Determination of rock stress-Code of practice, Part 2: Using USBM-type drill hole deformation gauge

Scope & Key Specifications from IS 13946 Part 2 (1994):

• Purpose: Measurement of in-situ rock stresses by overcoring method using deformation gauges.

Key Report Contents (Clause 7.1 & 7.2)

• General Info: Drillhole location, direction, length; geotechnical core log; equipment & procedure description with diagrams/photos.
• Detailed Data:
  • Field data sheets or plots showing overcoring deformation (Fig. 3).
  • Estimated principal strains: U1, U2, U3.
  • Radial pressure vs borehole deformation or stress-strain curves.
  • Tabulation of hole number, bearing, inclination, depth, Young’s modulus, Poisson’s ratio.
  • Principal stress magnitudes, directions, and statistical parameters.
  • Explanation of discrepancies in test results.

Equipment Requirements (Clauses 4.2 & 4.3)

• Deformation Gauge:
  • Measures pilot hole diameter changes in 1-3 orientations.
  • Sensitivity: Detect changes as small as 1 part in 10.
  • Must be waterproof, firmly held, detachable from cable.
  • Example: USBM gauge with strain gauges on cantilever arms.
• Others:
  • Strain indicator readout bridge and switchgear.
  • Placement rods marked for depth and orientation.
  • Calibration device for gauge sensors.
  • Biaxial modulus chamber for Young’s modulus measurement.

Typical Plot (Fig. 3)

• X-axis: Overcoring depth (mm)
• Y-axis: Diameter deformation (mm)
• Multiple curves representing deformation in different measurement planes.

Summary Table of Principal Outputs

flowchart TD
    A[Drillhole Preparation] --> B[Insert Deformation Gauge]
    B --> C[Overcoring & Data Acquisition]
    C --> D[Measure Diameter Changes (U1,U2,U3

IS 13946 Part 2 Key Formulas & Specifications for Overcoring Stress Measurement

1. Stress-Deformation Relationship (Plane Strain Isotropic Elasticity)

[ U = d(1 - \nu^2) \frac{1}{E} \left[ (P + Q) + 2(P - Q) \cos 2\theta \right] ]

• ( U ): Strain in axial direction along drillhole
• ( d ): Diameter of pilot hole
• ( E ): Young's modulus
• ( \nu ): Poisson's ratio
• ( P, Q ): Major and minor secondary principal stresses
• ( \theta ): Angle between ( P ) and measured deformation direction

2. Young's Modulus from Biaxial Chamber Test

[ E = \frac{D^2 - d^2}{2dP} U ]

• ( D ): Diameter of overcore
• ( d ): Diameter of pilot hole
• ( P ): Applied radial pressure
• ( U ): Measured change in pilot hole diameter

3. Principal Stresses & Orientation from USBM-type Gauge (3 sensors at 60°)

[ P = \frac{E}{6d} (U_1 + U_2 + U_3) + \frac{E}{2d} \sqrt{(U_1 - U_2)^2 + (U_2 - U_3)^2 + (U_3 - U_1)^2} ]

[ \theta_p = \frac{1}{2} \tan^{-1} \left[ \frac{\sqrt{3} (U_2 - U_3)}{2U_1 - U_2 - U_3} \right] ]

• ( U_1, U_2, U_3 ): Diameter changes measured by sensors
• ( \theta_p ): Angle of major principal stress from ( U_1 )

4. Reporting Requirements (Clause 7.2)

• Field data sheets with ( U_1, U_2, U_3 ) values
• Plots of radial pressure vs borehole deformation
• Tabulation of hole number, inclination, depth, Young's modulus, Poisson's ratio
• Stress

Key Definitions & Formulas from IS 13946 Part 2 (1994)

1. Diameteral Deformation (U) in Zero Axial Stress Condition (Clause 6.2):

[ U(\theta) = (P + Q) + 2(P - Q) \cos 2\theta ]

• (P, Q): Major and minor secondary principal stresses (perpendicular to hole axis)
• (d): Diameter of pilot hole
• (\theta): Angle between (P) and measured deformation (U)

For USBM-type gauges with three sensors (U_1, U_2, U_3) spaced 60° apart:

[ P = \frac{E}{6d} \left(U_1 + U_2 + U_3 + 2 \sqrt{(U_1 - U_2)^2 + (U_2 - U_3)^2 + (U_3 - U_1)^2} \right) ]

[ Q = \frac{E}{6d} \left(U_1 + U_2 + U_3 - 2 \sqrt{(U_1 - U_2)^2 + (U_2 - U_3)^2 + (U_3 - U_1)^2} \right) ]

Orientation angle (\theta_p):

[ \theta_p = \frac{1}{2} \tan^{-1} \left(\frac{\sqrt{3}(U_2 - U_3)}{2U_1 - U_2 - U_3}\right) ]

Ranges for (\theta_p) depend on relative magnitudes of (U_i).

2. Young's Modulus from Biaxial Chamber (Clause 4.3 & 6.3):

[ E = \frac{D^2 - d^2}{2 d P} \times \frac{\Delta d}{d} ]

• (D): Diameter of overcore
• (d): Diameter of pilot hole
• (P): Applied radial pressure
• (\Delta d): Change in pilot hole diameter

3. 3D Case with Axial Strain (Clause 6.3):

[ U = d (1 - \nu^2

IS 13946 Part 2: Equipment for Overcoring Stress Measurements

Key Equipment (Clause 4.3 & 4.2)

• Calibration device: For periodic calibration of gauge sensors to ensure accuracy.
• Biaxial modulus chamber: Includes pressure gauge and hand pump to measure Young's modulus of large diameter cores.
• Deformation gauge: Measures pilot hole diameter changes (usually 3 orientations). Detects changes as small as 1 part in 10. Must be waterproof, slip-resistant, and detachable from signal cable (e.g., USBM gauge with strain gauges).
• Strain indicator readout bridge and switchgear: For signal processing.
• Placement rods: Marked for depth and orientation to position deformation gauge inside pilot hole.

Measurement & Reporting (Clause 7.2)

Reports must include:

• Field data sheets or plots of overcoring data (U1, U2, U3 values).
• Radial pressure vs borehole deformation plots or stress/strain curves.
• Tabulation of hole details, stress orientations, Young's modulus, and Poisson's ratio.
• Principal stress magnitudes, directions, and statistical data (standard deviation, error, correlation).
• Explanation of discrepancies with other data.

Typical Formula for Young's Modulus from Biaxial Modulus Chamber Tests:

[ E = \frac{\sigma}{\epsilon} ] Where:

• (E) = Young's modulus
• (\sigma) = Applied radial pressure
• (\epsilon) = Measured strain (deformation)

Illustration: Equipment Setup for Overcoring

graph TD
    A[Deformation Gauge] --> B[Placement Rods]
    B --> C[Pilot Hole]
    C --> D[Strain Indicator Readout]
    D --> E[Calibration Device]
    F[Biaxial Modulus Chamber] --> G[Pressure Gauge & Hand Pump]
    G --> H[Rock Core Sample]

Summary:

• Use calibrated deformation gauges to measure pilot hole diameter changes.
• Employ a biaxial modulus chamber for Young's modulus.
• Record detailed data including stress directions, magnitudes, and mechanical properties.
• Follow calibration and measurement protocols strictly for reliable stress analysis.

IS 13946 Part 2: Drilling, Gauge Insertion and Overcoring Key Points

Drilling & Overcoring (Clause 55.6 & 5.2)

• Pilot hole diameter: 38 mm, drilled with single/double-tube core barrel (~2 m length).
• Overcoring bit diameter: ~150 mm (3-4 times pilot hole diameter).
• Drill rods: BQ wireline rods (55.6 mm o.d., 46 mm i.d.) or equivalent with large internal diameter for gauge passage.
• Stabilizers: Installed every ~3 m along drill string to minimize vibration.
• Water swivel: Connects drill rods, allows signal cable passage.
• Core retrieval tools: Core-breaking wedge, shovel, puller.

Gauge Insertion (Clause 4.2)

• Deformation gauge: Measures pilot hole diameter changes in 3 orientations; sensitivity ~1 part in 10.
• Gauge features: Firmly held, waterproof, detachable from signal cable.
• Readout: Strain indicator bridge and switchgear.
• Placement rods: Marked for depth and orientation.

Overcoring Procedure (Clause 5.2.8)

• Chuck speed: ~120 rpm.
• Penetration rate: ~20 mm/min.
• Water pressure: Low but sufficient for clearing cuttings; maintain steady.
• Data recording: Gauge readings every 10-20 mm penetration.

Typical Overcoring Setup Diagram

graph LR
  A[Drill Rig] --> B[Drill Rods with Stabilizers]
  B --> C[Pilot Hole (38 mm)]
  C --> D[Deformation Gauge Inserted]
  B --> E[Overcoring Bit (150 mm)]
  D --> F[Signal Cable to Readout]
  B --> G[Water Swivel]
  E --> H[Rock Core Extraction Tools]

This summary ensures correct gauge placement, drilling parameters, and equipment for accurate in-situ stress measurement by overcoring per IS 13946 Part 2.

Key Formulas & Specifications for Calculation of Stresses (IS 13946 Part 2)

1. Two-Dimensional Case (Stress perpendicular to drillhole axis, axial stress = 0)

[ U = d \left[(P + Q) + 2(P - Q) \cos 2\theta \right] ]

• (U): Change in pilot hole diameter at angle (\theta)
• (d): Diameter of pilot hole
• (P, Q): Major and minor principal stresses perpendicular to hole axis
• (\theta): Angle between direction of (P) and (U)

2. Three Sensor Axes (USBM-type gauge, spaced 60° apart)

• Principal stresses: [ P, Q = \frac{E}{6d}(U_1 + U_2 + U_3) \pm \frac{E}{6d} \sqrt{2(U_1^2 + U_2^2 + U_3^2) - (U_1 U_2 + U_2 U_3 + U_3 U_1)} ]
• Orientation of (P): [ \theta_p = \frac{1}{2} \tan^{-1} \left( \frac{\sqrt{3}(U_2 - U_3)}{2U_1 - U_2 - U_3} \right) ]

3. Three-Dimensional Case (Plane strain isotropic elasticity)

[ \varepsilon = d(1 - \nu^2)/E \cdot \left[(P + Q) + 2(P - Q) \cos 2\theta\right] ]

• (\varepsilon): Axial strain along drillhole
• (E): Young’s modulus
• (\nu): Poisson’s ratio
• Iteration possible if axial stress (\sigma_2) is known: [ \varepsilon_2 = \frac{-\sigma_2 - \nu (P + Q)}{E} ]

4. Young's Modulus from Overcoring Data

[ E = \frac{D^2 (D^2 - d^2)}{2dP} U ]

• (D\

IS 13946 Part 2: Reporting of Results - Key Points

1. Report Content (Clause 7.1 & 7.2)

• General Info:

  • Drillhole location, direction, and length.
  • Geotechnical core log with depth and rock characteristics.
  • Description and diagrams/photos of equipment and procedures.

• Detailed Data per Measurement Location:

  • Field data sheets or plots (Fig. 3) showing overcoring runs with estimated strains U1, U2, U3.
  • Radial pressure vs borehole deformation plots or stress/strain curves.
  • Tabulation including:
    • Hole number, bearing, inclination, overcore depth.
    • Measured strains (U1, U2, U3), Young’s modulus (E), Poisson’s ratio (ν).
  • Secondary principal stresses magnitudes/directions with statistical data (std. dev., error, correlation).
  • Discussion on discrepancies with explanations.

2. Key Formulas (Clause 6.3)

• Strain in axial direction:

[ \varepsilon = d(1 - \nu^2) / E \times (P + Q) + 2(P - Q) \cos 2\theta ]

• Young's modulus from biaxial chamber test:

[ E = \frac{D^2 - d^2}{2 d P} \times U ]

Where:

• (D) = overcore diameter
• (d) = pilot hole diameter
• (P) = applied radial pressure
• (U) = measured pilot hole diameter change
• (\nu) = Poisson’s ratio

3. Typical Plot (Fig. 3)

• Displays diametral deformation (mm) vs overcoring depth (mm) in the plane of measurement.

flowchart TD
    A[Drillhole Data Collection] --> B[Field Data Sheets & Plots]
    B --> C[Calculate U1, U2, U3]
    C --> D[Measure E & ν from biaxial tests]
    D --> E[Tabulate Hole Info + Strains + Elastic Properties]
    E --> F[Compute Principal Stresses & Directions]
    F --> G[

Typical Field Data Sheet - Key Formulas, Tables & Specifications (IS 13946 Part 2)

1. Data to Record (Clause 7.2 & 7.1)

• Drillhole ID, bearing, inclination, depth of overcore.
• Measured diameter changes: U1, U2, U3 (three orthogonal directions).
• Young’s modulus (E) and Poisson’s ratio (ν) at test location.
• Radial pressure (P) and borehole deformation (U) from biaxial chamber tests.
• Stress magnitudes and directions (σ1, σ2, σ3) with statistical errors.
• Geological and structural logs of rock core.
• Equipment and procedure details.

2. Key Formulas

• Young’s Modulus from Biaxial Chamber Test:

[ E = \frac{D^2 - d^2}{2dP} \times U ]

Where:

• (D) = Diameter of overcore

• (d) = Diameter of pilot hole

• (P) = Applied radial pressure

• (U) = Measured change in pilot hole diameter

• Strain in axial direction (ε):

[ \varepsilon = \frac{1}{E} \left[(1 - \nu^2)(P + Q) + 2(P - Q) \cos 2\theta \right] ]

• Axial strain with axial stress σ2:

[ \varepsilon_2 = -\frac{\sigma_2 - \nu (P + Q)}{E} ]

3. Typical Plot (Fig. 3)

• Plot of diametral deformation (mm) vs overcoring depth (mm) for each measurement plane.

4. Measurement Procedure (Clause 5.2.8)

• Chuck speed: ~120 rev/min
• Penetration rate: ~20 mm/min
• Record gauge readings every 10-20 mm penetration
• Maintain steady low water pressure for clearing cuttings

Mermaid Diagram: Data Flow in Field Data Sheet Preparation

flowchart LR
    A[Drillhole Info] --> B[Overcoring Measurements (U1,U2,U3)]
    B --> C[

Committee Composition - IS 13946 Part 2 (1994)

Rock Mechanics Sectional Committee, CED 48

• Chairman: Dr. Bhawani Singh, University of Roorkee
• Members: Experts from Irrigation Departments, CSIR Institutes, Geological Survey of India, Central Water & Power Research Station, IIT Delhi, National Thermal Power Corporation, BIS, and others.
• Member-Secretary: Dr. R. P. Kulkarni, Central Board of Irrigation & Power

Field Testing of Rock Mass and Rock Mass Classification Subcommittee, CED 48:1

• Convener: Shri U. S. Rajvanshi, U.P. Irrigation Research Institute
• Members: Professors from University of Roorkee, Indian School of Mines, experts from Government Irrigation Departments, CSIR, IIT Delhi, National Hydroelectric Power Corporation, and other research institutes.

Key Specifications from IS 13946 Part 2

• Field Data Sheet: Records hole number, date, orientation, calibration factors, deformation, temperature, water presence, and remarks.
• Measurement Data: Includes three deformation readings (U1, U2, U3), hole orientation, depth, and Young’s modulus & Poisson’s ratio.
• Young’s Modulus Calculation:

[ E = \frac{D^2 - d^2}{2dP} \times U ]

Where:

• (D) = diameter of overcore

• (d) = diameter of pilot hole

• (P) = applied radial pressure

• (U) = measured change in pilot hole diameter

• Stress-Strain Relation (3D isotropic elasticity):

[ \varepsilon = d (1 - \nu^2) E (P + Q) + 2(P - Q) \cos 2\theta ]

Where:

• (\varepsilon) = axial strain
• (\nu) = Poisson’s ratio
• (P, Q) = stress components
• (\theta) = orientation angle

Summary Diagram of Committee Structure

graph TD
    A[Rock Mechanics Sectional Committee] --> B(Chairman: Dr. Bhawani Singh)
    A --> C(Members: Experts from various Govt &