Methods of test for soils, Part 13: Direct shear test

IS 2720 Part 13: Scope & Key Specifications

• Scope:
  This part covers the direct shear test for determining the shear strength parameters of soils using a shear box apparatus.

• Normal Stress Selection (Clause 5.4):

  • Normal stresses (σ) applied during tests must simulate field conditions and design requirements.
  • Typical normal stresses range from low to high values depending on soil type and expected in-situ stresses.

• Weights (Clause 3.3):

  • Weights or loads are applied to provide required normal stresses on the soil sample.

• Rounding Off (Clause 6):

  • Final test results should be rounded as per IS 2-1960 rules, matching the significant figures of the standard.

• Reference Definitions (Clause 2.1):

  • Terminology follows IS 2809-1972 (Glossary for soil engineering terms).

Typical Shear Strength Parameters from Direct Shear Test

Shear Strength Equation (Mohr-Coulomb):

[ \tau = c + \sigma \tan \phi ]

flowchart LR
    A[Apply Normal Load (σ)] --> B[Shear Box with Soil Sample]
    B --> C[Apply Shear Force]
    C --> D[Measure Shear Stress (τ)]
    D --> E[Plot τ vs σ]
    E --> F[Determine c and ϕ from failure envelope]

For detailed apparatus specs and procedures, refer to IS 2720 Part 13 (1986).

IS 2720 Part 13 - Key References, Formulas & Tables

1. Weights & Normal Loads (Clause 3.3, 5.4)

• Normal stresses for shear tests should replicate field conditions.
• Weights or loads applied must correspond to design requirements.

2. Proforma for Recording Test Results (Clause 6.1.1)

3. Important Calculations:

• Shear Stress (τ):
  [ \tau = \frac{F}{A} ]
  where (F) = shear force, (A) = corrected specimen area.

• Cohesion (c) and Angle of Internal Friction (φ):
  From shear stress-normal stress plot (Mohr-Coulomb criterion):
  [ \tau = c + \sigma \tan \phi ]

4. Summary Table for Results

5. Additional Notes

• Rate of shearing and consolidation times must be analyzed before test.
• Plot shear stress vs. displacement to find max shear stress and displacement.
• Plot shear stress vs. normal stress to find cohesion and friction angle.

flowchart TD
    A[Apply Normal Load] --> B[Shear Box Test]
    B --> C[Record Vertical Dial Readings]
    B --> D[Record Displacement Dial Readings]
    C -->

IS 2720 Part 13: Definitions and Key Specifications

• Reference for Definitions:
  All terms related to soil engineering in IS 2720 Part 13 refer to IS 2809-1972 (Glossary of terms and symbols relating to soil engineering).

• Weights (Clause 3.3):
  Weights are used to apply required normal loads during shear box tests to simulate field stress conditions.

• Normal Stresses (Clause 5.4):
  Selected normal stresses for testing must reflect field conditions and design requirements to ensure relevant test results.

• Rounding Off (Clause 6):
  Test results or calculations shall be rounded off as per IS 2-1960 rules, maintaining the same number of significant digits as specified in the standard.

Key Notes:

This ensures consistency and accuracy in soil shear testing per IS 2720 Part 13.

IS 2720 Part 13: Preparation of Specimen - Key Points

1. Specimen Size (Clause 4.1)

• Specimens must be undisturbed and of required size as per Clause 5.1 (typically cylindrical or rectangular blocks).
• Preparation follows IS 2720 Part 1 (1983) guidelines for sampling.

2. Tools for Preparation (Clause 3.9)

• Use a spatula and a straight edge for trimming and leveling the specimen surface to the required dimensions.

3. Weights (Clause 3.3)

• Use calibrated weights to apply the required normal loads during testing.

4. Rounding Off Results (Clause 0.5)

• Final test values must be rounded off as per IS 2-1960.
• Retain the same number of significant digits as specified in the standard.

Typical Specimen Dimensions (Example from IS 2720 Part 13)

Summary Diagram: Specimen Preparation Steps

flowchart TD
    A[Sampling] --> B[Cutting Undisturbed Specimen]
    B --> C[Trimming with Spatula & Straight Edge]
    C --> D[Measuring Dimensions]
    D --> E[Applying Weights for Load]
    E --> F[Testing & Recording Results]

For detailed specimen size and preparation methods, refer to Clause 5.1 of IS 2720 Part 13 and IS 2720 Part 1.

IS 2720 Part 13 (1986) focuses on the direct shear test for soils. Regarding Apparatus:

• Equipment requirements are not detailed here; instead, refer to IS 11229:1985, which covers apparatus specifications comprehensively.
• Key apparatus includes:
  • Shear box (as per IS 2720 Part 13 and IS 11229)
  • Loading device to apply normal and shear loads
  • Weights (Clause 3.3) to provide required normal loads
  • Dial gauges or displacement transducers for measuring shear displacement

Important Notes:

• Test results must be recorded using the proforma in Appendix B of IS 2720 Part 13.
• Final values should be rounded as per IS 2:1960.
• For detailed apparatus dimensions and specifications, consult IS 11229:1985.

Summary Table: Apparatus Components

flowchart LR
    A[Soil Sample] --> B[Shear Box]
    B --> C[Normal Load (Weights)]
    B --> D[Shear Load (Loading device)]
    D --> E[Displacement Measurement]
    E --> F[Record Results (Appendix B)]

For full apparatus specs, always refer to IS 11229:1985.

IS 2720 Part 13: Test Procedure Key Points

• Clause 5.4: Select normal stresses for testing based on actual field conditions and design needs to ensure relevance.

• Clause 6.1.1: Test results must be properly recorded; use the recommended proforma in Appendix B for consistency.

• Rounding Off (Clause 0.5): Final test values should be rounded per IS 2-1960, matching the precision of the standard's specified values.

Typical Test Procedure Summary:

1. Sample Preparation: Prepare soil samples as per standard guidelines.
2. Stress Application: Apply normal stresses reflecting field conditions.
3. Measurement: Record deformation, pore pressure, or other parameters.
4. Data Recording: Use Appendix B proforma for systematic data capture.
5. Calculations: Compute strength parameters, ensuring rounding as per IS 2-1960.

Example: Rounding Rule (IS 2-1960)

If you need the Appendix B proforma or specific formulas (e.g., shear strength calculations), please specify.

IS 2720 Part 13 - Calculations and Reporting Key Points

1. Selection of Normal Stresses (Clause 5.4)

• Choose normal stresses reflecting actual field conditions and design requirements.
• Ensures test relevance to real soil behavior under load.

2. Recording Test Results (Clause 6.1.1)

• Use the proforma in Appendix B for consistent documentation.
• Record all relevant parameters: normal load, shear load, displacement, and failure conditions.

3. Rounding Off Values (Clause 0.5)

• Follow IS 2-1960 for rounding off test results.
• Retain the same number of significant digits as specified in the standard.

Typical Calculation Formula for Shear Strength Parameters:

[ \tau = c + \sigma \tan \phi ]

Where:

• (\tau) = Shear strength at failure (kPa)
• (c) = Cohesion intercept (kPa)
• (\sigma) = Normal stress applied (kPa)
• (\phi) = Angle of internal friction (degrees)

Recommended Report Contents:

• Sample identification and preparation details
• Normal stress applied
• Shear stress at failure
• Calculated (c) and (\phi)
• Observations and anomalies
• Compliance statement per IS 2720 Part 13

flowchart TD
    A[Test Setup] --> B[Apply Normal Stress]
    B --> C[Apply Shear Load]
    C --> D[Record Shear Stress at Failure]
    D --> E[Calculate c and φ]
    E --> F[Prepare Report using Appendix B]

This ensures clarity, repeatability, and compliance with IS 2720 Part 13 standards.

IS 2720 Part 13: Precision and Accuracy - Key Points

• Rounding Off (Clause 0.5):
  Final test results must be rounded per IS 2-1960 rules.

  • Retain significant figures equal to those specified in the standard.
  • Ensures consistency and comparability of results.

• Measurement Accuracy (Clause 3.5):
  Use Micrometer Dial-Gauges with accuracy of 0.01 mm.

  • One gauge for horizontal movement
  • One gauge for vertical compression of specimen

• Recording Results (Clause 6.1.1):
  Test results should be recorded in a recommended proforma (Appendix B), ensuring clarity and uniformity.

Summary Table: Measurement Accuracy

Rounding Off Rules (IS 2-1960):

• If digit to be dropped < 5 → round down
• If digit to be dropped > 5 → round up
• If digit = 5 → round to nearest even number

flowchart LR
    A[Test Measurement] --> B[Record Raw Value]
    B --> C[Apply IS 2-1960 Rounding]
    C --> D[Final Rounded Value]
    D --> E[Record in Proforma (Appendix B)]

This ensures precision (accurate instruments) and accuracy (correct rounding and recording) in soil testing per IS 2720 Part 13.

Rate of Shear for Consolidated Drained Test (IS 2720 Part 13, Clause 5.3 & Appendix A)

• Test Setup: Use shear box with perforated grid plates and saturated porous stones at top and bottom.
• Consolidation: Apply normal stress incrementally (refer IS 2720 Part 15) and allow complete consolidation.
• Shear Rate: Shear at a slow rate ensuring ≥ 95% pore pressure dissipation during the test.
• Time Factor (T_v): Calculate using consolidation theory to ensure drainage; shear rate corresponds to time factor allowing pore pressure dissipation.
• Number of Tests: Minimum 3-4 tests on specimens of same density at different normal stresses.
• Final Step: Measure final moisture content after test.

Key formula for Time Factor ( T_v ):

[ T_v = \frac{C_v \cdot t}{H^2} ]

Where:

• ( C_v ) = coefficient of consolidation (m²/s)
• ( t ) = time (s)
• ( H ) = drainage path length (m) (half or full thickness depending on drainage conditions)

For 95% consolidation, ( T_v \approx 0.848 ).

Summary Table for Shear Rate Control:

flowchart TD
    A[Apply Normal Stress] --> B[Allow Complete Consolidation]
    B --> C[Calculate Time Factor \(T_v\)]
    C --> D{Is \(T_v \geq 0.848\)?}
    D -- No --> B
    D -- Yes --> E[Shear at slow rate ensuring drainage]
    E --> F[Measure final moisture content]

Note: Use IS 2720 Part 15 for consolidation parameters and test procedure details.

IS 2720 Part 13 – Proforma for Recording Test Results

The recommended proforma (Appendix B) includes these key sections:

Key Calculations:

• Shear Stress (τ):
  [ \tau = \frac{\text{Shear Force}}{\text{Corrected Area}} ]

• Plot & Determine:

  • Maximum shear stress and corresponding displacement
  • Cohesion intercept (c) and angle of shearing resistance (φ) from shear stress vs normal stress plot

Notes:

• Rate of strain/shearing must reflect field conditions (Clause 5.4).
• Use IS 2-1960 rounding rules for final values.
• Calibration and lever ratios must be accurately recorded for load measurements.

flowchart TD
    A[Start Test] --> B[Record Sample Details]
    B --> C[Measure Specimen Dimensions]
    C --> D[Apply Normal Stress]
    D --> E[Apply Shear Stress]
    E --> F[Record Dial & Load Readings]
    F --> G[Calculate Shear Stress & Displacement]
    G --> H[Plot Curves & Determine c, φ]
    H --> I[Complete Summary Table]
    I --> J[Report Results]

This structured proforma ensures consistent, comprehensive recording of shear test data per IS 2720 Part 13.