Method for the quantitative description of discontinuities in the rock mass, Part 2: Spacing

Scope & Key Specifications from IS 11315 Part 2 (1987):

• Purpose: Defines methods to measure and present spacing of rock discontinuities (joints, fractures).

• Spacing Classification (Clause 5.1):

• Modal Spacing (Clause 4.4):
  The most common spacing ( S_m ) is calculated from measured distances ( d_m ) and presented as histograms (Fig. 2).
  Note: Use measuring tape and compass for accuracy; visual judgement acceptable for experienced geologists.

• Measurement Notes:

  • Exclude blast damage fractures.
  • Variability in spacing can be shown by block diagrams or histograms.
  • Where exposures are limited, seismic refraction or borehole techniques can estimate spacing.

flowchart LR
    A[Rock Exposure] --> B[Measure Discontinuity Spacing]
    B --> C[Record Min, Modal, Max Spacing]
    C --> D[Classify Spacing (Very Close to Very Wide)]
    D --> E[Present Results as Histograms or Block Diagrams]
    E --> F[Use for Rock Mass Characterization]

Summary: IS 11315 Part 2 standardizes discontinuity spacing measurement, classification, and presentation to aid rock mass evaluation for engineering purposes.

IS 11315 Part 2 - Key References and Specifications

1. Spacing Classification (Clause 5.1, Table 5)

• Record minimum (S_min), modal (S), and maximum (S_max) spacing for each discontinuity set.
• Present spacing distributions as histograms (see Fig. 2 in the standard).

2. Importance of Spacing (Clause 3.3)

• Spacing critically affects deformation potential.
• Low shear strength + multiple joint sets + appropriate spacing → slip and deformation likely.

3. Definitions (Clause 2.1)

• Terms as per IS 11358-1986 apply.

Summary Diagram: Discontinuity Spacing Categories

graph LR
    A[Discontinuity Spacing] --> B[Very Close < 60 mm]
    A --> C[Close 60-200 mm]
    A --> D[Moderate 200-600 mm]
    A --> E[Wide 600-2000 mm]
    A --> F[Very Wide > 2000 mm]

Note: Use these spacing categories to assess rock mass behavior and deformation risks in structural design.

Significance of Spacing in Rock Mass (IS 11315 Part 2)

• Spacing refers to the distance between discontinuities (joints, fractures) in a rock mass.
• According to Clause 3.3, spacing is critical when:
  • Shear strength is low.
  • Multiple joint sets exist, enabling slip or deformation.
• Smaller spacing → more discontinuities → weaker rock mass behavior.
• Larger spacing → more intact rock behavior.

Key Points:

• Spacing influences rock mass deformability and strength.
• Essential for mechanical behavior prediction and design.
• Must be measured accurately and rounded per IS rules.

Typical Parameters:

Formula:

[ S = \frac{L}{N} ]

• (L) = length of rock mass segment measured
• (N) = number of discontinuities counted

flowchart LR
    A[Rock Mass] --> B[Discontinuities]
    B --> C[Spacing (S)]
    C --> D[Mechanical Behavior]
    D --> E{Low Shear Strength + Multiple Joint Sets}
    E --> F[Increased Deformation Risk]

Summary: Proper quantification of spacing is vital for rock stability analysis and design in geotechnical engineering.

Measurement Procedure for Discontinuity Spacing (IS 11315 Part 2)

• Tape Positioning (Clause 4.1):
  Hold the measuring tape approximately perpendicular to the discontinuity set on the rock exposure to measure spacing ( S ). Correct for directional bias if tape is not perpendicular.

• Spacing Categories (Clause 5.1):

• Modal Spacing Calculation (Clause 4.4):
  The most common (modal) spacing ( S_{model} ) is calculated from measured distances ( d_m ). Present data as histograms (Fig. 2) showing minimum, modal, and maximum spacing.

• Additional Notes:

  • Exclude blast damage fractures.
  • Average modal spacings represent typical block size if persistence is assumed.
  • Use seismic refraction or borehole techniques if surface exposure is limited.

• Rounding Off Results:
  Follow IS:2-1960 for rounding final values.

flowchart LR
    A[Measure spacing with tape] --> B{Is tape perpendicular?}
    B -- Yes --> C[Record distances between discontinuities]
    B -- No --> D[Apply directional bias correction]
    D --> C
    C --> E[Calculate modal spacing \(S_{model}\)]
    E --> F[Classify spacing category]
    F --> G[Present histogram of spacing]

This ensures standardized, precise reporting of discontinuity spacing per IS 11315 Part 2.

IS 11315 Part 2 - Presentation of Results for Discontinuity Spacing

Key Specifications:

• Spacing categories (Clause 5.1):

Important Formulas:

• Modal spacing (Sₘ) is the most common spacing measured in the set (Clause 4.4):

[ S_m = d_m ]

where (d_m) is the mode of the measured spacing distribution.

• Average block size can be approximated by the average of modal spacings (S_1, S_2, \ldots) assuming persistence.

Presentation Methods:

• Use histograms for each discontinuity set showing frequency distribution of spacing (Fig. 2).
• Overlay frequency curves to visualize modal values and dispersion.
• Mean spacing may be used instead of modal spacing for samples with multiple or poorly defined modes.

Notes:

• Exclude blast-induced fractures.
• Measurement tools: tape and compass recommended but visual judgement allowed.
• For limited exposure, use seismic refraction or borehole techniques to estimate spacing.

Diagram Concept (Histogram Example):

barChart
    title Discontinuity Spacing Histogram
    x-axis Spacing (mm)
    y-axis Frequency
    VeryClose <60 : 15
    Close 60-200 : 30
    Moderate 200-600 : 40
    Wide 600-2000 : 10
    VeryWide >2000 : 5

This approach ensures clarity in reporting discontinuity spacing for rock mass characterization per IS 11315 Part 2.

IS 11315 Part 2: Notes on Measurement and Interpretation of Discontinuity Spacing

Key Terminology for Spacing (Clause 5.1)

Modal Spacing Formula (Clause 4.4)

[ S_{\text{model}} = d_m ]

• (d_m) = most common (modal) distance measured.
• Modal spacing is best presented via histograms for clarity.

Presentation of Results (Clause 5.2)

• Use histograms for each discontinuity set.
• Plot frequency curves on the same diagram for comparison.
• Mean spacing can substitute modal spacing to handle multiple/poorly-defined modes.

Additional Notes

• Use measuring tape and compass for accuracy (Note 1).
• Average modal spacing approximates typical rock block size (Note 2).
• Exclude blast damage fractures (Note 4).
• For limited exposure, use seismic refraction or borehole techniques to estimate spacing (Notes 5 & 6).

Visualization Concept (Histogram)

graph LR
A[Discontinuity Set] --> B[Spacing Measurements]
B --> C[Histogram]
C --> D[Modal Spacing (dm)]
C --> E[Min Spacing (Smin)]
C --> F[Max Spacing (Smax)]

This approach ensures consistent measurement, interpretation, and presentation of discontinuity spacing in rock masses per IS 11315 Part 2.