Methods of test for soils, Part 11: Determination of the shear strength parameters of a specimen tested in unconsolidated undrained triaxial compression without the measurement of pore water pressure

IS 2720 Part 11: Scope Overview

• Purpose: Specifies methods for determining the liquid limit of soils.
• Applicability: Used for classification and engineering behavior assessment of fine-grained soils.
• Units: All quantities and dimensions are expressed in SI units.
• Rounding Off: Final test results must be rounded per IS 2:1960 rules, keeping the same significant figures as specified values.

Key Specifications:

Important Notes:

• The standard includes additional items for specific procedures (Clause 3.3).
• Metal scale use is referenced (Clause 3.2.5) for measurement purposes.
• First published in 1971, revised to incorporate amendments and SI units.

For detailed test procedures and formulae (e.g., water content calculation), refer to the full IS 2720 Part 11 document.

IS 2720 Part 11 (1993) - Referenced Standards & Key Specifications

• Referenced Standard:

  • IS 2720 Part 2 (1973) — Determination of water content is a necessary adjunct for Part 11 testing procedures.

• Rounding Off Results:

  • Follow IS 2:1960 for rounding numerical values.
  • Retain significant figures equal to those specified in the standard.

• Key Test Data Reporting (Clause 7.3 & Annex A):

  Parameter	Description
  Initial length & diameter	Specimen dimensions before testing
  Initial weight & moisture	For sample characterization
  Load & Compression Gauge	To measure applied stresses
  Cell pressure (G3)	Confining pressure in triaxial test
  Deviator Stress (σ1 - σ3)	Difference between axial & cell pressure
  Vertical Stress (σ1)	Axial applied stress
  Strain	Change in length/original length
  Mode of failure	Shear plane angle & failure type

• Formula for Modulus of Rubber Membrane (Fig.1):
  [ M = \frac{\text{Load per 25 mm width}}{\text{Strain (extension)}} ]

• Specimen Preparation:

  • Use a 25 mm wide circumferential rubber membrane strip.
  • Mean length formula:
    [ L = 2(1 - d - 21) + (d + 1) ]

Summary Diagram of Test Setup (Simplified):

flowchart LR
    A[Specimen] --> B[Rubber Membrane (25mm wide)]
    B --> C[Load Application]
    C --> D[Compression & Load Gauges]
    D --> E[Measure Strain & Stress]
    E --> F[Calculate Deviator Stress & Modulus]

Note: IS 2720 Part 11 focuses on Unconsolidated Undrained Triaxial Compression Test without pore water pressure measurement. Use referenced IS parts for water content and other soil properties.

IS 2720 Part 11 — Apparatus Required (Summary)

1. General Preparation (Clause 3.1):

   • Standard sample preparation tools: cutting rings, trimming tools, mixing bowls, spatulas.

2. Moisture Content Determination (Clause 3.2.10):

   • Follow apparatus as per IS 2720 Part 2 (1973):
     • Moisture cans, oven (105 ± 5°C), balance (accuracy 0.01 g), desiccators.

3. Triaxial Test Apparatus (Clause 3.4):

   • Triaxial cell for sample placement.
   • Pressure applying system (Clause 3.4.2):
     • Must maintain cell fluid pressure within ±10 kPa (preferably ±5 kPa).
     • Includes a calibrated pressure gauge to measure confining pressure accurately.

Key Specification Table for Pressure Control (Clause 3.4.2)

Conceptual Diagram of Triaxial Test Setup

graph LR
A[Triaxial Cell] --> B[Sample]
B --> C[Confining Fluid]
C --> D[Pressure Application System]
D --> E[Pressure Gauge (±5-10 kPa accuracy)]

Summary: Use calibrated, precise pressure control apparatus for triaxial tests, and standard moisture content apparatus per IS 2720 Part 2.

IS 2720 Part 11: Preparation of Specimens - Key Points

Procedures for Specimen Preparation (Clauses 3.2 & 4.1.1)

• Procedure 1: Specimen from sampler tube same internal diameter as required specimen.
• Procedure 2: Specimen from sampler tube larger diameter than required specimen.
• Procedure 3: Specimen from block sample.

Procedure 1 Details (Clause 4.1.1.1)

• If tube ends are not flat/normal, extrude sample length, cut specimen, place in split mould, trim ends flat and normal.
• If tube ends are flat and normal, prepare specimen inside tube and then extract.

Additional Axial Load Measurement (Clause 3.4.4)

• Use proving rings:
  • 1 kN capacity, sensitivity 2 N for low strength soils.
  • 10 kN capacity, sensitivity 10 N for high strength soils.

Unit Conversion

• 1 kPa = 100 kgf/m²

Summary Table: Specimen Preparation Procedures

flowchart TD
    A[Sample Received] --> B{Sample Type}
    B -->|Tube same dia| C[Procedure 1]
    B -->|Tube larger dia| D[Procedure 2]
    B -->|Block sample| E[Procedure 3]
    C --> F[Cut or prepare specimen in tube]
    D --> G[Core/trim specimen]
    E --> H[Trim & shape specimen]

This concise overview aligns with IS 2720 Part 11 for specimen preparation.

IS 2720 Part 11: Test Procedure Key Points

Specimen Preparation (Clauses 3.2 & 3.3)

• Procedure 1: Specimen from sampler tube of same diameter.
• Procedure 2: Specimen from sampler tube larger than required.
• Procedure 3: Specimen from block sample.

Rounding Off (Clause None)

• Round results per IS 2:1960 rules.
• Retain significant figures equal to those specified in the standard.

Reporting of Results (Clause 7.1 & 7.3)

• Record:
  • Specimen dimensions (length, diameter)
  • Bulk density, moisture content
  • Cell pressure (G3)
  • Maximum principal stress difference (σ1 - σ3)
  • Corresponding strain, time to failure
  • Strain rate during test
• Use proforma in Annex A for test records.

Key Formulas

• Deviator Stress:
  [ \sigma_d = \sigma_1 - \sigma_3 ]
• Modulus (M) of rubber membrane:
  [ M = \frac{\text{Load per 25 mm width}}{\text{Strain extension}} ]

Typical Test Data Table Columns (Clause 7.3)

Summary Diagram: Specimen Preparation Procedures

flowchart TD
    A[Start: Sample Collection] --> B{Sampler Tube Diameter?}
    B -- Same as Specimen --> C[Procedure 1: Direct Sampling]
    B -- Larger than Specimen --> D[Procedure 2: Core Cutting]
    B -- Block Sample --> E[Procedure 3: Block Sampling]
    C --> F[Specimen Preparation]
    D --> F
    E --> F
    F --> G[Test Execution & Recording]

This ensures consistent specimen preparation, test execution, and reporting aligned with IS 2720 Part 11 requirements.

IS 2720 Part 11 — Key Formulas & Corrections for Stress Measurement

• Correction for Rubber Membrane Restraining Effect (Clause 6.1.1.2):

  [ \text{Correction} = 4M \times (1 - \varepsilon_D) ]

  Where:

  • ( M ) = Membrane modulus (material property)
  • ( \varepsilon_D ) = Strain in the membrane (dimensionless)

• Applying Correction (Clause 6.1.1.3):

  [ \text{Corrected Maximum Principal Stress Difference} = \text{Measured Value} - \text{Correction} ]

• Rounding Off (per IS 2:1960):

  • Final results must be rounded to the same number of significant figures as specified in IS 2720.
  • Follow standard rounding rules to maintain consistency.

Summary Table for Correction Application

This ensures accurate stress values by accounting for membrane stiffness effects during testing.

IS 2720 Part 11: Reporting of Results - Key Points

• Rounding Off: Final test values must be rounded per IS 2:1960, retaining the same significant figures as specified.

• Specimen Details to Report (Clause 7.1):

  • Dimensions (length, diameter)
  • Bulk density & moisture content
  • Cell pressure (σ3)
  • Maximum principal stress difference (σ1 - σ3)
  • Corresponding strain, time to failure, and strain rate

• Sampling Method (Clause 7.1.2):

  • Type of sampler used
  • Field sampling method

• Test Data Table (Clause 7.3, Table 7) includes columns for:

  1. Compression Gauge Reading
  2. Load Gauge Reading
  3. Compression of Sample
  4. Strain
  5. Corrected Area
  6. Load
  7. Deviator Stress (σ1 - σ3)
  8. Vertical Stress (σ1)
  9. σ1/σ3 ratio

• Specimen Preparation and Observations:

  • Initial weight, moisture content
  • Description and sketch after failure
  • Mode of failure & shear plane angle

Common Formulas:

• Deviator Stress:
  [ \sigma_d = \sigma_1 - \sigma_3 ]

• Modulus (M) of Rubber Membrane (Fig.1):
  [ M = \frac{\text{Load}}{\text{Strain Extension}} ]

Example Reporting Table (Simplified):

This structured reporting ensures clarity, repeatability, and compliance with IS standards for triaxial compression tests.

IS 2720 Part 11: Proforma for Test Record (Annex A & Clause 7.3)

The proforma captures essential test data for the Unconsolidated Undrained Triaxial Compression Test without pore water pressure measurement:

Key Sections in Proforma:

• Identification:

  • Operator, Date, Depth, Borehole No., Site, Sample No.

• Specimen Details:

  • Initial length (L₀), diameter (d₀), weight, moisture content
  • Description and sketch of specimen after failure
  • Mode of failure and angle of shear plane with vertical

• Test Conditions:

  • Cell pressure (σ₃)
  • Load gauge constant

• Test Data Table:

Important Formulas:

• Deviator Stress:
  [ \sigma_d = \sigma_1 - \sigma_3 ]

• Strain:
  [ \text{Strain} = \frac{\Delta L}{L_0} ]

• Corrected Cross-sectional Area:
  Accounts for specimen deformation, often calculated from diameter changes.

Notes:

• Record moisture changes before and after testing to check membrane impermeability.
• Sketch failure mode promptly to avoid moisture loss.
• Use proving rings with appropriate sensitivity (1 kN with 2 N sensitivity for low strength soils; 10 kN with 10 N sensitivity for high strength soils).

flowchart TD
    A[Start Test] --> B[Record Initial Specimen Data]
    B --> C[Apply Cell Pressure σ₃]
    C --> D[Measure Load & Compression]
    D --> E[Calculate Deviator Stress σ₁ - σ₃]
    E --> F[Record Strain & Corrected Area]
    F --> G[Note Mode of Failure & Sketch]