Highways are critical to a country’s growth and development. Roads connect more people and places, increasing land value, playing a key role in defence activities and contributing to the overall development of a country. Hence it is also important that sufficient measures are adopted during the construction of these highways to ensure consistent quality and performance over the years. In this article, we will go over the different evaluations that are carried out on highway materials.
Subgrade Soil
The undisturbed soil under the pavement is called the natural subsoil, and the supporting soil under the pavement with special layers is called the subsoil. Soil compacted by the controlled movement of heavy compactors is called compacted subsoil. The subsoil directly receives the traffic load from the road layers, thus becoming an integral part of the road structure.
Desirable properties of soil as a road material are:
- Stability
- Incompressibility
- Permanency of strength
- Minimum changes in volume and stability under adverse conditions of weather and groundwater
- Good drainage
- Ease of compaction
Soil Classification
Grain Size Analysis:
The purpose of this test is to determine the particle size distribution of soils. The analysis can be carried out with the following methods:
- Sieve grain size analysis: for determining particle size between 0.075mm to 100mm
- Hydrometer method: for particle sizes smaller than 0.075mm
The limits of grain size according to the Indian Standard classification is as follows:
- Larger than 2.00mm size – Gravel
- Between 2.00mm – 0.06 mm size Sand
- Between 0.06mm – 0.002 mm size Silt
- Smaller than 0.002 size Clay
Soil Tests
The subgrade should possess sufficient stability under adverse climatic and loading conditions for it to be able to provide support to the pavement. Hence, it is very important to evaluate the subgrade by conducting tests. They are classified into 3 groups:
- Shear tests
- Bearing tests
- Penetration tests
Shear Test
A good understanding of the shear strength of soil helps to calculate the ground-bearing capacity while designing foundations or pavements.
The 3 commonly used shear tests are:
- Direct shear test
- Triaxial compression test
- Unconfined compression test
California Bearing Ratio Test (CBR)
The CBR test works by testing the pressure required to penetrate a soil sample with a standard area piston and measuring it. This measurement is divided by the pressure required to achieve the same penetration on standard, high-grade crushed rock material. The harder the surface, the higher the CBR value.
Plate Bearing Test
The objective of the plate bearing test is to calculate the bearing capacity of the underlying subgrades and in some cases the complete roadway. It determines soil characteristics such as ultimate bearing capacity, soil stiffness, and soil settlement tendency.
Road Aggregates
Stone aggregates form the main part of road construction and form the basic materials used in the construction of road layers. Stone aggregates must withstand different magnitudes of stress due to wheel loads. The aggregate of the road surface must withstand wear due to the abrasive action of traffic, and deterioration due to wear and wheel loads.
Desirable properties of the aggregate are:
- Resistance to impact or toughness.
- Resistance to crushing.
- Resistance to weathering or durability.
- Resistance to abrasion or hardness.
- Resistance from getting polished
- Good adhesion or affinity with bituminous materials in the presence of water.
Tests On Road Aggregate
- Aggregate Impact Test
The aggregate impact test is carried out to evaluate the resistance to the impact of aggregates to fracture under repeated impacts; the test has been standardised by the Bureau of Indian Standards (BIS). The aggregate impact testing machine is used to carry out this test. The aggregate impact value should not normally exceed 30% for the aggregate to be used in the wearing course of pavement.
The toughness property of the aggregate is reported as below:
| Aggregate impact value % | Toughness property |
| Less than 10 | Exceptionally tough |
| 10 to 20 | Very tough/strong |
| 20 to 30 | Good for pavement surface course |
| Above 35 | Weak for pavement surface course |
- Aggregate Crushing Value Test
Aggregate strength is assessed by an aggregate crushing test. This value provides a relative measure of resistance to crushing under an applied compressive load. Aggregates that have a high crush resistance or low aggregate crush value are preferred for use in high grade pavements. The aggregate crushing value for good quality aggregates to be used for the base course must not exceed 45% and the value for the surface course must be less than 30%.
- Los Angeles Abrasion Test
The principle of this test is to determine the percentage of wear due to the relative friction between the aggregate and the steel balls used as an abrasive charge. This test is more reliable for evaluating the suitability of coarse aggregates for pavements because both abrasion and impact occur during the test similar to field conditions.
- Shape Tests
- Flakiness Index: The flakiness index of aggregate is the percentage by weight of aggregate particles the least dimension of which is less than three-fifth or 0.6 of their mean dimension.
- Elongation Index: The elongation index of an aggregate is the percentage by weight of particles, the greatest dimension of which or its length is greater than one and four fifth or 1.8 times their mean dimension. The elongation index is not applicable for sizes smaller than 6.3mm. A standard length gauge is used to gauge the greatest dimension or length of the aggregates.
Highway Drainage
Highway drainage is the process by which we remove and control excess surface and groundwater, which involves capturing and draining water from the road surface and subgrade. The installation of surface and subsurface drainage is the most important part of highway design and construction. It was also seen that the design of highway drainage systems is relatively easy and simple and the drainage works are relatively cheap.
Significance Of Drainage:
- The excess moisture that is present in the soil subgrade tends to lower its stability.
- The same increase in moisture content can also cause the reduction in strength of many other pavement materials like stabilised soil and water bound macadam.
- Considerable damage can be caused due to excess water on shoulders and pavement areas.
- Increase in weight and in stress due to high moisture content and also simultaneous reduction in the soil mass strength.
- In colder areas, the presence of water in subgrades and continuous groundwater supply can cause damage to the pavement because of frost action.
Requirements Of Highway Drainage Systems:
- The carriageway and shoulder should be drained off effectively from the surface water so that it cannot percolate into the subgrade.
- Surface water from the adjacent areas or lands shall also be prevented from entering the roadways.
- The capacity of the side drain should be good enough and with a longitudinal slope to carry away all the surface water that has been collected.
- In waterlogged areas, special care has to be taken due to presence of some detrimental salts or risk of flooring that is likely to occur.
- Formation of cross ruts and erosion should not be caused due to the flow of surface water across shoulders, roads and along slopes.
Surface Drainage:
- When it rains, a part of that rain water flows on the surface and the other part will penetrate through the soil as gravitational water until it reaches the ground water below the water table.
- Now this diversion and removal of excess soil water from the subgrade are known as Surface Drainage.
Methods Of Surface Drainage
- By longitudinal side drains.
- By providing a damp proof course.
- Giving sufficient slopes to the sides.
- Catch basins and inlets in urban areas.
- Providing a proper chamber.
- By keeping the level of carriage way at least 60 cm above HFL.
Side drains for a road in embankment:
- It is of utmost importance to provide a side drain on one or both sides when the road is built in an embankment.
- They should be at least 2 m away from the bottom edge of the embankment and the depth is to be kept 1 to 1.5 m to block the entry of drain water into the embankment.
Conclusion
For a highway to serve its purpose efficiently, it requires good quality materials and proper construction techniques for it to have consistent quality. Moreover, for it to be functional throughout its design life period, proper yearly maintenance needs to be conducted.
For example, proper drainage systems on highways can reduce yearly maintenance costs and increase the life of the pavement. Maintenance of the roads from time to time prevents the accidents caused due to faulty and damaged roads thus ensuring safe and comfortable movement of passengers from place to place and goods at all times.