bearing capacity of soil

What Is SBC of Soil | Safe Bearing Capacity of Soil6 min read

What Is SBC of Soil?

Bearing Capacity 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.


Values of Safe Bearing Capacity for Different Soils

Following are safe bearing capacity of soil for various soil types,

Rock Type Soil

SBC of Soil
Rocky Soil
Sr. No.Type of Soil/RockSafe Bearing
Capacity
KN/m²
Minimum
Depth of
Foundation (m)
Type of
Foundation
1Rock without lamination and defects granite & diorite32400.6Spread
Footing
2Laminated rock, as sandstone or limestone in sound condition16200.75Spread
Footing
Residual deposits of shattered and broken bedrock and shale880
0.90
Spread Footing
Soft rock
440
1.20Spread Footing

Non – Cohesive Soils

SBC of Soil
Non-Cohesive Soil
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

Cohesive Soils

SBC of Soil
Cohesive Soil

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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
Bearing Capacity Of Soil

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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