Standard Penetration Test as per IS code

The Standard Penetration Test (SPT) is an in-situ test used to assess the geotechnical properties of soil. It is a widely used test in India and is governed by the IS 2131:1981 code of practice.


Split-spoon sampler

Driving weight (63.5 kg)

Cathead assembly

Tripod and lifting frame

Recording sheets


Drill a borehole to the desired depth.

Lower the split-spoon sampler into the borehole.

Drive the sampler 300 mm into the soil using the driving weight falling from a height of 750 mm at a rate of 30 blows per minute.

Record the number of blows required for each 150 mm interval of penetration. The last 300 mm of penetration is considered for calculating the N-value.

Extract the sampler and recover the soil sample.

Visually classify the soil sample and record its description.


The N-value is the number of blows required to drive the sampler the last 300 mm. It is an indicator of the relative density and bearing capacity of the soil.


The N-value obtained in the field needs to be corrected for various factors such as:

Overburden pressure: The N-value increases with increasing depth due to the increasing overburden pressure.

Borehole diameter: The N-value is influenced by the diameter of the borehole.

Energy ratio: The N-value is affected by the efficiency of the hammer blow.

The corrected N-value can be used for various geotechnical engineering applications such as:

Foundation design

Slope stability analysis

Liquefaction assessment

Image of Standard Penetration Test:

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Standard Penetration Test Procedure 

Here’s a detailed explanation of the SPT procedure:

Before the Test:

  1. Planning: Define the desired depths for testing and choose appropriate equipment based on expected soil conditions.
  2. Equipment: Ensure all equipment, including the drilling rig, sampler, hammer, cathead, rods, and accessories, are in good condition and meet IS 2131:1981 specifications.
  3. Safety: Implement proper safety procedures, including wearing personal protective equipment and following safe drilling practices.

Test Procedure:

  1. Drilling: Drill a borehole to the desired depth, ensuring minimal disturbance to the soil. Remove spoil materials and clean the borehole bottom.
  2. Sampler Assembly: Attach the split-spoon sampler to the drill rods and lower it into the borehole.
  3. Driving the Sampler:
    • Use the standard 63.5 kg (140 lb) hammer with a 750 mm (30″) free fall.
    • Drive the sampler 300 mm (12″) into the soil, recording the number of blows for each 150 mm (6″) interval.
    • Discard the blows for the first 150 mm (seating drive).
    • The N-value is calculated using the blows for the second and third 150 mm (300 mm total).
    • If exceeding 50 blows for any 150 mm interval, stop the test and record a refusal (N = 50+).
  4. Sample Recovery: Retrieve the sampler and extract the soil sample.
  5. Logging: Record detailed information about the test, including:
    • Location and depth of the test
    • Soil description based on visual and tactile examination
    • Number of blows for each interval and calculated N-value
    • Other relevant observations (groundwater, etc.)

Standard Penetration Test Results

Standard Penetration Test (SPT) results consist primarily of the N-value, which represents the number of blows required to drive a standard split-spoon sampler the final 300mm (12 inches) into the ground. However, the test report also includes other valuable information.

Key Components of SPT Results:

  • Location and Depth: This identifies the specific borehole and depth where the test was conducted.
  • Soil Description: A thorough description of the soil encountered, including its visual and tactile properties (color, texture, moisture content, etc.).
  • N-value: The number of blows required for each 150mm (6 inches) interval of penetration, with the final calculated N-value based on the second and third 150mm sections.
  • Corrected N-value (N60): The N-value adjusted for various factors like overburden pressure, borehole diameter, and energy ratio to provide a more standardized value.
  • Other Observations: Any notable observations during the test, such as groundwater level, changes in soil behavior, or equipment issues.

Interpreting SPT Results:

The N-value is primarily used to estimate the relative density and bearing capacity of the soil. However, correlations exist to derive other geotechnical properties like:

  • Friction angle: An indicator of the soil’s internal shear strength.
  • Modulus of elasticity: A measure of the soil’s stiffness.
  • Liquefaction potential: The likelihood of the soil losing its strength under seismic loading.


While the SPT report itself is textual, several charts and graphs can visually represent the results:

  • N-value vs. Depth: A plot showing the variation of N-value with depth in the borehole, providing an overall profile of soil density.
  • Soil Classification Chart: Plotting the N-value and another parameter (e.g., grain size) on a standard chart to classify the soil type.
  • Liquefaction Assessment Charts: Using the N-value and other factors to estimate the soil’s liquefaction susceptibility.


SPT results are just one piece of the puzzle in geotechnical engineering. Combining them with other tests, visual observations, and engineering judgment is crucial for accurate characterization and safe design.

Standard Penetration Test Sampler

The standard penetration test (SPT) sampler is a crucial component of the SPT, an in-situ test used to assess the geotechnical properties of soil. It’s essentially a hollow, thick-walled steel tube driven into the ground to collect a disturbed soil sample while simultaneously measuring the resistance encountered.

Design and Components:

Here’s a breakdown of the sampler’s design and key components:

  • Material: Made of high-strength steel to withstand impact and penetration through various soil types.
  • Dimensions: Usually 50.8mm (2 inches) in outer diameter and 34.9mm (1.375 inches) in inner diameter, with a standard length of 610mm (24 inches).
  • Split Barrel: The sampler consists of two halves that can be screwed together, allowing for easy opening and retrieval of the soil sample.
  • Cutting Shoe: A hardened steel tip attached to the bottom of the sampler to facilitate penetration and minimize soil disturbance.
  • Check Valve: Located near the top of the sampler to prevent soil loss during retrieval.
  • Coupling: Connects the sampler to the drill rods for driving.

Standard Penetration Test Sampler


Depending on the specific soil conditions and project requirements, different types of SPT samplers might be used:

  • Heavy-duty sampler: For tougher soils or deep investigations.
  • Thin-walled sampler: For recovering less disturbed samples in sensitive soils.
  • Continuous sampler: Designed for collecting longer, continuous soil profiles.

Importance of SPT Sampler:

The design and quality of the SPT sampler directly impact the quality and reliability of the test results. A properly functioning sampler ensures:

  • Adequate penetration: Reaching the desired depth and obtaining a representative soil sample.
  • Minimal disturbance: Preserving the soil’s characteristics for accurate interpretation.
  • Accurate N-value: Reflecting the true resistance of the soil to penetration.

standard penetration test advantages and disadvantages

The SPT is a popular in-situ test for assessing soil properties, but it has both advantages and limitations to consider:


  • Cost-effective: SPT is relatively inexpensive compared to other geotechnical tests, making it suitable for various projects.
  • Quick and simple: The test can be performed quickly and requires minimal equipment, making it efficient for site investigations.
  • Widely applicable: SPT works in various soil types, including sands, silts, and clays, offering flexibility for diverse projects.
  • Standardized procedure: Following established procedures ensures consistency and allows for data comparison across different projects.
  • Direct in-situ information: Unlike lab tests, SPT provides direct information about the soil’s behavior in its natural state.
  • N-value provides valuable data: The N-value, a key output of the test, offers insights into soil density, bearing capacity, and other properties.
  • Established correlations: Many established correlations exist between N-value and other soil properties, aiding in comprehensive analysis.


  • Disturbed samples: The split-spoon sampler retrieves disturbed soil samples, limiting the analysis of certain properties like undrained shear strength.
  • Limited to granular soils: SPT results are less reliable in cohesive soils like clays.
  • Depth limitations: While depths of 50-60 meters are attainable, deeper investigations might require alternative methods.
  • Operator influence: Variations in driving technique can affect the N-value, demanding skilled and consistent operators.
  • Environmental limitations: Water table presence or hard layers can complicate penetration and data interpretation.
  • Requires interpretation: N-value needs to be interpreted with expertise and alongside other data for accurate conclusions.


The SPT is a valuable tool for geotechnical investigations due to its simplicity, affordability, and wide applicability. However, acknowledging its limitations and interpreting results within context is crucial to ensure its effectiveness. Combining SPT with other tests and expert judgment provides a more comprehensive understanding of subsurface conditions.

Is code used for standard penetration test?

Yes, there are several codes and standards used for the standard penetration test (SPT), depending on your location and specific requirements. Here are a few of the most common:


  • ISO 22476-3: This international standard specifies the general requirements for the performance of the SPT and the determination of the N-value.


  • IS 2131:1981 (India): This Indian Standard code of practice describes the procedure for performing the SPT and for interpreting the results.
  • ASTM D1586/D1586M-18 (USA): This American standard test method covers the standard penetration test and split-barrel sampling of soils.
  • BS 1377-9:1998 (UK): This British Standard method includes requirements for the site investigation and describes the procedure for the SPT, including equipment, materials, and reporting.

It’s important to note that these are just a few examples, and the specific code or standard that applies to your project will depend on several factors, such as:

  • Location: Different countries and regions may have their own established codes and standards.
  • Project requirements: Certain projects might have specific regulations or guidelines that dictate the testing procedure.
  • Soil conditions: Depending on the specific soil types encountered, different variations of the SPT procedure might be used.

What is the standard penetration test is 2131?

IS 2131:1981 is an Indian Standard code of practice that specifies the procedure for performing the Standard Penetration Test (SPT) and interpreting the results. This code is widely used in India for geotechnical investigations.

What is the standard penetration test SPT?

The term “standard penetration test” (SPT) itself is the standard for penetration testing in geotechnical engineering. However, depending on your specific context or region, there are various codes and standards that govern its implementation and interpretation. Here’s a breakdown:


  • ISO 22476-3: This international standard sets the general framework for performing the SPT and determining the N-value (key indicator of soil resistance).


  • IS 2131:1981 (India): This Indian Standard establishes the procedure for conducting and interpreting the SPT in India.
  • ASTM D1586/D1586M-18 (USA): This American standard specifies the methodology for SPT and split-barrel sampling in the United States.
  • BS 1377-9:1998 (UK): This British Standard guides the site investigation process and includes SPT requirements like equipment, materials, and reporting.

Choosing the Right Standard:

The appropriate standard depends on several factors:

  • Location: Different countries or regions might have their own established codes.
  • Project requirements: Specific regulations or guidelines might dictate the testing procedure for your project.
  • Soil conditions: Variations in the SPT procedure might be necessary for different soil types.

Always consult a qualified geotechnical engineer or relevant authorities to determine the most suitable standard for your specific project. They can ensure the SPT is performed and interpreted correctly according to the applicable regulations and your project’s needs.

Standard penetration test as per is code pdf

Here are some options:

1. Purchase the standard:

  • The official source for purchasing Indian Standards is the Bureau of Indian Standards (BIS). You can buy it directly from their website: <invalid URL removed>
  • Look for authorized BIS sales agents in your area.

2. Access online libraries:

  • Many academic and technical libraries subscribe to online databases containing national and international standards. If you have access to such a library, you can search for the standard there.
  • Some libraries might offer remote access to their resources for registered users.

3. Search for free resources:

  • While rare, you might find freely downloadable PDFs of the standard online. However, be cautious of the source and ensure it’s a reputable and trustworthy website.

Important Reminders:

  • Using outdated versions of standards is not recommended. Make sure you access the latest revision of IS 2131:1981, which might have modifications compared to the original 1981 version.
  • Always use official or trusted sources for standards to ensure their accuracy and reliability.

standard penetration test number of blows

The number of blows in a standard penetration test (SPT) is crucial for assessing the relative density and bearing capacity of the soil. However, it’s not just the total number of blows that matters, but how those blows are distributed and interpreted.

Here’s a breakdown:

Number of blows recorded:

  • During the SPT, blows are delivered at a rate of 30 per minute.
  • The total number of blows required to drive the sampler the final 300 mm (12″) is recorded.
  • However, only the blows for the second and third 150 mm intervals (total 300 mm) are used to calculate the N-value.

Calculating the N-value:

  • The N-value is the sum of the blows required for the second and third 150 mm intervals.
  • For example, if 8 blows were required for the second interval and 12 for the third, the N-value would be 8 + 12 = 20.

Interpreting the N-value:

  • Higher N-values indicate denser and stronger soils with higher bearing capacity.
  • Lower N-values correspond to looser and weaker soils with lower bearing capacity.
  • However, the interpretation needs to be done carefully, considering:
    • Corrections: N-values may be adjusted for factors like overburden pressure, borehole diameter, and energy ratio.
    • Soil type: Correlations between N-value and soil properties like density and bearing capacity depend on the specific soil type.
    • Experience and judgment: Interpreting SPT results effectively requires engineering expertise and consideration of other factors influencing soil behavior.


  • The number of blows in an SPT is only one piece of information. Proper interpretation requires considering the entire test procedure, corrections, soil type, and geotechnical expertise.

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