Low Cost Roads

“Roads Constructed at low cost and capable of being maintained at low cost are termed as low cost roads”

The low cost roads are meant for low intensity of traffic. This road are provided in localities where traffic volume does not

exceed 500 tonnes /day with a maximum of one third bullock carts or other slow moving vehicles and a truck load of 3 tonnes with a maximum of 5 tonnes depending upon local conditions.

The low cost roads are constructed with the maximum use of locally available road materials.

• The standards for low cost roads:

Route Right of way Formation level Geometrics, i.e. grades, curves, visibility etc.

• The standards for carriage way width , formation width and nature of crust for low cost roads should be such that

No alteration may be required when these roads are upgraded in future.

With the increase in traffic, it should be possible to strengthen the pavement in stages with the maximum utilization of the existing pavement

ECAFE(Economic commission for Asia and Far East) suggested the Desirable and minimum width for Right of way are 30m and 18m respectively for low cost roads.

• Classification of low cost roads:

The low cost roads may be broadly divided in the following five categories

1. Earth roads

2. Kankar roads

3. Gravel Roads

4. Water bound macadam roads (WBM)

5. Wet mix macadam roads (WMM)

• Earth Roads

General : Earth road is a type of road whose whole pavement section is

constructed with the locally available earth material preferably. Borrow pits are located at the nearby sites preferably outside the land width  where, the required earth is available.

Sub-grade and the surface of the earth roads are given larger camber of 1in 33 to 1 in 25 because they need faster drainage to be safe from the moisture. A maximum value of camber of 1 in 25 is the limit because higher camber will result in the formation of cross ruts and corrosion of pavement soils

• Specifications of Materials: The soil survey is carried out and suitable borrow pits are located within

nearer distances. The trees ,grass roots and other organic matter are removed before

excavating earth for construction The earth material used for the construction of earth roads are termed as

satisfactory if they possess the following properties:                                    Base Course                         Wearing Course

Clay content                       <5%                                      10 to 18% Silt content                       9 to 32%                                  5 to 15% Sand content                     60 to 80%                               65 to 80% Liquid limit                          <35%                                      <35% Plasticity Index                     <6%                                      4 to 10%

• Location of the centre line: 

The centre line and the road boundaries are marked on the ground by driving the wooden pegs. To follow the desired vertical profile of the road, reference pegs are also driven at a certain spacing which depends upon the estimated length of the road construction per day.

Centre line pegs are fixed at 500 m interval.

construction procedure of earth road 1. Preparing the subgrade :- The subgrade is prepared and the surface is

brought to the required camber and gradient.

2. Rolling an watering :- The surface is rolle properly and well wetted with water.

3. Spreading the soil :- After watering, a layer of graded soil, about 10 cm thick is evenly spread.

4. Rolling :- The layer of soil is rolled at optimum moisture content (OMC ) with a suitable roller and finally finished with a light roller.

The type of roller for compaction is decided based on soil type, desired amount of compaction and availability of equipment. Atleast 95 per cent of dry density of its light compaction is considered desirable.

Cont…

5. Checking :-The camber of the finished surface is checked an corrected, if necessary.

6. Curing :- The rolled surface is watered for four or five days for curing. No traffic should be allowed to pass over the road surface during this period.

7. Opening to traffic :- After curing, the traffic is allowed to pass but the surface is sprinkled over with water for a period of 10 to 1 days

• Kankar Roads Kankar is an impure limestone and it is available as a natural

deposit. A good quality kankar will show a bluish surface on being fractured. Kankar is found as “Block Kankar” or as “Nodular Kankar”. In the

past block kankar has been used as building material while nodular kankar is even today used for VR and ODR as a subgrade material for WBM roads

Cont… Some times road is built in stages starting from earth formation

which is allowed to settle for two to three monsoons and subsequently when the surface is to improved, the kankar forms the subgrade material for such roads

For road work kankar is broken into sizes varying from 20 to 63 mm.

The largest size are used for bottom layer and the smallest size is used for top layer

Kankar is normally spread at the 15 mm layer

Cont… The size supplied varies from 12 mm to 80 mm as specified below

1. Passing 80 mm sieve 100%

2. Retrained of 63 mm sieve 20%

3. Passing 25 mm sieve…Not more than 10%

4. Passing 12 mm sieve NIL

• The following sequence is followed in the construction of the kankar roads

Spreading of kankar over the section Ramming over stretch of 3.66 mm road at a time with the help of at

least 16 workers Ramming to continue till the surface has been thoroughly

compacted Surface to be kept well watered during the process of ramming Road to be opened traffic such that the traffic is spread over the

whole road width

• Gravel Roads A gravel road consists of a carriageway made of a layer of

compacted gravel. The gravel used consists of irregular stones mixed with a varying

amount of sand, silt, and clay, which can act as a binder. Following consideration govern the construction of gravel roads

(A) The carriageway of the gravel road should be provided with a camber of 1 in 25 to 1in 30 to achieve good drainage of the road surface

(B) It should be composed of pebbles which are hard, tough and durable rock particles to resist abrasion

Cont…(C) For making a good road surface, the pebbles should be held together by some cementing material such as clay which is the most generally available binder.

Following procedure is adopted in the construction of a gravel road

1. Material Gravel to be used for the construction is staked along the sides of the proposed road

2. The pegs are driven to show the limits of excavation, The site is cleared and the subgrade is then prepared to receive the layer of gravel

Cont…3. The first layer of gravel is spread in the prepared trench. The

compacted depth of gravel road is generally 20 cm and it is obtained in two layers. The layers is rolled by using smooth wheeled light rollers starting from the edges and proceeding towards entre.

4. Some quantity of water is then sprayed and rolling is done again.

5. The final rolling is carried out when the moisture content is at its optimum so that completed surface which is formed is firm.

6. A few days after the final rolling and drying out, the road is opening the road traffic.

c

Water bound macadam roads:

The water bound macadam road construction technique was given by the John Macadam. This technique in present day is used as given below.

For WBM construction we use three materials:

1. Aggregates

2. Screeners

3. Binders.

• Construction of water bound macadam road involves the following 6 basic steps as given below

1. Preparation of sub-grade

2. Preparation of sub-base

3. Preparation of base

4. Preparation of wearing course

5. Preparation of shoulders

6. Opening to traffic

• CONSTRUCTION OF SUB-GRADE

Sub-grade act as a cushion for other layers i.e. In order to achieve durable road sub-grade should be strong.

The sub-grade is thoroughly compacted by rollers weighing 8 tonnes by sprinkling water one night before.

In rocky regions the sub-grades are not rolled whereas in region of clay soils, a layer to natural sand, moorum or gravel, is provided over sub-grade.

Sub grade preparation

• CONSTRUCTION OF SUB-BASE

On a well compacted sub-grade, aggregates having size of 90 mm to 40 mm are preferred for the sub-base material, or over burnt bricks in layers of 15 cm thickness

total width of the sub-base to be kept 60 cm wider than pavement width, projecting 30 cm on each side.

The sub-base should be compacted by a roller to provide an even surface.

• CONSTRUCTION OF BASE

The specified materials of the base course is spread and proper grade, thickness and cross sections maintained as per design shown on the supplied drawings.

Aggregates having size of 63 to 40 mm are preferred for the base material

For economic considerations IRC has suggested non plastic materials such as, crushed over burnt bricks, moorum, gravels, etc. provided the liquid limit of the material is less than 20%, plasticity index is less than 6.0% and the portion of fines passing 0.075 mm sieve is less than 10%.

This course may be laid in one or two layers according to the total designed thickness and the thickness of each layer should not exceed 10 cm

Provide either bricks on end edging or earthen kerbs strong enough to prevent the new road material from spreading outward and also to retain water used in consolidation of the wearing course

Spread the road metal evenly over the prepared base to the specified thickness and hand pack them so that the finished surface is brought to the required camber.

Cont…

Spread the coarse aggregate over the surface and roll it dry with a suitable roller till interlocking of the aggregate is achieved with sufficient void space. The rolling is started from the edges and gradually shifted towards the centre.

After dry rolling, spread the screening materials (stones up to 12 mm size) with uniform rate so that voids of coarse aggregates get filled properly.

After spreading the screening material, sprinkle sufficient quantity of water, sweep the surface and roll it with roller again

Preparation of wearing course

Now apply the binding material in two to three thick layers at a slow and uniform rate. Each layer of binding material is rolled after adding sufficient water. The slurry is swept in with brooms to fill the void properly. The moving wheel of the roller should be cleaned with water.

After proper compacting allow it to dry over night. Spread a layer of sand or earth, about 6 mm thick and roll the surface again after sprinkling water lightly.

The surface may be allowed for 7 to 10 days of curing.

Spreading binder material

• CONSTRUCTION OF SHOULDERS

While curing the pavement surface, prepare the shoulders by filling earth to the specified cross slope and compact them properly by rolling or by tamping.

Width and thickness of the shoulder should be as per specification.

• OPENING TO TRAFFIC

After properly drying, the road pavement may now be opened to traffic, ensuring that the traffic is distributed uniformly over the full width of the pavement.

Wet mix macadam roads: Laying and compacting clean, crushed, graded aggregate and

granular material, premixed with water, to a dense mass on prepared sub-grade or existing pavement

Thickness of single compacted Wet Mix Macadam layer shall not be less than 75 mm

Coarse aggregate shall be crushed stone If crushed gravel is used, not less than 90% by Wt. of gravel

pieces retained on 4.75 mm sieve shall have at least two fractured faces

If water absorption value of coarse aggregate is greater than 2%, the soundness test shall be carried out as per IS:2386(Part-5)

Physical Requirement of CoarseAggregate for WMM

1. Los Angeles Abrasion IS:2386(Part-4) 40% Value orAggregate Impact Value IS:2386(Part-4) or IS:5640 30%

2. Combined Flakiness and Elongation indices (Total) IS:2386(Part-1) 30%

Grading Requirement for WMM

IS Sieve Destination Percent by Wt. Passing IS Sieve Destination the IS Sieve

53.00 mm45.00 mm26.50 mm22.40 mm11.20 mm4.75 mm2.36 mm

600 micron75 micron

10095-100

-60-8040-6025-4015-308-220-8

Materials finer than 425 micron shall have Plasticity Index (PI) notexceeding 6.

Construction of wet mix macadam road involves the following basic steps as given below.

1. Weather

2. Preparation of Base

3. Provision of Lateral Confinement of Wet Mix

4. Spreading of Mix and Compaction

5. Setting and Drying

6. Opening to Traffic

• weather The work of laying of wet mix macadam is not done during rainy

season

• Preparation of Base The surface of the sub-grade/sub-base/base to receive the WMM course shall be prepared to the specified lines and cross-fall (camber) and made free of dust and other extraneous matter. Any ruts or softy yielding places shall be corrected and rolled until firm surface is obtained, if necessary by sprinkling water.

• Provision of Lateral Confinement of Wet Mix This shall be done by laying materials adjoining shoulders along with

that of wet mix layer. The sequence of operations shall be such that the construction of the shoulder is done in layers each matching the thickness of the adjoining pavement layer.

Spreading of Mix and compaction

• Preparation of mix WMM shall be prepared in an approved mixing plant of suitable capacity

having provision for controlled addition of water Optimum moisture for mixing shall be determined in accordance with IS:

2720 (Part-8) after replacing the aggregate fraction retained on 22.4 mm sieve with material of 4.75 mm to 22.4 mm size

• Spreading of mix Immediately after mixing, the aggregate shall spread uniformly and

evenly upon the prepared sub-grade in required quantities by paver or motor grader

• CompactionAfter the mix has been laid to the required thickness, grade andcross fall/camber the same shall be uniformly compacted to the fulldepth with suitable roller

• Setting and Drying After final compaction of wet-mix macadam course, the road shall be

allowed to dry for 24 hours

• Opening to Traffic Preferably no vehicular traffic of any kind should be allowed on the

finished wet-mix macadam surface till it has dried and the wearing course laid

• References S.G. RANGWALA “Highway engineering” by Charotar Publication House S.P. Chandola “A text book of Transportation engineering” by S.Chand

Publication L.R. Kadiyali “principles and practices of Highway Engineering “by Khanna

Publishers IRC-19-1977 , IRC-109-1997 Guidelines

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