Method for standard penetration test for soils
1981 Edition

Overview of IS 2131 Scope

• Objective: Establishes uniform procedures and specifications for soil sampling and testing, including the use of split spoon samplers.
• Usage: Applied for soil profile development and bearing capacity estimation.
• Rounding Rules: Test outcomes should be rounded according to IS 2-1960, maintaining the significant figures consistent with standard values.
• Deliverables: Soil data is presented as a profile illustrating the nature and extent of soil layers (Clause 4.2).
• Correction Factors: Included for bearing capacity computations as detailed in the code.
• Standardization: Provides exact specifications for split spoon sampler design.

Highlights:

Note:

For detailed formulas and correction factors related to bearing capacity, refer to the specific clauses in IS 2131.

flowchart LR
    A[Soil Sampling] --> B[Testing Phase]
    B --> C[Data Evaluation]
    C --> D[Soil Profile Compilation]
    D --> E[Bearing Capacity Assessment]
    E --> F[Application of Correction Factors]
    F --> G[Final Design Parameters]

Consult the full IS 2131 for precise formulas and tables regarding bearing capacity and sampler specifications.

Essential Points on Drilling Equipment per IS 2131

2.1 Shell and Casing Dimensions (Clause 3.2.2.1)

• Maximum outer diameter of shell: Must be less than or equal to the inner diameter of the casing minus 25 mm.
• Clearance between the casing base and borehole bottom:
  • Not exceeding 150 mm when stabilized with water.
  • Not exceeding 300 mm when bentonite slurry is employed.

2.2 Water Level Monitoring (Clause 3.5.1)

• Document water table levels, water losses, and zones of excess pressure.
• Timing for water level measurements after casing removal:
  • Sands: immediately and after 30 minutes.
  • Silts: after 24 hours.
  • Clays: after 24 hours unless permeable layers exist.
• When using drilling mud, employ perforated casing and bail down. Measure water levels at 30 minutes and 24 hours until mud clears.

2.3 Drill Rod Rigidity (Clause 41.3)

• Utilize rods with stiffness equivalent to an A-rod (41.3 mm outer diameter) to minimize whipping.
• For boreholes deeper than 10 meters:
  • Install centering spacers at intervals of 10 meters or less.
  • Employ stiffer rods to maintain vertical alignment and dependable N-values.

Summary Table: Shell and Casing Parameters

flowchart TD
    A[Begin Drilling] --> B[Stabilize Borehole with Casing]
    B --> C{Stabilizing Medium?}
    C -->|Water| D[Shell OD ≤ Casing ID - 25 mm]
    C -->|Bentonite| D
    D --> E[Maintain Clearance Between Casing End and Borehole Bottom]
    E -->|Water| F[≤ 150 mm]
    E -->|Bentonite| G[≤ 300 mm]
    F & G --> H[Measure Water Levels per Soil Type]
    H --> I[Use Stiff Drill Rods & Spacers for Depths > 10 m]

These guidelines ensure borehole integrity, precise water level measurements, and consistent SPT data.

Key Procedures for Standard Penetration Test (SPT) as per IS 2131

• Purpose: Defines the uniform procedure for performing SPT to evaluate soil bearing capacity (Clause 1.1).

• Borehole Specifications:

  • Diameter ranging from 100 to 150 mm (Clause 2.1.1).
  • Must remain clean and undisturbed to allow proper sampler insertion.

• Sampling: Employ a standard split spoon sampler as per prescribed specifications.

• Test Execution:

  • Drive the sampler into soil using a 63.5 kg hammer dropped from a height of 75 cm.
  • Count and record the number of hammer blows for each 150 mm penetration.
  • The sum of blows for the last 300 mm penetration (excluding the initial 150 mm seating drive) is the N-value (SPT blow count).

• Rounding Off: Final numerical results should be rounded according to IS 2:1960 rules, matching the significant figures of reference values (Clause 0.4).

Example of SPT N-value Recording

flowchart TD
    A[Drill Borehole (100-150 mm diameter)] --> B[Insert Split Spoon Sampler]
    B --> C[Drive Sampler with Hammer]
    C --> D[Ignore Blows for First 150 mm]
    D --> E[Count Blows for Next 150 mm (N-value)]
    E --> F[Document N-value and Extract Sample]

Following this ensures consistent and dependable soil resistance data critical for foundation engineering.

Guidelines for Driving Casing as per IS 2131

• Clause 3.1:

  • The casing must not be driven beneath the intended test or sampling depth.
  • In cohesionless soils, advance casing by gradual rotation instead of driving, to prevent soil disturbance and changes in density.

• Clause 3.2.2.1:

  • When boring through sandy soils with casing:
    • Outer diameter of sampler shell should be ≤ (casing inner diameter - 25 mm).
    • Clearance between casing end and borehole bottom:
      • ≤ 150 mm if stabilized using water.
      • ≤ 300 mm if bentonite slurry is used.

• Clause 2.1.2:

  • Use casing or drilling mud when drilling in soils prone to collapse (sands, soft clays).
  • Prefer drilling mud below water table in cohesionless soils; use casing if mud alone is insufficient.

• Clause 3.5.1:

  • Record groundwater levels before and after casing installation.
  • Measure water levels after casing removal at specified intervals depending on soil type:
    • Sands: immediate and 30 minutes later.
    • Silts: after 24 hours.
    • Clays: after 24 hours with less accuracy.
  • For drilling mud, use perforated casing and bail down; monitor water levels at 30 minutes and 24 hours until mud clears.

Summary Table: Casing and Clearance Parameters

Conceptual Flowchart for Casing Advancement

flowchart TD
    A[Casing Positioned at Ground Level] --> B{Advance Casing}
    B -->|Rotate Slowly| C[Minimize Soil Disturbance]
    B -->|Do Not Drive Below Test Level| D[Stop at Sampling Depth]
    C --> E[Maintain Clearance ≤ 150 mm (water) or ≤ 300 mm (bentonite)]
    D --> F[Proceed with Sampling and Testing]

This practice minimizes soil disruption and ensures accurate in-situ test outcomes.

Borehole Preparation & Water Table Management under IS 2131

Important Specifications:

• Water Level in Borehole:

  • Above the water table: No water should be introduced to prevent soil disturbance.
  • Below the water table: Maintain water level at least 1.5 m above the natural water table.
  • Bentonite slurry can be used to stabilize the water level if needed.

• Casing and Shell Clearance (Clause 3.2.2.1):

  • Shell outer diameter must be ≤ casing inner diameter minus 25 mm.
  • Distance from casing bottom to borehole base:
    • ≤ 150 mm when only water is used for stabilization.
    • ≤ 300 mm if bentonite slurry is utilized.

• Water Table Measurement (Clause 3.5.1):

  • Sands: measure during casing removal and 30 minutes thereafter.
  • Silts: measure after 24 hours.
  • Clays: difficult to measure accurately unless permeable layers exist; record after 24 hours.
  • When drilling mud is used: employ perforated casing and bail down; measure at 30 minutes and 24 hours until mud is cleared.

Field Log Requirements (Clause 4.1):

• Record date, job identification, boring and sample numbers, sampler type, drilling method.
• Document sample elevation, recovery percentage, soil layer boundaries.
• Note water table details, soil classification, penetration data, casing particulars, and weather conditions.

Table Summarizing Casing and Shell Dimensions

flowchart TD
    A[Drilling Borehole] --> B{Is Borehole Above Water Table?}
    B -- Yes --> C[No Water Added]
    B -- No --> D[Maintain Water Level ≥ 1.5 m Above Water Table]
    D --> E{Use Bentonite Slurry?}
    E -- Yes --> F[Stabilize Water Level with Bentonite]
    E -- No --> G[Use Water Only for Stabilization]
    F & G --> H[Apply Casing with Proper Shell OD Clearance]

These measures ensure borehole stability and accurate groundwater level assessment.

Soil Sampling Procedures as per IS 2131

Key Requirements (Clause 3.3 & 3.4.1):

• Sampler Retrieval: Carefully bring the sampler to the surface and open it.
• Handling Samples: Transfer representative soil samples from the split spoon into jars without compacting or ramming.
• Sample Containers:
  • Must have airtight sealing lids or be sealed with wax to avoid moisture loss.
  • Sized adequately to hold the sample without deformation.
• Sample Quantity: Trim samples to fill jars fully, minimizing exposure to air and moisture loss.
• Use of Liners:
  • When packing material is not available, liners can be placed inside the sampler.
  • Adjust the sampler’s internal diameter so that combined internal diameter with liner equals 35 mm.
  • Seal both ends of liners with wax to preserve the sample's natural moisture during transport.

Additional References:

• Follow IS 9640-1980 for split spoon sampler specifications.
• Round off test results in accordance with IS 2-1960 for uniformity.

Summary Table: Sample Preservation Standards

flowchart LR
    A[Sampler Retrieved] --> B[Sampler Opened]
    B --> C[Remove Sample Without Ramming]
    C --> D{Is Packing Available?}
    D -->|Yes| E[Place Sample in Jar]
    D -->|No| F[Use Liner of 35 mm Internal Diameter]
    F --> G[Seal Both Ends of Liner with Wax]
    E & G --> H[Seal Jar (Airtight or Wax)]
    H --> I[Transport Sample Maintaining Moisture]

This process preserves the sample’s natural state, ensuring reliable laboratory analysis.

Procedures for Sampler Removal and Labelling as per IS 2131 (Clause 3.4)

Essential Steps:

• Sampler Removal (3.4.1):

  • Retrieve the sampler carefully to the surface.
  • Open the sampler and transfer soil samples gently into jars without compacting.
  • Use jars with airtight lids or wax seals to prevent moisture evaporation.
  • Ensure jar dimensions avoid sample deformation.
  • If liners are used, maintain a total internal diameter of 35 mm including the liner.
  • Seal both liner ends with wax to preserve moisture.

• Labelling Requirements (3.4.2):

  • Attach labels on jars or lids indicating:
    • Origin of the sample
    • Project or job reference
    • Borehole identification number
    • Sample sequence number
    • Depth at which sample was taken
    • Penetration resistance data
    • Length of soil recovered
    • Date of sampling

• Storage and Shipment (3.4.3):

  • Store jars in appropriate containers.
  • Prevent exposure to sunlight during transit.

Summary Table: Sample Label Details

flowchart TD
    A[Sampler Retrieved] --> B[Opened Carefully]
    B --> C[Transfer Soil to Jar Without Ramming]
    C --> D{Use Jar or Liner?}
    D -->|Jar| E[Seal Jar (Airtight or Wax)]
    D -->|Liner| F[Use 35 mm Internal Diameter Liner]
    F --> G[Seal Both Ends of Liner with Wax]
    E & G --> H[Apply Labels with Required Information]
    H --> I[Store Jars in Suitable Containers, Avoid Sunlight]

This ensures both sample integrity and traceability throughout handling and testing.

Key Field Observation Points in IS 2131

3.5.1 Data to be Recorded (Clause 4.1)

During boring and sampling, document:

• Date of operation, job identification, borehole and sample numbers.
• Reference benchmark and sample elevation.
• Type of sampler used and drilling method.
• Sample recovery percentage and boundaries of soil strata.
• Groundwater table information (see Clause 3.5).
• Soil classification and condition of samples.
• Penetration resistance data (N-values).
• Details of casing employed and prevailing weather conditions.

3.5.2 N-Value Correction for Cohesionless Soil (Clause 3.6.1)

• Adjust the observed SPT N-value for the effective vertical overburden pressure.
• Use the correction curve (Fig. 1 in IS 2131) to derive the corrected N' from the raw N.

3.5.3 Effective Vertical Overburden Pressure Calculation:

[ \sigma' = \sigma - u ] Where:

• (\sigma) is total vertical stress (overburden pressure).
• (u) is pore water pressure due to the water table.

3.5.4 Correction Formula Concept:

[ N' = \text{Correction Factor} \times N ] The correction factor depends on (\sigma') and is obtained from the standard graph.

Summary Table: Minimum Field Data to Capture

flowchart TD
    A[Start Boring] --> B[Record Date and Job Details]
    B --> C[Drill and Collect Samples]
    C --> D[Document All Field Observations]

Thorough recording ensures data reliability and proper interpretation.

Correction for Overburden Pressure on SPT N-Values (IS 2131)

Principal Formula (Clause 3.6.1)

• The raw SPT N-value (N) requires adjustment to a corrected value (N') accounting for the effective vertical overburden stress.
• The correction factor is a function of the effective vertical stress, (\sigma'_v) (kgf/cm²).

Correction Steps:

• Refer to Fig. 1 in IS 2131, which provides a curve relating the effective vertical stress to the correction factor.
• Calculate corrected N-value:

[ N' = N \times C_o ]

where (C_o) is the correction factor obtained from the figure based on (\sigma'_v).

Typical Correction Factor Overview:

Additional Notes:

• Correction applies exclusively to cohesionless soil types.
• Effective overburden pressure is computed as:

[ \sigma'_v = \gamma \times H - u ]

where:

• (\gamma) is soil unit weight (kgf/cm³),
• (H) is depth (cm),
• (u) is pore water pressure (kgf/cm²).

Process Flow Diagram:

graph LR
A[Raw N-Value] --> B[Calculate Effective Overburden Pressure \(\sigma'_v\)]
B --> C[Determine Correction Factor \(C_o\) from Fig. 1]
C --> D[Compute Corrected N-Value: N' = N × C_o]

Reference: For exact correction curves, see IS 2131 Clause 3.6.1 and corresponding Fig. 1.

IS 2131: Guidelines for Calculating and Interpreting Test Data

Key Points:

• Rounding (Clause 0.4): Final test or analysis outcomes must be rounded following IS 2-1960 guidelines, preserving the significant figures of the specified values.
• Data Recording (Clause 4.1): Essential details to document during boring include:
  • Date, job ID, boring and sample numbers
  • Sampler type, drilling technique
  • Sample elevation, recovery ratio
  • Soil strata boundaries, groundwater information
  • Soil classification, penetration data, casing details, weather conditions
• Data Presentation (Clause 4.2): Generate a soil profile illustrating the characteristics and depth of soil layers.

Important Computations:

• Effective Vertical Overburden Pressure ((\sigma'_{v})): [ \sigma'_{v} = \gamma \times H - u ] Where:

  • (\gamma): unit weight of soil (kgf/cm³)
  • (H): depth (cm)
  • (u): pore water pressure (kgf/cm²)

• N-Value Correction for Cohesionless Soil: Apply correction charts or factors from Fig. 1 in IS 2131 to adjust raw N-values for overburden pressure effects.

Summary Table: Field Data to Record

Guidelines for Soil Sample Storage and Transportation per IS 2131

Sample Storage (Clauses 3.4.1 & 3.4.3)

• Sample containers must:
  • Have airtight lids or be sealed with wax to prevent moisture evaporation.
  • Be of appropriate size to avoid sample deformation; fill jars gently without compacting.
• For liners inside the sampler:
  • Adjust sampler internal diameter so that total internal diameter including liner equals 35 mm.
  • Seal both ends of liners with wax to maintain natural moisture content.
• Store jars within suitable containers for shipment.
• Protect samples from exposure to sunlight during storage and transit.

Labelling Standards (Clause 3.4.2)

Each sample container should be labelled with:

• Sample origin location
• Job or project identification
• Borehole number
• Sample sequence number
• Sampling depth
• Penetration resistance data
• Length of recovered soil
• Date of sampling

Summary Table: Sample Jar Specifications

flowchart LR
  A[Sample Collection] --> B[Fill Jars Gently]
  B --> C[Seal Jars (Airtight or Wax)]
  C --> D[Label Jars with Required Information]
  D --> E[Store in Appropriate Containers]
  E --> F[Transport Shielded from Sunlight]

This protocol preserves sample moisture and integrity, avoiding contamination during handling and shipping.

Rounding Guidelines for Numerical Values as per IS 2131 Referencing IS 2-1960:

• Final test or analytical numbers must be rounded following IS 2-1960 standards.
• The number of significant digits in the rounded result must correspond with those in the specified reference values in IS 2131.
• This practice guarantees uniformity and comparability of results.

Rounding Rules from IS 2-1960:

Example:

• If the specified value is 12.3 (3 significant figures), a result of 12.345 is rounded to 12.3.
• If the specified value is 0.012 (2 significant figures), a result of 0.01267 is rounded to 0.013.

Additional Correction Formula (Clause 3.6.2):

For cases where N' > 15 in fine sand or silt below the water table:

[ N'' = 15 + (N' - 15) \times \text{correction factor} ]

(The specific correction factor depends on soil conditions; refer to IS 2131 for precise values.)

flowchart LR
    A[Observed or Calculated Value] --> B{Check Significant Figures Against IS 2131}
    B -->|Match| C[Apply IS 2-1960 Rounding Rules]
    C --> D[Final Rounded Value]

Summary: Always round numerical results in accordance with IS 2-1960, ensuring consistency with IS 2131 specified values for standard compliance.