Subgrade:

• The top soil, also known as the subgrade, is a natural soil layer that has been prepared to withstand loads from the layers above.
• It’s crucial that soil subgrade is never overstressed. The ideal moisture content should be present, and it should be compacted to the desired density.
• Red gravel soil or moorum makes up its composition.

A good subgrade should have these qualities.

• Sufficient compressive strength to bear the imposed wheel load.
• Effective pore space is limited, which limits the amount of water that can enter.
• having the ability to endure abrasive and impact stresses from traffic and weathering

Sub base course:

• The sub base course is the layer of material below the base course, and its main purposes are to maintain the pavement structure structurally, enhance drainage, and lessen the incursion of fines from the subgrade.
• The sub base course with additional fines can act as a filler between the sub grade and the base course if the base course is open graded.
• The usage of a sub base course is not always necessary. For instance, the extra qualities provided by a sub base course may not be necessary for a pavement built over a high-quality, firm subgrade. In certain circumstances, sub base course might not be offered.
• Broken stones and bound or unbound aggregates make up this layer.

Base course:

• The layer of material immediately above the sub base is known as the base course, and it helps to subsurface drainage and additional load distribution.
• It serves as a medium for dispersing stresses to distribute the surface wheel loads.
• Crushed stone, crushed slag and other untreated or stabilised materials could be included.

Binder course:

The binder course typically consists of aggregates having less asphalt and doesn’t require quality as high as the surface course, so replacing a part of the surface course by the binder course results in a more economical design. This layer provides the bulk of the asphalt concrete structure. Its main function is to distribute load to the base course.

Surface course:

• It is the layer directly in contact with the traffic loads and is usually constructed with superior quality materials. They are usually constructed with dense graded bituminous concrete.
• The functions and requirements of this layer are,
  • It provides characteristics such as friction, smoothness, drainage, etc., and it will also prevent the entrance of excessive quantities of surface water into the underlying pavement layers.
  • It must be impervious, firm, strong and skid proof to provide resistance to abrasion, compression, tension, repetitive action of wheel loads and weathering action.
  • It must provide smooth riding surface.
  • It should offer water tight layer against surface infiltration.

Seal coat:

It is a thin surface treatment used to water proof the surface and to provide skid resistance.

Tack coat:

A very thin layer of asphalt emulsion diluted with water is used as a “tack coat” to promote bonding between two layers of binder course. It must be extremely thin, evenly cover the entire area, and dry quickly.

Prime coat:

In order to bond two layers together, a binder layer is placed on an absorbent surface, such as granular bases, and low viscosity cutback bitumen is applied to it. Prime coat, in contrast to tack coat, penetrates into the layer beneath, fills in the gaps, and creates a watertight surface.

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Highway components and materials used in highway construction. Highway constructions are typically built above ground level and include the following elements.

1. Fill or embankment
2. Subgrade
3. Flexible and rigid layers of pavement construction

If the highway’s vertical alignment necessitates lowering the formation level below the general. The highway could be built-in cutting at some points, and the highway structure would be different including the following elements.

1. Preparation of cutting
2. Subgrade
3. Flexible and rigid layers of pavement construction

Highway Embankment and Cutting Materials

Organic waste materials such as garbage and other biodegradable materials should not be used in embankments because of undesirable characteristics such as high compressibility, settlement, and high variations instability and volume with water content. Local materials make up the road stretches in cutting; these are often diverse types of soils present along the alignment; nevertheless, rocky and other forms of strata are occasionally encountered along the cut stretches.

Layers of Flexible Pavement Materials

1. In the drainage layer, select granular soils or crushed aggregates with suitable permeability.
2. Granular base course with stone aggregates and fine aggregates
3. Bitumen binder, coarse aggregates, and fine aggregates in the binder and surface course

Rigid Pavement Layer Materials

1. In the drainage layer, select granular soils or crushed aggregates with suitable permeability.
2. For dry lean concrete in the sub base course, coarse aggregates, fine aggregates, and Portland cement are used.
3. Coarse and fine aggregates, as well as Portland cement, are used to make a high-quality concrete pavement slab that acts as both a foundation and a surface course.

Subgrade Soil

The subgrade soil is an essential component of the road pavement structure, receiving traffic loads directly from the pavement layers. In the construction of a pavement structure, the subgrade soil and its qualities are critical. The major purpose of the subgrade is to provide appropriate support for the pavement, and it must be stable under poor weather and loading conditions to do so.

Poor subgrade conditions are sometimes blamed for the production of waves, corrugations, rutting, and pushing in black top pavements, as well as the phenomena of pumping, blowing, and subsequent cracking in cement concrete pavements.

Desirable Features

• Stability
• Incompressibility
• Permanency of strength
• Minimum changes in volume and stability under severe weather and ground water conditions
• Good drainage
• Ease of compaction are all desired features of soil as a highway material.

The soil should be stable or resistant to permanent deformation under load, as well as weathering resistance, in order to maintain the desired subgrade support. A minimum variance in volume will ensure a minimum variation in the subgrade’s differential strength values. To avoid excessive moisture retention and subsequent frost action, good drainage is important. Under certain types and amounts of compaction, ease of compaction provides increased dry density and strength.

Index Properties of Soil

The soil properties on which their identification and classification are based are known as index properties.

• Grain size distribution: It is found by mechanical analysis. That is Coarse grained soil by sieve analysis and fine soil by sedimentation analysis or Hydrometer method. The grain size analysis is carried out to determine the percentage of individual grain size present in a soil sample.
• Liquid limit: It is defined as the minimum water content at which the soil will start flowing under the application of very small shearing force. It can be determined in the laboratory using a mechanical device like Casagrande apparatus or cone penetration method.
• Plastic limit: It is the minimum moisture content at which the soil remains in a plastic state or undergoes plastic or non recoverable deformation under applied load.
• Plasticity index: It is the numerical difference between the liquid limit and plastic limit. It indicates the range of moisture content over which the soil is in plastic condition.
• Shrinkage limit: It is the maximum moisture content at which the further reduction in water content does not cause reduction in volume. It is the lowest water content that can occur in clayey soil sample which is completely saturated.

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