Have you ever wondered, how important is soil for any construction? It in fact plays a critical role in withstand the loads of any construction. It is always necessary to determine the soil, to maintain the structural integrity, without causing any damage or even collapse. To identify the safety and load bearing capacity, the Safe Bearing Test (SBT) can be used. Since each soil varies from one another, the SBC test can help identify and design safe and durable structures.
What is the SBC Test of Soil?
The Safe Bearing Capacity (SBC) Test of soil is a test that helps in determining the capacity of the soil to withstand loads without failure, cracks or excessive settlement. This test is done to prevent the foundation of a structure from being weak or to prevent it from settling unequally. Knowing the safe bearing capacity of soil helps the engineer to design a foundation strong enough to handle the proposed loads of a building, thus making it safe and stable for years to come.
Different Methods of SBC Testing
Standard Penetration Test (SPT)
It is one of the most commonly used methods for assessing soil bearing capacity of fine and coarse-grained soils. This test involves driving a split-barrel sample into the bottom of a borehole using a series of hammer blows. The number of blows it takes for the sampler to penetrate to a certain depth is noted (N-value). This method may not be suitable for the clay and gravel fill method.
Cone Penetration Test (CPT)
It is a dynamic testing method that measures the resistance of the soil against penetration. This method uses a cone-shaped probe, which is pushed into the ground at a constant rate, and the resistance is recorded. This method is highly reliable, providing a detailed profile of soil properties and resistance.
Plate Load Test
This method is used to determine the bearing capacity and strength of the ground by applying an increasing load through a circular steel plate. This method induces settlement and observes the soil’s behavior under the applied pressure. It is one of the commonly used testing methods for solids of larger particle sizes with a coarse or firmer texture.
Pressure Meter Test
This method assesses the amount of deformation of soil under pressure. This test involves inserting a cylindrical probe into a borehole and inflating the pressure meter to apply pressure to the surrounding soil.
Vane Shear Test
This testing method involves a vane shear tip inserted into a borehole, which is continuously rotated at a constant rate. The torque required to rotate the vane, and the bearing capacity of the soil is calculated. Using this, the shear strength of the soil is measured. This method is best suited to evaluate soft, cohesive soils that have a shear strength less than 0.5 kgf/cm².
SBC Values for Different Types of Soil (As Per IS 1904:1961)
| Type of Soil | Permissible Safe Bearing Capacity (kN/m²) |
| Rocks | |
| Hard rocks without laminations and defects:GraniteTrapDiorite | 3300 |
| Laminated Rocks:SandstoneLimestone | 1650 |
| Residual deposits of shattered and broken bed rock and hard shale | 900 |
| Soft Rock | 450 |
| Non-Cohesive Soils | |
| Gravel, sand, and ground materials that are compact and offer high resistance to penetration | 450 |
| Coarse sand, compact, and dry | 450 |
| Medium sand compact and dry | 250 |
| Fine sand compact and dry | 250 |
| Loose Gravel or sand-gavel mixture | 250 |
| Fine Sand, loose and dry | 100 |
| Cohesive Soils | |
| Soft shale, hard or still clay in a deep bed | 450 |
| Medium clay, readily independent with a thumbnail | 250 |
| Moist clay and sand clay mixture | 150 |
| Soft clay intended with moderate thumb pressure | 100 |
| Very soft clay which can be penetrated several cm with the thumb | 50 |
| Black cotton soil or other shrinkable or expensive clay in a dry condition | To be determined only after investigation |
| Peat | |
| Peat | To be determined only after investigation |
| Made-up Ground | |
| Filler or made-up ground | To be determined only after investigation |
Factors Affecting SBC of Soil
Types of Soil Structure
Soils vary based on different parameters like cohesion, friction, shear strength, etc. They have a direct impact on the bearing capacity of the soil. For cohesionless soil and mixed types of soils, it increases proportionally with an increase in the angle of friction. In contrast, cohesive soil varies linearly with cohesion value.
Water Table and Variations
Water tables near the foundation can adversely affect the bearing capacity of the soil due to seepage and uplift problems. However, the water table at a considerable depth below the base of the foundation does not have any significant impact on the bearing capacity of the soil.
Density of the Soil
Density of the soil plays a critical role in the bearing capacity. The denser the soil, the greater will be its bearing capacity.
Void Ratio and Porosity
The presence of subsurface voids tends to decrease the bearing capacity of the soil. Voids allow water to penetrate the surface, which affects the soil strength.
How To Improve Soil Beating Capacity?
Soil Compaction
This is the process of compaction of soil by removing air gaps to make the soil denser and stronger. The denser the soil, the more weight it can support. Compaction is usually done by heavy machinery like rollers to put the soil in a position where it can carry loads without movement or settlement.
Proper Draining
Adequate drainage can stop the soil from ingesting water, thereby maintaining its bearing capacity. Waterlogged soil can become soft and unstable. French drains or proper site grading would allow water to flow away from the area near the foundation.
Confining the Soil
This is the process of limiting a soil mass pressure by surrounding it with some constraints, such as concrete or steel structures, so that the soil cannot spread or shift under load. The displacement of soil can be prevented by confining the soil, thereby soil stabilizing and improving its strength to carry heavier loads.
Grouting
Grouting is process of injecting a cement or other mixture into the ground to stabilise the voids and strengthen it. This method is highly suitable for loose or sandy soil, where it helps bind the particles and increases the load-bearing capacity of the soil.
Chemical Treatment
Chemical stabilisation is a method where chemicals are applied to treat and improve soil. The chemical interaction between soil and stabilizer can improve soil properties by decreasing moisture content, increasing cohesion and ability to carry weight.
Conclusion
In summary, SBC tests play a pivotal role in ensuring the safety and stability of any construction. To accurately assess the soil’s capacity, SPT, CPT and plate loading tests can be helpful. In case of poor ground surface, certain precautionary measures can be done to stabilise the ground. This is essential for any construction project to design and plan the construction and stabilise the foundation for long-term safety, and durability.