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Bearing capacity from SPT numbers

    One of most commonly method 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 based on one inches 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) / K                                                                         [1.12]

For footing width, greater than 4 ft:                   

  Qa = (N/6)[(B+1)/B]2 / K                                                    [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]2 / K                                                        [1.15]

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.

 

Example 7: 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 = 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 8: Determine soil bearing capacity by SPT numbers

Given:

  • Soil SPT number: 20

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

RequirementEstimate allowable soil bearing capacity based on Meryerhof’s equation.

Solution:

Meryerhof’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