Coarse-grained soil and fine-grained soil are two different types of soil that can be identified based on their texture or ‘feel’ and particle size. It is very crucial to observe the range of particle sizes in soil as it heavily influences the key colloidal properties and has great many engineering implications.

Compaction of soil, plasticity, shear strength, surface area, packing density et cetera vary with the variations in sizes and so does the behavior of soils. The soil particle size generally ranges from 20cm in boulders to 0.1 microns in clay particles.

Physical and mechanical properties of coarse-grained and fine-grained soil are different and they are summarized in Table 1.

Table 1 Differences between Coarse-grained soil and fine-grained soil

Coarse-grained soilFine-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 angularSilt 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 sieveFine-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 qualitiesIt 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 freelyFine-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 stateVolume change occurs as moisture content varies.
Coarse-grained soil feels gritty when touched by handIt feels smooth, greasy and sticky when touched by hand

Madeh Izat Hamakareem

Madeh Izat Hamakareem

Madeh is a Structural Engineer who works as Assistant Lecturer in Koya University. He is the author, editor and partner at theconstructor.org.