Road Construction and Geometric Design Part 2

Road Construction and Geometric Design of Roads

Road construction and geometric design of roads is very important subject for the civil engineer, In this article we will discuss about road construction and steps involved in road construction.

Read more; Road Construction and Geometric Design of roads (part 01)

Flexible pavements construction

The main construction activities involved with flexible pavement are as follows;

↝ Site Clearing
↝ Sub grade preparation
↝ Embankment Construction
↝ Roadway Excavation
↝ Sub Base Construction
↝ Shoulder Construction

↝ Base Construction
↝ Surfacing
↝ Drainage Construction

Site Clearing

Site Clearing involves the clearing, grubbing, removing and disposing of trees, stumps, roots, debris, bushes and all the vegetation from the area of proposed right of way. Clearing refers to the removal of material above the existing ground surface. Grubbing is the removal of objects to a nominal depth below the surface. Also this work consists of removing top soil of the ground surface.

Excavator, Bulldozer & motor grader can be effectively used in this activity.

Sub grade preparation

The ‘‘sub grade’’ is the in situ material upon which the pavement structure is placed. Before starting the construction of sub base, the existing surface should be prepared to the required shape & levels & should be properly compacted. A good sub grade must be able to support loads transmitted from the pavement structure & not undergo the large volume changes due to moisture. Poor sub grade should be avoided if possible, but when it is necessary to build over weak soils there are several methods used to improved sub grade performance:

o Removal and replacement (over-excavation) – Poor sub grade soil removed & replaced with higher quality fill.

o Stabilization [ස්ථාවර කිරීම] with a cementitious or asphaltic binder – The addition of an appropriate binder (such as lime, Portland cement or emulsified asphalt) can increase sub grade stiffness [දැඩි බව] &/or reduce swelling [ඉදිමීම] tendencies.

o Additional base layers – Add additional base layers. These layers spread pavement loads over a larger sub grade area.

Frist two methods will be costly and could be applied for small sections. Commonly used method is the third method.

Embankment Construction

Whenever it is required to raise the grade line of a highway above the ground level an embankment is needed. When constructing embankment, following factors has to be considered;

⇘ Top width – Top width of the embankment is the roadway width.

⇘ Height – Decided on the basis of selected vertical profile.

⇘ Side Slopes – A flatter slope is to be preferred from the view point of erosion & safety. Slope to be provided will depend on the quality of the material used & normally 3:1 (3 horizontal to one vertical) is satisfactory
Turfing or rip-rap method can be used to protect the embankment.

Construction Procedure

o Compaction of original ground – The original ground should be levelled and compacted as required.

o Benching – When the embankment is constructed on hillside slopes, it is necessary to bench the surface of the hill slope with benches approximately having a height of 0.5m & width of 1.5 – 3.0m. After benching, the surface is well compacted & tested.

o Earth filling – The fill material will be placed in layers normally < 225mm loose thickness & will be compacted using rollers to the optimum moisture content. Soil for the next layer should be spread after testing and confirming the compaction.

Roadway Excavation

This work includes all excavation within the limit of the road way, required for the purpose of road way construction. After excavation, unsuitable material have to be transported and disposed and trimming and exposed faces of excavation have to be shaped.

Sub Base Construction

Sub base is the layer immediately below the base & it will be constructed above the sub grade. The material using for sub base should be a permeable material. It should drain water outside. The commonly using material for sub base is gravel. But crushed rock or stabilized sub grade material also could be used. Sub base material will be laid over the sub grade in layers not more than 225mm in loose thickness & will be compacted to achieve 100% maximum dry density at optimum moisture content. The compacted sub base will be finished to levels, grades and cross sections to the required levels.

Shoulder Construction

Shoulder is between the edge of the pavement and the edge of the embankment. Shoulders are provide lateral support to pavement layers, space for pedestrians, help drain out the surface water & provide clearance for road signs. The shoulder may be surfaced with gravel, Asphalt Concrete, Cement Concrete, or any other paving material depending on requirement.

Construction Procedure

The shoulder material will normally be spread in layers not exceeding 225mm & will be compacted to achieve 100% of maximum dry density. Where the shoulder is built up of more than one layer, the layers to follow will be placed only after the degree of compaction of the previous layer has been tested and found satisfactory. The shoulder will be finished to the required levels, lines and slopes.

Base Construction

The base is the main load-bearing and load-spreading layer of the pavement & normally consists of bitumen base, crushed aggregate or gravelly soil. There are several methods for Base construction. Some of the commonly used methods are;





System of all traditional methods are same. The difference is in single size aggregate base choker stone (smaller size aggregate) is used, but in macadam base quarry dust is used instead of choker stone. In dry bound method after compaction dust will move into the voids. For that vibrating roller is used. In water bound method steel wheel roller is used & spread water. In penetration macadam base method the difference is bitumen is applied before applying the quarry dust after that quarry dust is applied & compacted.

Single sized Aggregate Base

Construction Materials

~ Aggregate – nominal size 50mm or 37.5mm and 19mm or 12.5mm
~ Compacted thickness of the base – 55mm or 75mm, depending on the aggregate size

Construction Method

The selected aggregate will be spread evenly on the prepared surface. The aggregate will be compacted using a steel wheeled roller and rolling will be done from the edge towards center. The 19mm or 12.5mm choker stone will be spread evenly & rolled to ensure that the stones are well embedded in the surface of the base course.

While the rolling is being done, the surface shall be wetted and lightly brushed so as to ensure that the finer aggregate are worked in to the interstices. There will be application of two coats of hot bitumen over the single size aggregate base.

~ First coat at the rate of 2 Ltrs/Sqm
~ Second/Subsequent coat 1 Ltr/Sqm

Each coat will be blinded with sand. Application of the blinder can be done manually or using mechanical sprayer.

Dense Graded Aggregate Base

Construction Materials

Aggregate – Aggregate Base Course (ABC). ABC is a graded [] crushed rock with nominal maximum size of 50mm or 37.5mm. It is used for large road construction projects. The thickness of the ABC layer may vary from 100 – 225mm.

Construction Procedure

The aggregate will be spread over the prepared surface by using motor grader without segregation. The material should be kept wet during transporting & spreading in order to avoid segregation. The thickness should not be less than 75mm in any case. If layer thickness exceeding 150mm, it should be done in two or more layers. ABC will be compacted by using a roller to 100% of maximum dry density at its optimum moisture content.

In bitumen bound base method aggregate mixed with bitumen at once & spread.

Road surface/wearing course construction

The surfacing is the uppermost layer of the pavement and forms an interface with traffic and the environment. It may consist of some kind of non-structural, impermeable bituminous surface treatment or a structural layer of premixed bituminous material (asphalt concrete).
Road surfacing are applied to:

> To make the road surface waterproof.
> To control dust.
> To make a smooth and non-skid surface.
> To stop disintegration of the pavement.

After completion of base, depending on the surfacing method, intermediate courses will be applied. Basically there are two type of intermediate courses;

Prime Coat – Prime coat is applied over the prepared ABC surface. This will penetrate the surface, plug capillary voids & bond. Traffic will not be allowed on the primed surface for a period of minimum 3 hrs until the binder has penetrated & dried up. Prior to open for traffic, sand can be spread over the primed surface, as blotting material. Cut back bitumen MC 30 or bituminous emulsion CSSI can be used for prime coat.

Tack Coat – A tack coat is an application of asphalt to an existing paved surface to provide bond between the existing surface and the asphalt material to be placed on it. Tack coat must be thin & uniformly cover the entire surface to be treated. A thin tack coat does no harm to the pavement, and it will properly bond the course. Prior to application of the tack coat, the surface should be dried, thoroughly swept so that the surface would be free of dust. The application can be manually, using hand sprayer or bitumen distributor. Cut back bitumen MC 30 or emulsion CSS I Can be used.

Read more; Road Construction and Geometric Design of roads (part 01)

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