Standard Penetration Test (SPT) for Drillers

British Drilling Association
30 Mar 202315:44

TLDRThe Standard Penetration Test (SPT) is a critical method for assessing soil characteristics, essential in structural design. The test involves dropping a 63.5 kg weight from a height of 760 mm onto an anvil to drive a drill string into the borehole. Proper equipment, calibration, and maintenance are crucial for accurate results. The SPT requires specific drill rods, hammer types, and adherence to safety protocols. Results are used to define ground material properties and must be meticulously recorded, including depth, water level, and any encountered refusals during the test. Regular checks and calibration of the equipment ensure the reliability and accuracy of the SPT.

Takeaways

  • 🔨 The Standard Penetration Test (SPT) is a method used for sampling sands, developed in the early 1900s and standardized in the late 1940s.
  • 📈 The test is governed by a dedicated standard, BS EN ISO 22476-3, which dictates procedures for equipment, calibration, maintenance, and reporting.
  • 🏗️ Equipment for SPT includes a 63.5 kg weight dropped over a 760 mm height onto an anvil, with the entire assembly not exceeding 115 kg.
  • 🔧 Commonly used equipment in the UK includes the automatic triphammer (standard drop hammer) and the hydraulic hammer, used with various drilling machines.
  • 🛠️ Correct size of drill rods is crucial, with AB-sized rods of around 50 mm diameter being ideal, and rods must not exceed 10 kg per meter in weight.
  • 🔍 Regular inspection and maintenance of SPT equipment is essential for safety and accuracy, with daily checks for hydraulic hammers and inclusion in equipment inspection schemes for automatic hammers.
  • 📏 The straightness and verticality of the test string affect the results, and measures like centralizers or top clamps help maintain alignment.
  • 📝 Detailed recording of test parameters such as depth, blows per increment, and the N value is required, along with the condition of the SPT shoe or cone.
  • 🔄 The rods should be checked for straightness every 20 tests, and any bent rods must not be used.
  • 🔋 For design purposes, the energy ratio of the hammer must be determined and recorded, with recalibration required at least every 12 months.

Q & A

  • What is the Standard Penetration Test (SPT)?

    -The Standard Penetration Test (SPT) is a method developed in the early 1900s for sampling sands, and later evolved into a test used for all materials to define the ground's material characteristics, aiding in the design of structures and foundations.

  • What is the dedicated standard for carrying out the SPT?

    -The dedicated standard for carrying out the SPT is BS EN ISO 22476-3.

  • What are the two common types of equipment used for SPT in the UK?

    -The two common types of equipment used for SPT in the UK are the automatic triphammer, also known as a standard drop hammer, and the hydraulic hammer.

  • What is the weight and height specification for the SPT anvil and weight drop?

    -The SPT requires a 63.5 kilogram weight to be dropped over a 760 millimeter height onto an anvil.

  • What is the ideal size of the drill rod for SPT?

    -The ideal drill rod for SPT is an 'A' or 'B' sized rod of around 50 millimeters diameter, with a weight not exceeding 10 kilograms per meter.

  • How often should the straightness of the rods be checked?

    -The straightness of the rods should be checked at least every 20 tests.

  • What is the purpose of using a centralizer at the top of the casing during SPT?

    -A centralizer at the top of the casing helps to keep the rod centrally aligned within the borehole, preventing side to side movement or wobbling of the drill string for more consistent and reliable results.

  • What should be recorded for each SPT increment?

    -For each SPT increment, the number of blows, the depth of penetration, and whether the increment was achieved or met refusal should be recorded.

  • What happens if the hammer bounces on impact during SPT?

    -If the hammer bounces on impact, the test should be terminated immediately, the depth penetrated should be measured, and the refusal noted as bouncing, with no N value recorded.

  • How often should the energy ratio of the SPT hammer be determined and an energy ratio report completed?

    -The energy ratio of the SPT hammer should be determined and an energy ratio report, often called an SPT calibration report, must be completed as a minimum every 12 months.

  • What should be done with the disturbed sample obtained from using a split spoon during SPT?

    -The disturbed sample obtained from using a split spoon should be placed into a tub or jar and correctly labeled, with the test depth and other relevant information recorded.

Outlines

00:00

🏗️ Introduction to Standard Penetration Testing (SPT)

This paragraph introduces the Standard Penetration Test (SPT), a method developed in the early 1900s for sampling sands. It explains the evolution of SPT into a widely used test with a dedicated standard (BS EN ISO 22476-3). The video aims to examine the standard's requirements regarding equipment, calibration, maintenance, pre-test checks, test procedure, and reporting. It begins by discussing the equipment used for SPT, including the 63.5 kg weight, anvil, and various drilling machines. The importance of using the correct size and weight of drill rods is emphasized, as well as the safety and maintenance of the SPT hammer.

05:03

🛠️ Equipment Maintenance and Test Execution

This paragraph delves into the maintenance and inspection of SPT equipment, highlighting the importance of using well-maintained equipment to ensure accurate results. It discusses the consequences of poorly maintained equipment, such as a tripping mechanism becoming dry, worn, or damaged. The paragraph also covers the proper use of centralizers to maintain the rod's alignment within the borehole and the necessity of checking rod straightness. Detailed procedures for conducting and recording the test, including measuring depths and marking rods, are provided. The paragraph emphasizes the need to record all relevant information accurately for each test.

10:04

🔍 Conducting the Standard Penetration Test

This section outlines the step-by-step process of conducting the SPT, starting with the seating drive and followed by the main or test drive. It explains how to record the number of blows for each 75 mm increment and how to handle refusal situations during the test. The paragraph also addresses the possibility of the hammer bouncing on impact in strong materials and the need to terminate the test in such cases. The importance of using appropriate SPT shoes or cones depending on the material encountered is highlighted, along with the need to replace any worn or damaged equipment. The paragraph concludes with instructions on how to safely remove the test string from the borehole after the test.

15:05

📋 Recording and Reporting of Test Results

The final paragraph focuses on the recording and reporting of SPT results. It emphasizes the need to document all relevant information, such as test depth, self-weight penetration, number of blows per increment, and the N value. The paragraph also discusses the use of split spoons for obtaining disturbed samples and the importance of labeling and recording these samples. It mentions the necessity of using SBT equipment for design purposes and the requirement for an energy ratio report for the hammer. The paragraph concludes by encouraging viewers to seek further information or guidance and wishing them success in conducting SPTs.

Mindmap

Keywords

💡Standard Penetration Test (SPT)

The Standard Penetration Test (SPT) is a geotechnical engineering method used to obtain information about soil properties through a dynamic penetration process. It involves driving a split spoon sampler or a cone into the ground using a standardized hammer drop. The resistance encountered during the penetration is measured by the number of blows required to advance the sampler a specific distance. This test is crucial for understanding the ground's material characteristics which are then considered in the design of structures and foundations. In the video, the SPT is described as having a dedicated standard, BS EN ISO 22476-3, which outlines the procedures for carrying out the test.

💡Anvil

An anvil is a heavy block of metal or other hard material on which another object is struck for various purposes, such as shaping or cutting. In the context of the Standard Penetration Test, the anvil is the device that is struck by the falling weight of the SPT assembly, transferring the energy to drive the drill string into the borehole. The anvil is a critical component as it directly influences the effectiveness of the energy transfer and the accuracy of the test results. The script specifies that the anvil must be fitted with a manufacturer's approved role or cell lock pin with rubber dampeners in place, ensuring a secure and efficient connection during the test.

💡Drill String

A drill string is a column of drill pipes or rods that are connected end-to-end and lowered into a borehole during drilling operations. In the Standard Penetration Test, the drill string is used to support the SPT equipment, including the anvil and the sampler, and to transmit the energy from the hammer to the ground. The drill rods must not exceed 10 kilograms per meter in weight and are ideally of a specific size, such as the AB-sized rod of around 50 millimeters diameter, to comply with the test standards. The straightness of the drill string is crucial for the accuracy of the SPT, as it ensures the proper alignment and penetration of the sampler into the soil.

💡Hydraulic Hammer

A hydraulic hammer is a type of impact tool that uses hydraulic pressure to generate repetitive blows. In the context of the Standard Penetration Test, a hydraulic hammer is one of the two common types of equipment used in the UK for performing the test. It is more commonly found on rotary and dynamic sampling machines and operates by using a chain-driven mechanism to lift and drop the weight. The hydraulic hammer's efficiency and maintenance are essential for the reliability and safety of the SPT, as poorly maintained equipment can compromise the test results and pose risks to the operators.

💡Split Spoon

A split spoon is a type of sampler used in geotechnical drilling, particularly for retrieving disturbed samples of soil. In the SPT, when silt or clay layers are encountered, a split spoon with cutting shoes is used to collect samples. The spoon is driven into the ground by the SPT hammer and then extracted, containing a sample of the soil. This sample can provide valuable information about the soil's properties, such as its consistency and strength. The video script mentions that when a split spoon is used, the driller should label the sample correctly and record it as a separate disturbed sample over the depth range of the test.

💡Casing

Casing refers to the tubular lining installed in a borehole during drilling operations to prevent the hole from collapsing and to provide structural support. In the context of the Standard Penetration Test, the casing serves to stabilize the borehole and maintain its integrity during the testing process. The depth of the casing is an important measurement recorded during the SPT, as it relates to the actual depth of the borehole and the positioning of the test equipment. The casing also plays a role in ensuring that the test string remains centrally aligned within the borehole, which is crucial for achieving consistent and reliable test results.

💡Borehole

A borehole is a narrow shaft drilled into the ground, typically for the purpose of extracting water, oil, or gas, or for geotechnical investigations. In the Standard Penetration Test, the borehole is the hole into which the SPT equipment is inserted and where the actual penetration testing takes place. The condition of the borehole, such as its cleanliness and depth, is critical for the accuracy of the SPT. The borehole must be prepared to the specified depth and kept free of obstructions to ensure that the test can be conducted properly and that the results accurately reflect the soil conditions.

💡Calibration

Calibration refers to the process of checking and adjusting a device or instrument to ensure it provides accurate and consistent readings or measurements. In the context of the Standard Penetration Test, calibration is essential for ensuring that the SPT hammer delivers the correct energy per blow and that the test results are reliable. The video script mentions that SPT equipment used for obtaining N values for design purposes must have the energy ratio of the hammer determined, and a calibration report must be completed at least every 12 months. This ensures that the hammer is operating within the specified tolerances and that the test results can be trusted for engineering design purposes.

💡Dynamic Sampling Machine

A dynamic sampling machine is a type of drilling equipment used in geotechnical investigations to collect soil samples dynamically, often through the use of a hammering action. In the context of the Standard Penetration Test, a dynamic sampling machine can be utilized to perform the test, particularly when a hydraulic hammer is used. These machines are equipped with features that allow for the efficient and accurate execution of the SPT, such as mechanisms for lifting and dropping the hammer weight, and systems for measuring and recording the penetration resistance. The script highlights the importance of using the correct type of equipment for the SPT to ensure the validity of the results obtained.

💡N Value

The N value, also known as the penetration resistance, is a numerical result obtained from the Standard Penetration Test. It represents the average number of blows required to advance the sampler 300 millimeters into the soil, and is used as an indicator of soil strength and compressibility. The N value is calculated by adding up the number of blows for each 75-millimeter increment of the test drive and is a critical piece of data used in the design of foundations and other geotechnical structures. The video script emphasizes the importance of recording the N value accurately and the necessity of using properly calibrated equipment to ensure reliable N values.

💡Maintenance

Maintenance refers to the activities performed to keep equipment in good working order, ensuring safety, efficiency, and longevity. In the context of the Standard Penetration Test, regular maintenance of the SPT hammer and other equipment is crucial. The script outlines that hydraulic hammers should be inspected as part of the daily machine pre-use check, and that any worn or damaged components, such as the tripping mechanism, should be replaced or refurbished immediately. Proper maintenance helps to prevent accidents, ensures that the SPT is carried out safely and correctly, and maintains the accuracy and reliability of the test results.

Highlights

Standard Penetration Test (SPT) is a method developed in the early 1900s for sampling sands, and later evolved into a test used for all materials.

The dedicated standard for SPT is BS EN ISO 22476-3.

SPT requires a 63.5 kg weight dropped over a 760 mm height onto an anvil, with the entire assembly not exceeding 115 kg.

Two common types of equipment used in the UK for SPT are the automatic triphammer and the hydraulic hammer.

Correct size of drill rod must be used for SPT, with the ideal being a B-sized rod of around 50 mm diameter.

SPT hammers should be well-maintained, checked for safety, and inspected daily.

Hydraulic hammers should have a manufacturer's approved role or cell lock pin with rubber dampeners.

The test results can be affected by variables such as equipment maintenance, driller skill, and the straightness of the test string.

A centralizer or closing the top clamps can help keep the rod centrally aligned within the borehole for consistent results.

Rod straightness should be checked at least every 20 tests and documented in the driller's records.

The test string should be lowered to the base of the borehole with at least 450 mm above the datum point.

Mark the rods with 75 mm increments for the seating drive and main/test drive.

Record the number of blows for each 75 mm increment and calculate the N value if the main drive is completed.

If the hammer bounces on impact, the test should be terminated immediately and the refusal noted as bouncing.

After the test, the hammer should be safely removed and the test string can be removed from the borehole.

SPT split spoons with cutting shoes should be used in silts and clays, while a 60-degree cone is used in gravels or rocky materials.

The driller's log must record all relevant information including test depth, self-weight penetration, number of blows, casing depth, and water level.

SPT equipment used for design purposes must have the energy ratio of the hammer determined and an energy ratio report completed every 12 months.

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