Method for the quantitative description of discontinuities in the rock mass, Part 3: Persistence

IS 11315 Part 3 - Introduction: Key Formulas, Tables & Specs

1. Persistence of Failure Surfaces (Clause 5.4)

• Persistence (%) of potential failure surfaces, including stepped surfaces, should be estimated conservatively.
• Round persistence upwards to nearest 10% (e.g., 92% → 100%).
• Assume 100% persistence when in doubt for safety.

2. Persistence Classification (Clause 4.3)

3. Shear Strength Estimation of Intact Rock Bridges

Using Mohr envelope (linear shear strength):

[ C = \frac{( \sigma_c - \sigma_t )}{6} ]

Where:

• (C) = cohesion due to intact rock bridges
• (\sigma_c) = uniaxial compressive strength (UCS)
• (\sigma_t) = tensile strength (assumed (\sigma_c / \sigma_t = 9))

Interpretation: Cohesion ≈ 1/6 of UCS.

4. Units (SI Units)

5. Important Notes

• Persistence estimates are engineering judgement-based; bias towards conservatism.
• Stepped failure surfaces (2D & 3D) may form when persistence < 100%.
• Intact rock bridges significantly increase shear strength.

flowchart TD
    A[Discontinuity Persistence] --> B{Persistence Level}
    B -->|<1 m| C[Very Low]
    B -->|1-3 m| D[Low]
    B -->

IS 11315 Part 3 — Scope: Key Specifications & Tables

1. Scope Summary

• Defines persistence of potential failure surfaces (including stepped surfaces) in rock mass investigations.
• Persistence should be estimated and rounded upwards to the next multiple of 10% (e.g., 92% → 100%).

2. Persistence Classification (Clause 4.3)

3. Units (SI Units)

• Length: metre (m)
• Force: newton (N) = 1 kg·m/s²
• Pressure/Stress: pascal (Pa) = 1 N/m²
• Energy: joule (J) = 1 N·m
• Power: watt (W) = 1 J/s

4. Rounding Rule (Clause 5.4)

• Final values must be rounded according to IS 2-1960 (standard rounding).

Visual Summary: Persistence Levels

graph LR
    A[Very Low <1m] --> B[Low 1-3m]
    B --> C[Medium 3-10m]
    C --> D[High 10-20m]
    D --> E[Very High >20m]

Use this scope to estimate and classify rock discontinuity persistence consistently.

IS 11315 Part 3 — Key Definitions & Specifications

Persistence of Discontinuities (Clause 4.3 & 5.4)

Persistence refers to the length of potential failure surfaces in rock masses, categorized as:

• Persistence estimates should be rounded upwards to the nearest 10% (e.g., 92% → 100%).

Units & Symbols (SI Units)

Notes:

• Definitions align with IS 11358-1986.
• Rounding of results follows IS 2-1960.
• Persistence relates to rock discontinuities affecting stability.

flowchart LR
    A[Discontinuity Persistence] --> B{Length (m)}
    B -->|<1| VeryLow[Very Low Persistence]
    B -->|1-3| Low[Low Persistence]
    B -->|3-10| Medium[Medium Persistence]
    B -->|10-20| High[High Persistence]
    B -->|>20| VeryHigh[Very High Persistence]

This concise summary covers key definitions, units, and persistence classification per IS 11315 Part 3.

IS 11315 Part 3: General Principles — Key Formulas, Tables, and Specifications

1. Persistence of Failure Surfaces (Clause 5.4):

• Estimate persistence of potential failure surfaces (including stepped surfaces).
• Round estimates upwards to the next multiple of 10%.
  • Example: 92% → 100%.

2. Persistence Classification (Clause 4.3):

3. SI Units and Definitions:

4. Rounding Off (Clause 0.6):

• Follow IS 2-1960 for rounding final values.

flowchart TD
    A[Estimate Persistence] --> B{Round Up?}
    B -- Yes --> C[Round to next 10%]
    B -- No --> D[Use exact value]
    C --> E[Classify Persistence]
    D --> E
    E --> F{Persistence Level}
    F -->|<1 m| G[Very Low Persistence]
    F -->|1-3 m| H[Low Persistence]
    F -->|3-10 m| I[Medium Persistence]
    F -->|10-20 m| J[High Persistence]
    F -->|>20 m| K[Very High Persistence]

Summary:

IS 11315 Part 3: Procedure Key Points

Persistence Estimation (Clause 5.4)

• Persistence of potential failure surfaces (including stepped surfaces) should be estimated.
• Round the estimate upwards to the next multiple of 10% (e.g., 92% → 100%).

Persistence Classification (Clause 4.3)

Units and Symbols (SI Units)

• Length: m (metre)
• Force: N (newton) = 1 kg·m/s²
• Pressure/Stress: Pa (pascal) = 1 N/m²
• Energy: J (joule) = 1 N·m
• Power: W (watt) = 1 J/s

Rounding Off (Clause None: 0.6)

• Follow IS 2-1960 for rounding off test or analysis results.

Summary Diagram: Persistence Classification

graph LR
A[Very Low Persistence] -->|<1 m| B[Low Persistence]
B -->|1-3 m| C[Medium Persistence]
C -->|3-10 m| D[High Persistence]
D -->|10-20 m| E[Very High Persistence]
E -->|>20 m| F[Failure Surface]

This classification helps in assessing the continuity and potential impact of discontinuities in rock or soil mass during stability analysis.

IS 11315 Part 3 — Presentation of Results: Key Points

1. Persistence Estimation (Clause 5.4)

• Persistence of potential failure surfaces (including stepped) should be estimated.
• Round persistence upwards to the nearest multiple of 10% (e.g., 92% → 100%).
• Conservative approach: assume 100% persistence if uncertain, as intact rock bridges contribute significantly to shear strength.

2. Persistence Categories (Clause 4.3)

3. Shear Strength of Intact Rock Bridges

• Cohesion, C, estimated from Mohr envelope assuming linear shear strength:

  [ C = \frac{F_c - \sigma_t}{6} ]

  where:

  • (F_c) = uniaxial compressive strength
  • (\sigma_t) = tensile strength (assumed (F_c / \sigma_t = 9))

• This implies cohesion ≈ 1/6 of unconfined compressive strength.

4. Units (SI System)

• Length: metre (m)
• Force: newton (N) = kg·m/s²
• Pressure/Stress: pascal (Pa) = N/m²

5. Reporting

• Round off results per IS 2-1960 rounding rules.

flowchart LR
    A[Discontinuity Persistence Estimation] --> B{Persistence Level?}
    B -->|<1 m| C[Very Low Persistence]
    B -->|1-3 m| D[Low Persistence]
    B -->|3-10 m| E[Medium Persistence]
    B -->|10-20 m| F[High Persistence]
    B -->|>20 m| G[Very High Persistence]
    A --> H[Round Persistence to nearest 10%]
    H --> I[Assume 100% if uncertain]
    I --> J[Calculate

Key Notes & Recommendations from IS 11315 Part 3 (1987):

• Persistence Estimation (Clause 5.4):

  • Persistence of failure surfaces (including stepped) should be estimated and rounded up to the nearest 10%.
  • Example: 92% persistence → assume 100%.

• Discontinuity Data (Clause 4.5):

  • Record termination data ((x, r \text{ or } d)) for each discontinuity end, plus length in meters.

• Persistence & Shear Strength:

  • Persistence estimates should be conservative (close to 100%) because intact rock bridges contribute significantly to shear strength.
  • Shear strength due to intact bridges can be approximated by:

  [ C = \frac{F_c - \sigma_t}{6} ]

  where:
  (F_c) = uniaxial compressive strength,
  (\sigma_t) = tensile strength (assumed (F_c / \sigma_t = 9)).

• Units:

  • Use SI units as per the table (e.g., length in meters, force in newtons, stress in pascals).

Summary Table: Persistence & Cohesion Estimation

flowchart LR
    A[Discontinuity Data] --> B[Record termination data (x, r or d)]
    B --> C[Estimate persistence]
    C --> D{Is persistence < 100%?}
    D -- Yes --> E[Consider stepped failure surfaces]
    D -- No --> F[Assume full persistence]
    E --> G[Estimate shear strength using Mohr envelope]
    F --> G
    G --> H[Calculate cohesion