Soil can be classified into two main types, namely coarse-grained soil and fine-grained soil. The identification of these soil types is based on their texture or ‘feel’ as well as the size of their particles. The range of particle sizes in soil is an important factor to consider as it affects many key colloidal properties and has significant engineering implications.
The behavior of soils varies depending on the particle size, which can range from 20cm in boulders to 0.1 microns in clay particles. Factors such as compaction of soil, plasticity, shear strength, surface area, and packing density also vary with the variations in sizes. As a result, the physical and mechanical properties of coarse-grained and fine-grained soil are different, and these differences are summarized in Table 1.
Understanding the differences between coarse-grained and fine-grained soil is crucial for various engineering and construction applications. For instance, the selection of soil type is important when building structures such as foundations, embankments, and retaining walls. It is also essential in the design of drainage systems, roadways, and pavements. In summary, the study of soil properties is an important aspect of civil engineering and construction, and knowledge of the different types of soil is necessary for successful construction projects.
Table 1 Differences between Coarse-grained soil and fine-grained soil
| Coarse-grained soil | Fine-grained soil |
| Coarse-grained soil is frequently identified based on the particle size or grain size. | It is identified on the basis of its plasticity |
| Individual particles are visible by naked eye. | Individual particles are not visible by the naked eye. |
| It is divided into two classes sand and gravel. | It is divided into silt and clay. |
| Shape of grains varies from rounded to angular | Silt tend to have spherical shape and clay particles are flaky |
| Particles with a diameter larger than 4.75 mm are termed gravel and particles with diameter range between 4.75 mm to 75 micron is known as sand. | Fine-grained soil particles diameter that fallen between 75 micron to 2 micron are known as silt and particles having a diameter smaller than 2 micron is called clay |
| Coarse-grained soils have 50% or less material passing the No. 200 sieve | Fine-grained soils have 50% or more material passing the No. 200 sieve. |
| Engineering properties such as strength and compressibility of coarse-grained soil are governed by the grain-size of the particles and their structural arrangement. | Engineering properties of fine grained soil are controlled by mineralogical factors and moisture content. |
| There is no crucial change in strength with the change in moisture condition. | Strength changes with a change in moisture condition |
| Coarse-grained soil is described on the basis of its gradation (well or poor), particle shape (angular, sub-angular, rounded or sub-rounded) and mineralogical components. | Fine-grained soil is described depend on its dry strength, dilatancy, dispersion and plasticity. |
| It has good load-bearing qualities | It has good-load bearing qualities when dry; however, it possesses little or no load-bearing strength if it is wet. This characteristic is especially true with clay. |
| Coarse-grained soil drains freely | Fine-grained soil is impermeable due to its small particles size. |
| Moisture condition variation does not affect its volume; however, vibration accentuates volume change in loose state | Volume change occurs as moisture content varies. |
| Coarse-grained soil feels gritty when touched by hand | It feels smooth, greasy and sticky when touched by hand |