Standard Penetration Test (SPT) Procedure and Applications , limitations, advantage and disadvantage, methods in Soil Exploration

 Standard Penetration Test (SPT) Procedure and Applications in Soil Exploration.

             Standard penetration test 

The Standard Penetration Test (SPT) is a widely used in-situ test for determining the geotechnical properties of soil. It is standardized under ASTM D1586 and IS 2131 in various regions. SPT provides data that is critical for evaluating soil strength, density, and consistency, and is commonly employed in geotechnical site investigations. Below is a detailed, technical explanation of the procedure and its applications.

SPT Procedure

1. Test Equipment

• Drilling Rig: A rotary or percussion drill is used to advance the borehole to the desired depth.
• Split-Barrel Sampler: A thick-walled steel tube, typically 50.8 mm (2 inches) in outer diameter, 35 mm (1.375 inches) in inner diameter, and 600 mm (24 inches) in length.
• Drive Rods: Hollow rods used to transmit the impact force to the sampler.
• Hammer Assembly: A 63.5 kg (140 lb) hammer dropped from a fixed height of 760 mm (30 inches) to drive the sampler.
• Borehole Casing: To stabilize the borehole and prevent collapse in loose soils.

2. Drilling the Borehole

• A borehole is drilled to the required test depth using auger drilling, rotary drilling, or percussion drilling techniques. If necessary, the borehole is stabilized with casing or drilling mud.

3. Setting the Split-Barrel Sampler

• The sampler is attached to the drive rods and lowered to the bottom of the borehole. Care is taken to ensure the sampler is properly seated.

4. Conducting the Penetration Test

• The hammer is raised to a height of 760 mm (30 inches) and allowed to fall freely, delivering energy to drive the sampler into the soil.
• The penetration depth is measured after every 150 mm (6 inches) of sampler advancement. The test is divided into three increments:
  • First Increment: Penetration of the first 150 mm (used to ensure seating of the sampler; results are usually discarded).
  • Second Increment: Penetration of the second 150 mm (counted as part of the N-value).
  • Third Increment: Penetration of the final 150 mm (also counted as part of the N-value).
• The number of blows required to penetrate the second and third 150 mm increments is recorded. This sum is called the Standard Penetration Resistance (N-value)

5. Retrieving the Sampler

• After the test, the sampler is extracted, and the soil sample inside is recovered for classification and laboratory testing.

6. Depth Interval and Frequency

• SPT is generally performed at regular depth intervals (e.g., every 1.5 m or at changes in soil strata) to profile the subsurface conditions.

7. Energy Efficiency Corrections

• The actual energy transferred to the sampler varies due to equipment and operational differences. The N-value is corrected for hammer energy efficiency using the formula:

  N' = N * (Em / Er)

  Where:

  • N' = Corrected N-value
  • N = Measured N-value
  • Em = Measured energy transferred to the rod
  • Er = Reference energy (typically 60% efficiency)

Applications of SPT in Soil Exploration.

1. Soil Classification

• Soil samples recovered from the split-barrel sampler are classified based on grain size distribution, plasticity, and other properties.
• The N-value is correlated with the consistency of cohesive soils (e.g., very soft, soft, firm, stiff) and the relative density of granular soils (e.g., loose, medium-dense, dense).

2. Determination of Soil Strength

• Empirical correlations between N-values and soil parameters (e.g., undrained shear strength, angle of internal friction) are used to estimate strength properties.

3. Estimation of Settlement

• SPT results are used to calculate immediate and long-term settlements of foundations.

4. Bearing Capacity of Foundations

• N-values are used to derive allowable bearing pressures for shallow and deep foundations.

5. Liquefaction Potential Assessment

• SPT data, combined with seismic and geotechnical analysis, helps evaluate soil susceptibility to liquefaction during earthquakes.

6. Subsurface Profiling

• Continuous N-value profiles provide insights into soil stratigraphy and changes in material properties with depth.

7. Suitability for Pile Foundations

• SPT results are employed to estimate pile capacity through empirical correlations and to assess driving resistance during pile installation.

. Limitations of the SPT.

• Disturbance of Samples: The test does not provide undisturbed samples, limiting its use for detailed laboratory testing.
• Energy Losses: Variability in hammer efficiency and rod length can introduce errors in N-values.
• Soil Type Dependence: SPT is less effective in very soft clays and silts, gravelly soils, and dense granular materials.

Despite its limitations, the Standard Penetration Test remains a cornerstone of geotechnical site investigations due to its simplicity, cost-effectiveness, and extensive empirical correlations.

The Standard Penetration Test (SPT) is a widely used in-situ test in geotechnical engineering for determining soil properties. It involves driving a split-barrel sampler into the soil using a drop hammer and recording the number of blows required to achieve a standard penetration depth. Below are the methods and variations for performing the SPT:

What are the various methods to perform spt test.

1. Standard Method

This is the conventional approach to performing the SPT:

• Procedure:
  1. A borehole is drilled to the desired depth.
  2. The split-barrel sampler (also known as the split-spoon sampler) is lowered to the test depth.
  3. A 63.5 kg (140 lbs) hammer is dropped from a height of 760 mm (30 inches) repeatedly to drive the sampler into the soil.
  4. The number of blows is recorded for every 150 mm (6 inches) of penetration, and the total blows for the final 300 mm (12 inches) are noted as the SPT-N value.
• Standardization: This method follows standards such as ASTM D1586 or IS 2131.

2. Automatic Hammer Mechanism

To improve repeatability and reduce human error:

• Description: An automatic trip-hammer mechanism is used instead of a manual hammer to ensure consistent energy delivery.
• Advantages:
  • Reduced variability due to operator errors.
  • More uniform energy transfer.

3. Energy Correction Method

Since energy losses occur in manual and mechanical setups, corrections are applied to account for variations:

• Description: The actual energy delivered to the sampler is measured and corrected to a standard energy efficiency (usually 60% of theoretical energy).
• Corrections:
• Adjustments for rod length, borehole diameter, or overburden pressure.
• Used to obtain the corrected (N₆₀) value.

4. Continuous SPT (Modified Approach)

To obtain more detailed soil profiles:

• Description: The sampler is driven continuously or at closer depth intervals (e.g., every 150 mm) rather than at fixed depths.
• Use: Provides a better understanding of soil stratification and variability.

5. Rotary Drilling with SPT

Used when encountering challenging soils like gravel or cobbles:

• Description: Rotary drilling methods are used to advance the borehole to the test depth. Drilling fluid (bentonite slurry) or casing may be used to stabilize the hole.
• Advantage: Prevents borehole collapse in loose or cohesionless soils.

6. SPT with Dynamic Cone Penetration (DCPT)

Sometimes combined with other penetration methods:

• Description: After the SPT, dynamic cone penetration tests are performed using a similar setup but with a cone tip instead of the split-spoon sampler.
• Use: Provides additional data for cross-comparison.

7. SPT with Borehole Stabilization Techniques

Used in unstable soils:

• Description: Stabilization techniques (e.g., using drilling mud or casing) are applied to prevent collapse during SPT.
• Application: Essential in loose sands, silts, or saturated soils.

8. Refusal Testing

Performed when very hard layers are encountered:

• Procedure: The SPT is terminated if the sampler cannot penetrate the soil due to excessive blow counts (e.g., 50 blows for 150 mm or less penetration).
• Use: Helps identify bedrock or dense gravel layers.

9. Auger-Assisted SPT

This method involves using an auger to pre-drill the borehole:

• Procedure: The auger drills to a depth close to the test level, and the SPT is then performed.
• Advantage: Reduces time and effort in soft soils.

10. Offshore SPT

Performed for marine or offshore projects:

• Description: SPT equipment is modified for underwater use, often using a seabed frame or platform.
• Challenges: Dealing with wave action, currents, and logistics.

Advantage and disadvantage of spt test.

Advantages of SPT:

1. Simplicity and Cost-Effectiveness:
   The test is straightforward to perform, requires relatively simple equipment, and is cost-efficient compared to other methods.

2. Widely Accepted:
   It is a standardized and globally recognized method, making it easier to compare results across projects.

3. Simultaneous Sampling:
   It provides disturbed soil samples, allowing for further laboratory analysis of soil type and properties.

4. Field Correlations:
   SPT data can be correlated with various soil properties, such as bearing capacity, density, and shear strength.

5. Applicability:
   It is suitable for various soil types, including sands, silts, and clays, and can be performed at different depths.

6. Rapid Data Collection:
   The test can provide real-time data during site investigations, aiding quick decision-making.

Disadvantages of SPT:

1. Disturbed Samples:
   The test provides only disturbed samples, which are not suitable for advanced laboratory tests requiring undisturbed samples.

2. Inaccuracy in Cohesive Soils:
   It may not provide accurate results in cohesive or very soft soils, where the resistance is low and penetration may not reflect actual properties.

3. Operator Dependence:
   The results can vary significantly depending on the skill and experience of the operator, leading to inconsistencies.

4. Energy Loss Variability:
   Variations in hammer efficiency and energy transfer to the sampler can cause discrepancies in the results.

5. Limited Information:
   It only provides a measure of resistance (N-value) without direct measurement of other soil properties like permeability or compressibility.

6. Depth Limitations:
   Performing the test at great depths can be challenging, and accuracy might decrease with increasing depth.

7. Insensitive to Soil Structure:
   It does not account for the layering or anisotropy in soils, which can affect the results.

Some important questions answer related to spt test.

1. What is the purpose of the Standard Penetration Test?

The SPT is performed to determine the relative density, strength, and other properties of soil at a specific depth. It is particularly used to evaluate the bearing capacity of soil and assess its suitability for foundations.

2. What is the basic principle of the SPT?

The SPT involves driving a split-spoon sampler into the ground using a standard weight (63.5 kg) dropped from a height of 760 mm. The number of blows required to drive the sampler 30 cm into the soil is recorded as the N-value, which indicates soil resistance.

3. What equipment is used in the SPT?

The main equipment includes:

• Split-spoon sampler
• Drive hammer (63.5 kg weight)
• Guide rod
• Drill rig for boring holes
• Measuring tape for depth readings

4. How is the SPT test conducted?

1. A borehole is drilled to the desired depth.
2. The split-spoon sampler is placed at the bottom of the borehole.
3. The hammer is dropped repeatedly to drive the sampler 450 mm into the soil.
4. The number of blows required for each 150 mm is recorded.
5. The total blows for the last 300 mm is taken as the N-value.

5. What is the significance of the N-value?

The N-value is used to estimate:

• Soil density (loose, medium, or dense)
• Bearing capacity of soil
• Liquefaction potential in seismic zones
• Foundation design parameters

6. What are the limitations of the SPT?

• It provides approximate values and is less precise compared to other tests.
• The results may vary due to operator errors, equipment conditions, or soil disturbances.
• It is not suitable for very soft soils or cohesive soils.

7. What corrections are applied to the raw N-value?

• Overburden pressure correction (CN): Accounts for soil pressure due to depth.
• Energy correction (CE): Adjusts for the efficiency of the hammer system.
• Dilution or rod length correction: Considers the effects of rod length on energy delivery.

The corrected N-value is called the N₆₀ or N₇₀ depending on the energy level.

8. What is the typical range of N-values for different soil types?

• Loose sand: 0–10
• Medium dense sand: 10–30
• Dense sand: 30–50
• Very dense sand: >50
• Soft clay: 0–4
• Stiff clay: 8–15

9. What are the advantages of the SPT?

• Simple and cost-effective.
• Provides in-situ soil resistance.
• Can be performed at various depths.
• Widely accepted and standardized globally.

10. What are the applications of the SPT?

• Determining the depth of suitable soil for foundations.
• Designing shallow and deep foundations.
• Assessing soil liquefaction during earthquakes.
• Estimating pile load capacity.