Method for the determination of unconfined compressive strength of rock materials
1979 Edition

Overview of the Standard's Scope

IS 9143 describes the procedure for testing rock specimens under axial compression to determine their strength properties. It aligns globally accepted methods with Indian field conditions.

Essential Specifications:

• Specimen Type: Cylindrical rock cores with diameter-to-height ratio typically 1:2.
• Loading Equipment: Must apply axial force with adequate capacity and maintain a controlled loading rate as specified.
• Report Details: Must include specimen count, dimensions, failure mode, rock type, anisotropy orientation, sampling data, storage conditions, testing details, moisture, temperature, loading rate, and other physical characteristics.

Formula for Compressive Strength Calculation:

[ \sigma_c = \frac{P}{A} = \frac{4P}{\pi d^2} ]

where (P) is the peak load in Newtons and (d) is specimen diameter in millimeters.

Typical Parameters:

flowchart TD
    A[Sample Preparation] --> B[Dimension Verification]
    B --> C[Load Application]
    C --> D[Load & Deformation Measurement]
    D --> E[Failure Mode Documentation]
    E --> F[Test Reporting]

For complete procedural details, refer to the standard.

Summary of Apparatus Requirements

Rock Specimen (Clause 3.1)

• Preferably a right circular cylinder.
• Tolerances as defined in Clause 3.3.
• Other regular shapes accepted if reported.
• Preparation guided by IS 9179-1979.

Loading Apparatus (Clause 2.1)

• Must apply precise axial load.
• Capacity aligned with specimen size.
• Load application rate controlled as per Clause 4.3.
• Calibration and verification at regular intervals.

Reporting Details (Clause 6.2)

Include specimen count, shape, failure mode, rock lithology, anisotropy orientation, sampling and storage details, testing date and equipment, specimen dimensions, moisture, ambient temperature, testing duration, stress rate, and additional physical properties.

Compressive Strength Calculation:

[ \sigma_c = \frac{P}{A} = \frac{4P}{\pi d^2} ]

where (\sigma_c) is compressive strength, (P) is load at failure, and (d) is specimen diameter.

Apparatus Flow Diagram

flowchart LR
    A[Load Application Machine] -->|Axial Load| B[Rock Specimen]
    B --> C[Load Measurement]
    B --> D[Failure Monitoring]

Refer to IS 9179-1979 for specimen prep and ensure calibration adherence.

Preparation and Dimension Guidelines

Specimen Shape and Size (Clauses 3.1 and 3.3)

• Shape: Preferably a right circular cylinder.
• Length-to-Diameter Ratio: Ideally between 2 and 3.
  • If less than 2, apply correction assuming L/D=2.
• Diameter:
  • Greater than 10 times the largest grain size.
  • Preferably 45 mm, with a minimum of 35 mm.

Dimensional Tolerances

• End flatness: Within 0.05 mm.
• Ends parallelism: Within 0.002 times diameter.
• Ends perpendicularity: Within 0.001 radians (~0.05 mm for 45 mm diameter).
• Cylindrical surface: Smooth and straight within 0.3 mm over length.
• Diameter variation: Not exceeding 0.2 mm along specimen length.
• Diameter measurement: Average of two diameters at three heights (top, middle, bottom) measured at right angles, with 0.1 mm accuracy.

Measurement Method

• Take diameters at three different heights.
• Each height: two perpendicular diameters.
• Average all six measurements for final diameter.

Length Correction Formula (if L/D < 2):

[ \text{Corrected Strength} = \text{Measured Strength} \times \frac{L/D}{2} ]

flowchart LR
    A[Specimen Prep] --> B[Shape: Right Circular Cylinder]
    B --> C[Diameter ≥ 35 mm, Preferably 45 mm]
    B --> D[Length/Diameter Ratio: 2–3]
    D --> E{If L/D < 2}
    E -->|Yes| F[Apply Correction]
    E -->|No| G[No Correction]
    A --> H[End Flatness ≤ 0.05 mm]
    A --> I[Ends Parallel ≤ 0.002 D]
    A --> J[Ends Perpendicular ≤ 0.001 radians]
    A --> K[Smooth and Straight Cylinder Surface]

Procedure for Conducting UCS Test

• Specimen (Clauses 3.1 & 2.3): Preferably right circular cylinders prepared as per IS 9179-1979, with required tolerances.
• Loading Machine (Clause 2.1): Must accurately apply axial load with capacity suited to specimen, maintaining loading rate per Clause 4.3 to avoid shocks.
• Result Rounding (Clause 0.4): Follow IS 2-1960 for numerical rounding.

Loading Rate Guidelines (Clause 4.3):

Testing Flowchart

flowchart TD
    A[Prepare Specimen] --> B[Set Up Load Machine]
    B --> C[Apply Axial Load at Specified Rate]
    C --> D[Record Load and Deformation]
    D --> E[Compute Results]
    E --> F[Round Data as per IS 2-1960]
    F --> G[Compile Test Report]

Refer to Clauses 3.3, 4.3 of IS 9143 and IS 9179 for detailed specimen and loading specifications.

Calculation of Rock UCS per IS 9143

Main Formula (Clause 5.1)

[ \text{UCS} = \frac{P_{max}}{A} ]

where:

• (P_{max}) = Peak axial load the specimen sustains (N)
• (A) = Cross-sectional area of the specimen (mm²), calculated based on specimen diameter.

Details

• Specimens typically have regular shapes, mainly cylindrical.
• UCS is an index test used for rock strength classification.
• Portable apparatus enables testing in laboratory or field conditions.

Notes

• Cross-sectional area is commonly measured at specimen mid-height.
• Specimen dimensions usually range from 38 to 54 mm diameter and length 2–2.5 times diameter.
• UCS is expressed in megapascals (MPa).

flowchart TD
    A[Rock Specimen] --> B[Measure Diameter & Height]
    B --> C[Compute Cross-sectional Area: A = πd²/4]
    C --> D[Apply Axial Load Until Failure]
    D --> E[Record Maximum Load P_max]
    E --> F[Compute UCS = P_max / A]

This approach provides rapid rock strength classification for engineering applications.

Key Points on Reporting According to IS 9143

1. Rounding: Apply IS 2-1960 rules for rounding numerical values.
2. Strength Reporting: Document UCS for each specimen to three significant digits and calculate average strength.
3. Mandatory Report Elements (Clause 6.2):

Average Strength Calculation

[ f_{avg} = \frac{1}{n} \sum_{i=1}^n f_i ]

where (f_i) is individual specimen strength and (n) is the specimen count.

graph TD
  A[Start Reporting] --> B[Record Individual UCS Values]
  B --> C[Calculate Mean UCS]
  C --> D[Include Specimen and Test Details]
  D --> E[Incorporate Physical Property Data]
  E --> F[Apply Rounding Rules]
  F --> G[Complete Report]

This ensures test data clarity, traceability, and consistency.

Essential Precautions and Specifications for Test Specimens

Specimen Dimensions (Clause 3.3)

• Length-to-diameter ratio: Preferably between 2 and 3.
  • If less than 2, apply corrections based on a slenderness ratio of 2.
• Diameter: Greater than 10 times the largest grain size, preferably 45 mm, minimum 35 mm.
• End flatness: Not exceeding 0.05 mm.
• Ends parallelism: Within 0.002 times the diameter.

Loading Machine Requirements (Clause 2.1)

• Adequate capacity for the specimen size.
• Load applied at the rate specified in Clause 4.3.
• Regular calibration and verification depending on usage.

Additional Notes

• For specimens smaller than 45 mm diameter, tolerances should be adjusted accordingly.
• Numerical values should be rounded following the updated code provisions.

flowchart TD
    A[Test Specimen] --> B{Dimension Checks}
    B --> C[Length/Diameter 2-3]
    B --> D[Diameter > 10× Grain Size]
    B --> E[End Flatness ≤ 0.05 mm]
    B --> F[Ends Parallel ≤ 0.002×D]
    A --> G[Loading Machine]
    G --> H[Capacity as per Clause 2.1]
    G --> I[Controlled Load Rate per 4.3]
    G --> J[Regular Calibration]

Following these ensures dependable and repeatable test outcomes in line with IS 9143.