Bearing Capacity of Soil
The safe bearing capacity of soil is the safe extra load soil can withstand without experiencing shear failure. The Safe Bearing Capacity (SBC) is considered unique at a particular site. But it also depends on the following factors:
• Size of footing
• Shape of footing
• Inclination of footing
• Inclination of ground
• Type of load
• Depth of footing etc.
SBC alone is not sufficient for design. The allowable bearing capacity is taken as the smaller of the following two criteria
• Limit states of shear failure criteria (SBC)
• Limit states of settlement criteria
Based on ultimate capacity, i.e., shear failure criteria, the SBC is calculated as
SBC = Total load / Area of footing
Usually the Allowable Bearing Pressure (ABP) varies in the range of 100 kN/m2 to 400 kN/m2. The area of the footing should be so arrived that the pressure distribution below the footing should be less than the allowable bearing pressure of the soil. Even for symmetrical Loading, the pressure distribution below the footing may not be uniform. It depends on the Rigidity of footing, Soil type and Conditions of soil. In case of Cohesive Soil and Cohesion less Soil the pressure distribution varies in a nonlinear way. However, while designing the footings a linear variation of pressure distribution from one edge of the footing to the other edge is assumed. Once the pressure distribution is known, the bending moment and shear force can be determined and the footing can be designed to safely resist these forces.
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