Code of practice for the in-situ shear test on rock

IS 7746: Scope - Key Formulas, Tables & Specifications

Scope:
IS 7746 covers the in-situ shear test on rock joints, focusing on specimen preparation, test execution, and reporting.

Key Specifications & Reporting Requirements (Clause 9.1)

• Test Report Must Include:
  • Methods of specimen preparation.
  • Geological description of intact rock, shear surface, filling, and Joint Roughness Coefficient (JRC).
  • Test equipment details with sketches/photos.
  • Layout plan with test block locations and shear directions.
  • Photographs of sheared surfaces.
  • Data sheets (Annex A) with consolidation curves and shear stress vs displacement graphs.
  • Summary of peak & residual shear strengths with corresponding normal stresses.
  • Graph of shear strength vs normal stress (Fig. 5) to derive shear parameters.

Important Formulas & Parameters

• Shear Strength Parameters from Fig. 5:

• Relationship:
  [ \phi_a = \phi_u + i ] where
  ( \phi_u ) = basic friction angle,
  ( i ) = asperity angle.

Typical Data Sheet (Annex A)

Graphical Interpretation (Clause 8.5 & 8.6)

• Shear stress vs shear displacement shows peak or smooth curves depending on joint type.
• Normal displacement vs shear displacement slope estimates asper

IS 7746: Referenced Standards & Key Specifications Summary

Referenced Standard:

• IS 11315 (Part 4): 1987 — Method for quantitative description of rock discontinuity roughness, essential for characterizing shear surfaces.

Key Formulas & Parameters from IS 7746

Shear Strength Envelope (Fig. 5)

[ \tau = c' + \sigma_n \tan \phi_b \quad \text{(Peak shear strength)} ]

• ( c' ) = cohesion intercept
• ( \phi_b = \phi_u + i ) (basic friction angle + asperity angle)
• Residual strength corresponds to asperities sheared off.

Test Reporting Requirements (Clause 9.1)

• Geological description of rock and discontinuities
• Test equipment details with sketches/photos
• Layout plan with block IDs and shear directions
• Photographs of sheared surfaces
• Data sheets as per Annex A
• Consolidation and shear stress-displacement curves (Figs. 3 & 4)
• Peak and residual shear strength vs. normal stress graphs (Fig. 5)

Typical Data Sheet (Annex A)

| Time (min) | Applied Normal Force (P_{na}) (kN) | Normal Displacement (mm) | Applied Shear Force (kN) | Shear Displacement (mm) | Lateral Displacement (mm) | ( \sigma_n ) (MPa) | ( \tau ) (MP

IS 7746: Principle, Key Formulas & Specifications for In-Situ Shear Test

Principle (Clause 8.3)

• Total Normal Load on Shear Plane (Pn):
  [ P_n = P_{na} + P_{sa} \sin 15^\circ ] where:

  • (P_{na}) = Normal load from jack
  • (P_{sa}) = Applied inclined load on base

• Total Shear Force on Shear Plane (Ps):
  [ P_s = P_{sa} \cos 15^\circ ]

• For non-horizontal shear planes, resolve forces perpendicular and parallel to shear plane.

• Normal force (P_n) is reduced after each shear increment by (P_{sa} \sin 15^\circ) to keep normal stress approx. constant.

• Further normal force reduction to compensate for area change:
  [ \Delta P_n = A_s \times P_n ] where (A_s) = width change of block (mm).

Stress Calculations

• Normal Stress:
  [ \sigma_n = \frac{P_n}{A} ]

• Shear Stress:
  [ \tau = \frac{P_s}{A} ]

Shear Strength Parameters (Clause 8.6 & Fig. 5)

• Plot Peak & Residual Shear Strength vs Normal Stress to derive:
  • ( \phi_r ) = Residual friction angle
  • ( \phi_a ) = Apparent friction angle
  • ( c' ) = Cohesion intercept (peak shear)
  • ( c ) = Apparent cohesion (at (\phi_b))

Typical Data Sheet (Annex A)

IS 7746: Location of Test Site - Key Points

• Clause 4.1: Determine the intensity and direction of forces from the structure to locate the test site appropriately.

• Clause 4.2:

  • Select a test area representative of the site's geology.
  • Preferably test where maximum shear stresses are expected.
  • Align test shearing direction with the anticipated shearing direction during construction.

• Clause 4.3:

  • Prefer testing in drifts or tunnels for convenience.
  • If unavailable, test at design level or in open excavations within fresh rock.
  • Avoid zones with induced fractures from blasting.

Summary Table: Test Site Location Criteria

Conceptual Diagram: Test Site Selection Process

flowchart TD
    A[Determine Forces & Directions] --> B[Identify Max Shear Stress Zones]
    B --> C[Select Representative Geological Area]
    C --> D{Drifts/Tunnels Available?}
    D -- Yes --> E[Conduct Test in Drifts/Tunnels]
    D -- No --> F[Conduct Test at Design Level/Open Excavation]
    F --> G[Avoid Blasted Fracture Zones]

This ensures test results reflect actual field conditions for reliable design application.

IS 7746: Preparation of Test Site – Key Points

1. Location & Setup (Clauses 4.3, 6.1.1, 6.2)

• Preferred site: Inside drifts or tunnels for convenience.
• Alternative site: At design level or open excavations in fresh rock (foundation/abutment).
• Avoid: Zones with induced fractures from blasting to ensure test integrity.

2. Preparation Guidelines (Clause 5)

• Ensure the test site surface is smooth, clean, and free of loose material.
• Maintain natural in-situ stress conditions as much as possible.
• Avoid disturbance that may alter rock properties or pore water pressures.

3. Important Considerations (Clause 3.3)

• Assess pore water pressure differences between test and design conditions.
• Consider progressive failure potential—test results may vary under actual design stresses.

Summary Table: Test Site Preparation Checklist

flowchart TD
    A[Test Site Selection] --> B{In Drift/Tunnel?}
    B -- Yes --> C[Prepare smooth rock face]
    B -- No --> D[Open excavation at design level]
    D --> C
    C --> E[Avoid blasted/fractured zones]
    E --> F[Maintain in-situ stress & pore pressure]
    F --> G[Conduct test]

This ensures reliable, representative test results per IS 7746 standards.

IS 7746 - Types of Set-up for Shear Tests on Rock Discontinuities

1. Types of Set-up (Clause 6.1)

• Two types depending on test location:
  • Open Excavation (Fig. 1)
  • Drift or Tunnel (Fig. 2)
• Load application: Normal and tangential forces must pass through the centroid of the shear area at the base of the block.

2. Set-up in Open Excavation (Clause 6.1.1)

• Uses:
  • Anchor beam arrangement
  • Roller arrangement
• Components:
  • Hydraulic jacks for normal and shear loading
  • Dial gauges for shear and normal displacement
  • Steel casing, cement concrete, mortar/concrete layer

3. Set-up for Test in Drift or Tunnel (Clause 6.2)

• Similar to open excavation setup
• Normal load jack reacts against the roof of the tunnel (prepared as per 5.5.1)

Key Points Summary

Load Application Principle

• Forces must be aligned to pass through the centroid of the shear area to avoid eccentric loading.

flowchart LR
    A[Normal Load Jack] --> B[Shear Area Centroid]
    C[Shear Load Jack] --> B
    B --> D[Discontinuity Under Test]
    E[Dial Gauges] --> D

This setup ensures accurate shear strength measurement by controlling load application and displacement measurement as per IS 7746.

IS 7746: Test Procedure - Key Formulas, Tables & Specifications

1. Test Data Sheet (Annex A)

• Records normal force (Pna), shear force (Ps), normal displacement (Δn), shear displacement (Δs), and lateral displacement over time.
• Calculate stresses: [ \sigma_n = \frac{P_n}{A}, \quad \tau = \frac{P_s}{A} ] where:
  • (P_n = P_{na} + P_{sa} \sin \theta)
  • (P_s = P_{sa} \cos \theta)
  • (A) = initial contact area

2. Shear Strength Interpretation (Clause 9.1 & Fig. 5)

• Peak and residual shear strength parameters are derived from plots of shear strength vs normal stress.
• Key parameters:
  • (c') = cohesion intercept (peak shear strength)
  • (\phi_r) = residual friction angle
  • (\phi_a) = apparent friction angle (below asperity shearing)
  • (i) = asperity angle
• Typical relationship: [ \tau = c' + \sigma_n \tan \phi' ]

3. Test Report Must Include (Clause 9.1)

• Geological description of rock and joint surface.
• Equipment details with sketches/photos.
• Location and orientation of test blocks.
• Photographs of sheared surfaces.
• Consolidation curve and shear stress vs displacement graphs.
• Summary table of peak/residual shear strength with normal stresses.

4. Test Setup (Clause 6.2)

• Proper orientation and spacing of test blocks.
• Measure joint roughness as per IS 11315 (Part 4).
• Collect infilling materials for index testing.

flowchart TD
    A[Specimen Preparation] --> B[Test Setup in Drift/Tunnel]
    B --> C[Conduct Shear Test]
    C --> D[Record Data (Annex A)]
    D --> E[Plot Shear Stress vs Normal Stress]
    E --> F[Determine c', φ', φ_r]
    F --> G[Prepare Detailed Test Report]

IS 7746: Calculations & Interpretation - Key Points

1. Data Recording (Annex A)

• Record Applied Normal Force (Pna), Shear Force (Ps), and corresponding displacements (normal, shear, lateral).
• Calculate stresses: [ \sigma_n = \frac{P_n}{A}, \quad \tau = \frac{P_s}{A} ] where (A) = initial surface area.

2. Graphs to Plot (Clause 8.5)

• Shear Stress vs Shear Displacement (Fig. 4):
  • Curve A: Tight joints (peak and sudden drop).
  • Curve B: Filled joints (no distinct peak).
• Normal Displacement vs Shear Displacement:
  • Curve C: Tight joints (steeper slope).
  • Curve D: Filled joints (gentler slope).
• Asperity angle (i) estimated from slope of normal vs shear displacement.

3. Shear Strength Parameters (Clause 8.6 & Fig. 5)

• Plot Peak & Residual Shear Strength vs Normal Stress.
• Determine parameters:
  • ( \phi_r ) = residual friction angle
  • ( \phi_a ) = apparent friction angle (below asperity failure)
  • ( c' ) = cohesion intercept (peak)
  • ( c ) = apparent cohesion at ( \phi_b )

4. Test Report (Clause 9.1)

Include:

• Specimen preparation, geological description, equipment details.
• Layout plan & photos.
• Data sheets, consolidation curves, shear stress vs displacement graphs.
• Summary of peak/residual shear strengths and parameters.

Summary Table of Key Formulas:

IS 7746: Key Formulas, Tables & Specifications for Report (Clause 9.1 & Annex A)

Report Contents (Clause 9.1)

• Specimen preparation methods
• Geological description: intact rock, sheared surface, filling, JRC
• Test equipment details: description + sketch/photo + orientation
• Layout plan: test block locations, ID numbers, shearing directions
• Photographs: sheared surfaces with dimensions & shearing direction
• Data sheets (Annex A): consolidation curve (Fig. 3), shear stress vs displacement graphs (Fig. 4)
• Abstract: blockwise peak & residual shear strength, normal stress, displacements
• Shear strength graph (Fig. 5): peak & residual shear strength vs normal stress with parameters

Shear Strength Parameters (Fig. 5)

• Peak shear strength:
  [ \tau = c' + \sigma_n \tan \phi_r ]
• Residual shear strength:
  [ \tau = c + \sigma_n \tan \phi_b ]
• Where:
  • (c') = cohesion intercept (peak)
  • (c) = apparent cohesion (residual)
  • (\phi_r) = residual friction angle
  • (\phi_b = \phi_u + i) (apparent friction angle below stress da; (i) = asperity angle)

Typical Data Sheet (Annex A)

Typical Data Sheet for In Situ Shear Test (IS 7746: Clause 9.1 & Annex A)

Key Components of Data Sheet (Annex A):

Graphs to Plot (Clause 8.5 & 8.6):

• Shear Stress vs Shear Displacement (Fig. 4)

  • Curve A: Tight joints (peak & sudden drop)
  • Curve B: Filled joints (no distinct peak)

• Normal Displacement vs Shear Displacement (Fig. 4)

  • Trend C: Tight joints
  • Trend D: Filled joints

• Peak & Residual Shear Strength vs Normal Stress (Fig. 5)

  • Determine parameters:
    • ( \phi_r ): Residual friction angle
    • ( \phi_a ): Apparent friction angle
    • ( c' ): Cohesion intercept (peak)
    • ( c ): Apparent cohesion

Important Formulae:

• Normal Stress: [ \sigma_n = \frac{