Summary
- Coarse-grained soil mainly includes sand and gravel and is classified by particle size and grain-size distribution.
- It is typically permeable, incompressible, well-drained, and has good load-bearing strength when properly compacted.
- Coarse-grained soils are employed in various civil engineering applications in India, such as foundations, roads, embankments, backfilling, drainage systems, and agricultural drainage.
Coarse grained soil mainly contains larger particles such as sand and gravel. It usually has good drainage, low cohesion, and high permeability, which makes it important in construction, roadwork, drainage systems, and agriculture in India.
This guide describes the important types, characteristics, classification, and application of coarse-grained soil.
What is Coarse-Grained Soil?
Coarse-grained soil is classified based on particle size. Under the Indian Standard Soil Classification System, more than 50% of the soil by mass is retained on a 75-micron IS sieve.
Their behaviour depends on grain size, grading, density, and particle shape. In construction, engineers assess coarse-grained soil for applications such as foundations, embankments, backfilling, and pavement layers.
Read our types of sand used in construction guide to understand the differences between M-sand, river sand, P-sand, and other construction sands.
What Are the Types of Coarse-Grained Soils?
The main types of coarse-grained soil are gravel and sand. They are classified mainly by particle size, grain-size distribution, and the amount of fine material present.
Here are their key characteristics:
Gravel
Gravel contains particles larger than sand. Under Indian soil classification, gravel particles generally range from 4.75 mm to 80 mm. Gravel has large voids between particles. This allows water to drain quickly.
Based on particle size, gravel can be classified as:
- Coarse gravel: 20-80 mm
- Fine gravel: 4.75-20 mm
Gravel may be well graded or poorly graded. Well-graded gravel contains particles of different sizes, which helps achieve better packing and compaction.
Gravel is commonly used in road and pavement layers, drainage systems, backfilling, foundations, and embankments.
Sand
Sand contains smaller particles than gravel but larger particles than silt. Sand particles generally range from 0.075 mm to 4.75 mm.
Based on particle size, sand can be classified as:
- Fine sand: 0.075 mm to 0.425 mm
- Medium sand: 0.425 mm to 2 mm
- Coarse sand: 2 mm to 4.75 mm
These cut-offs come from soil classification. Construction-sand grades used for concrete and plaster are drawn differently, as explained in our coarse sand vs fine sand guide.
Sand usually allows water to pass through easily. Its engineering behaviour depends on grading, density, particle shape, and moisture conditions.
In India, sand is widely assessed for construction, foundation work, drainage, road projects, and other geotechnical applications.
Properties of Coarse-Grained Soil
Coarse-grained soil in general has high permeability, good shear strength, low compressibility, and good drainage. These properties have made it suitable for various construction, drainage, and geotechnical applications.
The following are the key characteristics of coarse-grained soil:
Compressibility
The compressibility of coarse-grained soils is generally low. The majority of settlement occurs immediately after loading as the particles settle in a more dense location. They generally show less long-term consolidation than clayey soils.
Drainage Capacity
Coarse-grained soils usually drain water quickly because of their large voids. This means that they can be used in drainage layers, backfill, filters, roadwork, and embankments.
Density
Compaction and grain size distribution are the principal factors affecting density. Well-graded soils can be denser as smaller particles can be placed in the voids between larger particles. Higher density generally improves stability and load-bearing capacity.
Frost Resistance
Clean and well-drained coarse soils generally resist frost action better than fine soils because they retain less water. However, higher fine content can reduce this resistance.
Shear Strength
Coarse-grained soils have greater shear strength primarily due to friction and interlocking of the particles. Particles that are dense, angular, and well-graded generally offer the greatest shear resistance. Compacted coarse soil can be used for foundations, slopes, and embankments.
Stability with Changes in Moisture
Coarse-grained soils are less responsive to wet/dry conditions as compared to clayey soils. Their strength is primarily due to friction and interlocking. The effective stress and stability, however, can also become saturated under saturated conditions.
Uses of Coarse-Grained Soils in India
Coarse-grained soils are widely used in Indian construction and infrastructure because they drain well, compact effectively, and can support loads when properly selected.
The most important applications of coarse grained soils in India are:
- Road and Highway Work: Sand and gravel are employed in embankments, sub-base, and drainage. Adequate compaction results in greater stability and minimises water-induced damage.
- Railway Track Support: Coarse aggregates are used to provide support for railway tracks, distribute loads, and provide good drainage around the tracks.
- Drainage Systems: Sand and gravel are used in road drainage, foundation drainage, retaining walls, and filter layers as water flows through easily.
- Backfilling: Coarse soil is commonly used for trenches, pits, foundations, and retaining walls. Good compaction helps control settlement.
- Foundations: Dense sand and gravel can support shallow foundations where bearing capacity and groundwater conditions are suitable.
- Dams and Embankments: Coarse materials are used in drainage and filter zones to control seepage and reduce fine-soil movement.
- Agricultural Drainage: Sand and gravel can be used to enhance the drainage of waterlogged fields and subsurface drainage.
Read our building materials checklist for house construction to understand where sand, aggregates, and other materials are used at different construction stages.
Coarse-Grained vs Fine-Grained Soil
Coarse-grained soil mainly contains sand and gravel, while fine-grained soil mainly contains silt and clay. The two groups differ in particle size, drainage, plasticity, and response to moisture.
Here are the main differences between coarse-grained and fine-grained soil:
| Factor | Coarse-Grained Soil | Fine-Grained Soil |
|---|---|---|
| Basis of identification | Identified mainly by particle size and grain-size distribution. | Identified mainly by particle size and plasticity characteristics. |
| Particle visibility | Individual particles are usually visible to the naked eye. | Individual particles are generally too small to see clearly. |
| Main soil types | Includes gravel and sand. | Includes silt and clay. |
| Particle size | Gravel is larger than 4.75 mm, while sand ranges from 0.075 mm to 4.75 mm. | Silt and clay particles are smaller than 0.075 mm. |
| Effect of moisture | Strength usually changes less with moisture variation. | Strength can change significantly with moisture variation. |
| Permeability | Generally higher, allowing faster drainage. | Generally lower, resulting in greater water retention. |
How Does Coarse-Grained Soil Affect Construction in India?
In the construction industry, coarse-grained soil can be used effectively when it is graded and compacted, as it can enhance stability, drainage, and load-bearing properties.
The major effects of coarse-grained soil in construction are as follows:
- Enhances drainage along foundations, roads, and retaining walls.
- Provides good load-bearing support when densely compacted.
- Generally has low compressibility, which results in reduced long-term settlement.
- Stabilizes road bases, embankments, and backfills.
- Compacts readily, unlike many fine-grained soils.
Next Steps: Using Coarse-Grained Soil Effectively
Coarse-grained soil includes sand and gravel, and has many applications in the fields of construction, drainage, infrastructure, and agriculture. It is graded, dense, compacted, and affected by moisture.
Check particle size, soil density, groundwater, and project requirements before use. Good testing results can be used to ensure suitability of foundations, roads, embankments, backfilling, and drainage.
Frequently Asked Questions
What are the index properties of coarse-grained soil?
Key index properties include grain-size distribution, density, water content, specific gravity, and relative density. These help classify the soil and assess its behaviour.
How is water content determined in coarse-grained soil?
The oven-drying method is used to measure water content. The test method will be dependent on soil type and test conditions.
What happens when fine soil mixes with coarse soil?
Fine particles fill the voids between coarse particles and can reduce permeability. The mixture may also show different compaction, drainage, and shear-strength behaviour.
How does water content affect shear modulus?
Higher water content can reduce effective stress and stiffness, especially when the soil becomes saturated. The effect depends on density, drainage conditions, and confining pressure.
What are the engineering properties of coarse-grained soil?
The most important engineering properties are permeability, shear strength, density, compressibility, compaction behaviour, and drainage capacity.
What is coarse soil?
Coarse soil is soil in which more than half the mass is retained on a 75-micron IS sieve. It is made mostly of sand and gravel.
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