What is soil engineering?

Soil engineering is a part of geotechnical engineering that deals with the characteristics of soil. It also helps to understand the behavior of the soil in different conditions. The basic concepts of soil are studied to check the strength, compressibility, and porosity of the soil. This makes soil engineering an important topic for civil engineering.

The soil in highway design construction is the basic ingredient that forms intermediate support for the embankment. It is used extensively for highways so that it can support the road, and all the other load heavy transportation can get distributed over the pavement design. In the structure of the highway, the soil is used in the embankment and subgrade of the pavement.

Highway pavement

  • Pavements can be categorized into 3 types
  • Rigid pavement
  • Flexible pavement
  • Semi-rigid pavement

Components of a pavement system

  • Subgrade- It provides a foundation to the pavement with subgrade soil and comprises natural soil. The transportation load eventually passes through all the layers and is taken up by subgrade soil.
  • Subsurface course- This layer overlays between subgrade and base course in a highway. Subsurface comprises poor-quality granular material as slag.
  • Base course- This is the most important layer because it bears the impact of the traffic flow. It comprises good quality materials of boulders and gravel. The base course is situated between the subsurface course and the surface course. The top layer of the pavement design is overlaid on this layer
  • Surface course- It is the topmost layer of the pavement that comprises a bituminous or concrete slab. This layer should not have water retaining properties and should be weather resisting.
  • Binder- Asphalt is used as a binding material along with cement in road construction by a civil engineer because it comprises mixtures of aggregate as sand, gravel, slag, and asphalt concrete.
  • Cement – It is used in layering of the top seal. Cement is mixed with the aggregates and overlaid on the top of all the layers. In concrete slab road pavement, cement is used in large amounts with mixtures of gravel and sand to reduce the infiltration of the pavement system.
Hierarchy of Highway pavement

Classification of soil

Soils in geotechnical engineering are classified based on their profiles that are used in highway construction work. Common soil tests are conducted to understand the structural properties of the soil at the subgrade level.

Some common types of soil used in the pavement design work are:

Gravel: These are rock-like materials with particle sizes less than 2.36 mm. It contributes to the aggregate of the material and contains more granular particles.

Sands: In these, particle size ranges from 2 mm to 0.6 mm. Murom comes under this category. It contains finer particles than gravel.

Silts: In these, particle sizes are finer than sand, and ranges from 0.6 to 0.002 mm. They have little cohesion and infiltration properties.

Clay: These are finer than silty soil, less than 0.002 mm particle size. When clay is dry, it holds very high strength. It can be exemplified by black cotton soil, which is clayey soil with expansive properties.

Organic matter: These are partially decomposed plants or living matter. For instance, peat, organic clay, and so on.

Properties of soil

In geotechnical engineering, the soil suitable for highway pavement should have the following structural properties:

  • Type of soil: The size of the particles in a soil defines the strength of the soil. For instance, the granular size of the particle is always going to provide less strength to the pavement.
  • Compressibility: This property in soils enables it to compress, when the load is applied on it by road rollers and when the voids present in the soil decrease, which takes less area than the given area.
  • Permeability: This is a very important property of soil for pavement construction, as it allows water to percolate through it, and that eases the compaction in the soil. If the soil is not permeable, then it can saturate the pavement. For instance, the case of heavy clay. The soil in the subgrade should provide proper drainage to the pavement.
  • Stability: Soil should remain stable at all adverse conditions such as weathering, storms, heavy traffic loads. It should not be elastic. Soil stabilization is a process of transforming soil properties to increase its long-lasting strength for a structural design.

Soil tests for road construction

Soil tests in geotechnical engineering are done to analyze the quality of soil, its moisture content, compaction, nature of the soil, and strength of the subgrade for highway construction.

Some tests for soils are:

Moisture content test

In this test, bearing capacity and settlement of the soil are understood. It measures the amount of water present in soil with the percentage of dry mass.

The moisture content can be calculated by the following formula:

W = (W2 - W3)(W3 - W1)×100


W= Weight of the container

W2 = Weight of a container with wet sample

W3 = Weight of a container with dry sample

Particle size distribution test

This test is conducted in geotechnical engineering to check the grain size distribution in a soil sample. It checks the suitability of soil for road construction. Sieve analysis is done for fine soils and along with that, other hydrometer tests are done. Ingrain size distribution clay is the best component to use because it is classified by AASHTO or UCS classification system.

Procter test

This test is done to check the compaction of the soil. It is used to determine the dry density per cubic meter in soil. It establishes a relationship between dry density and optimum moisture content by compacting a mold of soil with a hammer of size, that is, 2.5 kg dropped from a height of 30 cm.

Wet density = Weight of compacted soil

Yd = Y1+w


Specific gravity test

This test is important to compute the void ratio, degree of saturation, and so on.

Specific gravity =(W2-W1)[(W2 -W1) -(W3 -W4)]



W1 = Weight of a bottle

W2=Weight of a bottle with dry soil

W3=Weight of a bottle with soil and water

W4=Weight of a bottle with water

California bearing ratio test

It checks the strength of the subgrade pavement in a highway design. It is used for the design of a flexible pavement by penetrating soil mass with a piston of 1.25mm/min.

CBR ratio = Test LoadStandard load × 100


Plate load test

This test is conducted to calculate the modulus of subgrade reaction for wheel load stresses in rigid pavements. It evaluates subgrade in-situ used in pavement design.

Modulus of subgrade reaction- It is the pressure sustained by a unit deformation of the subgrade with a specific plate size. It is denoted by K.

Importance of soil testing

It helps to calculate the thickness and load-bearing capacity of the pavement on highway. By testing soil, compute moisture content, nature, permeability, and type of pavement can be constructed. It also helps to analyze the strength and avoid any kind of deformation required for highway design.

Context and Applications

Soil is of great importance in the construction of pavements for structural engineers in highway design. a geotechnical engineer tests the soil engineering and physical properties are changed to make it more advantageous for the building pavements, rigid, or flexible. In civil engineering, many dissertations are based on the mitigation of roadways as the soil is an important ingredient in the construction of a road. It is of great importance in both undergraduate and postgraduate courses such as,

  • Bachelors in Technology (Civil engineering)
  • Masters in Technology (Civil engineering)

Practice Problems

Question 1) Which of the following is the best roadway pavement?

  1. cement
  2. bituminous
  3. asphalt
  4. Water Bound Macadam (WBM)

Answer- Option c

Explanation- Asphalt pavements are considered the best pavements because they are cheaper and provide better safety.

Question 2) What is the thickness of a roadway pavement?

  1. 9-12 inches
  2. 5-6 inches
  3. 6-12 inches
  4. 12-18 inches

Answer- Option c

Explanation- A pavement thickness is 6-12 inches.

Question 3) Why is a slope provided in a pavement?

  1. Safety
  2. Drainage
  3. Avoid skidding
  4. Aesthetic

Answer- Option b

Explanation- The slope is provided in the pavement for drainage purposes.

Question 4) Why are geosynthetic used?

  1. Binder
  2. Drainage
  3. Reinforced base
  4. Grain size distribution

Answer- Option c

Explanation- Geosynthetics are used for the reinforced base.

Question 5) Which of the following tests helps to calculate the modulus of subgrade reaction?

  1. CBR test
  2. Specific gravity test
  3. Plate load test
  4. Moisture content test

Answer- Option c

Explanation- Modulus of subgrade reaction is calculated by the plate load test.

  • Geosynthetics
  • Geomechanics
  • Asphalt
  • Subgrade

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