Determination of rock stress-Code of practice, Part 4: Using flat jack technique

IS 13946 Part 4 — Scope & Key Definitions

This part elaborates on the flat jack method for rock stress measurement, enhancing IS 7292:1974 with calibration and SI units.

Key Symbols & Parameters (Clause 3.1)

Important Notes

• All dimensions are in millimetres.
• Calibration of pressure and displacement devices is mandatory before each test series.
• Spacer bar length: 300 mm (Fig.4).
• The method supersedes IS 7292:1974.

Conceptual Relationships

The observed displacement ( W ) at gauge points relates to stresses ( S ) and ( Q ), and flat jack pressure ( P ), corrected for slot geometry:

[ W = W_o + W_1 + W_2 - W_i ]

Where:

• ( W_o ) = displacement due to normal stress ( S )
• ( W_1 ) = finite slot width correction
• ( W_2 ) = displacement due to parallel stress ( Q )
• ( W_i ) = displacement due to jack pressure ( P )

flowchart LR
    S[Rock Stress Normal to Slot (

IS 13946 Part 4: Definitions and Symbols (Clause 3.1)

Key symbols used in rock stress measurement with flat jacks:

Important Notes:

• All dimensions in millimeters (mm).
• Gauge lengths: i, k = 250 mm, j, l = 500 mm.
• Calibration of pressure and displacement devices is mandatory before tests.
• Stress relation example (Clause 16.87):

[ Q = 0.815 \times 16.87 + 0.064 \times S ]

Visual Concept (Slot & Gauge Points):

graph LR
A[Slot Axis]
B[Gauge Point y]
C[Displacement W, Wo, W1, W2, Wi]
A --> B
B --> C

This concise summary helps in understanding measurements and calculations related to rock stress using flat jack tests per IS 13946 Part 4.

Principle of Flat Jack Technique (IS 13946 Part 4)

• Concept:
  A slot is cut in rock; the displacement caused is measured. A flat jack is inserted and pressurized until displacement cancels. This cancellation pressure is used to determine rock stresses.

• Key Parameters:

  • Slot length, (2c = 33,cm)
  • Flat jack length, (2c_0 = 30,cm)
  • Slot width, (2y_0 = 4.0,cm)
  • Gauge length, (2y = 25,cm)
  • Poisson's ratio, (\nu = 0.15 \text{ to } 0.25) (typical value 0.167)

• Calibration:
  Flat jack suppliers provide calibration factors to account for edge effects and welding-induced pressure differences.

• Formulas (Clause 4.3):
  Two unknowns: (S) and (Q) (rock stress parameters)
  Cancellation pressures: (P_h) (horizontal), (P_v) (vertical)

  [ S = F_1 P_h + F_2 Q \quad (9) ] [ Q = F P_v + F S \quad (10) ]

  Where constants for given dimensions:
  [ F_1 = 0.815, \quad F_2 = 0.064 ]

• Example:
  Given (P_h = 21.10,kg/cm^2), (P_v = 16.87,kg/cm^2), (\nu = 0.167), solve equations (9) and (10) simultaneously for (S) and (Q).

Summary Table of Constants and Dimensions

IS 13946 Part 4: Calculation of Rock Stress Using Flat Jack Technique

Key Symbols (Clause 3.1)

Basic Formula for Rock Stress (Normal to Slot)

[ S = \frac{P}{W_i} (W_o + W_1 - W) ]

• P = pressure applied in flat jack
• Wi = displacement caused by flat jack pressure P
• W, Wo, W1 = measured and corrected displacements at gauge points

Procedure Summary:

• Cut a slot of length 2c in rock.
• Insert flat jack of length 2Co.
• Measure displacements (W) at gauge points at distances (y).
• Calculate corrected displacement (W_o + W_1).
• Apply pressure (P) in flat jack and measure (W_i).
• Use formula above to find stress (S).

Notes:

• Correction (W_1) accounts for finite slot width (modeled as elongated ellipse).
• Poisson's ratio (u) and deformation modulus (E) influence displacement calculations.
• Gauge lengths typically 250 mm and 500 mm as per Fig. 1.

flowchart LR
    A[Cut Slot in Rock (length 2c)] --> B[Insert Flat Jack (length 2Co)]
    B --> C[Measure Displacement W at Gauge Points (distance y)]
    C --> D

IS 13946 Part 4 — Test Equipment Key Points

• Dimensions: All dimensions are in millimetres (Clause 10.5).
• Spacer Bar: Fig.4 shows a 300 mm long spacer bar and gauge pin details for precise measurement setup.
• Calibration:
  • All pressure and displacement measuring devices must be calibrated before each test series.
  • Calibration to be done by an independent testing laboratory.
• Test Site Requirements (Clause 1.5):
  • Site must be flat and on sound rock.
  • Soundness checked by striking rock with a 35 mm diameter, 1–1.5 m long steel rod.
  • Avoid sites with drumminess, weathered rock, or near large excavations/stress concentrations.
• Reporting (Clause 6.6.1): Report must include:
  • Test site and location description
  • Rock type and geological structures
  • Flat jack specifications and calibration details
  • Displacement gauge type, manufacturer, and calibration

Calibration & Measurement Setup Summary

flowchart TD
    A[Test Site Selection] --> B[Check Rock Soundness]
    B -->|Strike with 35mm rod| C{Sound Rock?}
    C -->|Yes| D[Proceed with Test Setup]
    C -->|No| E[Reject Site]
    D --> F[Calibrate Equipment]
    F --> G[Flat Jack & Displacement Gauge]
    G --> H[Conduct Test & Record Data]
    H --> I[Report with Calibration & Site Details]

This ensures accurate, reliable test results per IS 13946 Part 4.

IS 13946 Part 4 - Test Procedure Key Points

• Dimensions: All dimensions are in millimetres (Clause 10.5).

• Spacer Bar: Use a 300 mm long spacer bar with gauge pin as per Fig. 4 (details in code).

• Calibration:

  • All pressure and displacement measuring devices must be calibrated before each test series.
  • Calibration should be done by an independent testing laboratory.

• Reporting Requirements (Clause 6.6.1): Test reports must include:

  • Test site location and exact test location within site
  • Rock type and geological structures
  • Flat jack specification and calibration details
  • Displacement gauge type, manufacturer, and calibration data

• Reference: The method elaborates on IS 7292:1974, adding calibration and SI unit usage. IS 7292:1974 will be withdrawn after this part's publication.

Summary Table: Calibration & Reporting

flowchart LR
    A[Start Test Procedure] --> B[Calibrate Pressure Devices]
    B --> C[Calibrate Displacement Gauges]
    C --> D[Setup Spacer Bar (300 mm)]
    D --> E[Conduct Test]
    E --> F[Record Data]
    F --> G[Prepare Report with Required Details]
    G --> H[End]

This ensures accuracy and traceability in flat jack testing per IS 13946 Part 4.

IS 13946 Part 4: Site Preparation and Selection Key Points

Site Selection (Clause 4.5 & 1.5)

• Site Characteristics:
  • Should be as flat as possible without pronounced irregularities.
  • Located on sound rock; test soundness by striking with a 35 mm diameter, 1–1.5 m long steel rod.
  • Avoid sites with drumminess, weathered/damaged rock, or near large excavations/blasting zones.
  • Avoid points of stress concentration (e.g., corners).
  • Test drifts may be excavated inside tunnels if necessary.

Site Preparation & Jig Fixing (Clause 6.3.1)

• Drill eight holes for gauge points:
  • Diameter: 20 mm
  • Depth: 10 cm
• After drilling, fill holes with 1:3 cement-sand mix (water/cement ratio 0.4–0.5).
• Insert gauge pins and secure with spacer bars.
• Jig slot must be parallel to the test plane.

Reporting Requirements (Clause 6.6.1)

Include:

• Test site and location description.
• Rock type and geological structures.
• Flat jack specs and calibration.
• Displacement gauge type, manufacturer, and calibration data.

Summary Table: Hole Drilling for Gauge Pins

flowchart TD
    A[Select Flat, Sound Rock Site] --> B[Test Soundness with Steel Rod]
    B -->|No Drumminess| C[Mark Test Plane]
    C --> D[Drill 8 Holes (20mm dia, 10cm depth)]
    D --> E[Fill Holes with Cement-Sand Mix]
    E --> F[Insert Gauge Pins & Fix Jig]
    F --> G[Conduct Test & Record Data]

This ensures reliable site selection and preparation for flat jack testing per IS 13946 Part 4.

IS 13946 Part 4: Calibration Key Points

• Calibration Requirement:
  All pressure and displacement measuring devices must be calibrated before each test series by an independent laboratory (Clause 10.5, Clause 6.2).

• Calibration Factors:
  Flat jack suppliers must provide calibration factors accounting for edge effects (welding-induced pressure differences), especially for small flat jacks (Clause 6.2).

• Units & Dimensions:
  All dimensions are in millimetres (Clause 10.5, Figures 3 & 4).

• Calibration Report Must Include (Clause 6.6.1):

  • Test site location and specific test point
  • Rock type and geological structures
  • Flat jack specifications and calibration details
  • Displacement gauge type, manufacturer, and calibration data

Calibration Concept: Edge Effect Correction

Hydraulic pressure in flat jacks (P_hydraulic) is higher than actual pressure on slot walls (P_actual) due to welding edge effects. Calibration factor (K) corrects this:

[ P_{actual} = K \times P_{hydraulic} ]

Summary Table: Calibration Data to Record

flowchart LR
    A[Start Test Series] --> B[Calibrate Devices]
    B --> C{Independent Lab?}
    C -- Yes --> D[Obtain Calibration Factors]
    C -- No --> E[Repeat Calibration]
    D --> F[Apply Calibration Factors]
    F --> G[Record Calibration Data]
    G --> H[Proceed with Testing]

Note: Refer to IS 7292:1974 for original methods; IS 13946 Part 4 elaborates calibration and reporting in SI units.

IS 13946 Part 4 (1994) — Installation and Testing Key Points

Installation & Testing (Clause 6.3, 6.5, 6.6.1)

• Test Site Report Must Include:

  • Location & description of test site and specific test location.
  • Rock type & geological structures.
  • Flat jack specs and calibration details.
  • Displacement gauge type, manufacturer & calibration info.

• Calibration:

  • All pressure and displacement devices must be calibrated before each test series by an independent lab.

• Observations (Clause 6.5):

  • Continuous gauge readings (i, j, k, /) from gauge pin fixing through slot drilling and flat jack fixing.
  • Plot gauge readings vs. time.
  • Plot displacement recovery vs. flat jack pressure to find cancellation pressure.
  • Use average cancellation pressure if recovery is not simultaneous.
  • Cancellation pressure used to compute rock properties (see Clause 4.3).

Dimensions & Equipment (Clause 10.5, Fig. 4)

• All dimensions in millimetres.
• Spacer bar length: 300 mm.
• Use gauge pins as per Fig. 4 for accurate measurement.

Summary Table for Gauge Readings

Formula for Rock Property Calculation (from Clause 4.3)

[ \text{Rock Modulus} = \frac{\text{Applied Pressure (Cancellation Pressure)}}{\text{Measured Deformation}} ]

flowchart TD
    A[Test Site Selection] --> B[Installation of Gauge Pins]
    B --> C[Drilling Slot & Installing Flat Jack]
    C --> D[Calibration of Devices]
    D --> E[Continuous Gauge Reading (i,j,k,/)]
    E --> F[Plot Readings vs Time & Pressure]
    F --> G[Determine Cancellation Pressure]
    G --> H[Compute Rock Properties]

Note: Refer to IS 13946 Part 4 for detailed calibration procedures and exact gauge installation methods.

Fixing the Flat Jack (IS 13946 Part 4, Clause 6.4 & 6.4.1)

• Slot Preparation:

  • Fill the slot with 1:3 cement-sand mortar (water/cement ratio = 0.4 to 0.5).
  • Grout under pressure or use tamping rods to eliminate air gaps.

• Flat Jack Placement:

  • Align the flat jack axis with the slot axis.
  • Embed the flat jack in the mortar-filled slot.

• Curing:

  • Allow mortar to gain a compressive strength ≥ 10 MPa before loading.
  • Typical curing time: 3 to 5 days.

• Typical Flat Jack Size:

  • Example: 300 mm × 300 mm (as per Fig. 5 in the code).

Summary Table for Mortar and Curing

flowchart TD
    A[Prepare Slot] --> B[Fill with 1:3 Cement-Sand Mortar]
    B --> C[Grout under pressure / Tamping]
    C --> D[Place Flat Jack aligned with slot axis]
    D --> E[Cure for 3-5 days until 10 MPa strength]
    E --> F[Flat Jack Ready for Use]

This ensures proper embedment and reliable flat jack operation for stress measurements or rock behavior monitoring.

IS 13946 Part 4: Observations and Measurements Key Points

Gauge Readings (Clause 6.5)

• Gauge readings i, j, k, l at specified points (Fig. 1) must be continuously recorded from:
  • Fixing gauge pins
  • Drilling the slot
  • Fixing the flat jack
• Plot readings vs. time.
• Record displacement recoveries vs. flat jack pressure to find cancellation pressure (P) (Fig. 6).
• Use average cancellation pressure if recovery is not simultaneous.

Symbols & Dimensions (Clause 3.1)

Key Formulas (from Clause 4.3 & related)

• Rock deformation modulus (E):

[ E = \frac{P}{W_i} ]

Where:

• (P) = cancellation pressure

• (W_i) = displacement caused by flat jack pressure

• Displacement corrections:

[ W = W_o + W_1 + W_2 ]

Where:

• (W_o) = displacement due to stress on an infinitely thin slot
• (W_1) = correction for finite slot width
• (W_2) = displacement due to stress parallel to slot

Calibration (Clause 10.5)

• All pressure and displacement devices must be calibrated before testing.
• Calibration by an independent lab is mandatory.

Summary Diagram: Gauge Points and Slot Setup

graph LR
  A-D["500 mm gauge points"]
  B-C["250 mm gauge points"]
  Slot["Slot (width 2y₀)"]
  FlatJack["Flat Jack (pressure P)"]
  GaugePins["Gauge Pins at i, j, k, l"]
  
  Slot --> A-D
  Slot --> B-C
  Slot --> FlatJack
  FlatJack --> GaugePins

Reporting of Results as per IS 13946 Part 4 (Clause 6.6 & 6.6.1)

The report must include:

• Test Site Location: Precise geographical description.
• Test Location within Site: Exact position details.
• Geological Information: Rock type and local geological structures.
• Flat Jack Details: Specification and calibration data.
• Displacement Gauge: Type, manufacturer, and calibration details.

Units:

• All dimensions and measurements in millimetres (mm) (Clause 10.5).

Calibration Requirements:

• Pressure and displacement devices must be calibrated before each test series.
• Calibration must be performed by an independent testing laboratory.

Summary Table for Reporting

Calibration Flow (Mermaid Diagram)

flowchart TD
    A[Start Test Series] --> B[Calibrate Pressure Devices]
    B --> C[Calibrate Displacement Gauges]
    C --> D[Verify Calibration by Independent Lab]
    D --> E[Proceed with Testing]

This ensures accuracy and traceability in reporting test results.

IS 13946 Part 4 — Limitations and Assumptions (Clause 4.2)

The flat jack method for in-situ stress measurement assumes:

• Stress redistribution occurs due to slot cutting, causing strain/displacement at gauge points.
• Cancellation pressure (P) applied in the flat jack produces strain equal and opposite to slot cutting strain.
• This pressure restores initial rock stress (S, Q) before slot cutting.
• Rock properties: Deformation modulus (E) and Poisson's ratio (u) are isotropic (same in all directions).
• Stress concentration effects from tunnel driving are negligible.

Key Symbols (Clause 3.1)

Fundamental Relation

The cancellation pressure P equals the initial rock stress normal to the slot:

[ P = S ]

The displacement at gauge points relates to stresses and flat jack pressure by:

[ W = W_o + W_1 + W_2 = W_i \quad \Rightarrow \quad \text{(at cancellation pressure)} ]

Where:

• (W_o) = displacement due to slot stress,
• (W_1) = correction for finite slot width,
• (W_2) = displacement due to parallel stress

IS 13946 Part 4 (1994) – Key References, Symbols & Specifications

Key Symbols (Clause 3.1)

Specifications & Notes

• All dimensions are in millimeters (Clause 10.5).
• Gauge lengths: 250 mm (i,k), 500 mm (j,l).
• Calibration of pressure and displacement devices mandatory before each test series by an independent lab.
• The method extends IS 7292:1974 with calibration and SI units; IS 7292:1974 withdrawn post-publication.

Conceptual Relation of Displacements

graph LR
    P[Flat Jack Pressure P] --> Wi[Displacement Wi]
    S[Stress Normal to Slot S] --> Wo[Displacement Wo]
    Wo --> W1[Correction W1]
    Q[Stress Parallel to Slot Q] --> W2[Displacement W2]
    Wi & Wo & W1 & W2 --> W[Observed Displacement W]

This forms the basis for interpreting slot deformation and rock stress measurements in situ.