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How to Convert SPT to Ground Bearing Pressure

By Maria Kielmas ; Updated January 09, 2018
Construction sites need detailed soil investigations.
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The bearing pressure of a soil is the maximum load that it can bear before its fails by shearing or liquefaction. This value is the primary consideration in the design of foundations. The standard penetration test is a simple method for estimating the mechanical properties of a soil at a potential construction site. Geotechnical engineers have accumulated data over several decades to provide a very empirical relation between SPT results and a soil’s bearing pressure.

Choose the precise site for the test. This may be on the surface of the ground or at the bottom of a bore hole if information is required for the soil characteristics at greater depth in the ground.

Place the sample tube into the ground and hammer it into the ground so that its lower end is six inches below the surface. Raise the hammer to a height of 30 inches above the top of the tube and allow to drop onto the tube. Count the number of hammer blows required for the tube to penetrate a further six inches into the ground.

Lift the sample tube out of the ground and empty its contents into moisture-proof containers. This soil will be analyzed in later laboratory tests. Reassemble the sample tube and replace it into its last position in the ground. Repeat the hammer blows from a 30-foot height above the tube until it penetrates a further six inches into the ground. Count the number of blows. Lift the sample tube out of the ground again and empty its contents into another moisture-proof container for future analysis.

Calculate the average number of hammer blows, N, required per six inches of soil penetration from the two tests. This is the sum of each number of blow counts divided by two. Each single hammer blow is equivalent to a bearing pressure of 2.1 bar, or 30.45 pounds per square inch, according to empirical engineering calculations. Multiply the N value by either of the preceding factors to obtain the bearing pressure in the required units of measurement.


Things You Will Need

  • Soil sample tube
  • 140-pound manual or auto hammer
  • Waterproof bags or containers
  • N-value and pressure correlation charts

About the Author


Based in London, Maria Kielmas worked in earthquake engineering and international petroleum exploration before entering journalism in 1986. She has written for the "Financial Times," "Barron's," "Christian Science Monitor," and "Rheinischer Merkur" as well as specialist publications on the energy and financial industries and the European, Middle Eastern, African, Asian and Latin American regions. She has a Bachelor of Science in physics and geology from Manchester University and a Master of Science in marine geotechnics from the University of Wales School of Ocean Sciences.