Method for the quantitative description of discontinuities in rock masses, Part 10: Block size

IS 11315 Part 10 - Scope Summary

• Scope Reference: IS 11315 Part 10 refers to definitions and terms as per IS 11358-1986.
• Purpose: Specifies testing and analysis procedures related to the material or product covered.
• Rounding Off: Final test results must be rounded according to IS 2-1960 (Rules for rounding off numerical values).

Key Points:

• No direct formulas or tables are provided in the Scope clause.
• Definitions and terminologies are governed by IS 11358-1986.
• Reporting and rounding of values must comply with IS 2-1960.

Additional Notes (General Practice):

• Always refer to IS 11358-1986 for term definitions.
• Follow IS 2-1960 for rounding:
  • If the digit after the last significant figure is >5, round up.
  • If <5, round down.
  • If exactly 5, round to the nearest even digit.

flowchart LR
  A[IS 11315 Part 10] --> B[Definitions per IS 11358-1986]
  A --> C[Rounding per IS 2-1960]

For detailed formulas or tables, consult respective parts of IS 11315 or related IS codes.

IS 11315 Part 10: Definitions & Key Specifications

• Definitions:
  As per Clause 2.1, definitions follow IS 11358-1986 (general rock mass terminology).

• Block Size Classification (Clause 4.3):
  Based on Jv (joints per m³), which quantifies block size via joint density:

• Additional Note:
  • Jv > 60 indicates crushed rock, typical of a clay-free crushed zone.

Summary

• Use Jv to classify rock block sizes.
• Refer to IS 11358-1986 for detailed terminology.
• This classification aids in rock mass characterization for design and analysis.

flowchart TD
    A[Joint Density (Jv)] --> B{Jv Value}
    B -->|<1| C[Very Large Blocks]
    B -->|1-3| D[Large Blocks]
    B -->|3-10| E[Medium-sized Blocks]
    B -->|10-30| F[Small Blocks]
    B -->|>30| G[Very Small Blocks]
    B -->|>60| H[Crushed Rock Zone]

Significance of Block Size in Rock Mass Behaviour (IS 11315 Part 10)

• Block Size Definition (Clause 3.1):
  Determined by discontinuity spacing, number of joint sets, and persistence of discontinuities.

• Mechanical Behaviour (Clause 3.3):

  • Large blocks → less deformable rock mass, favorable arching & interlocking in underground works.
  • Small blocks → soil-like behaviour; failure modes shift from translational/toppling to circular/rotational.
  • Extremely small blocks → may behave like a flow zone (e.g., "sugar-cube" shear zone).

• Quantitative Measures (Clause 3.5):

  • Block Size Index (Ib): Average dimension of typical blocks.
  • Volumetric Joint Count (Jv): Total number of joints per unit volume.

Key Formulas

• Volumetric Joint Count (Jv):

  [ J_v = \sum_{i=1}^{n} \frac{1}{S_i} ]

  where:
  ( S_i ) = mean spacing of joints in the ( i^{th} ) set
  ( n ) = number of joint sets

• Block Size Index (Ib):
  Approximate average block dimension, inversely proportional to ( J_v ):

  [ I_b \approx \frac{1}{J_v} ]

Summary Table

flowchart LR
    A[Discontinuity Spacing & Sets] --> B[Block Size (Ib)]
    B --> C[Rock Mass Behaviour]
    C --> D{Block Size}
    D -->|Large| E[Less deformable, arching]
    D -->|Small| F[Soil-like failure modes]
    D -->|Very Small| G[Flow-like behaviour]

In essence: Block size critically controls deformation and failure modes in rock masses,

IS 11315 Part 10 focuses on quantitative description of block size and shape formed by intersecting discontinuities in rock masses.

Key Points & Formulas:

• Block Size Measurement:

  • Blocks are characterized by their linear dimensions (length, width, height) measured along discontinuity intersections.
  • The mean block size (L) is often taken as the average of these dimensions.

• Block Shape Description:

  • Shape is described by ratios of block dimensions:
    • Shape Ratio 1: ( \frac{L_{min}}{L_{max}} )
    • Shape Ratio 2: ( \frac{L_{mid}}{L_{max}} )
  • These ratios help classify blocks as cubic, tabular, or elongated.

• Quantification Method:

  • Measure block sizes in the field using tape or laser distance meters.
  • Record multiple blocks for statistical reliability.
  • Calculate average block size and shape ratios.

Typical Table Format for Block Size:

Summary:

• Measure block dimensions along discontinuity intersections.
• Calculate shape ratios for block shape classification.
• Use statistical averages for engineering design inputs.

flowchart TD
    A[Field Measurement] --> B[Measure Block Dimensions]
    B --> C[Calculate Mean Block Size]
    B --> D[Calculate Shape Ratios]
    C --> E[Quantitative Description]
    D --> E

This method ensures consistent, quantitative characterization of rock blocks per IS 11315 Part 10.

IS 11315 Part 10: Recording and Reporting Block Size Indices

Key Formulas and Specifications

• Block Size Index (Ib):
  Estimated by averaging typical block dimensions selected visually.

  • Accuracy: ±10% is sufficient.
  • Purpose: Represents average block dimensions realistically.

• Formula for Sedimentary Rocks (3 sets of joints):
  [ I_b = \frac{S_1 + S_2 + S_3}{3} ]
  where (S_1, S_2, S_3) = modal spacings of joint sets.

• Important Notes:

  • For >3 joint sets, averaging spacings can overestimate (I_b) due to widely spaced sets.
  • Record the modal block size index (I_p) for largest, smallest blocks and domain details (number of sets, persistence).

Data to Record (Clause 5.1)

• Modal block size index (I_p) (typical, largest, smallest)
• Number of joint sets
• Persistence and description of joints

Summary Table

flowchart TD
    A[Select typical block sizes] --> B[Measure average dimensions (Ib)]
    B --> C{Number of joint sets > 3?}
    C -- Yes --> D[Use caution; wide sets may skew Ib]
    C -- No --> E[Calculate Ib = (S1 + S2 + S3)/3]
    E --> F[Record Ip (modal block size indices)]
    D --> F
    F --> G[Report number of sets and persistence]

This approach ensures realistic block size representation for engineering and geological assessments.

IS 11315 Part 10: Classification of Rock Mass Blockiness

Key Formulas:

• Block Size Index (Ib):
  Average dimension of typical rock blocks, estimated visually with ~10% accuracy.
  For multiple joint sets (especially 3 or more):
  [ I_b = \frac{S_1 + S_2 + S_3}{3} ]
  where (S_1, S_2, S_3) = average spacings of the three principal joint sets.

Classification Terms (Clause 5.3 & 4.4):

Notes:

• Avoid using average modal spacings if more than three joint sets exist, as widely spaced sets can skew (I_b).
• Sedimentary rocks commonly have cubic/prismatic blocks due to two cross joints plus bedding planes.

flowchart LR
    A[Rock Mass] --> B{Blockiness Type}
    B --> C[Massive]
    B --> D[Blocky]
    B --> E[Tabular]
    B --> F[Columnar]
    B --> G[Crushed]

This classification aids in understanding rock stability and excavation behavior.

IS 11315 Part 10: Notes on Joint Sets and Sampling Procedures

Key Points & Formulas

• Sampling length: Use 5 or 10 m perpendicular to joint sets for counting joints (Clause 4.2.1).
• Joint count per metre:
  [ \text{Joint count per metre} = \frac{\text{Number of joints counted}}{\text{Sampling length (m)}} ]
• Volumetric Joint Count (Jv): Sum of joints per metre for all joint sets present (Clause 4.2).
  [ J_v = \sum \text{(joints per metre for each set)} ]
• Random discontinuities can be included but usually have minimal effect unless systematic joint spacing is wide (1-10 m).

Block Size Classification (Clause 4.3)

• Note: ( J_v > 60 ) indicates crushed rock (clay-free crushed zone).

Sampling Procedure Summary

• Face the direction of strike for each joint set.
• Count joints perpendicular to strike to avoid angular corrections.
• Use paced or estimated lengths (±10% accuracy).
• Use mean spacing, not modal spacing, for calculations.

flowchart TD
    A[Select Joint Set] --> B[Face Direction of Strike]
    B --> C[Count Joints Perpendicular to Strike]
    C --> D[Divide Count by Sampling Length (5 or 10 m)]
    D --> E[Calculate Jv = Sum of Joints/m for all Sets]
    E --> F[Classify Block Size based on Jv]

This ensures rapid, reliable field mapping and block size estimation.

IS 11315 Part 10 (1987) - Application Guidelines: Key Points

IS 11315 Part 10 refers to prestressed concrete, and its application guidelines align with IS 11358-1986 definitions.

Key Specifications & Formulas

• Prestressing Force (P):
  [ P = A_p \times f_{pu} ]
  Where:

  • (A_p) = Area of prestressing steel
  • (f_{pu}) = Ultimate tensile strength of prestressing steel

• Losses in Prestress:
  Consider initial prestress force minus losses due to:

  • Elastic shortening
  • Creep and shrinkage of concrete
  • Relaxation of steel
  • Anchorage slip

• Allowable Stresses:
  Follow limits as per IS 1343 for concrete and prestressing steel.

Tables (Typical)

Application Notes

• Use IS 11358 for term definitions and IS 1343 for prestressed concrete design.
• Ensure proper anchorage and stress transfer.
• Account for all prestress losses in design calculations.

flowchart LR
    A[Prestressing Steel Area \(A_p\)] --> B[Calculate Initial Force \(P\)]
    B --> C[Subtract Losses]
    C --> D[Effective Prestress Force]
    D --> E[Check Against Allowable Stresses]

For detailed design, always cross-reference IS 1343 and IS 11358 along with IS 11315 Part 10.

Rounding Off Numerical Values as per IS 11315 Part 10

• Reference Standard: IS 11315 Part 10 mandates rounding off numerical results according to IS 2:1960 (Rules for rounding off numerical values).

Key Points from IS 2:1960 (Rounding Rules)

Summary

• Always round off final test or analysis results in accordance with IS 2:1960.
• This ensures uniformity and accuracy in reporting.
• No specific rounding rules are detailed in IS 11315 Part 10 except this reference.

flowchart TD
    A[Final Value] --> B{Digit to be dropped}
    B -->|<5| C[Leave last digit unchanged]
    B -->|>5| D[Increase last digit by 1]
    B -->|=5 followed by non-zero| D
    B -->|=5 no following digits| E{Is last digit odd?}
    E -->|Yes| D
    E -->|No| C

Use IS 2:1960 for consistent rounding in rock mechanics test results per IS 11315 Part 10.

IS 11315 Part 10 (1987) - References and Related Standards

• Primary Reference:

  • Definitions and terms are as per IS 11358-1986 (Terminology related to the standard).

• Rounding Off Results:

  • Follow IS 2-1960 for rounding off numerical values in test reports.
  • Typically, rounding is done to a specified number of decimal places based on measurement precision.

• Related Standards:

  • IS 11315 series covers various test methods; Part 10 refers to specific procedures within this series.
  • IS 11358-1986 provides fundamental definitions ensuring consistency across tests.

Summary Table of References

If you need formulas or tables for a specific test in IS 11315 Part 10, please specify the test type for detailed data.