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How to Convert SPT to Ground Bearing Pressure

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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.


Recognize that up to 65 percent of the energy in the hammer fall is lost during the fall is lost through bending, heat noise and friction. The empirical values obtained from a simple SPT test must be refined by further investigations.


Beware that the SPT sample tube may encounter rocks in the soil that are larger than its own diameter. This introduces inaccuracies into the measurements.

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