Method for the quantitative description of discontinuities in rock masses, Part 5: Wall strength

IS 11315 Part 5 - Scope: Key Formulas, Tables & Specifications

Scope Summary:

Defines classification and assessment of rock mass weathering, rock material weathering, and rock strength for engineering purposes.

1. State of Weathering of Rock Mass (Table 1)

2. State of Weathering of Rock Material (Table 2)

3. Strength of Rock by Manual Index (Table 3)

4. Schmidt Hammer Rebound Number & Corrections

• Rebound number (r) correlates with rock strength (10-60 range).
• Lowest for weak rocks (<20 MPa), highest for very strong (>150 MPa).
• Use vertical downward hammer application for standard correlation.
• Corrections apply for other hammer directions (Table 4).

Key Terminology & Definitions from IS 11315 Part 5 (1987)

1. Weathering State of Rock Mass (Table 1)

2. Weathering State of Rock Material (Table 2)

3. Strength of Rock by Manual Index (Table 3)

IS 11315 Part 5 – Factors Affecting Wall Strength

Key Factors Affecting Wall Strength:

• Weathering & Alteration of rock mass significantly reduce strength.
• Strength depends on rock mass continuity and rock material properties.

Important Tables & Tests:

Schmidt Hammer Test (Fig. 1):

• Non-destructive test to estimate surface hardness.
• Correlates rebound number to compressive strength.

Typical Strength Estimation Formula:

[ \sigma_c = k \times R ]

• (\sigma_c): Compressive strength
• (R): Rebound number from Schmidt hammer
• (k): Empirical constant from calibration

flowchart LR
    A[Rock Mass Weathering] --> B[Strength Reduction]
    C[Rock Material Type] --> B
    D[Manual Index Tests] --> E[Strength Estimation]
    F[Schmidt Hammer Test] --> E
    B --> E

Summary: Wall strength depends on rock weathering, rock type, and test-based strength indices (manual and Schmidt hammer). Use Tables 1-3 and Fig.1 for assessment.

IS 11315 Part 5 (1987) - Equipment and Test Methods Key Points

Equipment for Wall Strength Evaluation (Clause 3.7)

• Geological hammer with a tapered point.
• Sharp tool (strong pen knife or similar).
• Schmidt Hammer (L-type) with:
  • Manufacturer’s conversion table to correct rebound number for orientation.
  • Graph (Fig. 1) to convert corrected rebound number to estimated uniaxial compressive strength (UCS).
• Equipment for dry density measurement of small rock samples:
  • Oven
  • Balance
  • Pyknometer
  • Beaker

Reporting Results (Clause 0.6)

• Final test or analysis values must be rounded off as per IS 2-1960 standard.

Typical Schmidt Hammer Use Formula:

[ \text{UCS} = f(\text{Corrected Rebound Number}) ]

• Use manufacturer’s table for correction.
• Use graph (Fig. 1) for UCS estimation.

Summary Table Example (from Manufacturer)

flowchart LR
    A[Geological Hammer] --> B[Rock Surface Preparation]
    C[Schmidt Hammer] --> D[Rebound Number Measurement]
    D --> E[Correction using Table]
    E --> F[Estimate UCS using Graph]
    G[Sample] --> H[Dry Density Measurement]
    H --> I[Oven, Balance, Pyknometer]

This ensures standardized, reproducible rock strength evaluation and reporting per IS 11315 Part 5.

IS 11315 Part 5 - Test Procedures and Data Collection: Key Formulas, Tables, and Specifications

1. Rounding Off Results

• Follow IS 2-1960 for rounding final observed or calculated test values.

2. Schmidt Hammer Test (Clause 4.4.5)

• Used to estimate Joint Compressive Strength (JCS) on discontinuities, including mineral coatings.
• Rebound number (r) ranges:
  • Weak rocks: ~10 (Fc < 20 MPa)
  • Very strong rocks: ~60 (Fc > 150 MPa)
• Corrections needed when hammer is not vertical downwards.

3. Rock Weathering Classification (Table 1 & 2)

4. Manual Index Strength of Rock (Table 3)

IS 11315 Part 5: Data Analysis & Strength Estimation Summary

1. Strength Governed by Weathering (Clause 3.6)

• Rock Mass Weathering Grades (Table 1):
  • Fresh (Grade I) to Residual Soil (Grade VI)
• Rock Material Weathering (Table 2):
  • Fresh, Discoloured, Decomposed, Disintegrated

2. Strength Estimation by Manual Index Tests (Table 3)

• Note: S1-S6 for cohesive soils; RO-R6 for rock discontinuity walls.

3. Schmidt Hammer Test (Clause 3.6, Fig. 1)

• Use L-type Schmidt hammer.
• Measure rebound number (r) and dry density (γ).
• Use manufacturer’s correction table and Fig. 1 graph to estimate uniaxial compressive strength (UCS).

4. Equipment (Clause 3.7)

• Geological hammer (pointed)
• Pen knife
• Schmidt hammer (L-type)
• Oven, balance, pycnometer (for density)

5. Estimation Formula (Clause 4.4.3)

• Joint Compressive Strength (JCS) estimated from mean rebound number (r) and rock density (γ) using Fig. 1 correlation chart.

Example: Manual Index to UCS

If rock is "Medium strong" (R3), fractured by single hammer blow:
UCS ≈ 25-50 MPa

Visual: Schmidt Hammer Correlation (Simplified)

graph LR
A[Rebound Number (r)] --> B[Correct for Orientation]
B --> C[Use Fig. 1 Chart]
C --> D[Estimate UCS (MPa)]
E[Rock Density (γ)] --> C

This process ensures a practical

IS 11315 Part 5 (1987) - Presentation of Results: Key Points

1. Rounding Off Results

• Final values must be rounded as per IS 2:1960 standard.

2. Weathering Classification of Rock Mass (Table 1)

3. Weathering of Rock Material (Table 2)

4. Strength of Rock by Manual Index (Table 3)

• Note: S1-S6 apply to cohesive soils; R0-R6 apply to rock discontinuities.

5. Schmidt Hammer Testing Notes & Corrections

• Rebound number r ranges 10-60 (weak to very strong rock).
• Use vertical downward hammering for standard correlation.
• Apply corrections if hammer used at other angles (Table 4

IS 11315 Part 5 (1987) - Limitations and Precautions Summary

Key Points on Schmidt Hammer Testing (Clause 4.4.5):

• Mineral Coatings: Thin mineral coatings (e.g., calcite, chlorite, talc) can affect rebound values; test directly on coating surface.
• Rebound Number Range: 10 (weak rock, <20 MPa) to 60 (very strong rock, >150 MPa).
• Testing Direction: Rebound number varies with hammer orientation.
  • Vertical downwards: baseline.
  • Corrections needed for other directions (see Table 4).
• Weathering Effect: Joint wall compressive strength (JCS) may be ~25% of intact rock strength due to weathering.
• Block Movement: May cause low rebound values; high values are rare.

Table 4: Schmidt Hammer Rebound Correction (r)

Weathering Classification (Clause 3.6, Table 1 & 2)

IS 11315 Part 5 (1987) — Key References Summary

1. Weathering States of Rock Mass (Clause 3.6, Table 1)

2. Weathering States of Rock Material (Table 2)

3. Strength of Rock by Manual Index (Clause 3.6, Table 3)

4. Schmidt Hammer Testing (Clause 4.4.5 & Fig.1)

• Rebound number (r): 10 (weak rock, <20 MPa) to 60 (very strong rock, >150 MPa).
• Use vertical downward hits for standard correlation.
• Apply correction factors (Table 4) for other orientations:

| Rebound (r) | -45° | +45° | Horizontal (0°) |

IS 11315 Part 5 (1987) primarily deals with the presentation of results for concrete tests, including annexures and conversion tables.

Key Points on Annexures & Conversion Tables:

• Annexures typically include:

  • Sample calculation formats.
  • Standardized reporting templates.
  • Graphs or charts for quick reference.

• Conversion Tables cover:

  • Unit conversions (e.g., psi to MPa, lb/ft³ to kg/m³).
  • Strength equivalence tables for different curing periods.
  • Aggregate size conversions.

Common Useful Conversions (Indicative):

Typical Formula for Strength Conversion:

[ f_{c,28} = f_{c,t} \times \frac{f_{c,28,ref}}{f_{c,t,ref}} ]

Where:

• ( f_{c,28} ) = compressive strength at 28 days.
• ( f_{c,t} ) = compressive strength at age t.
• ( f_{c,28,ref}, f_{c,t,ref} ) = reference strengths from standard curves.

For detailed annexures and exact tables, refer to the official IS 11315 Part 5 document.