How Is Spt N Value Calculated

What Is SPT N Value?

The SPT N value, also known as the standard penetration number, is a measure of the resistance encountered when a split-spoon sampler is driven into the ground. It is a key parameter in geotechnical investigations and is used to classify soils and assess their engineering properties.

The SPT test is performed by driving a 50.8 mm (2-inch) diameter split-spoon sampler into the ground using a 63.5 kg (140 lb) hammer dropped from a height of 760 mm (30 inches). The number of blows required to drive the sampler 300 mm (12 inches) into the ground is recorded as the SPT N value.

The SPT N value is influenced by various factors, including soil type, moisture content, overconsolidation, and the energy of the hammer. It is a relative measure of soil strength and is used to correlate with other soil properties such as shear strength, relative density, and liquefaction potential.

How to Calculate SPT N Value

Calculating the SPT N value involves performing a standard penetration test and recording the number of blows required to drive the sampler into the ground. The SPT N value is then calculated based on the recorded data.

The standard procedure for calculating the SPT N value is as follows:

1. Prepare the test site by clearing the area and removing any obstructions.
2. Drill a borehole to the desired depth using a drilling rig.
3. Insert the split-spoon sampler into the borehole and secure it in place.
4. Drive the sampler into the ground using a 63.5 kg (140 lb) hammer dropped from a height of 760 mm (30 inches).
5. Record the number of blows required to drive the sampler 300 mm (12 inches) into the ground.
6. Repeat the test at different depths to obtain a profile of the SPT N values.
7. Calculate the SPT N value by dividing the total number of blows by the number of 150 mm (6-inch) increments driven.

The SPT N value is typically reported as the average number of blows per 150 mm (6-inch) increment. It is important to follow standard test procedures and record accurate data to ensure reliable results.

SPT N Value Formula

The SPT N value is calculated using the following formula:

Where:

• SPT N is the standard penetration number.
• Number of blows for 300 mm penetration is the total number of blows required to drive the sampler 300 mm into the ground.
• Number of 150 mm increments is the number of 150 mm increments driven into the ground.

For example, if 50 blows are required to drive the sampler 300 mm into the ground, the SPT N value would be calculated as follows:

The SPT N value is typically reported as the average number of blows per 150 mm increment. It is important to follow standard test procedures and record accurate data to ensure reliable results.

SPT N Value Chart

The SPT N value chart provides a visual representation of the relationship between the SPT N value and soil properties. The chart is based on empirical correlations between the SPT N value and other soil parameters such as shear strength, relative density, and liquefaction potential.

The chart shows the variation of the SPT N value with depth for a typical soil profile. The SPT N value is highest in dense sands and gravels and lowest in soft clays and silts. The chart helps engineers interpret the SPT N values and make informed decisions about foundation design and soil improvement techniques.

SPT N Value Table

The SPT N value table provides a summary of the SPT N values obtained from a standard penetration test. The table includes the depth of the test, the number of blows required to drive the sampler into the ground, and the calculated SPT N value.

The table shows the variation of the SPT N value with depth for a typical soil profile. The SPT N value is highest in dense sands and gravels and lowest in soft clays and silts. The table helps engineers interpret the SPT N values and make informed decisions about foundation design and soil improvement techniques.