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
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:
Requirement: Estimate 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
|
