What Is SBC of Soil?
SBC stands for Safe Bearing Capacity, which is a term used in civil engineering to refer to the maximum amount of load a soil can safely support without undergoing excessive settlement or shear failure. The safe bearing capacity of soil is typically determined through a series of geotechnical investigations and tests, which take into account factors such as the soil type, moisture content, density, and structure, as well as the nature and intensity of the loads that the soil will be subjected to.
The SBC of soil is a crucial factor in the design and construction of foundations, retaining walls, and other structures that rely on the soil for support.
Significance Of Safe Bearing Capacity Of Soil
The safe bearing capacity (SBC) of soil is a crucial factor in the design and construction of any structure that rests on or is embedded in the soil. The SBC determines the maximum load that the soil can safely support without undergoing excessive settlement or shear failure. If a structure is built without considering the SBC of the soil, it may lead to structural failure, endangering the safety of the occupants and causing damage to the property.
The SBC of soil is determined through a series of geotechnical investigations and tests, which take into account factors such as the soil type, moisture content, density, and structure, as well as the nature and intensity of the loads that the soil will be subjected to. These tests help engineers and architects to design the foundation of the structure and determine the size, depth, and type of foundation required to ensure the safety and stability of the structure.
Overall, the significance of the SBC of soil lies in its ability to ensure the safety and stability of the structure by providing a reliable foundation. A proper understanding of the SBC of soil helps in the selection of an appropriate foundation type and size, which can significantly reduce the risks associated with construction and ensure the longevity of the structure.
Safe Bearing Capacity of Different Types of Soil
The magnitude of load a soil could withstand transfer from the foundation is called the Safe Bearing Capacity of Soil.
The Pressure exceeding this value might cause the failure of soil and subsequently that of structure.
Similarly, the gross pressure at which the soil failure occurs is called the ultimate bearing capacity of the soil. And dividing it with a factor of safety gives the safe bearing capacity of the soil.
During designing the foundation is important to know the bearing capacity of the soil through different field tests.
Standard values for safe bearing capacity are calculated and verified for different types of soils are made available and are used when test data is limited or rapid construction is required.
minimum sBC of soil
The minimum safe bearing capacity (SBC) of soil required for construction varies depending on the type of structure and the intensity of the load it will bear. In general, the SBC of soil should be sufficient to ensure that the structure does not experience excessive settlement or shear failure, which can lead to structural damage and compromise the safety of the occupants.
The minimum SBC of soil required for typical residential construction projects ranges from 100 kN/m² to 200 kN/m². However, for high-rise buildings, bridges, or other structures that experience heavy loads, the required SBC can be much higher, often exceeding 500 kN/m². The actual SBC for a particular construction project is determined through geotechnical investigations and tests conducted by a qualified engineer.
It’s essential to ensure that the SBC of soil meets or exceeds the minimum requirements to ensure the safety and stability of the structure. Failure to do so can lead to costly repairs and compromise the safety of the occupants.
Safe Bearing Capacity of Soil
here are some safe bearing capacity of soil as per IS Code
1. Rock Type Soil
| Sr. No. | Type of Soil/Rock | Safe Bearing Capacity KN/m² | Minimum Depth of Foundation (m) | Type of Foundation |
| 1 | Rock without lamination and defects granite & diorite | 3240 | 0.6 | Spread Footing |
| 2 | Laminated rock, such as sandstone or limestone in sound condition | 1620 | 0.75 | Spread Footing |
| 3 | Residual deposits of shattered and broken bedrock and shale | 880 | 0.90 | Spread Footing |
| 4 | Soft rock | 440 | 1.20 | Spread Footing |
2. Non–Cohesive Soils
| No. | Type of Rock/Soil | Safe Bearing Capacity KN/m² | Minimum Depth of Foundation (m) | Type of Foundation |
| 1. | Gravel, Sand, and gravel are compact and offering high resistance to penetration when excavated tools | 440 | 1.2 | Spread Footing |
| 2. | i) Coarse compact and dry sand ii) Medium compact and dry sand | 440 245 | 1.2 1.2 | Spread Footing |
| 3. | Fine Sand, Silt | 150 | 1.2 | Spread Footing & Combine Footing |
| 4. | Loose gravel or sand-gravel mixture, loose coarse to medium sand dry | 245 | 1.5 | Spread Footing & Combine Footing |
| 5. | Fine Sand | 100 | 1.8 | Spread Footing & Combine Footing |
3. Cohesive Soils
4. Bearing Capacity of Soil – Cohesive Soils
| No. | Type of Rock/Soil | Safe Bearing Capacity KN/m² | Min. Depth of Foundation (m) | Type of Foundation |
| 1. | Soft shale, hard or stiff clay in the deep dry bed | 440 | 1.5 | Spread Footing & Combine Footing, strip footing |
| 2. | Medium clay, readily indented with a thumbnail | 245 | 1.8 | Combined or Mat Footing |
| 3. | Moist clay and sand-clay mixture which can be indented with slight thumb pressure | 150 | 1.8 to 3m depth varies depending on moisture movement zone | Combined or Mat Footing |
| 4. | Soft clay indented with moderate thumb pressure | 100 | 3 | Deep (Pile) foundation |
| 5. | Very soft clay which can be penetrated several centimeters with a thumb | 50 | 5 | Pile Foundation |
Methods For Determining Soil Bearing Capacity
Many Methods have got approval for estimating the bearing capacity of the soil. Procedures for Some of them are discussed below,
Step 1: A well of dimension 2 m X 2m is drugged for collecting samples. In which the bottom of the well is leveled by pouring the soil manually and then compacted.
Step 2: Place a plate of dimensions 60 cm x 60 cm x 5 cm at the center. The plate used is mostly a steel plate.
Step 3: After placing a steel plate centrally, erect about a 40 cm X 40 cm wall, with bricks, stone blocks, or concrete up to the height of 50 cm above ground level.
Step 4: Next Step involves loading the wall using cargo and subsequently measuring the load imposed on the wall.
Step 5: The used cargo may consist of sandbags, beams, or R.S.J. The bearings are subsequently increased by a specific amount in that respective time interval, generally, 0’5 tonnes loading in a time interval of about 20 to 30 minutes.
Step 6: An increase in each bearing is followed by measuring the difference in levels between the pins and the top of the wall.
Note: The difference observed in the level will be completely the same until the soil gives away.
Step 7: The increase in the difference of bearing increase must be stopped at the failure point and the bearing of soil is calculated, which is equal to the total bearing divided by the area of the steel plate.
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Bearing Capacity of Soil