The bearing capacity of soil refers to the ability of soil to support the loads applied to the ground. It is the maximum load per unit area that the soil can carry without experiencing shear failure or excessive settlement. Understanding what is bearing capacity of soil is crucial for designing safe foundations for any structure. If the soil under a foundation is weak, the structure may settle unevenly or even collapse.
Why is Bearing Capacity Important?
Bearing capacity determines whether the ground is suitable to hold a building or other structures. Choosing the right type of foundation depends on the bearing capacity of the underlying soil. Low bearing capacity can lead to structural failures, cracks in buildings, and dangerous settlements. It also affects the cost of construction, as poor soil may require deep foundations or soil improvement.
Types of Bearing Capacity of Soil
1. Ultimate Bearing Capacity
This is the maximum pressure the soil can withstand before failure occurs. It includes both safe and unsafe zones of loading.
2. Net Ultimate Bearing Capacity
It is calculated by subtracting the self-weight of the soil above the foundation from the ultimate bearing capacity.
3. Safe Bearing Capacity
Safe bearing capacity is the maximum pressure the soil can safely bear without risk of shear failure. It includes a factor of safety, usually ranging from 2.5 to 3.
4. Net Safe Bearing Capacity
It refers to the safe load that can be applied on the foundation after removing the effects of overburden pressure.
5. Allowable Bearing Pressure
This is the pressure that includes considerations of both shear failure and permissible settlement. It is used in actual design practice.
Factors Affecting Bearing Capacity of Soil
Several key factors influence the bearing capacity:
- Soil Type: Cohesive soils like clay behave differently from cohesionless soils like sand.
- Moisture Content: Water presence can either improve or reduce strength based on soil type.
- Density and Compaction: Well-compacted soils have higher strength and bearing capacity.
- Depth of Foundation: Deeper foundations generally have more bearing capacity.
- Size and Shape of Footing: Larger and well-shaped footings distribute loads better.
- Load Duration: Short-term loads might be carried better than sustained loads.
- Presence of Water Table: A high water table reduces soil strength.
- Soil Layering: Weak layers below strong surface soils reduce overall capacity.
Common Soil Types and Their Bearing Capacities
| Soil Type | Approximate Bearing Capacity (kN/m²) |
|---|---|
| Soft clay | 50 – 100 |
| Medium clay | 100 – 200 |
| Dense sand | 200 – 600 |
| Gravel | 300 – 600 |
| Hard rock | 1000+ |
Note: Actual values depend on field conditions and must be verified by soil tests.
Methods of Improving Bearing Capacity
When soil is found to be weak, engineers use several techniques to enhance its strength:
- Compaction: Densifying the soil using rollers or compactors.
- Soil Replacement: Removing weak soil and replacing it with granular material.
- Drainage Improvement: Reducing water content through proper drainage.
- Grouting: Injecting cement or chemicals to fill voids and bind particles.
- Reinforced Soil: Using geotextiles, geogrids, or layers of concrete.
Bearing Capacity Testing Methods
1. Plate Load Test
A steel plate is placed at the foundation level and load is gradually applied to measure settlement. This test gives direct insight into the safe bearing capacity of soil on site.
2. Standard Penetration Test (SPT)
A split spoon sampler is driven into the soil using a hammer, and the number of blows required is counted. It helps in estimating soil density and strength.
3. Cone Penetration Test (CPT)
A cone-shaped device is pushed into the soil at a constant rate to measure resistance. It gives continuous data about soil layers and strength.
4. California Bearing Ratio (CBR) Test
Mostly used for pavement subgrade design, this test evaluates the resistance of soil to penetration.
5. Laboratory Shear Strength Tests
Tests like Direct Shear Test, Triaxial Test, and Unconfined Compression Test are used to assess the strength parameters of soil samples in the lab.
How to Increase Bearing Capacity on Site
- Excavate and backfill with granular soil or crushed stone.
- Install sand or gravel piles to reinforce the subsoil.
- Use raft or mat foundations for better load distribution.
- Design deeper foundations to reach stronger strata.
- Use soil stabilization techniques such as lime or cement mixing.
Practical Guidelines for Foundation Design Based on Bearing Capacity
- Never design foundations without conducting site-specific soil tests.
- Use allowable bearing pressure values with proper factors of safety.
- For residential buildings, isolated footings may suffice if the soil is good.
- In poor soils, prefer combined footings, raft foundations, or piles.
- Consider future loading conditions like water tanks, extensions, or seismic forces.
FAQs About Bearing Capacity of Soil
What is bearing capacity of soil?
It is the maximum load per unit area the soil can support without failing or causing excessive settlement.
What is safe bearing capacity?
Safe bearing capacity is the load the soil can safely bear after applying a safety factor, typically 2.5 to 3.
How can I test bearing capacity on-site?
The plate load test is the most reliable field method. Other tests include SPT and CPT, depending on site conditions and budget.
What are the typical bearing capacities of different soils?
Soft clay has the lowest (50–100 kN/m²), while gravel and rock have high capacities (300–1000+ kN/m²).
Can the bearing capacity of soil be increased?
Yes, by using soil compaction, replacing poor soil, grouting, or using deeper foundations and soil reinforcement techniques.
How does water affect bearing capacity?
Water reduces effective stress and weakens soil, especially in sandy and clayey soils. High water tables significantly lower bearing strength.
Conclusion
Understanding what is bearing capacity of soil is essential for safe, economical, and efficient construction. It forms the foundation of all structural designs. From choosing the right type of foundation to preventing future settlement issues, bearing capacity plays a central role. By knowing the influencing factors and proper testing methods, engineers can design structures that last longer and perform better. Always consult with geotechnical engineers and conduct soil tests before beginning any construction project.
