Method for the quantitative description of discontinuities in rock masses, Part 5: Wall strength
1987 Edition

Scope Overview: Defines the classification system for weathering states of rock masses and rock materials, and methods for evaluating rock strength for engineering applications.

Weathering State of Rock Mass (Table 1):

Weathering State of Rock Material (Table 2):

Manual Index Strength Classification (Table 3):

Schmidt Hammer Rebound Numbers & Corrections:

• Rebound numbers range from 10 (weak rock) to 60 (very strong rock).
• Standard application is vertical downward; corrections apply for other orientations (see Table 4).

Principal Terms and Definitions (IS 11315 Part 5 - 1987)

Weathering States of Rock Mass (Table 1):

Weathering States of Rock Material (Table 2):

Manual Index Strength Classification (Table 3):

Key Influences on Rock Wall Strength (IS 11315 Part 5):

• Weathering and alteration processes considerably diminish strength.
• Strength is influenced by rock mass continuity and intrinsic rock material properties.

Reference Tables:

Strength Testing Methods:

• Manual index tests provide uniaxial strength estimates.
• Schmidt hammer rebound correlates with compressive strength (non-destructive testing).

Typical Strength Estimation Equation: [ \sigma_c = k \times R ] Where:

• (\sigma_c) = compressive strength
• (R) = Schmidt hammer rebound number
• (k) = empirical calibration constant

Summary: Wall strength depends primarily on weathering degree, rock type, and test indices from manual and Schmidt hammer methods.

Required Instruments and Methods (IS 11315 Part 5, Clause 3.7):

• Geological hammer with pointed tip.
• Sharp knife or pen knife for manual index tests.
• L-type Schmidt hammer with manufacturer-supplied correction tables and strength conversion graph (Fig. 1).
• Equipment for measuring dry density of rock samples: oven, weighing balance, pycnometer, beakers.

Usage Notes:

• Schmidt hammer rebound values must be corrected for hammer orientation.
• Manual index tests are preferred on fresh, hand-sized rock samples.
• All results should be rounded as per IS 2-1960.

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

This systematic approach ensures consistent and reproducible rock wall strength assessments.

Test Methodology and Data Recording Highlights (IS 11315 Part 5):

• Adhere to IS 2-1960 for rounding off test results.
• Schmidt hammer testing involves measuring rebound numbers on discontinuity walls, including surfaces with mineral coatings.
• Rebound number ranges approximately 10 (weak rock) to 60 (strong rock), with corrections for non-vertical hammering.

Weathering Classifications:

Manual Index Strength (Table 3):

Data Processing & Strength Estimation (IS 11315 Part 5):

• Rock mass weathering grades range from fresh (I) to residual soil (VI).
• Rock material weathering states: fresh, discoloured, decomposed, disintegrated.
• Manual index tests classify strength from soft clay to extremely strong rock.
• Schmidt hammer (L-type) rebound numbers combined with dry density and correction tables estimate UCS.

Estimation Formula for Joint Compressive Strength (JCS):

• Calculated from mean rebound number and rock density using correlation graphs.

Illustration: For example, a rock classified as "medium strong" (R3) with fractured surface by hammer can have UCS approximately 25–50 MPa.

This methodology enables practical and reliable wall strength estimation.

Guidelines for Presenting Test Results (IS 11315 Part 5):

• Final strength values should be rounded according to IS 2:1960.

Weathering Classification of Rock Mass (Table 1):

Rock Material Weathering (Table 2):

Manual Index Strength (Table 3):

Schmidt Hammer Test Notes:

• Rebound number ranges from 10 to 60.
• Use vertical downward hits for standard reading.
• Apply corrections for other hammer orientations.

Limitations and Precautions in Field Testing (IS 11315 Part 5):

• Thin mineral coatings (e.g., calcite, chlorite, talc) affect rebound readings; tests on coatings must be documented.
• Rebound values vary with hammer orientation; vertical downward is baseline, others require correction.
• Weathering can reduce joint wall compressive strength to about 25% of intact rock strength.
• Loose or fractured rock masses pose challenges; tests may not represent actual strength due to discontinuities.

Schmidt Hammer Rebound Correction Table (Excerpt):

This highlights the need for careful interpretation and complementary geological assessments.

Reference Summaries for IS 11315 Part 5 (1987):

• Weathering classifications for rock mass and rock material (Tables 1 and 2).
• Manual index strength classification for soils and rocks (Table 3).
• Schmidt hammer testing procedures, rebound ranges, and correction guidelines (Clause 4.4.5, Fig. 1).

These references form the backbone for standardized rock wall strength evaluation.

IS 11315 Part 5 includes annexures and conversion charts to aid in test result interpretation and unit conversions.

Annexures typically provide:

• Sample calculation procedures.
• Standard formats for reporting.
• Graphs/charts for quick reference.

Conversion Tables cover:

• Unit conversions, e.g., psi to MPa, lb/ft³ to kg/m³.
• Strength equivalence for different curing durations.
• Aggregate sizing conversions.

Typical conversion factors:

Common strength conversion formula: [ f_{c,28} = f_{c,t} \times \frac{f_{c,28,ref}}{f_{c,t,ref}} ] Where (f_{c,28}) is strength at 28 days, and (f_{c,t}) is strength at age t.

For detailed annexure content, consult the full IS document.