Generally, soils are classified into two main categories: fine and coarse. Fine soils comprise clay and silt, while coarse soils consist of sand and gravel. Each type of soil has its role in construction; however, from the points of view of strength, stability, and drainage, coarse-sized soil dominates the scene. Highway construction and road maintenance processes, embankments, foundation constructions, and drainage are applications where this soil is used. In this blog, we will explore everything about coarse-grained soil,types of soil, what it is and why it is important, and where it does all the work.
What is Coarse-Grained Soil?
Coarse-grained soil refers to the soil that has grains between 0.075 mm (75 microns) and 60 mm, accounting for more than 50% of its total mass. It has gravel fill (particles ranging from 4.75 mm to 60 mm) and sand (particles ranging from 0.075 mm to 4.75 mm). Due to its relatively large particle size and low fines content, coarse-grained soil exhibits high permeability, low compressibility, and low cohesion. When properly compacted, it shows excellent engineering properties such as high shear strength, good drainage, and stability, so that it is often used as a filling material in embankments and highway construction.
Importance of Coarse-Grained Soil in Construction
Coarse-grained soils are considered essential for construction works because of their strength, stability, and drainage ability. These soils, hardly affected by moisture changes, have been regarded as the most reliable bases in road and railway construction. With coarse-grained sands located around a structure, water holding capacity will decrease, hydrostatic pressure will decrease, and there will be fewer chances of damage.
Characteristics of Coarse-Grained Soil
High Permeability
Being coarse-grained, these soils have large particles and very wide void spaces. This structure allows water to flow through them very easily; hence, they have a very high permeability. They can have a hydraulic conductivity ranging from 10⁻⁴ up to 10¹ cm/s and can be used as drainage layers and filter placements wherever speedy removal of water is of concern.
Low Cohesion
The soils do not show any cohesion or very little due to their large size, thus not allowing any electrochemical inter-particle attraction, unlike clays. Therefore, the strength depends mainly on inter-particle friction rather than cohesive bonding. Nonetheless, the frictional resistance increases with particle angularity and compaction.
Good Load-Bearing Capacity
When compacted properly, such soils can bear heavy loads with high bearing capacity of soil. On the contrary, the high shear strength and low compressibility of these soils make them the best choice for foundations, retaining walls, and embankments. Their capacity to maintain structural properties when bearing loads makes coarse soils popular in geotechnical engineering.
Relatively Non-Plastic
Coarse-grained soils are non-plastic. This means that under an imposed stress, coarse soils do not undergo any permanent deformation. They have no Atterberg limits and do not change their behavior with moisture content, unlike fine-grained soils such as clays. Coarse soils, therefore, tend to be more stable and predictable for engineering applications.
Properties of Coarse-Grained Soil
Shear Strength
Coarse-grained soils possess very high shear strength, primarily due to interparticle friction. Their strength parameters are dependent on particle size, angularity, density, and confining pressure. Angle of internal friction (ϕ) usually varies between 30° and 45°, being higher for well-compacted gravels. They are thus suitable for load-bearing uses, such as foundations and slopes.
Compressibility
Coarse-grained soils remain largely non-compressible under normal loading conditions. Most of the settlement occurs immediately after loading due to the rearrangement of particles, whereas clayey soils settle over time through consolidation. This property ensures the long-term stability of a structure either built on or composed of these soils.
Density
Depending on compaction and gradation, the dry density of coarse-grained soils usually ranges between 1.6 and 2.2 g/cm³. High-grade ones tend to yield higher densities and better performance enhancement. Density enables good load-bearing quality and resistance to deformation.
Drainage Capacity
Because of the large voids and open structure of coarse soils, they provide very good drainage. They are used for road pavement on layers, around foundations, and for filter systems. Good drainage helps reduce the damage caused by water, such as frost heaving and surface erosion.
Frost Resistance
Generally, soils resist frost action more when clean and well-drained, as they cannot hold water to expand or contract during freeze-thaw cycles. Hence, they are suitable for zones where frost heave is a concern.
Stability with Change in Moisture
Coarse-grained soils are those which undergo strength change with fluctuations in moisture content. Their strength is mostly due to mechanical interlocking and not bonds formed due to the presence of moisture. Hence, such soils are good for use in situations like embankments and flood zones where water content can vary.
Applications of Coarse Grained Soil in Construction Projects
- Embankments of Highways and Roads: Structural stability and quick drainage.
- Railway Ballast: The foundation layer for railway tracks is coarse gravel due to its bearing and drainage.
- Drainage Layers: For building drainage systems, road drainage systems, and retaining wall drainage.
- Backfilling: Used for trench filling or pits and behind retaining walls.
- Foundations: Shallow foundations where soil bearing capacity has to be considered.
Key Differences of Coarse-Grained Soil vs Fine-Grained Soil
| Coarse-Grained Soil | Fine-Grained Soil |
| Identified based on the particle size or grain size. | Identified based on its plasticity. |
| Individual particles are visible to the naked eye. | Individual particles are not visible to the naked eye. |
| Classified into sand and gravel.Gravel: > 4.75 mm; Sand: 4.75 mm to 75 microns. | Classified into silt and clay.Silt: 75 to 2 microns; Clay: < 2 microns. |
| Grain shapes vary from rounded to angular. | Silt particles are spherical, and clay particles are flaky. |
| 50% or less material passes through the No. 200 sieve. | 50% or more material passes through the No. 200 sieve. |
| Engineering properties depend on grain size and arrangement. | Properties are influenced by mineralogy and moisture content. |
| Minimal strength change with moisture variations. | Strength varies significantly with moisture changes. |
Conclusion
Because of their high strength, stability, and favourable drainage characteristics, coarse-grained soils have an application in many civil and construction engineering projects. Since they provide a reliable working condition regardless of environmental changes, they are suitable for the bases, embankments, and other structural applications. A sound knowledge of the properties of coarse soils and their distinction from fine soils is necessary for the correct application of geotechnical engineering and construction planning.