Bearing from SPT Number

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Introduction

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.

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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/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

Standard Penetration Test (SPT)

Soil Bearing Capacity Failure Demonstration

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/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:

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/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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