According to the construction and materials used the roads may be grouped into two types;

1. The flexible road made up of layes of various materials such as lime stone, gravel, kanker, red earth etc.

2. The rigid road made up of cement concrete.

FARM STEAD ROAD :

The farmstead road generally carries traffic of greater weight and density than the field road.  Therefore it should have a smooth, hard surface requiring minimum maintenance and easy cleaning. For this, the concrete is most suitable.  If any existing flexible road is to be made use of, then it should be reconditioned with a wearing surface.

FIELD ROAD

Field road should have sufficient hardness and not necessarily smoothness.  Hence all that is required is a good hard base with a surface of gravel of broken stone.  The base may be constructed using rubble, broken brick, late rite or any other cheap material available in the locality.

1.3 THE REQUIREMENTS OF A GOOD ROAD:

An ideal road should be perfectly straight, level, smooth hard and dry.  Such perfection can of course be rarely reached and hence in practice a compromise of all the above requirements is to be made.

STRAIGHTNESS :

Straight road will facilitate easy plying of vehicles and good visibility to avoid accident.  Wherever curves occur they should be smooth and easy.  When vehicles are moving along curves of a rod, a side thrust is created and it tends to dislodge the vehicle from the road causing slipping or skidding and overturning. To avoid this and maintain equilibrium of the vehicle moving on a curve, the outside edge of the road is kept at a higher level to counter act the centrifugal force.  This rise is known as super elevation.  Super elevation is the inclination given to the cross section of road on curves in order to reduce the centrifugal force on a running vehicle.  It is expressed as a fraction of the difference in level between the outer and the inner edges of the road to its width.

The generally adopted super elevation for the various kind of roads are given in the table.  Table showing the super elevation for various kinds of roads.

Kind of roads	Super elevation
Water bound macadam road	1 in 48
Tar macadam	1 in 60
Cement concrete	1 to 72

 

LEVELNESS :

The road should be level or flat as far as possible to give maximum efficiency of the vehicles. In forming new roads easy gradients are to be maintained so that the road is suitable for all kinds of vehicle and to achieve this, much cutting and embankment may be necessary.  But, for the sake of economy, the amount of cutting and filling should be equal.  This must be achieved without sacrificing the gradient.  The importance of work and the reasonable expenditure that can be incurred will be the main factor in deciding the gradient.

Gradient is the slope (rise or fall) of a road along its longitudinal section.  It is expressed as a ratio of the difference in level between two points to their longitudinal distance.  It is also expressed as a percentage of slope (i.e.) the rise or fall for 100 meters (100 ft.) distance.  As the steepness of roads beyond a certain limit will make it difficult for vehicular traffic, a gradient practically found useful called ruling gradient is adopted in road construction.  The ruling gradient adopted for roads in plains is 1 in 40, where as in hilly tracts it is 1 in 20, In road making, a level road would be ideal from the attractive and efficiency point of view.  In practice it is often impracticable due to unevenness of ground.  Further, roads with appreciable gradient will help the drainage.

SMOOTHNESS :

It is important for two reason.  In the case of smooth roads the wear and tear on vehicles in greatly reduced.   Secondly, it helps to drain off water since on smooth water proof surface water will not stick but will be shed to ditches at the sides.  Otherwise the road will never be satisfactory and maintenance cost will be heavy.

HARDNESS :

The necessity for hardness is self evident, since its resistance to wear and tear and durability depends on the hardness of road itself.

DRYNESS :

It is the native soil that really supports the weight of the traffic, and that, while the soil is preserved dry, it will carry any weight without sinking.  Hence the removal of excess water falling on the finished surface by providing chamber, intercepting water from the adjoining land which drains naturally across  th3e road and removing sub soil water by providing drains is essential.  To effect drainage on the surface, the roads are constructed with sufficient camber.  Camber is the rise of the center of roads with reference to the edge.

The camber adopted for different kinds of roads are given in the table.  The tables shows the camber for different kinds of roads.

Kind of roads	Camber
Earthen roads	1 in 20 to 1 in 24
Water bound macadam road	1 in 30 to 1 in 48
Tar or bitmin	1 in 36 to 1 in 48
Cement concrete	1 in 60 to 1 in 72

 

Camber should be suitably adopted so as to facilitate easy traffic, since steep camber causes slipping of high speed vehicles and erosion on berms.  The usual shape of camber adopted for roads is a curved contour flat at the central half and slightly steeper at the sides.

LOCATION OF ROADS :

The location and setting out the road is the first problem in road making.  While determining the location it is essential to be clear as to the purpose of the road and the points it has to serve.  A rod in a farm should serve as may economical one.  The topography of the land, the existing obstacles requiring road crossing and the nature of the soil underlying are all factors to be examined.  Too many crossing of waterways will increase the cost.  Heavy cutting and embankments should generally be avoided.  The location of firm soil underneath will to reduce the cost of construction and maintenance.  Existing lanes may also be considered.

After making a reconnaissance survey, the alignment of the road is roughly fixed.  Then a detailed survey should be conducted to prepare a plan showing a strip of sufficient width of either side of the proposed alignment to judge the deviations etc.  In this plan, the alignment, sites of culvert and crossings are marked.  The levels in the centre line at regular intervals together with cross sections should be taken and the levels are noted on the plan.  On studying the levels and taking into consideration the various factors given under the requirement of a farm road, the centre line of the road is suitably altered and marked on the plant to effect economy.

With the aid of the detailed plan, the alignment on ground is peg marked at regular intervals for taking again the longitudinal and cross section levels to design and prepare the estimate.  The formation level of road must be at least 30 cm (1 ft.) above the level of land, so that the road may be fit for traffic even during rains and the road surface may not be eroded.

WIDTH OF ROADS:

If passing places or berms known as shoulders are provided 2.4 m to 3.0 m (8 ft. to 10 ft.) wide roads are sufficient for running through fields.  However, ner the farmstead still wider roads are necessary, since vehicles are housed here and the traffic will be denser.  For long vehicles and trailers, the width should be increased on curves to negotiate.  The increase in width depends on the radius of curves; 1 m (3.5 ft.) for 30 m radius and 2.5 m (8.5 ft.) for 18 m. radius.

Fig. 5.1. Cross section of a farm road.

The berm is the space available from the road edge to the edge of the drain and it should be a minimum of 1.2m (4 ft.).  Hence the width of roads including the drains wil come to a minimum of 7.2 m to 9.0 m (24 ft. to 30 ft.) for ordinary roads.

1.4 DESIGN OF ROAD:

The expected strength of the road is determined primarily by the nature or frequency of traffic expected, the stability of the sub soil and the designed life time of the road.  The design is also influenced by the available facilities for maintenance.  For low maintenance the strength should be more an vice versa.

Table 5.1. Dimensions of various structures of a farm road for different traffic in different soils

Type of soil	Heavy (traffic including a high proportion of heavy lorries pay load exceeding 3 tons or traffic the maximum load of which is 4 tons)
	Sub base	Base	Surfacing
Soft soil (loam, soft clay; new unconsolidated embankments	15 cm (6” gravel or kunker	22.5 cm (9”) hardoore or stone pitching or 15-20 cm (6 to 8 in 1:2:4 cement concrete	Bituminous surfacing
Medium soil (sand, stiff clay, lime stone, gravel or rock embankments or old consolidated embankments)	Hard 7.5 cm (3”) gravel or kunker	15 cm (6”) hard core or stone pitching or 12.5 cm (5”) 1:2:4 cement concrete	7.5 cm (3”) gravel or Bituminous surfacing
Hard soil, gravel, kunker rock.	7.5 cm (3”) gravel or kunker	15 cm (6”) hard core or stone pitching or 12.5 cm (5”) 1:2:4 cement concrete	7.5 cm (3”) gravel or Bituminous surfacing

 

Table 5.2     Dimensions of various structures of a farm road for different traffic in different soils.

Type of soil	Medium (traffic consisting of mainly of medium weight lorries pay load not exceeding 3 tons or traffic the maximum wheel load of which is 2 tons)
	Sub base	Base	Surfacing
Soft soil (loam, soft clay; new unconsolidated embankments	15 cm (6” gravel or kunker	15 to 2.5 cm (6 to 9”) hard core or stone pitching or 12.5 to 15 cm (5 to 6 in 1:2:4 cement concrete	Bituminous surfacing
Medium soil (sand, stiff clay, lime stone, gravel or rock embankments or old consolidated embankments)	7.5 to 15 cm (3 to 6”) gravel or kunker	15 cm (6”) hard core or stone pitching  or 12.5 cm (5 in  1:2:4 cement concrete	7.5 cm (3”) gravel or Bituminous surfacing
Hard soil, gravel, kunker rock	2.5 cm (1”) sand to 7.5 cm (3”) gravel or kunker	7.5 cm (3”) gravel or kunker to 15 cm (6”) hard core or stone pitching or 12.5 cm (5”) 1:2:4 cement concrete	7.5 cm (3”) gravel or bituminous surfacing

 

Table 5.3. Dimensions of various structures of a farm road for different traffic in different soils

Type of soil	Light (traffic consisting of mainly of cars and light vans or traffic the maximum wheel load of which is ½ ton)
	Sub base	Base	Surfacing
Soft soil (loam, soft clay; new unconsolidated embankments	15 cm (in) gravel or kunker	15 cm (6”) hard core or stone pitching or 12.5  cm (5”) 1:2:4 cement concrete	7.5 cm (3”) gravel or Bituminous suffering
Medium soil (sand, stiff clay, lime stone, gravel or rock embankments or old consolidated embankments)	7.5 cm (3”) gravel or kunker	15 cm (6”) hard core or stone pitching  or 12.5 cm (5”) 1:2:4 cement concrete	7.5 cm (3”) gravel or Bituminous surfacing
Hard soil, gravel, kunker rock	0 to 7.5 cm (0 to 3”) sand.	0 to 7.5 cm (0 to 3”) gravel or kunker	7.5 cm (3”) gravel or bituminous surfacing.

 

The table 5.1, 5.2, 5.3 show the approximate sizes of the structure of a road worked out based on the subsoil and type of traffic.

1.5 PARTS OF ROAD STRUCTURES:

The road is a structure composed of the following layers :

1. Sub-soil or road bed

2. Sub grade

3. Foundation course

4. Base course and

5. Wearing course or surface

Where natural conditions are favourable the sub soil and sub grade may be combined into one so also the base and foundation courses.

Fig. 5.2. Cross section of a road structure.

SUB SOIL:

It is the ground underlying the road after the removal of top soil.  This may be natural or prepared bed on which to lay the road.  This should be sufficiently strong so as to offer resistance to the movement and distortion caused by the vehicles.

SUB GRADE :

Sub grade is the formation which supports the foundation and helps to keep the foundation at the designed level.  This is a compacted layer of granular permeable material covering the road bed so as to prevent the entry of water into the road foundation.

FOUNDATION COURSE:

This is a layer laid over the sub grade to distribute the load coming over the road uniformly over a sufficient area of sub grade.  This is also known as ‘soling’ or ‘bottoming’.  This should be rigid and should offer resistance to the movement or distortion due to moving loads to a maximum extent.

BASE COURSE:

This is the layer providing the main supporting strength of the road structure.  This consists of a minimum depth of consolidated material which resist the distortion and helps to distribute the load to the foundation.  This should afford good bond to the wearing course.

WEARING COURSE:

The main functions of the wearing course are to provide a smooth running surface and to protect the base from abrasion and from the effects of rain.  Hard base cannot be rolled to true surface and this has not resistance to wear and tear.  Therefore, they require some surfacing.  The flexible roads require simplest form of surfacing whereas rigid roads require water proof surfacing either by i. water binding or ii. Applying tar or bitumen.

DRAINAGE:

Water stagnation on the surface of earthern roads often the road, leads to formation of ruts and pot holes and finally to erosion of the surface.  The stagnation on paved road is also not desirable, since it hastens the erosion by traffic and makes it unpleasant.  In the case of roads having the water table very near to the formation level, the increase in water table results in wetting the soil and thereby reducing the stability of the road surface.  Hence for the removal of both surface and subsurface water and to intercept water from the adjoining lands, side ditches are essential on either side of the road or on one side as the topography permits. The drains should be located at a suitable distance not less than 1.2 m (4 ft.) from the edge of the road and to a depth of about 0.3m (1 ft.) below the formation.  The verge should fall from the road to the ditch.  The ditches should have sufficient cross sectional area to discharge the entire water collected in them.

CULVERTS:

At such places as field gateways and road junctions, culverts are to be provided for discharging the drain water through pipes.  The length of pipes depends on the length of road and atmount of water to be discharged.  The minimum diameter of the pipe should be 22.5 cm (9 inches).  Culverts are also provided to enable passage over roads running across narrow streams, drains or small waterways.

1.6 CLASSIFICATION OF ROADS:

According to the materials used for construction, roads may be classified as

1.Earthen roads

2. Gravel roads

3. Water bond macadam roads

4. Tar or bitumin roads and

5. Cement concrete roads

Earthen roads are temporary roads up of earth available in the locality.  They are suitable for low speed vehicles such as bullock drawn carts.  Gravel roads are better than earthen roads and are stable for light traffic. Water bound macadam roads are still better, and suited for heavy traffic of low speed vehicles.  Bitumen or tar roads are roads that are coated with bitumen or tar and they are suitable for rubber tyred vehicles of both slow and high speed.  Cement concrete roads are stable and suited for all kinds of traffic.  Each of the roads has its own qualities which influence the choice and design.  The choice of road depends mainly on the traffic, finance and local conditions.

EARTH WORK FOR ROADS:

For all type of roads, earth work is a preliminary work to be carried out.  The earth work may be cutting or forming embankment.  Further, even where the ground level itself becomes the formation level, the top surface should be cleared of the bushes, vegetation, prickly pear etc.  Hence the earth work is essential for the entire length of the road.  The operation involved in earthwork are:

1. Clearing the road way

2. Excavation and

3. Embankment

1. Clearing the road way

The roadway is cleared of the vegetations, shrubs and interrupting trees etc., so that the natural, compact surface is obtained to act as a key to the materials laid over it.  In removing the shrubs and trees care must be taken to seed that they are routed out completely without leaving any trace of the roots under ground.

2. Excavation

When the ground level is higher than the formation level extra earth is to be removed and transported to the low lying areas to forman embankments.  This can be done by employing manual labour or using bullocks drawn scrappers or farm machineries such as bulldozers.

The cross section of the road as designed is peg marked on the ground.  The centre line of the road, edges of the formations or cuttings are marked on ground with pegs.  The slopes to be adopted will depend on the angle of repose of the soil.  For shallow depths a vertical cut is made and then the sides are cut to the required slope.   But in deep cuttings, the excavation is done step by step, the top vertical cut being wider and the width decreases towards the bottom.  The sides are then trimmed by dressing the steps.  The earth so excavated is transported to the places of embankments by carts, lorries or trailers.  If it is not economical or the soil is unsuitable for forming embankments, they are used for forming spoil banks leaving a marging from the edge of cutting known as berm.

4. Embankment

Embankment are formed when the ground level is lower than the formation level.  The soil or earth used for forming embankment should be clay, sandy loam or gravel.  The top surface of the roady way si clared of vegetations etc., then the surface is scraped and the soil laid and spread evenly to the full width in layers of 15 cm (6 inches) thick.  Each layer is rolled and consolidated if necessary by adding water when the soil is not wet.  The embankment is brought up to the formation level giving due allowance for settlement.  This sides are then neatly trimmed to the required slops.

When excavation of the roadway at places of cutting does not provide adequate material for embankments, the exra eath to make up the deficiency is obtained by excavating pits brown as ‘borrow pits’.  The borrow pits are selected in the vicinity of embankment to reduce the cost of transport.  The borrow pits are of rectangular shaped.  In between borrow pits a space of 0.9 m to 1.2 m  (3 ft. to 4 ft.) is left to enable the workman to go over it.