Bearing from SPT Number
Courses > Foundation Analysis and Design > Bearing Capacity of Shallow Foundations > Bearing from SPT NumberIntroduction
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.
» Try our free powerful online bearing capacity calculator «
Concepts and Formulas
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/ft^{2}.
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
Solved sample problems
Example 1: Determine soil bearing capacity by SPT numbers (English units)
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/ft^{2}
Bowles’ equation:
Qa = (N/2.5) / K = (10 /2.5) /1.22 = 3.3 kips/ft^{2}
Example 2: Determine soil bearing capacity by SPT numbers (English units)
Given:

Soil SPT number: 20

Footing type: 8 feet wide square footing, bottom of footing at 4 ft below ground surface.
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/ft^{2}
Bowles’ equation:
Qa = (N/4)[(B+1)/B]2 / K = (20/4)[(8+1)/8]2 /1.17 = 5.4 kips/ft^{2}
Download Files
No files available for this topic. Suggest one!
Read also
 How do you calculate the number of dowel bars?
 What is the purpose of dowel bars?
 Effect of Ground Water Table
 How many stringers should be there in a staircase
 Terzaghi's Method