Scope & Key Specifications from IS 13946 Part 3

• Scope: Measurement of in-situ rock stresses using overcoring strain gauge techniques.

Key Specifications:

• Drilling Equipment (Clause 4.1):

  • Drill with NXC (86 mm dia) or larger core barrels.
  • Pilot hole drilling with 38 mm dia coring bits and centralizers.
  • Strain cell with three rosettes (each 3-4 gauges) glued close to minimize rock volume influence.
  • Gauge length: ≥ 10 mm.
  • Orientation of rosettes as per Fig.1 & Fig.2 for six independent strain components.

• Measurement Procedure (Clause 5.3.2):

  • Take two readings per gauge and average.
  • Minimum two complete rounds of readings.

• Data Reporting (Clause 7.2):

  • Tabulate orientation & strain relief readings.
  • Provide Young's modulus (E) and Poisson's ratio (ν) with determination method.
  • Report six components of strain relief tensor (nearest 0.1 MPa).
  • Include standard deviation and error estimates.

Typical Strain Relief Tensor Components (6 components):

Formula for Stress from Strain (Hooke's Law for isotropic rock):

[ \begin{bmatrix} \sigma_{xx} \ \sigma_{yy} \ \sigma_{zz} \ \tau_{xy} \ \tau_{yz} \ \tau_{zx} \end{bmatrix}

\frac{E}{(1+\nu)(1-2\nu)} \begin{bmatrix} 1-\nu & \nu & \nu & 0 & 0 & 0 \ \nu & 1-\nu & \nu & 0 & 0 & 0 \ \nu & \nu & 1-\nu

IS 13946 Part 3 (1994) – Key References & Formulas

1. Stress-Strain Relation (Clause 6.4)

For isotropic rock, relate measured strains to stress tensor components (σf, σp, σ2, τxy, τxz, τyz):

[ \begin{aligned} A_{xx} &= \frac{2E}{1-v^2} \left[ \cos^2 \theta - (1-v) \times (1-\cos^2 \theta) \cos^2 \theta \right] \ A_{yy} &= \frac{2E}{1-v^2} \left[ \cos^2 \theta - (1-v) \times (1-\cos^2 \theta) \cos^2 \theta \right] \ A_{zz} &= \frac{2E}{1+v} + \frac{2E}{1+v} \cos^2 \theta + \frac{2}{1+v} \sin(2\theta) \cos \theta \ A_{xz} &= \sin(2\theta) E \ A_{xy} &= (1 - \cos 2\theta) \sin 2\theta E \ \end{aligned} ]

• E = Young's modulus (from lab or biaxial/triaxial tests)
• v = Poisson's ratio
• θ, ω = angles defined per Fig.1 & 2 (orientation of gauges)

2. Data Reporting Requirements (Clause 7.2)

• Tabulate orientation & strain relief readings per gauge
• Specify E and v values and their derivation
• Present six components of strain relief tensor (nearest 0.1 MPa)
• Include standard deviation & regression error estimates (Clause 6.5)

3. Measurement Procedure (Clause 5.3.2)

• Take two readings per gauge; average them
• Perform at least two full rounds of readings
• Use typical data sheet format (Annex A)

4. Notes

• Applicable only for homogeneous, elastic rock.
• Unsuitable for water-saturated holes due to bonding issues.
• Requires intact core lengths ~2-3 times hole diameter.

5. Related

IS 13946 Part 3: Key Definitions & Formulas for Strain-Stress Relation

1. Definitions

• E: Young's Modulus of rock (from lab/core tests)
• v: Poisson's Ratio of rock (from lab/core tests)
• σf, σp, σ2, τxy, τxz, τyz: Components of stress tensor
• w, θ: Orientation angles of strain gauges (see Fig. 1 & 2 in code)
• Strain Relief Tensor: Six components computed at each measurement point (to 0.1 MPa accuracy)

2. Core Formula (Clause 6.4)

Relates measured strains to stress tensor components assuming isotropic rock:

[ \begin{aligned} A_{xx} &= \frac{2E}{1-v^2} \left[ \cos^2\theta - (1-v) \times (1-\cos 2\theta) \cos^2\theta \right] \ A_{yy} &= \frac{2E}{1-v^2} \left[ \cos^2\theta - (1-v) \times (1-\cos 2\theta) \cos^2\theta \right] \ A_{zz} &= \frac{2E}{1+v} + \frac{2E}{1+v} \cos 2\theta + \frac{2E}{1+v} \sin(2\theta) \cos \theta \ A_{xz} &= \sin(2\theta) E \ A_{xy} &= (1 - \cos 2\theta) \sin 2\theta E \end{aligned} ]

(Note: The above are symbolic; refer to IS 13946 Fig.1 & 2 for exact angle definitions and full expressions.)

3. Data Reporting Requirements (Clause 7.2)

• Tabulate gauge orientation & strain relief readings
• Provide E and v values with determination method
• Report six components of strain relief tensor (±0.1 MPa)
• Include standard deviation & error estimates from regression (Clause 6.5)

4. Measurement Procedure (Clause 5.3.2)

Key Specifications & Equipment for Drilling and Installation (IS 13946 Part 3)

1. Drilling Equipment (Clause 4.1)

• Drill & Core Barrels: NXC type, 86 mm diameter or larger for main hole.
• Pilot Hole: Core bits and centralizers for 38 mm diameter pilot hole concentric with main hole.
• Safety: Exhaust control (scrubbers) and anti-spark devices for underground gas hazard.
• Strain Cell:
  • Holds multiple strain gauges glued close to minimize rock volume effect.
  • Uses 3 rosettes (each with 3-4 gauges) oriented at 0°, 60°, 300° for 6 independent strain measurements.
  • Gauge length ≥ 10 mm.

2. Installation Equipment (Clause 4.2)

• Installing Tool:
  • Holds strain cell, enables electrical connection to multi-conductor cable.
  • Contains orienting device and gas-operated mechanism to press gauges against pilot hole walls.
  • Sized for easy insertion into pilot hole.
• Cleaning Tools: Alcohol spray for drying and priming pilot hole for adhesive bonding.
• Plug: To seal pilot hole during overcoring.

3. Measurement & Quality Control (Clause 5.1.4 & 5.3.1)

• Pilot hole core must be intact and free from weakness planes.
• If broken, drill 0.5 m deeper and repeat.
• Use periscope/TV camera to inspect fracturing.
• Initial strain readings taken before sealing pilot hole with plug.

Strain Gauge Rosette Orientation (Fig. 1 & 2)

Summary Diagram: Installation Setup

graph TD
    A[Drill Hole (90 mm dia)] --> B[Pilot Hole (38 mm dia)]
    B --> C[Installing Tool]
    C --> D[Strain Cell with Rosettes]
    C --> E[Electrical Cable &

IS 13946 Part 3: Installation and Measurement Procedures Key Points

Installation Equipment (Clause 4.2)

• Installing tool: Holds strain cell, plugs strain gauges electrically, orients cell, and pushes gauges into pilot hole walls (usually gas-operated).
• Cleaning tools: Alcohol spray for drying and priming pilot hole for good gauge adhesion.
• Sealing plug: To seal pilot hole during overcoring.

Measurement Equipment (Clause 4.3)

• Strain bridges: Accuracy to ±5 × 10⁻⁶ mm/mm.
• Electrical connectors/switches: For measuring strain on each gauge.
• Temperature device: Thermocouple/thermistor with ±1℃ accuracy at sensor location.
• Adhesives and solvents: Ensure strain gauges remain bonded during drilling and measurement.
• Calibration: All strain gauges must be calibrated before use.

Measurement Procedure (Clause 5.3.2)

• Take two readings per gauge per round; average these.
• Take at least two complete rounds.
• Use averaged readings for calculations.

Reporting Details (Clause 7.2)

• Orientation and strain relief tabulation.
• Young’s modulus (E) and Poisson’s ratio (ν) with determination method.
• Six components of strain relief tensor (nearest 0.1 MPa).
• Standard deviation and error estimates from regression.

Typical Strain Calculation Formula:

[ \epsilon = \frac{\Delta R / R}{G_F} ]

• (\epsilon): Strain
• (\Delta R / R): Change in resistance ratio
• (G_F): Gauge factor (usually ~2)

Diagram: Strain Cell Installation Concept

flowchart LR
    A[Pilot Hole] --> B[Installing Tool]
    B --> C[Strain Cell Inserted]
    C --> D[Strain Gauge Rosettes Pushed Out]
    D --> E[Electrical Connection via Multi-conductor Cable]
    E --> F[Measurement Equipment]

For detailed field data sheets, refer to Annex A of IS 13946 Part 3.

IS 13946 Part 3: Data Reduction & Stress Calculation Key Points

1. Stress-Strain Relationship (Clause 6.4)

For isotropic rock, the stress components (\sigma_x, \sigma_y, \sigma_z, \tau_{xy}, \tau_{xz}, \tau_{yz}) relate to measured strains via:

[ \begin{aligned} A_{xx} &= \frac{2E}{1-v^2} \left[\cos^2 \theta - (1 - v^2)(1 - \cos^2 \theta) \cos^2 \theta \right] \ A_{yy} &= \frac{2E}{1-v^2} \left[\cos^2 \theta - (1 - v^2)(1 - \cos^2 \theta) \cos^2 \theta \right] \ A_{zz} &= \frac{2E}{1+v} + \frac{2E \cos^2 \theta}{2(1+v)} \sin(2\theta) \cos \theta \ A_{xz} &= \sin(2\theta) \sin \phi E \ A_{xy} &= (1 - \cos 2\phi) \sin 2\theta E \ \end{aligned} ]

• (E) = Young's modulus
• (v) = Poisson's ratio
• (\theta, \phi) = angles defining gauge orientation (see IS 13946 Fig. 1 & 2)

2. Data to Report (Clause 7.2)

• Tabulate orientation and strain relief readings per gauge.
• Specify Young’s modulus (E) and Poisson’s ratio (v) with test method.
• Calculate and tabulate all 6 components of the stress tensor (to 0.1 MPa accuracy).
• Provide standard deviation and error estimates for regression fits.

3. Principal Stress Calculation (Clause 6.6)

• Compute principal stresses (\sigma_1, \sigma_2, \sigma_3) and their directions from the stress tensor using elasticity relations.
• Typically done via matrix eigenvalue analysis in software.

Summary Table for Elastic Constants

| Parameter |

IS 13946 Part 3 — Reporting of Results: Key Points & Formulas

1. Report Content (Clause 7.1 & 7.2)

• General Info:

  • Drillhole locations, directions, lengths
  • Geotechnical core log with depth & rock characteristics
  • Description of procedure, equipment, with diagrams/photos

• Detailed Depth Info per Measurement Location:

  • Tabulated orientation & strain relief readings per gauge
  • Young’s modulus (E) and Poisson’s ratio (ν) used, with determination method
  • Six components of the strain relief tensor (to 0.1 MPa precision)
  • Standard deviation & error estimates for regression (see Clause 6.5)

2. Data Handling (Clause 5.3.2)

• Take two balances per gauge, average them
• Minimum two complete rounds of readings
• Use average readings for calculations

3. Strain Relief Tensor Components

The six independent components of the stress or strain tensor typically are:

4. Regression & Error Estimation

• Use regression to fit strain data to stress model (Clause 6.5)
• Calculate standard deviation (σ) and error bounds to quantify reliability

5. Alternative Data Reduction (Clause 6.7)

• If rock is significantly anisotropic, consider alternative methods beyond isotropic assumptions

Summary Table for Reporting

Typical Field Data Sheet for Overcoring Results (IS 13946 Part 3)

Based on Clauses 5.3, 5.1.4, 6.1, and 7.1, key components and formulas include:

1. General Information (Clause 7.1)

• Drillhole details: Location, direction, length (typically up to 20 m, 90 mm dia pilot hole).
• Geotechnical log: Core description, depth of measurements, geological/structural features.
• Equipment & procedure: Description with diagrams/photos (e.g., CSIR triaxial strain cell).

2. Core Quality (Clause 5.1.4)

• Use solid, unbroken core free from planes of weakness.
• If core is broken, drill 0.5 m deeper and repeat until solid core is found.
• Examine fractured zones with periscope/TV camera before rejection.

3. Strain Measurement & Calculation (Clause 6.1)

• Strain relief values = Average strain before overcoring − Average strain after overcoring.

4. Key Data to Record

5. Formula Summary

[ \Delta \varepsilon = \bar{\varepsilon}{before} - \bar{\varepsilon}{after} ]

Where:

• (\bar{\varepsilon}_{before}) = Average strain reading before overcoring
• (\bar{\varepsilon}_{after}) = Average strain reading after overcoring

Committee Composition - IS 13946 Part 3 (1994)

Key Points:

• The Rock Mechanics Sectional Committee, CED 48 is responsible for this part.
• The committee includes experts from:
  • Universities (e.g., University of Roorkee, IIT Delhi)
  • Government departments (Irrigation, Power, Geological Survey)
  • Research Institutes (CSIR, NTPC, Central Water & Power Research Station)
  • Industry representatives (Hindustan Construction, Associated Instrument Mfrs)
  • BIS (Bureau of Indian Standards) officials (Ex-officio members)

Chairman:

• Dr. Bhawani Singh, University of Roorkee

Member-Secretary:

• Dr. R. P. Kulkarni, Central Board of Irrigation & Power

Composition Highlights:

Stress Measurement - Overcoring (Clause 6.4)

Stress-Strain Relation for Isotropic Rock:

[ \begin{align*} A_{xx} &= \frac{2E}{1 - v^2} \left[ (1+v) \cos^2 \theta - (1 - v) \times (1 - \cos^2 2\theta) \cos^2 \theta \right] \ A_{yy} &= \frac{2E}{1 - v^2} \left[ (1+v) \cos^2 \theta - (1 - v) \times (1 - \cos^2 2\theta) \cos^2 \theta \right] \ A_{zz} &= \frac{2E}{1+v} + \frac{2E}{1+v} \cos^2 2\theta - \frac{2E}{1 - v^2} \sin (2\theta) \cos \theta \ A_{xy} &= (1 - \cos 2\theta) \sin 2\theta \frac{E}{1 - v^2} \end