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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/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.

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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/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:

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}

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