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Bearing from SPT Number

Courses > Foundation Analysis and Design > Bearing Capacity of Shallow Foundations > Bearing from SPT Number


Introduction on Bearing from SPT Number :

One of 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 Meyerhof based on one inch of foundation settlement.  Bowles revised Meyerhof’s equations because he believed that Meyerhof’s equation might be conservative.

 


Concepts and Formulas of Bearing from SPT Number:

Meyerhof’s equations:

For footing width, 4 feet or less:                       

Qa = (N/4) / K

For footing width, greater than 4 ft:                   

Qa = (N/6)[(B+1)/B]2 / K

 

Bowles’ equations:

For footing width, 4 feet or less:                       

Qa = (N/2.5) / K

For footing width, greater than 4 ft:                    

Qa = (N/4)[(B+1)/B]2 / K

 

Where

Qa: Allowable soil bearing capacity, in kips/ft2.

N: SPT numbers below the footing.

B: Footing width, in feet

K = 1 + 0.33(D/B) £ 1.33

D: Depth from ground level to the bottom of footing, in feet.

 


Watch! (Videos):

Soil Bearing Capacity Failure Demonstration

 


Solved sample problems of Bearing from SPT Number:

Example 1: Determine soil bearing capacity by SPT numbers (English units)

Given

Requirement: 

Estimate allowable soil bearing capacity based on.

Solution:

Meryerhof's equation

K = 1+0.33(D/B) = 1+0.33*(2/3) = 1.22

Qa = (N/4) / K = (10 /4) /1.22 = 2 kips/ft2

Bowles’ equation:

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

 

Example 2: Determine soil bearing capacity by SPT numbers (English units)

Given:

Requirement

Estimate allowable soil bearing capacity based on Meyerhof’s equation.

Solution:

Meyerhof’s equation

K = 1+0.33(D/B) = 1+0.33*(4/8) = 1.17

Qa = (N/6)[(B+1)/B]2 / K = (20/6)[(8+1)/8]2  /1.17 = 3.6 kips/ft2

Bowles’ equation:

Qa = (N/4)[(B+1)/B]2 / K = (20/4)[(8+1)/8]2  /1.17 = 5.4 kips/ft2

 


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