# Bearing from SPT Number

Courses > Foundation Analysis and Design > Bearing Capacity of Shallow Foundations > Bearing from SPT Number

### 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.

### 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.

### 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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