Method for standard penetration test for soils

IS 2131: Scope Summary

• Purpose: Standardizes procedures and specifications for soil sampling and testing, including split spoon samplers.
• Application: Used for soil profile reporting and bearing capacity calculations.
• Rounding Off: Test results must be rounded per IS 2-1960, keeping significant figures consistent with standard values.
• Output: Soil data is presented as a soil profile showing nature and extent of strata (Clause 4.2).
• Correction Factors: Included for bearing capacity calculations (details in the standard).
• Standardization: Includes detailed specs for split spoon samplers.

Key Points:

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]
    B --> C[Data Analysis]
    C --> D[Soil Profile Report]
    D --> E[Bearing Capacity Calculation]
    E --> F[Apply Correction Factors]
    F --> G[Final Design Input]

For precise formulas and tables, consult the full IS 2131 document sections on bearing capacity and sampler specifications.

IS 2131 Key Points on Drilling Equipment

1. Shell & Casing Dimensions (Clause 3.2.2.1)

• Outer diameter of shell ≤ (Inner diameter of casing − 25 mm)
• Distance between casing end and borehole bottom:
  • ≤ 150 mm if water is used for stabilization
  • ≤ 300 mm if bentonite is used

2. Water Level Measurement (Clause 3.5.1)

• Record water table, water loss, and excess pressure zones.
• Water level measurement timing after casing removal:
  • Sands: Immediately and after 30 min
  • Silts: After 24 hours
  • Clays: 24 hours (unless pervious seams exist)
• For drilling mud, use perforated casing and bail down, measuring water levels at 30 min and 24 h intervals until mud clears.

3. Drill Rod Stiffness (Clause 41.3)

• Use rods with stiffness equal to A-rod (41.3 mm OD) to reduce whipping.
• For depths >10 m:
  • Use centering spacers every 10 m or more frequently.
  • Use stiffer rods to maintain vertical alignment and reliable N-values.

Summary Table: Shell & Casing Dimensions

flowchart TD
    A[Start Drilling] --> B[Use casing for stabilization]
    B --> C{Material used?}
    C -->|Water| D[Shell OD ≤ Casing ID - 25 mm]
    C -->|Bentonite| D
    D --> E[Maintain distance casing end to bottom]
    E -->|Water| F[≤ 150 mm]
    E -->|Bentonite| G[≤ 300 mm]
    F & G --> H[Measure water levels as per soil type]
    H --> I[Use stiff rods & spacers for depths >10 m]

This ensures borehole stability, accurate water level measurement, and reliable SPT results.

IS 2131: Standard Penetration Test (SPT) - Key Test Procedure Points

• Purpose: Standardized procedure for SPT to assess soil bearing capacity (Clause 1.1).

• Borehole Requirements:

  • Diameter: 100 to 150 mm (Clause 2.1.1)
  • Must be clean and undisturbed for sampler insertion.

• Sampling: Use a standardized split spoon sampler (detailed specs in a separate document).

• Test Execution:

  • Drive sampler into soil by a 63.5 kg hammer falling 75 cm.
  • Record number of blows for each 150 mm penetration.
  • Total blows for 300 mm penetration (excluding initial 150 mm) is the N-value (SPT blow count).

• Rounding Off: Final test values must be rounded per IS 2:1960 rules, matching significant figures of specified values (Clause 0.4).

Typical SPT N-value Calculation

flowchart TD
    A[Borehole Drilling (100-150 mm)] --> B[Insert Split Spoon Sampler]
    B --> C[Drive Sampler by Hammer]
    C --> D[Count Blows for 150 mm (Ignore)]
    D --> E[Count Blows for Next 150 mm (N-value)]
    E --> F[Record N-value and Sample]

This procedure ensures repeatable and reliable soil resistance data for foundation design.

IS 2131: Key Points on Driving the Casing

• Clause 3.1:

  • Casing shall not be driven below the test/sample level.
  • In cohesionless soils, advance casing by slowly turning rather than driving to avoid soil disturbance and density alteration.

• Clause 3.2.2.1:

  • When boring through sand with casing:
    • Outer diameter of sampler shell ≤ (Inner diameter of casing - 25 mm).
    • Distance between casing end and borehole bottom:
      • ≤ 150 mm if stabilized with water only.
      • ≤ 300 mm if bentonite is used.

• Clause 2.1.2:

  • Use casing or drilling mud in soils prone to cave-in (sand, soft clay).
  • Prefer drilling mud below water table in non-cohesive soils; use casing if mud alone fails.

• Clause 3.5.1:

  • Record water table levels before/after casing.
  • Measure water levels after casing removal:
    • Sands: after 30 min.
    • Silts: after 24 h.
    • Clays: 24 h (less accurate).
  • For drilling mud, use perforated casing and bail down; measure water levels at 30 min and 24 h intervals.

Summary Table: Casing Driving & Clearance

Conceptual Diagram of Casing Advancement

flowchart TD
    A[Casing at Ground Level] --> B{Advance Casing}
    B -->|Slowly turn| C[Minimize soil disturbance]
    B -->|Do not drive below test level| D[Stop at test/sample level]
    C --> E[Maintain clearance ≤ 150 mm (water) or 300 mm (bentonite)]
    D --> F[Proceed with sampling/testing]

This ensures minimal soil disturbance and accurate in-situ test results per IS 2131.

IS 2131: Borehole Preparation & Water Table Management

Key Specifications:

• Water level in borehole:

  • Above water table: No water added to avoid soil disturbance.
  • Below water table: Maintain water level ≥ 1.5 m above water table.
  • Use bentonite slurry to stabilize water level if needed.

• Casing and shell clearance (Clause 3.2.2.1):

  • Shell outer diameter ≤ casing inner diameter - 25 mm.
  • Distance from casing end to borehole bottom:
    • ≤ 150 mm (water stabilization only)
    • ≤ 300 mm (if bentonite slurry used)

• Water table measurement (Clause 3.5.1):

  • Sands: measure during casing pull & after 30 min.
  • Silts: measure after 24 h.
  • Clays: no accurate measurement unless pervious seams; record 24 h level.
  • With drilling mud: use perforated casing, bail down, measure water level at 30 min & 24 h intervals until mud cleared.

Field Log Data (Clause 4.1):

• Date, job ID, boring/sample number, sampler type, drilling method
• Sample elevation, recovery ratio, soil strata limits
• Water table info, soil ID, penetration records, casing details, weather data

Summary Table: Casing & Shell Dimensions

flowchart TD
    A[Borehole Drilling] --> B{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?}
    E -- Yes --> F[Maintain water level with bentonite slurry]
    E -- No --> G[Maintain water level with water only]
    F & G --> H[Use casing with shell OD ≤ casing ID - 

Obtaining Soil Samples as per IS 2131

Key Specifications (Clause 3.3 & 3.4.1):

• Sampler Retrieval: Raise the sampler to the surface and open carefully.
• Sample Handling: Transfer typical soil samples from the split spoon to jars without ramming.
• Jars:
  • Must have self-sealing tops or be sealed with wax to prevent moisture loss.
  • Size should allow filling without deforming the sample.
• Sample Size: Cut samples to fill jars completely, minimizing air exposure and water loss.
• Use of Liners:
  • If packing is unavailable, use liners inside the sampler.
  • Adjust internal diameter of the sampler so that total internal diameter with liner = 35 mm.
  • Wax both ends of the liner to preserve natural moisture during transit.

Additional Notes:

• Follow IS 9640-1980 for split-spoon sampler specifications.
• Round off test results as per IS 2-1960 for consistency.

Summary Table: Sample Preservation Requirements

flowchart LR
    A[Sampler Raised] --> B[Sampler Opened]
    B --> C[Sample Removed Without Ramming]
    C --> D{Packing Available?}
    D -->|Yes| E[Put Sample in Jar]
    D -->|No| F[Use Liner (35 mm ID)]
    F --> G[Wax Seal Both Ends]
    E & G --> H[Seal Jar (Self-sealing or Wax)]
    H --> I[Transport Sample Preserving Moisture]

This ensures natural moisture retention and sample integrity for accurate soil testing.

IS 2131: Removal of Sampler and Labelling (Clause 3.4)

Key Specifications:

• Removal of Sampler (3.4.1):

  • Raise sampler to surface and open carefully.
  • Transfer soil sample from split spoon to jars without ramming.
  • Use jars with self-sealing tops or seal with wax to prevent moisture loss.
  • Jars must be sized to avoid deforming the sample.
  • If liners are used, ensure total internal diameter = 35 mm after lining.
  • Wax both ends of liner to preserve natural moisture.

• Labelling (3.4.2): Labels must be fixed on jars or covers with:

  • Origin of sample
  • Job designation
  • Boring number
  • Sample number
  • Depth of sampling
  • Penetration record
  • Length of recovery
  • Date of sampling

• Storage and Shipment (3.4.3):

  • Store jars in suitable containers.
  • Avoid exposure to sunlight during transit.

Summary Table: Sample Labelling Information

flowchart TD
    A[Sampler raised to surface] --> B[Open sampler carefully]
    B --> C[Transfer soil to jar without ramming]
    C --> D{Use jar or liner?}
    D -->|Jar| E[Seal jar (self-sealing/wax)]
    D -->|Liner| F[Use liner with 35mm internal diameter]
    F --> G[Wax both ends of liner]
    E & G --> H[Label jar with required info]
    H --> I[Store jars in container, avoid sun]

This ensures sample integrity and traceability per IS 2131 standards.

IS 2131: Field Observations Key Points

1. Field Data Recording (Clause 4.1)

Record the following during borings:

• Date, Job ID, Boring & Sample numbers
• Reference datum & Sample elevation
• Type of sampler & Drilling method
• Sample recovery ratio & Stratum limits
• Water table info (Clause 3.5)
• Soil identification & sample condition
• Penetration records (N-values)
• Casing used & Weather data

2. Correction of N-Value for Cohesionless Soils (Clause 3.6.1)

• Correct N-value (Standard Penetration Test blow count) for effective vertical overburden pressure.
• Use Fig. 1 (Correction curve) to obtain corrected N' from observed N.

3. Effective Vertical Overburden Pressure

[ \sigma' = \sigma - u ]

• (\sigma) = total vertical stress (overburden)
• (u) = pore water pressure (water table effect)

4. Correction Formula (Conceptual)

[ N' = \text{Correction factor} \times N ]

• Correction factor depends on (\sigma') (see Fig.1 in IS 2131).

Summary Table: Minimum Field Data to Record

flowchart TD
    A[Start Boring] --> B[Record Date & Job ID]
    B --> C[Drill & Collect Sample]

IS 2131 - Correction Factors for SPT N-value in Cohesionless Soil

Key Formula: Correction for Overburden Pressure (Clause 3.6.1)

• The raw SPT N-value (N) must be corrected to N' to account for the effective vertical overburden pressure.
• The correction factor depends on the effective vertical stress, σ'v (kgf/cm²).

Correction Procedure:

• Use Fig. 1 (from IS 2131) which provides a curve or chart relating effective vertical overburden pressure to the correction factor.
• The corrected N-value is:

[ N' = \frac{N \times C_o}{1} ]

where ( C_o ) is the correction factor from Fig. 1 based on σ'v.

Typical Correction Factor Behavior (Summary):

Additional Notes:

• Correction applies only to cohesionless soils.
• Overburden pressure is calculated as:

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

where:

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

Summary Diagram:

graph LR
A[Raw N-value] --> B[Determine Effective Overburden Pressure σ'v]
B --> C[Find Correction Factor C_o from Fig.1]
C --> D[Calculate Corrected N-value N' = N × C_o]

Reference: IS 2131 Clause 3.6.1 and Fig. 1 for precise correction curves.

IS 2131: Calculation and Interpretation of Results

Key Points from IS 2131:

• Rounding Off (Clause 0.4): Final test or analysis values must be rounded as per IS 2-1960, retaining the same significant figures as the specified values.
• Data Recording (Clause 4.1): Essential data to be recorded during boring includes:
  • Date, job ID, boring/sample number
  • Sampler type, drilling method
  • Sample elevation, recovery ratio
  • Soil strata limits, water table info
  • Soil identification, penetration records, casing used, weather data
• Data Presentation (Clause 4.2): Prepare a soil profile showing soil strata nature and extent.

Important Calculations:

• Effective Vertical Overburden Pressure (σ’v):
  [ \sigma’_v = \gamma \times H - u ] where
  (\gamma) = unit weight of soil (kgf/cm³),
  (H) = depth (cm),
  (u) = pore water pressure (kgf/cm²).

• Correction of N-value for Overburden in Cohesionless Soil:
  Use correction charts (Fig. 1 in IS 2131) or apply correction factor (C_o) to raw N-values to account for overburden pressure.

Summary Table: Data to be Recorded in Field

IS 2131: Storage and Transportation of Soil Samples - Key Points

Sample Storage (Clauses 3.4.1 & 3.4.3)

• Sample jars must:
  • Have self-sealing tops or be sealed with wax to prevent moisture loss.
  • Be sized to avoid deforming the sample; fill jars without ramming.
• If liners are used inside the sampling spoon:
  • Adjust spoon’s internal diameter so that total internal diameter with liner = 35 mm.
  • Seal liner ends with wax to maintain natural moisture.
• Store jars in suitable containers for shipment.
• Avoid exposure to sunlight during storage and transport.

Labelling Requirements (Clause 3.4.2)

Each jar must be labelled with:

• Origin of sample
• Job designation
• Boring number
• Sample number
• Depth of sampling
• Penetration record
• Length of recovery
• Date of sampling

Summary Table: Sample Jar Specifications

flowchart LR
  A[Sampling] --> B[Fill jars without ramming]
  B --> C[Seal jars (self-sealing or wax)]
  C --> D[Label jars with required info]
  D --> E[Store in suitable containers]
  E --> F[Transport without sunlight exposure]

This ensures sample integrity by preserving moisture and preventing contamination during transport.

Rounding Off Numerical Values as per IS 2131 (ref. IS 2-1960):

• Final test or analysis values must be rounded off following IS 2-1960 rules.
• The number of significant figures retained in the rounded value should match the significant figures of the specified value in IS 2131.
• This ensures consistency and comparability of results.

Key Points from IS 2-1960 (Rounding Rules):

Example:

• If specified value = 12.3 (3 significant figures), then result = 12.345 → rounded to 12.3.
• If specified value = 0.012 (2 significant figures), then result = 0.01267 → rounded to 0.013.

Related Correction Formula (Clause 3.6.2):

For N' > 15 in fine sand/silt below water table:

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

(Note: The exact correction factor is context-specific; refer to IS 2131 for detailed values.)

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

Summary: Always round test results per IS 2-1960, maintaining significant figures consistent with IS 2131 specified values for valid compliance.