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What Is SBC of Soil?
The SBC of Soil is the maximum load-carrying capacity of soil per unit area which the soil or rock can carry without yielding or displacement.
It is a very known fact that all structures whether they are buildings, dams, bridges, etc. are built on soils. The Foundation of each structure must require the transfer of its load on large areas of soil.
So, that the foundation of every structure should be so designed that the soil below does not fail in shear nor there is excessive settlement of the structure.
Significance Of Safe Bearing Capacity Of Soil
Safe Bearing Capacity has the following significance regarding the soil,
- The Soil tends to deform when stressed due to loading. The soil deformation depends upon factors like water content, bulk density, angle of internal friction, and the manner in which load is applied on the soil.
- The Soil properties like density shear strength permeability etc., affect the bearing capacity of the soil.
- Normally, Dense sand will have more bearing capacity than loose sand as the unit weight of dense sand is more than loose sand.
- If the bearing capacity of soil is sufficient to take a load of the structure at shallow depth, then a shallow foundation is provided and the option for shallow are combined footing, Isolated footing, or strip footing.
- In case when soil immediately below the structure does not have adequate bearing capacity deep foundations are provided.
- Pile, piers, or well are the options for deep foundations. For the type of soil which is liable to differential settlement or where there is a wide variation in loading between adjacent columns Mat or Raft foundation is most suitable.
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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.
Safe Bearing Capacity of Soil
Following are safe bearing capacity of soil for various soil types 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, as sandstone or limestone in sound condition | 1620 | 0.75 | Spread Footing |
| Residual deposits of shattered and broken bedrock and shale | 880 | 0.90 | Spread Footing | |
| 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 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
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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 thumb nail | 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 thumb | 50 | 5 | Pile Foundation |
Methods For Determining Soil Bearing Capacity
Many Methods have got approval for estimating the bearing capacity of the soil. Procedure 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: 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: 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 total bearing divided by the area of the steel plate.
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