Method of determination of point load strength index of rocks

IS 8764 - Scope & Key Specifications Summary

• Scope:
  IS 8764 covers the point load test for rock strength, used for quick estimation of rock strength and classification.

• Key Limitation:
  Tests are unreliable if the point load strength index (Is or Is50) < 1 MPa (Clause 1.3).

• Referenced Standards:

  • IS 11358: Glossary of rock mechanics terms
  • IS 13030: Test methods for rock properties (water content, porosity, density)

• Typical Test Setup (Figures 1 & 2):

  • Loading platen with central welded nut and fixing hole
  • Vertical support with 25 mm hole
  • Bearing plate: 10 mm thick
  • Angle section: 40×40×5 mm
  • Diaphragm bolt for platen alignment check
  • Thin cross-wires for specimen positioning

• Load Gauges (Clause 4.3.3):

  Gauge No.	Max Capacity	Reading Range	Accuracy
  1	25 kN (2500 kgf)	up to 0.25 kN (25 kgf)	±2%
  2	100 kN (10,000 kgf)	up to 0.50 kN (50 kgf)	±2%

• Important Formula (Substituted in Clauses 7.1.1 & 7.3):
  Although specifics are not provided here, the standard uses:
  [ Is = \frac{P}{D^2} ]
  where:

  • ( Is ) = Point Load Strength Index (MPa)
  • ( P ) = Failure load (N)
  • ( D ) = Distance between loading points (mm)

flowchart TD
    A[Specimen] -->|Load applied| B[Loading Platen]
    B --> C[Load Gauge]
    C --> D[Read Load P]
    D --> E[Calculate Is = P / D²]
    E --> F{Is < 1 MPa?}
    F -- Yes --> G[Results

IS 8764 - Key References, Formulas & Specifications

1. Referenced IS Standards

• IS 11358:1987 — Glossary of terms & symbols in rock mechanics.
• IS 13030:1991 — Test methods for water content, porosity, density of rock.

These provide foundational definitions and test procedures referenced by IS 8764.

2. Point Load Strength Index (Is)

• Tests unreliable if Is < 1 MPa (Clause 1.3).
• Formula for Is (substituted in clauses 7.1.1 & 7.3, page 6) typically:

[ I_s = \frac{P}{D^2} ]

Where:

• (P) = failure load (N)
• (D) = core diameter or equivalent dimension (mm)

3. Loading Platens (Clause 4.5.1)

• Conical platens with 60° angle.
• Radius of curvature at cone truncation = 5 mm.
• Cone and spherical truncation meet tangentially (Fig. 1).

4. Specimen Shapes (Fig. 3)

• Shapes for:
  • (a) Diametral test
  • (b) Axial test
  • (c) Block test
  • (d) Irregular lump test
• Minimum cross-sectional area (W = W_1 + W_2 + W_3) (all in mm).

5. Equivalent Core Concept

• Used for irregular lumps to define a core diameter equivalent for testing.

flowchart LR
    A[Rock Sample] --> B[Specimen Preparation]
    B --> C{Test Type}
    C --> |Diametral| D[Diametral Test]
    C --> |Axial| E[Axial Test]
    C --> |Block| F[Block Test]
    C --> |Irregular| G[Irregular Lump Test]
    D & E & F & G --> H[Apply Load via 60° Conical Platens]
    H --> I[Measure Failure Load P]
    I --> J[Calculate Point Load Strength Index \(I_s = \frac{P}{D^2}\)]

**

IS 8764: Definitions & Key Specifications

• Point Load Strength Index (Is or /L):

  • Tests unreliable if Is < 1 MPa (Clause 1.3).
  • Used to estimate rock strength from point load tests.

• Loading Platens (Clause 4.5.1):

  • Conical shape with 60° angle.
  • Radius of curvature at cone truncation: 5 mm.
  • Cone and spherical truncation meet tangentially (Fig. 1).

• Equivalent Core Section (Fig. 3):

  • Minimum cross-sectional area W = (W1 + W2 + W3) / 3 (all dimensions in mm).
  • Applies to diametral, axial, block, and irregular lump tests.

• Referenced IS Standards:

  • IS 11358: Glossary of rock mechanics terms.
  • IS 13030: Test methods for water content, porosity, density of rock.

Typical Loading Platen Geometry (Fig. 1)

graph TD
    A[Conical Platen] -->|Angle 60°| B[Truncated Cone]
    B -->|Radius 5 mm| C[Spherical Truncation]
    C -->|Tangential meeting| A

These definitions ensure consistent rock strength testing and specimen preparation as per IS 8764.

IS 8764: Apparatus and Equipment Key Points

Gauges (Clause 4.3.3)

• Gauge No. 1:
  • Max capacity: 25 kN (2,500 kgf)
  • Readability: 0.25 kN (25 kgf)
• Gauge No. 2:
  • Max capacity: 100 kN (10,000 kgf)
  • Readability: 0.50 kN (50 kgf)
• Accuracy: ±2%

Important Specifications

• Point load strength index (Is or /L) tests are unreliable if Is < 1 MPa (Clause 1.3).
• Figures 1 & 2 depict typical loading platen and diaphragm bolt for alignment; dimensions include:
  • Angle: 40×40×5 mm
  • Bearing plate thickness: 10 mm
  • Fixing hole: 25 mm diameter
• Quick release coupling and thin cross-wires are part of the setup.

References to Other IS Codes

• IS 11358: Glossary for rock mechanics terms
• IS 13030: Test methods for rock properties (water content, porosity, density)

Typical Point Load Testing System Components (Fig. 1 & 2)

graph LR
A[Vertical Support] --> B[Central Welded Nut]
B --> C[Loading Platen]
C --> D[10 mm Bearing Plate]
D --> E[Angle 40x40x5 mm]
C --> F[Quick Release Coupling]
E --> G[Fixing Hole 25 mm]

This summary covers essential apparatus specs and gauge details for point load testing per IS 8764. For exact formulas (7.1.1, 7.3), refer to the latest standard edition.

IS 8764: Sampling and Specimen Preparation - Key Points

• Specimen Size & Shape (Clause 5.3):
  Samples (core or lumps) must meet size and shape requirements for different tests:

  • Diametral
  • Axial
  • Block
  • Irregular lump
    Refer Fig. 3 for detailed dimensions.

• Number of Specimens (Clauses 6.4.2 & 6.6.2):

  • Minimum 10 specimens per sample recommended.
  • Increase number if sample is heterogeneous or anisotropic for statistical reliability.

• Sample Identification (Clause 9.2):
  Record and report:

  • Sample ID
  • Location & depth of sampling
  • Date of sampling and testing

Typical Specimen Preparation Guidelines (from IS 8764 & practice):

* Exact sizes per Fig. 3 of IS 8764.

flowchart TD
    A[Sample Collection] --> B[Specimen Preparation]
    B --> C{Specimen Type}
    C -->|Diametral| D[Cut to Disc Shape]
    C -->|Axial| E[Cut Cylindrical Specimen]
    C -->|Block| F[Cut Rectangular Block]
    C -->|Irregular Lump| G[Use as Received]
    D & E & F & G --> H[Minimum 10 Specimens]
    H --> I[Testing]
    I --> J[Report: Sample ID, Location, Depth, Date]

Summary: Ensure specimens conform to dimensions in Fig. 3, prepare minimum 10 specimens per sample, and document all sampling details as per IS 8764.

IS 8764: Test Procedures - Key Points

1. Point Load Strength Index (Is or /L):

   • Tests are unreliable if Is < 1 MPa (Clause 1.3).
   • This index is crucial for rock strength assessment.

2. Referenced Standards:

   • IS 11358:1987 – Glossary of rock mechanics terms.
   • IS 13030:1991 – Laboratory methods for water content, porosity, density, etc.

3. Diametral Test (Clause 6.4):

   • Used to determine tensile strength of rock cores by applying load diametrically.
   • Typical formula for tensile strength, σ_t:
     [ \sigma_t = \frac{2P}{\pi d l} ]
     where:
     • (P) = Failure load
     • (d) = Diameter of core
     • (l) = Length of core

4. Design Figures:

   • Figures 1 & 2 (not shown here) provide typical test setups; alternative designs are acceptable if suitable.

Summary Table: Tensile Strength by Diametral Test

flowchart LR
    A[Apply Load P] --> B[Rock Core Diameter d]
    B --> C[Rock Core Length l]
    C --> D[Calculate Tensile Strength σ_t = 2P / (π d l)]
    D --> E[Evaluate Rock Tensile Strength]

This concise summary covers key test formulas and references for IS 8764 test procedures.

IS 8764: Point Load Strength Index (PLSI) - Key Formulas & Specs

1. Point Load Strength Index (Clause 7.1.1)

[ I_{s(50)} = \frac{P}{D_e^2} ]

Where:

• (I_{s(50)}) = Point Load Strength Index for standard core size (50 mm) in MN/m² (or kgf/cm²)
• (P) = Failure load in Newtons (N)
• (D_e) = Equivalent core diameter in mm
• (D = 50) mm (standard core diameter)

2. Equivalent Core Diameter, (D_e)

If core diameter (D \neq 50) mm, convert to standard size using:

[ D_e = \sqrt{\frac{4A}{\pi}} ]

Where (A) is the cross-sectional area of the core.

3. Point Load Strength Anisotropy Index (Clause 8.1)

[ \text{Anisotropy Index} = \frac{I_{s(50)}^{\perp}}{I_{s(50)}^{\parallel}} ]

• Ratio of point load indices measured perpendicular and parallel to the rock fabric.

Summary Table:

flowchart TD
    A[Measure Failure Load P (N)] --> B[Measure Core Diameter D (mm)]
    B --> C{Is D = 50 mm?}
    C -- Yes --> D[Calculate \(I_{s(50)} = P / D^2\)]
    C -- No --> E[Calculate Equivalent Diameter \(D_e\)]
    E --> D

IS 8764: Key Points on Anisotropic Rock Testing

1. Uniaxial Compressive Strength (qc) vs Point Load Strength Index (/L(50))

• For unweathered rocks:

  [ q_c = 15 \times /L(50) ]

2. Testing Directions for Anisotropic Rocks (Clause 8)

• Test in directions of greatest and least strength, generally:

  • Parallel to planes of anisotropy (weakest)
  • Normal to planes of anisotropy (strongest)

• Core drilling:

  • Preferably drill perpendicular to weakness planes.
  • Angle between core axis and normal to weakness planes ≤ 30°.

• Load application:

  • For least strength: load along a single weakness plane.
  • For greatest strength: load perpendicular to weakness planes.

3. Strength Anisotropy Index (Clause 8.1.1)

[ I_a(50) = \frac{/L_{\perp}(50)}{/L_{\parallel}(50)} ]

• (I_a(50) \approx 1) for quasi-isotropic rocks.
• Higher values indicate stronger anisotropy.

4. Additional Notes

• Tests unreliable if ( /L < 1 , \text{MPa} ).
• Shortest dimension of anisotropic lumps usually perpendicular to weakness planes.

Summary Table

flowchart LR
    A[Rock Sample] --> B{Is it Anisotropic?}
    B -- Yes --> C[Test in directions]
    C --> D[Parallel to weakness planes (Least strength)]
    C --> E[Perpendicular to weakness planes (Greatest strength)]
    D --> F[Apply

IS 8764: Reporting and Identification of Samples - Key Points

Identification (Clause 9.2)

• Report must include:
  • Sample ID
  • Location of sampling
  • Depth of sampling
  • Date of sampling and testing

Sample Requirements (Clause 5.3 & 8.1.2)

• Samples (core or lumps) must meet size and shape requirements as per Fig. 3 (typically diametral, axial, block, or irregular lump).
• For anisotropic rock lumps, the shortest dimension is perpendicular to weakness planes.

Reporting Flexibility (Clause 5.7)

• If ideal sample size/shape isn't possible (broken/slabby rock or limited quantity), relax requirements but clearly state modifications.
• For core logging, tests may be done at arbitrary depth intervals (e.g., every 1m or 3m).
• Strength values with limited reliability are preferable to none.

Summary Table for Sample Reporting

flowchart TD
    A[Sample Collection] --> B[Record ID, Location, Depth, Date]
    B --> C{Sample meets size & shape?}
    C -- Yes --> D[Proceed with standard testing]
    C -- No --> E[Modify procedure & record changes]
    D --> F[Report Results with full details]
    E --> F

This ensures traceability and clarity in rock strength testing per IS 8764.

IS 8764: Correlation of Uniaxial Compressive Strength (UCS) with Point Load Strength

Key Formulas:

• Uniaxial Compressive Strength (qc) from corrected point load strength (Is(50)):

[ q_c = 13 \times I_s(50) ]

• Alternate correlation (Clause 7.4) for unweathered rocks:

[ q_c = 15 \times I_s(50) ]

where:

• ( q_c ) = uniaxial compressive strength (MN/m² or kgf/cm²)
• ( I_s(50) ) = corrected point load strength index (MPa)

Important Specifications:

• Applicability:

  • Correlations valid only for unweathered rocks.
  • For anisotropic rocks (shaly, bedded, schistose), test in directions of maximum and minimum strength (parallel and normal to weakness planes).

• Core orientation:

  • Core axis should be drilled perpendicular to weakness planes (angle ≤ 30°) for best results.

• Reliability:

  • Tests unreliable if ( I_s ) or ( I_s(50) < 1 ) MPa.

Summary Table:

flowchart LR
    A[Point Load Test] --> B[Calculate \(I_s(50)\)]
    B --> C{Is rock unweathered?}
    C -- Yes --> D[Use \(q_c = 15 \times I_s(50)\)]
    C -- No --> E[Use \(q_c = 13 \times I_s(50)\)]
    D & E --> F[Estimate UCS]

IS 8764: Validity and Interpretation of Test Results

Key Specifications:

• Minimum specimens/sample: Preferably ≥10 (Clause 6.5.2), more if sample is heterogeneous or anisotropic.
• Validity of tests: Refer to Fig. 5 for valid/invalid failure modes:
  • Valid: Diametral, Axial, Block tests.
  • Invalid: Core test failure, invalid axial tests.
• Point Load Strength Index (Is or I/L): Tests unreliable if Is < 1 MPa (Clause 1.3).

Mean Strength Index Calculation (Clause 7.1.2):

• For ≥10 valid tests:
  1. Remove two highest and two lowest values.
  2. Calculate mean of remaining values.
• For fewer specimens:
  1. Remove highest and lowest values.
  2. Calculate mean of remaining.

Summary Table: Mean Strength Index Calculation

Additional References:

• IS 11358: Glossary of rock mechanics terms.
• IS 13030: Test methods for water content, porosity, density.

flowchart LR
    A[Test Specimens ≥10] --> B{Remove Extremes}
    B -->|Remove 2 highest + 2 lowest| C[Calculate Mean of Remaining]
    A2[Test Specimens <10] --> B2{Remove Extremes}
    B2 -->|Remove highest + lowest| C2[Calculate Mean of Remaining]

This ensures statistically reliable strength index values and filters out outliers or invalid tests.