Method for the quantitative description of discontinuities in rock masses, Part 6: Aperture
1987 Edition

Introduction to IS 11315 Part 6: Aperture in Rock Discontinuities

Scope Summary:

• Provides a method for quantitatively describing the aperture in rock mass discontinuities.
• Defines aperture as the perpendicular spacing between adjoining rock walls, which may be air or water filled.

Aperture Classification (Clause 5.1, Table 1)

SI Units and Derived Units (Clause 5.4)

Important Remarks

• Apertures classified as extremely wide (10–100 cm) and cavernous (>1 m) require photographic documentation.
• Classification assists in predicting rock mass behavior and supports engineering design.
• Measurement results should be rounded as per IS 2-1960.

IS 11315 Part 6: Defining the Scope and Aperture Terminology

Scope:

• Establishes aperture as the perpendicular distance between adjacent rock walls within a discontinuity, which may contain air or water.
• Provides a systematic approach to describe discontinuities influencing rock mass mechanical properties.
• Forms part of a comprehensive system addressing various discontinuity parameters such as orientation, spacing, roughness, and filling.

Aperture Categories (Table 1)

Relevant Units for Rock Mechanics

Notes on Data Presentation

• Photographic records are recommended for very wide and cavernous apertures.
• Final values must conform to rounding rules specified in IS 2-1960.

graph TD
    A[Rock Mass Discontinuity] --> B[Aperture Measurement]
    B --> C{Aperture Size}
    C -->|<0.25 mm| D[Tight (Closed)]
    C -->|0.25 - 0.5 mm| E[Partly Open]
    C -->|0.5 - 2.5 mm| F[Open]
    C -->|2.5 - 10 mm| G[Moderately Wide]
    C -->|> 10 mm| H[Wide]
    C -->|1 - 10 cm| I[Very Wide]
    C -->|10 - 100 cm| J[Extremely Wide]
    C -->|> 1 m| K[Cavernous]

Terminology and Definitions in IS 11315 Part 6

• Aperture: The orthogonal separation between adjacent rock surfaces forming an open discontinuity, potentially filled with air or water.
• Discontinuity: A mechanical break or weakness such as joints, faults, or shear zones characterized by limited tensile strength.

Aperture Size Classification (Clause 5.1, Table 1)

SI Units and Related Units (Clause 5.4)

Additional Notes

• Apply rounding as per IS 2-1960.
• Include photographic evidence for apertures exceeding 10 cm.

Procedures for Field Measurement of Aperture (IS 11315 Part 6)

• Instruments:

  • For small apertures (<10 mm): feeler gauges are preferred.
  • For larger apertures (>10 mm): steel rulers or tapes calibrated in millimeters.

• Methodology:

  • Document apertures of all discontinuities intersecting the measurement line.
  • For prominent discontinuities, note aperture variation along their trace.

• Special Techniques:

  • Borehole periscopes equipped with calibrated scales allow aperture measurement up to depths of approximately 30 meters.
  • Integral sampling with reinforced core extraction may be employed for specific scenarios.

• Considerations:

  • Surface aperture measurements may overestimate actual rock mass apertures due to weathering or excavation effects.
  • Aperture alone is insufficient to predict water flow; permeability tests are recommended.
  • Aperture dimensions can vary significantly in sheared or displaced joints.

Aperture Classification (Table 1)

Units (SI Units)

Diagram Overview

graph TD
    A[Perform Aperture Measurement] --> B{Aperture Size}
    B -->|<10 mm| C[Use Feeler Gauges]
    B -->|≥10 mm| D[Use Steel Rule or Tape]
    E[Borehole] --> F[Periscope (≤30 m depth)]
    E --> G[Camera and TV Inspection]
    E --> H[Pressure Packers]
    I[Special Cases] --> J[Integral Sampling Method]

Guidelines for Presenting Aperture Measurement Results (IS 11315 Part 6, Clause 5)

• Aperture Classification (Table 1):

• Measurement Notes:

  • Use feeler gauges for fine apertures.
  • Employ millimeter-scaled rulers for larger gaps.
  • Surface weathering or excavation may distort aperture sizes.
  • Borehole periscopes are recommended for depths up to 30 meters.
  • Correct measurements if boreholes intersect discontinuities at angles other than 90°.

• Photographic Documentation:

  • Include images for apertures classified as extremely wide and cavernous to support records.

• Measurement Units:

  • Length in metres (m)
  • Force in newtons (N) where 1 N = 1 kg·m/s²
  • Pressure in pascals (Pa) where 1 Pa = 1 N/m²

Aperture Classification Flowchart

graph LR
    A[Tight < 0.25 mm] --> B[Partly Open 0.25 - 0.5 mm]
    B --> C[Open 0.5 - 2.5 mm]
    C --> D[Moderately Wide 2.5 - 10 mm]
    D --> E[Wide > 10 mm]
    E --> F[Very Wide 1 - 10 cm]
    F --> G[Extremely Wide 10 - 100 cm]
    G --> H[Cavernous > 1 m]

Limitations and Precautions in Aperture Measurement (IS 11315 Part 6)

• Measurement Techniques:

  • Small apertures are estimated approximately using feeler gauges.
  • Larger apertures are measured with millimeter-graded rulers.
  • Borehole apertures are assessed via periscopes up to 30 meters depth, borehole cameras, TV inspection, or pressure-sensitive packers.

• Challenges:

  • Surface apertures often appear larger than actual due to weathering or excavation disturbances.
  • Machine-drilled tunnels and boreholes yield more accurate, undisturbed aperture data.
  • Optical distortions limit periscope use beyond 30 meters.
  • Aperture size varies along discontinuities because of asperities and debris accumulation.
  • Aperture dimensions alone do not reliably predict water permeability since surface roughness and flow paths affect conductivity.

• Accuracy Notes:

  • Correct aperture values for borehole angles if intersections with discontinuities are not perpendicular.
  • Integral sampling with reinforced core bars is advised for special cases.
  • Aperture measurements should be supplemented with in situ permeability testing for hydraulic assessments.

Aperture Size Classification (Table 1)

SI Units Reference