Road Construction and Geometric Design Part 1

Road Construction and Geometric Design of roads (part 01)

What’s is Civil Engineering Construction?

Any Construction work other than buildings is known as Civil Engineering Construction work.
Example: Roads, Bridges, Culverts etc.

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Road Construction and Geometric Design of roads (part 02)

Road Construction

The highway system is generally classified into various groups. The classification may very from country to country. General classification is given below;

Access roads

Access roads are the lowest level in the network hierarchy. Vehicular flows will be very light, need only be sufficient to provide appropriate access to the rural agricultural, commercial and population centers served.

Collector roads

Collector roads have the function of linking traffic to and from rural areas, either direct to adjacent urban centers, or to the arterial road network.

Arterial roads

Arterial roads are the main routes connecting national and international centers. Traffic on them is derived from that generated at the urban centers & from the inter urban areas through the collector and access road system. Levels of traffic flow & speeds relatively high. Geometric standards need to enable efficient traffic operation.

Expressways

These are defined as arterial highways with full or partial control of access. Their main function is to provide for movement of heavy traffic at high speeds. Parking, loading and unloading of goods and pedestrian traffic are not permitted on these facilities.

In Sri Lanka, the roads are divided in to types of A, B, C, D & E. A & B are the main roads and come under the purview of Road Development Authority. C & D come under the purview of Provincial Councils. Other categories of roads belong to the Local Government Authorities.

Geometric Design of roads

The safe, efficient & economic operation of a highway is governed to a large extent by the care with which the geometric design has been worked out. It includes the design elements of horizontal and vertical alignment, sight distance, cross section, intersection treatment etc.

Design Controls

The basic design controls of geometry include topography, traffic, speed, capacity, design vehicle etc.

Topography – The design elements such as alignment of the road should be related to the topographical features for economic and safe design.

Traffic – Information on traffic volumes, traffic composition and traffic loading is important in the determination of the appropriate standard of a road. The traffic has a major impact on the selection of road class. Traffic directly affects the geometric features such as width, no. of lanes etc.

Design Vehicle Dimensions – A ‘‘Design Vehicle’’ is a selected motor vehicle, weight, dimensions and operating characteristics of which are used to establish highway design controls such as width of pavements, parking geometrics etc.

Speed – The value of a highway is largely indicated by the speed, safety and convenience afforded by the facility for travel. Speed is important in economic operation and plays a vital role in determining the geometric design. The design speed has to be correlated with the terrain conditions and classification of the highway.

Capacity – Capacity is the maximum flow that can be accommodated in a highway facility.

Geometric Design elements

Horizontal alignment

In the design of horizontal alignment, following should be considered for economic & safe design;

⇘ Consistent with the topography

⇘ Providing good drainage

⇘ Minimizing earthworks

⇘ Achieving a uniform operating speed.
Therefore, the standard of alignment selected for a particular section of road should extend throughout the section with no sudden changes from easy to sharp curvature. Where sharper curvature is unavoidable, a sequence of curves of decreasing radius is recommended. In straight sections, camber shape will be maintained in order to provide proper drainage over the road surface.

When a vehicle is moving on a curved path, it is subjected to an outward force, commonly known as the centrifugal force. In order to resist this force, it is the usual practice to super elevate the roadway cross section at bends.

Vertical alignment

One of the important considerations in designing vertical alignment is the gradient. The cost of operation of vehicles, the speed of vehicles and the capacity of a highway are profoundly affected by the grades provided. Minimum grades are primarily related to the need for adequate drainage. For uncurbed pavements that are adequately drain laterally, relatively flat or even level profile grades may be used. With curbed pavement, the minimum longitudinal grade in usual cases should be 0.5 percent. Vertical Curves should be provided at points of change of grade. They serve as a gradual transition from one gradient to another without discomfort to riders. They provide adequate visibility for stopping and overtaking. The minimum lengths of crest vertical curves are determined mainly by sight distance requirements.

Cross-section,

Right of Way – The width of land secure & preserved in public interest for road development purpose.

Roadway width – The total width of the road including shoulders.

Carriageway width – The portion of the roadway for the movement of vehicles.

Median – The portion of a divided highway separating the two travelled ways for traffic in opposite directions. The median width includes both inside shoulders.

Shoulder – The portion of the roadway contiguous to the travelled way for the accommodation of stopped vehicles, for emergency use, and for lateral support of base and surface courses.

Curbs – A curb is a vertical or sloping member along the edge of pavement or shoulder, strengthening or protecting edge, & clearly defining the edge.

Road construction materials.

Both Portland cement concrete & asphalt concrete can be used to construct the road
Portland cement concrete – Binder cement (aggregate + cement)
Asphalt concrete – Binder bitumen (aggregate + bitumen)

Soil

Soil is the cheapest and the most widely used material in highway system. All road pavement structures eventually rest on soil foundation. The range of particle sizes encountered in soil is very large: from boulders to clay.

Aggregates

Aggregates is the major component of materials used in road construction. Natural aggregates for road making are obtained from rock. The aggregate should consist of clean, hard, durable particles of angular shape and rough surface. They should be free from soft or elongated particles, deleterious matter and free from clay and excess dust. Mostly used aggregates are quarry dust, rubble (150 X 225 & 100 X 150 mm) & metal (12.5mm, 19mm, 37.5mm, 50mm)

Asphalt Binders

Asphalt is one of the two principal constituents of Hot Mixed Asphalt (HMA). Asphalt functions as an inexpensive, waterproof adhesive. In other words, it acts as the glue that holds the road together. Asphalt binder is simply the residue left over from petroleum refining. There are 3 types of asphalt binders.

i. Penetration Grading (Hot Bitumen)
Asphalt Binders with high penetration are used for cold climates while asphalt binders with low penetration are used for warm climates. When Penetration Grade comes in barrels, it is in semi-solid state. Before using, it has to be heated approximately to 170`C.

ii. Asphalt Cutbacks
Asphalt Cutback which is in liquid form can be made by adding solvents (cutter stock (Kerosene) – volatile petroleum distillate products) to the asphalt cement. Asphalt cutbacks are used in various climate conditions & for prime/tack coats & for bituminous surface treatments. Not environmental friendly.

iii. Asphalt Emulsions
This is a non- flammable liquid substance. Emulsions are environmentally friendly, have the same basic uses as cutbacks & are commonly used in the field.

Asphalt emulsions are two type: that are negatively (anionic) charged & positively (cationic) charged. The setting rates are slow, medium and rapid. In Sri Lanka, the types that we mostly used are CRS I (60% bitumen), CRS II (65% bitumen) & CSS I. Emulsions are used for tack coat, prime coat, surface treatments and cold mixes.

Road Pavements

The aims in designing a pavement are to protect the natural ground (i.e. the sub grade) from the high and concentrated load stresses that, would be applied directly to the sub grade by the wheels of vehicles. At the same time it is also necessary to ensure that the layers of the pavement itself are strong enough to support the traffic loads.
The structural design of road pavements depends primarily on the following factors,

> Strength of the sub grade
> Traffic loading
> Properties of the materials
> Variability and uncertainty in the above three items and in the quality control of the construction process.

Types of pavements can be broadly categorized in to two types;

↝ Flexible pavement
↝ Rigid pavement

Flexible pavement

Here the total pavement structure deflects under loading.
A typical flexible pavement structure consists of;

Surfacing/Wearing course – The wearing course is the uppermost layer of the pavement. It provides the riding surface for the road users. The wearing course should be smooth and dust-free and have adequate skid [ලිස්සා යාම] resistance.


Base course – This is the layer immediately below the wearing course and main load bearing layer.

Sub base course – The sub-base is a secondary load-spreading layer and lies below the base course. It acts as a separation between the sub grade and the base, & provides a working platform during the construction of the upper layers in the pavement.

Rigid pavement

Rigid pavement structure deflects [ඇද වෙනවා] very little under loading. A Rigid pavement structure is typically composed of a Concrete surface course built on top of either;

                  Sub grade or

                  An underlying base course.

Because of its relative rigidity, the pavement structure distributes loads over a wide area with only one, or at most two, structural layers.



Almost all rigid pavements are made with Portland Cement Concrete.

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Road Construction and Geometric Design of roads (part 02)

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