Difference between Gross Bearing Capacity and Safe Bearing Capacity of Soil?

Gross bearing capacity of the soil is a bearing capacity due to gross loads.

In gross bearing loads building loads, foundation loads, overburden loads should be considered. For design, gross bearing capacity value not take in consideration due to its high value.

Safe bearing capacity does not include overburden pressure and foundation weight. It is anet soil pressure considering shear failure.

Safe bearing capacity value for different types of soil

1. Hard or stiff clay- 440 kN/m^2 
2. Medium clay - 245kN/m^2 
3. Moist clay or sand-clay mixture - 150kN/m^2 
4. Black cotton soil - 130 to 160kN/m^2 
5. Compact and dry coarse sand - 440kN/m^2 
6. Compact and dry medium sand - 245kN/m^2 
7. Fine sand and silt - 150kN/m^2 
8. Loose and dry fine sand - 100 kN/m^2
9.  Laminated rocks - 3240kN/m^2 
10. Soft rocks - 440kN/m^2

Hi,

In gross bearing capacity, the gross load is taken. Loads from the superstructure, self-weight of the foundation, and overburden pressure are the gross loads. Gross bearing capacity value is not taken in design as it is very high.

Safe bearing capacity is the bearing capacity value that neglects the overburden pressure and stress due to the footing weight. It is obtained by considering the factor of safety, i.e. net load is taken into consideration. More detailed explanation is given below.

GROSS BEARING CAPACITY OF SOIL (qu)

• It is also called as Ultimate Bearing Capacity of Soil 
• It is represented by qu 
• Minimum gross pressure or load on the soil that can cause shear failure of the soil just below the footing. 
• Gross bearing capacity of the soil is defined based on gross load. 
• Gross load includes: Loads from superstructure, self-weight of the foundation and overburden pressure.

NET ULTIMATE BEARING CAPACITY OF SOIL (qnu)

• Represented by qnu 
• Net ultimate bearing Capacity = Gross bearing Capacity – (Stress due to weight of the footing + Stress due to overburden or surcharge) 
• As shown in below figure, the depth of footing is Df, if the density of footing and soil are same, equal to γ. Then, qnu= qu - γDf

SAFE BEARING CAPACITY OF SOIL

Safe Bearing capacity of soil is obtained after applying the factor of safety. 

It is divided into two:

• Safe Net Bearing Capacity 
• Safe Gross Bearing Capacity

Safe Net Bearing Capacity is defined as the net soil pressure that can be safely applied to the soil considering shear failure alone. It is obtained as qns = Net Ultimate Bearing Capacity (qnu)/Factor of Safety, i.e. qns = qnu/FS 

Safe Gross Bearing Capacity or Safe bearing Capacity is defined as the maximum gross pressure that soil can carry without shear failure. It is given by safe gross bearing capacity qs = qns + γDf