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WHAT IS PRESUMPTIVE BEARING CAPACITY OF SOIL AND ITS VALUES?

What is Presumptive Bearing Capacity?

Building codes of various organizations in different countries gives the allowable bearing capacity that can be used for proportioning footing. These are called presumptive bearing capacity values. These values are based on experience with other structures already built.

What are The Values of Presumptive Bearing Capacity?

The table given below shows the presumptive bearing capacity values for different types of soils.

Type of Soil / Rock

Safe / Allowable Bearing Capacity (kN/m2)

Rock

3240

Soft rock

440

Coarse sand

440

Medium sand

245

Fine sand

440

Soft shell / Stiff clay

100

Soft clay

100

Very soft clay

50

Notes

  1. For non-cohesive soils the values should be reduced by 50% if the water table is above or near base of footing.
  2. These values should be used only for preliminary design. The actual bearing capacity of soil should be calculated as per the standard codes.

What are The Limitations of Using Presumptive Bearing Capacity Values?

  • Presumptive bearing capacity values are based only on visual classification of surface soil. So these values are not true representation of the actual capacity and not reliable.
  • These values don’t consider important factors affecting the bearing capacity; such as the shape, width, depth of footing, location of water table, strength and compressibility of the soil.
  • These values are conservative and can only be used for preliminary design or even for final design of small unimportant structures.
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