The quantitative classification system of rock mass - Guidelines, Part 1: RMR for predicting engineering properties

IS 13365 Part 1: Scope & Key Specifications

• Scope:
  This standard provides guidelines for classification and characterization of rock masses for engineering purposes, focusing on rock mass quality, strength, and support design.

• Key Parameters (Clause 3.1.1):

  • Uniaxial Compressive Strength (qc):
    Determined per IS 9143, IS 8764, or IS 10785 from rock cores. Preferred over point load strength.
  • Rock Mass Number (N):
    Used for predicting ground conditions:
    [ N = Q \times SRF ] where
    • ( Q ) = Rock Quality Designation
    • ( SRF ) = Stress Reduction Factor

• Rock Mass Behavior Classification (Fig. 4):
  Based on ( N ), tunnel depth, and width, ground conditions are categorized as:

  • Self-supporting
  • Non-squeezing
  • Mild, Moderate, or High Squeezing

• Referenced IS Codes (Annex A):

  IS No.	Title
  8764:1978	Point Load Strength Index of Rocks
  9143:1979	Unconfined Compressive Strength of Rock Materials
  9221:1979	Modulus of Elasticity and Poisson's Ratio of Rock
  11315 (Parts 1-3)	Quantitative Description of Discontinuities

Summary Table: Rock Mass Number & Ground Condition

flowchart TD
    A[Rock Quality (Q)] --> C[Rock Mass Number (N)]
    B[Stress Reduction Factor (SRF)] --> C
    C --> D{Ground Condition}
    D -->|High N| E[Self-supporting]
    D -->|Moderate N| F[Non-squeezing]
    D -->|Low N| G[Mild to High Squeezing]

Note: Use latest editions of referenced IS codes for accurate

IS 13365 Part 1 (1998) - Key References, Formulas & Tables

1. Rock Mass Number (N) and Ground Condition Prediction

• Rock Mass Number:
  [ N = Q \times SRF ] where Q = Rock Quality, SRF = Stress Reduction Factor.

• Tunneling Conditions (Clause 5.1.8 / Fig.4):

2. In-situ Modulus of Deformability (E in GPa)

• Empirical correlations with Rock Mass Rating (RMR):

[ E = 2 \times RMR - 100 ] [ E = 10 \times \frac{(RMR - 10)}{40} ] [ E = 10 \text{ to } 40 \times \log_{10} Q ]

3. Recommended Indian Standards (Annex A)

IS 13365 Part 1: Collection of Field Data (Clause 3.1 & 3.1.1)

• Key Parameter:

  • Uniaxial Compressive Strength (UCS), qc of intact rock cores is primary for rock strength assessment.

• Testing Standards:

  • UCS tests on rock cores should follow:
    • IS 9143 (Rock core sampling)
    • IS 8764 (Methods of rock testing)
    • IS 10785 (Rock testing procedures)
  • Point Load Strength Index is secondary; UCS preferred.

• Data Recording:

  • Use data sheet format as per Annex B of IS 13365 Part 1 for recording parameters including UCS and point load strength.

• Rating System (Annex B, Item I):

  • Rock strength classification based on UCS values, e.g.:

    UCS (MPa)	Rock Strength Rating
    > 250	Very High
    100 – 250	High
    50 – 100	Medium
    < 50	Low

• Important Note:

  • Correlations using UCS are for feasibility and preliminary designs only.
  • In-situ tests (e.g., plate load, pressuremeter) are essential for final design.

Summary Diagram of Data Collection Flow

flowchart TD
    A[Rock Core Sampling] --> B[UCS Testing (IS 9143/8764/10785)]
    B --> C[Record UCS in Data Sheet (Annex B)]
    C --> D{Use for Design Stage?}
    D -->|Preliminary| E[Use UCS Correlations]
    D -->|Final| F[Conduct In-Situ Tests]

This ensures structured, reliable rock mass characterization per IS 13365 Part 1.

IS 13365 Part 1: Key Formulas & Tables for Rock Mass Rating (RMR)

1. Rock Mass Rating (RMR) Overview

• RMR classifies rock mass based on:
  • Uniaxial compressive strength of rock material
  • Rock quality designation (RQD)
  • Spacing of discontinuities
  • Condition of discontinuities
  • Groundwater conditions
  • Orientation of discontinuities (excluded if considered separately)
• Used to predict engineering properties like support pressure, stand-up time, and modulus of deformability.

2. Estimation of In-Situ Modulus of Deformability (E)

Three empirical correlations (E in GPa):

3. Prediction of Tunnelling Conditions (Clause 23.4)

• (N) = Rock mass number (from 4.3)
• (H) = Overburden (m)
• (B) = Tunnel width (m)

Note: Avoid double counting pore water pressure or joint orientation in both RMR and stability analysis.

4. Rock Mass Quality Classification

| RMR Range | Rock Quality (Q) | Description | |-----------

Engineering Properties of Rock Masses (IS 13365 Part 1)

1. Rock Mass Rating (RMR) & Classification (Table 1)

2. Shear Strength of Jointed Rock Masses (Clause 5.1.6)

• For poor rock masses (RMR < 60):

  [ \tau = A(\sigma_n + T)^B, \quad \tau = 0 \text{ if } \sigma_n < 0 ]

• For hard rock masses (RMR > 60):

  [ \tau_n = A(\sigma_n + T)^B, \quad \tau_n = 0 \text{ if } \sigma_n < 0 ]

Where:

• (\tau) = shear strength
• (\sigma_n) = normal stress on the joint
• (A, T, B) = constants (from Table 2, varies with moisture and saturation)
• (q_c) = uniaxial compressive strength of intact rock

3. Mobilised Uniaxial Compressive Strength for Tunnels:

[ q_{c, \text{mass}} = 70 \text{ MPa (approx.) for } Q \leq 10, J_w = 1

Prediction of Tunnelling Conditions (IS 13365 Part 1, Clause 5.1.8)

Tunnelling ground conditions are predicted using the correlation between Rock Mass Number (N), Tunnel Overburden (H in m), and Tunnel Width (B in m):

• N = Rock Mass Number (from IS 13365 Clause 4.3)
• H = Overburden depth (m)
• B = Tunnel width (m)

Additional Key Formulas:

• In-situ Modulus of Deformability, E (GPa):

[ \begin{cases} E = 2 \times RMR - 100 \ E = 10 \times \frac{RMR - 10}{40} \ E = 10 \text{ to } 40 \times \log_{10} Q \end{cases} ]

• Overburden vertical stress:

[ \sigma_v = 0.27 \times H \quad (\text{kg/cm}^2) ]

Notes:

• Avoid double counting parameters in RMR and structural analysis (e.g., pore water pressure or joint orientation).
• Groundwater inflow should be measured or qualit

Estimation of Support Pressure (IS 13365 Part 1)

Key Formula (Clause 7.5):

[ \text{Proof} = \frac{7.5 \times B^{0.1} \times H^{0.5} - \text{RMR}}{2 \times \text{RMR}} \quad \text{(kg/cm}^2) ]

• B = Span of opening (m)
• H = Overburden/tunnel depth (m), typically > 50 m
• RMR = Rock Mass Rating

Allowable Bearing Pressure

• Related to RMR, estimated as per IS 12070 (refer IS 12070 for detailed tables).

Prediction of Tunnelling Conditions (Clause 5.1.8)

Notes:

• Use Q-system (IS 13365 Part 2) with correct Stress Reduction Factor (SRF) for more reliable support pressure.
• These empirical relations are for steel rib supports in conventional blasting tunnels.

flowchart TD
    A[Input: Span

Stand-up Time for Unsupported Span
(IS 13365 Part 1:1998)

Key Definitions:

• Effective Span = Minimum of (size of opening, distance between tunnel face & support)
• For arched openings, stand-up time is significantly higher than flat roofs.
• Controlled blasting increases stand-up time by reducing rock damage.

Stand-up Time Guidelines (Clause 2.5 & Fig.1):

• Stand-up time decreases sharply with increasing unsupported span.
• Applies primarily to saturated rock masses in slopes.

Influencing Parameters:

• Rock Mass Cohesion (kg/cm²)
• Internal Friction Angle (°)
• Rock Mass Quality (Q) [IS 13365 Part 2]
• Unit Weight (Y) in g/cc
• Joint Roughness (Jr) & Joint Alteration (Ja) Numbers

Conceptual Relationship (Fig.1):

graph LR
  A[Unsupported Span] --> B[Stand-up Time]
  C[Rock Mass Quality] --> B
  D[Arch Shape] --> B
  E[Controlled Blasting] --> B
  B --> F[Safe Excavation Time]

Practical Use:

• Determine effective span.
• Use Fig.1 or Table above for initial stand-up time.
• Adjust based on rock parameters and excavation method.
• Use stand-up time for planning support installation to avoid collapse.

Summary:
Stand-up time is crucial for tunnel safety, varying inversely with unsupported span and directly with rock quality and support methods. Use IS 13365 Part 1 guidelines combined with rock mass parameters for design.

Adjustment for Joint Orientation (IS 13365 Part 1)

Key Points from Clause 3.1.6 & Annex B, C:

• Orientation of discontinuities is defined by strike (angle from magnetic north) and dip (angle between horizontal and discontinuity plane).
• Record strike and dip for critical joint sets unfavorable to the structure.
• Compare joint orientation with tunnel axis, slope face, or foundation alignment.
• Use Annex C, Tables C1 & C2 to assess joint favourability for tunnels and dam foundations.
• Adjust the basic Rock Mass Rating (RMR) by applying orientation factors from Item VII, Annex B.

Procedure for Adjustment:

1. Determine joint strike and dip relative to tunnel axis.
2. Classify joint favourability using Tables C1 (tunnels) or C2 (foundations).
3. Apply orientation adjustment factor to the basic RMR:

[ RMR_{adjusted} = RMR_{basic} \times F_{orientation} ]

Where ( F_{orientation} ) is from Annex B (Item VII).

Typical Orientation Adjustment Factors (Example):

Summary Diagram:

flowchart TD
    A[Record Joint Strike & Dip] --> B[Compare with Tunnel Axis]
    B --> C{Use Tables C1/C2}
    C -->|Favorable| D[Factor = 1.0]
    C -->|Moderate| E[Factor = 0.85]
    C -->|Unfavorable| F[Factor = 0.7]
    D --> G[Calculate Adjusted RMR]
    E --> G
    F --> G

References:

• Annex B (Item VI & VII): Strike, dip recording & orientation adjustment.
• Annex C (Tables C1 & C2): Joint favourability classification.
• Clause 4.5: Separate RMR for different tunnel orientations.

This adjustment ensures RMR reflects realistic stability conditions based on

IS 13365 Part 1 (1998) — List of Referred Indian Standards

This standard references several IS codes related to rock mass characterization and design:

Key Notes:

• These referenced standards provide methods for testing rock strength, elasticity, discontinuities, and classification.
• IS 13365 Part 1 uses these for rock mass classification and design guidance.
• Always check for the latest editions of these standards.

Example: Rock Mass Number (N) Calculation (from IS 13365 Part 1)

[ N = Q \times SRF ]

Where:

• Q = Rock Quality Designation (from rock mass classification)
• SRF = Stress Reduction Factor (accounts for rock stress and excavation conditions)

flowchart LR
    A[Rock Mass Parameters] --> B[Point Load Strength Index (IS 8764)]
    A --> C[Unconfined Compressive Strength (IS 9143)]
    A --> D[Modulus of Elasticity & Poisson's Ratio (IS 9221)]
    B --> E[Rock Quality Designation (Q)]
    C --> E

IS 13365 Part 1: Data Sheet & Key Formulas for Geomechanical Classification of Rock Masses

1. Rock Mass Rating (RMR) & Engineering Properties (Clause 5.1)

• Rock mass rating (RMR) correlates with engineering properties (strength, deformability).
• Use Table 1 (Clause 5.1) for ranges of properties based on RMR.
• Interpret properties within recommended ranges for given RMR.

2. Prediction of Tunnelling Conditions (Clause 23.4)

• N = rock mass number (Clause 4.3), B = tunnel width (m), H = overburden (m).

3. In-situ Modulus of Deformability, ( E ) (GPa)

Three empirical formulas:

[ \begin{aligned} E &= 2 \times RMR - 100 \ E &= 10 \times \frac{RMR - 10}{40} \ E &= 10 \text{ to } 40 \times \log_{10} Q \end{aligned} ]

• (RMR): Rock Mass Rating
• (Q): Rock mass quality number

4. Recommended Mohr Envelopes for Jointed Rock Masses (Table 2, Clause 5.1.6)

• Rock mass quality classified by RMR and Q values:

IS 13365 Part 1: Assessment of Joint Favorability for Tunnels and Foundations

Key Formulas & Correlations for Tunneling Conditions (Clause 5.1.8)

Shear Strength of Jointed Rock Masses (Clause 5.1.6)

[ \tau = A(\sigma_n + T)^B, \quad \text{with } \tau=0 \text{ if } \sigma_n < 0 ]

• (\tau) = shear strength
• (\sigma_n) = normal stress on joint
• (A, T, B) = constants from Table 2 (vary with rock quality & saturation)
• (q_c) = uniaxial compressive strength of intact rock

In-situ Modulus of Deformability (E) (GPa)

• (E = 2 \times RMR - 100)
• (E = 10 \times \frac{RMR - 10}{40})
• (E = 10 \text{ to } 40 \times \log

IS 13365 Part 1: Committee Composition - Key Details

The Rock Mechanics Sectional Committee, CED 48 is responsible for formulating IS 13365 Part 1. The committee comprises experts from academia, government departments, research institutes, and industry.

Committee Structure (Annex D Summary):

Notes:

• The committee includes technical experts and government officials ensuring a comprehensive approach.
• The committee composition is detailed in Annex D of IS 13365 Part 1.
• This committee oversees rock mechanics standards including classification and rating systems.

If you need formulas or tables related to rock mass classification (RMR, RQD, discontinuity spacing), please specify!