How to Determine Safe Bearing Capacity of Soil

Bearing capacity is the power of foundation soil to hold the forces from the superstructure without undergoing shear failure or excessive settlement.

Foundation soil is that portion of ground which is subjected to additional stresses when foundation and superstructure are constructed on the ground. The following are a few important terminologies related to bearing capacity of soil.

How to Calculate Bearing Capacity From SPT Numbers

One of the most common methods for determining allowable soil bearing capacity is from standard penetration test (SPT) numbers. It is simply because SPT numbers are readily available from soil boring. The equations that are commonly used were proposed by Meryerhof’s based on one inch of foundation settlement. Bowles revised Meyerhof’s equations because he believed that Meryerhof’s equation might be conservative.

Meryerhof’s equations:

For footing width, 4 feet or less:
Qa=(N/4)IK                                    [1.12]
For footing width, greater than 4 ft:
Qa=(N/6)[(B+I)/B]’IK                       [1.13]

Bowles’ equations:

For footing width, 4 feet or less:
Qa=(N/2.5)/K                                   [1.14]
For footing width, greater than 4 ft:
Qa=(N/4)[(B+1)/B]’IK                       [l.l5]
Qa:Allowable soil bearing capacity, in kips/ft2.
N:SPT numbers below the footing.
B:Footing width, in feet
K=  I+0.33(0/B)        1.33
D:Depth from ground level to the bottom of footing, in feet.

Example7: Determine soil bearing capacity by SPT numbers Given

• Soil SPT number:10
• Footing type: 3 feet wide strip footing, bottom of footing at 2 ft below ground surface.

Requirement: Estimate allowable soil bearing capacity based on. Solution:

Meryerhof’s equation

K=I+0.33(0/B)=1+0.33*(2/3)=122

Qa=(N/4)IK=(10/4)/1.22= 2kips/ft2Bowles’equation:

Qa=(N/2.5)IK=(10/2.5)/1.22=3.3kips/ft2

Example8:Determine soil bearing capacity by SPT numbers Given:

• Soil SPT number:20
• Footing type:8 feet wide square footing, bottom of footing at 4ft below ground surface.

Requirement: Estimate allowable oil bearing  capacity  based  on  Meryerhof’s equation.