This standard outlines the procedure to evaluate the strength of cylindrical rock samples under triaxial compression conditions. It details methods to determine peak strength values and derive critical parameters like internal friction angle and apparent cohesion, which are vital for geotechnical and rock engineering analyses. The guidelines are intended for professionals engaged in assessing rock behavior for foundations, slope analysis, and subterranean construction.
Overview
This standard outlines the procedure to evaluate the strength of cylindrical rock samples under triaxial compression conditions. It details methods to determine peak strength values and derive critical parameters like internal friction angle and apparent cohesion, which are vital for geotechnical and rock engineering analyses. The guidelines are intended for professionals engaged in assessing rock behavior for foundations, slope analysis, and subterranean construction.
Audience
Contents
Structure
Overview of Scope & Essential Specifications
Must include:
[ \sigma_a = \frac{P}{A} = \frac{P}{\pi (d/2)^2} ]
Where:
| Specimen ID | Height (mm) | Diameter (mm) | Confining Pressure (MPa) | Axial Strength (MPa) |
|---|---|---|---|---|
| 1 | 100 | 50 | 5 | 120.5 |
| 2 | 100 | 50 | 10 | 135.7 |
graph TD
A[Rock Sample] --> B[Specimen Preparation]
B --> C{Specimen Shape}
C -->|Cylindrical| D[Dimension Measurement]
D --> E[Testing under Axial Load and Confining Pressure]
E --> F[Recording Axial Strength and Failure Mode]
Primary References, Formulae, and Tables
Include:
[ \sigma_1 = \frac{P}{A} ]
Where:
| Parameter | Specification |
|---|---|
| Hardness | Rockwell HRC ≥ 30 |
| Diameter | D to 1.02 × D |
| Thickness | ≥ 15 mm or D/3 |
| Surface Flatness | ±0.005 mm |
flowchart LR
P(Loading Piston) -->|Applies Axial Load| Specimen
Specimen -->|Confined by| C(Triaxial Cell)
C -->|Pressure Controlled by| HP(Confining Pressure System)
Equipment Specifications for Triaxial Compression Testing
Confining Pressure Generation & Measurement (Clause 3.4):
Testing Platens (Clause 1.02b):
flowchart LR
P[Loading Device] --> MC[Control Unit]
MC --> P
HP[Pressure Equipment] --> C[Triaxial Cell]
P --> C
C --> Specimen
Guidelines for Preparing Test Specimens
Specimen Fabrication (Clause 4.1)
Platens Specifications (Clause 1.02b)
Assembly for Testing (Figs. 1 & 2)
| Characteristic | Specification |
|---|---|
| Hardness | ≥ HRC 30 |
| Diameter | D to 1.02 × D |
| Thickness | ≥ 15 mm or D/3 |
| Surface Flatness | ±0.005 mm |
flowchart LR
P[Loading Piston] --> L[Load Applied]
L --> Specimen[Specimen in Membrane]
Specimen --> Platens[Platens with Spherical Seats]
Specimen --> C[Triaxial Cell]
C --> HP[Hydraulic Pressure System]
HP --> MC[Control Unit]
Key Steps in the Testing Process
Include:
graph LR
A[Confining Pressure (MPa)] --> B[Axial Stress (MPa)]
B --> C[Strength Envelope Curve]
Summary: Ensure accurate confining pressure control, proper specimen preparation and marking, detailed data recording, and comprehensive reporting for standard compliance.
Guidelines for Calculation and Result Interpretation
Rounding: Apply IS 2:1960 rules for rounding numerical results.
Axial Strength Usage: Use axial strength values (not stress) in analyses.
Report Requirements (Clause 7.2):
Strength Envelope Plotting (Clause 6.2): Plot confining pressure (x-axis) against axial strength (y-axis) to derive strength envelope (see Fig. 3).
[ \text{X-axis} = \text{Confining Pressure (MPa)}, \quad \text{Y-axis} = \text{Axial Strength (MPa)} ]
| Specimen ID | Height (mm) | Diameter (mm) | Confining Pressure (MPa) | Axial Strength (MPa) |
|---|---|---|---|---|
| 1 | 50 | 25 | 5 | 120 |
| 2 | 50 | 25 | 10 | 140 |
graph LR
A[Confining Pressure] --> B[Calculate Axial Strength]
B --> C[Tabulate Data]
C --> D[Plot Strength Envelope]
D --> E[Interpret Rock Strength]
Mandatory Details for Test Report (Clauses 7.1 & 7.2)
Reports must contain:
[ \sigma_1 = \sigma_3 \tan^2 \left(45^\circ + \frac{\phi}{2}\right) + 2c \tan \left(45^\circ + \frac{\phi}{2}\right) ]
Where:
| Specimen No. | Height (mm) | Diameter (mm) | Confining Pressure (MPa) | Axial Strength (MPa) |
|---|---|---|---|---|
| 1 | 100 | 50 | 5.0 | 45.3 |
| 2 | 100 | 50 | 10.0 | 60.7 |
graph LR
A[Sample Collection] --> B[Storage and Conditioning]
B --> C[Orientation Setup]
C --> D[Triaxial Testing]
D --> E[Data Measurement]
E --> F[Report Finalization]
Accuracy and Calibration Requirements Overview
Axial Displacement Measurement (Clause 5.8): Must achieve accuracy within ±0.5% of measured displacement.
Confining Pressure Control (Clause 3.4):
Numerical Rounding: Follow IS 2:1960 for consistent rounding of test data.
| Parameter | Accuracy Required | Clause Reference |
|---|---|---|
| Axial displacement | ±0.5% | 5.8 |
| Confining pressure control | ±2% | 3.4(a) |
| Pressure measuring device | ±2% | 3.4(b) |
flowchart LR
A[Hydraulic Pump] --> B[Pressure Control System]
B --> C[Pressure Gauge / Transducer (±2% Accuracy)]
C --> D[Confining Pressure Maintained Within ±2%]
E[Axial Displacement Measurement] --> F[Accuracy ±0.5%]
G[Test Data] --> H[Rounded per IS 2:1960]
Though IS 13047:1991 does not explicitly provide safety and handling clauses, general best practices and related IS codes suggest:
Specimen Handling:
Operation of Testing Equipment:
Specimen Preparation:
Data Reporting:
[ \text{Axial Strength} = \frac{P}{A} ]
Where:
flowchart TD
A[Specimen Preparation] --> B[Careful Handling and Transport]
B --> C[Setup of Testing Machine]
C --> D[Application of Load]
D --> E[Record Maximum Load]
E --> F[Calculate Axial Strength]
F --> G[Report Results per IS 2:1960]
For comprehensive safety instructions, refer to IS 456 and equipment manufacturer documentation.
Key Annexures and Figures in IS 13047
Reports should include:
| Specimen No. | Height (mm) | Diameter (mm) | Confining Pressure (MPa) | Axial Strength (MPa) |
|---|---|---|---|---|
| 1 | 100 | 50 | 5 | 120 |
| 2 | 100 | 50 | 10 | 140 |
graph LR
A[Sample Collection] --> B[Preparation]
B --> C[Orientation Setup]
C --> D[Triaxial Compression Test]
D --> E[Measurement of Axial Strength and Confining Pressure]
E --> F[Plotting Strength Envelope (Fig. 3)]
F --> G[Compilation of Test Report (Clause 7.2)]
Note: Always consult the most recent edition of IS 13047 for updates on formulas and illustrations.
Frequently Asked
As per IS 13047, the rock sample must be a right circular cylinder (Clause 4.3.1). The diameter should be at least ten times the largest mineral grain size, preferably no less than the NX core size (~54 mm) (Clause 4.3.3). While length is not explicitly specified, a length-to-diameter ratio of 2:1 is standard practice. At least five specimens per rock sample are recommended for testing (Clause 5.9). This ensures representativeness and reliability in results.
According to IS 13047 (Clauses 5.5 and 3.4), confining pressure and axial load are increased simultaneously, maintaining axial stress close to the confining pressure until the target confining pressure is reached. Thereafter, confining pressure is kept within ±2% of the set value using a hydraulic pump or pressure intensifier. Pressure is monitored continuously by a gauge or transducer with ±2% accuracy. The triaxial cell encloses the specimen, which is isolated by a flexible membrane, while platens transmit the axial load. Control units regulate both axial load and confining pressure to ensure precise testing.
IS 13047 Clause 5.6 prescribes a continuous, shock-free loading rate that causes specimen failure within 5 to 15 minutes. The stress rate should be between 0.5 to 1.0 MPa/s, with axial load measurement accuracy within ±1%. The loading machine must be calibrated with parallel loading faces and spherical seating. Axial load and confining pressure are increased simultaneously and approximately equally until the confining pressure setpoint is reached (Clause 5.5), which is then maintained within ±2%.
IS 13047 describes calculation of internal friction angle (φ) and apparent cohesion (c) from triaxial test results by plotting axial strength (σ₁) against confining pressure (σ₃) for multiple specimens (Clause 6.2, Fig. 3). A linear envelope is fitted: σ₁ = mσ₃ + b, where m and b are obtained from the best fit. Then, tan φ = (m - 1)/(m + 1) and c = b / (2√m), following Coulomb’s failure criterion: σ₁ = σ₃ tan²(45° + φ/2) + 2c tan(45° + φ/2). The test report should include the plot and specify confining pressure ranges used.
IS 13047 Clause 7.2 mandates comprehensive reporting including: lithologic description; loading axis orientation relative to rock fabric; sample source, location, depth, orientation, and sampling date; storage conditions; moisture content and testing temperature; test duration and stress rate; testing date and equipment type; number of specimens; failure mode descriptions with sketches; physical properties such as specific gravity, absorption, permeability, and porosity referencing IS standards; notes on specimen shape if non-cylindrical; tabulated data with specimen number, dimensions, confining pressure, and axial strength (to three significant digits); and any additional observations. This ensures thorough documentation and reproducibility.
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