Code of practice for in-situ determination of rock mass deformability using flexible dilatometer, Part 2: Radial displacement

Scope of IS 12955 Part 2:

IS 12955 Part 2 deals with the quantitative description and reporting of discontinuities in rock masses, essential for geotechnical investigations.

Key Points:

• Scope covers methods to describe discontinuities such as orientation, spacing, persistence, roughness, wall strength, aperture, filling, seepage, number of sets, block size, core recovery, and rock quality.
• It references IS 11315 Parts 1 to 11, each addressing a specific discontinuity characteristic.
• Reporting includes detailed drilling logs, geotechnical logs, discontinuity characteristics, and calibration/testing methods.

Relevant IS 11315 Parts:

Calibration & Testing (Clause 7.1 & 5.2):

• Calibration constants must be calculated using prescribed methods.
• Equipment and methods must be reported, including any deviations from the standard.

Summary Diagram: Rock Mass Discontinuity Description

graph LR
A[Rock Mass] --> B[Discontinuities]
B --> C[Orientation (IS 11315-1)]
B --> D[Spacing (IS 11315-2)]
B --> E[Persistence (IS 11315-3)]
B --> F[Roughness (IS 11315-4)]
B --> G[Wall Strength (IS 11315-5)]
B --> H[Aperture (IS 11315-6)]
B --> I[Filling (IS 11315-7)]
B --> J[Seepage (IS 11315-8)]
B --> K[Number of Sets (IS 11315-9)]
B --> L[Block Size (IS 11315-10)]
B --> M[Core Recovery & Rock Quality (IS 11315-11)]

**In essence, IS 12955 Part

IS 12955 Part 2 (1990) - Referenced Indian Standards Summary

This part of IS 12955 covers in-situ rock mass deformability measurement using flexible dilatometers with radial displacement measurement.

Key Referenced IS Codes (Annex A):

Important Notes:

• These standards provide quantitative methods for describing rock mass discontinuities, crucial for interpreting dilatometer test results.
• IS 12955 Part 2 focuses on direct radial displacement measurement by flexible dilatometers.
• Dilatometer tests yield local deformability moduli, which can be correlated with other tests (e.g., plate jacking) for design-scale extrapolation.
• The standard mark ensures compliance with BIS quality and inspection systems.

Typical Formula for Rock Mass Deformability Modulus (E):

[ E = \frac{p}{u_r / r} ]

Where:

• ( p ) = applied pressure (Pa)
• ( u_r ) = radial displacement (m)
• ( r ) = radius of the borehole

IS 12955 Part 2: Location of Test Site - Key Points

• Clause 3 (Location of Test Site): Specifies the need for proper site selection, referencing related standards (Annex A).

• Clause 8.1 (Site Reporting Requirements):
  For the entire test site, report:

  • Drilling details: agency, method, equipment.
  • Geotechnical logs: drill core data, casing/cementing, groundwater, rock types, test section locations.
  • Discontinuity characteristics within test sections ±0.5 m (per IS 11315 Parts 1-11).
  • Calibration/testing methods and equipment, noting deviations from IS 12955.
  • Complete calibration results.

• Clause 6.2.1 (Test Location Accuracy):
  The test probe position must be measured and recorded with an accuracy of ±5 cm after verifying hole clearance with a diameter gauge.

Summary Table for Location Accuracy

flowchart LR
    A[Drilling] --> B[Geotechnical Logging]
    B --> C[Discontinuity Survey ±0.5m]
    C --> D[Calibration & Testing]
    D --> E[Record Probe Location ±5cm]
    E --> F[Report All Details]

This ensures reliable, reproducible test results per IS 12955 Part 2.

IS 12955 Part 2: Preparation of Test Site – Key Points

1. Test Site Preparation (Clause 3.2:4)

• Ensure the test site is accessible and safe for drilling and testing operations.
• Clear the area of debris and provide stable working platforms.
• Mark test locations accurately based on site investigation.

2. Location and Reporting (Clause 8.1)

• Report drilling details: agency, method, and equipment.
• Provide geotechnical logs showing:
  • Cased/cemented sections
  • Groundwater levels
  • Rock types and characteristics
  • Test section locations
• Document discontinuities within ±0.5 m of test sections per IS 11315 (Parts 1-11).
• Include calibration method, equipment details, and deviations from the standard.
• Submit full calibration results.

3. Drilling Equipment (Clause 5.1)

• Use equipment suitable for the rock type and test method.
• Ensure proper maintenance for accurate test hole preparation.

Summary Table: Test Site Preparation Checklist

This structured preparation ensures reliable and reproducible test results as per IS 12955 Part 2.

IS 12955 Part 2 – Test Equipment: Key Points

Calibration Equipment (Clause 5.2)

• Calibration devices must ensure accuracy and traceability.
• Calibration constants are essential for converting raw data into meaningful results (see Clause 7.1).

Test Equipment (Clause 5.5.2)

• Pressure Measuring System:
  • Use Bourdon gauge or electric pressure transducer.
  • Range: as required by test (≥ 20 MPa for hard rocks).
  • Sensitivity: better than ±2% of full scale.
• Pressurizing Fluids: glycerin, ethylene glycol, water, or hydraulic oil.

Drill Core Logging (Clause 4.3)

• Record: core recovery, fracture frequency, rock characteristics, weathering, structural features (schistosity, foliation, bedding, joints).
• Cores must be available on-site for inspection.

Sample Formula for Calibration Constant (Clause 7.1)

[ K = \frac{P_{actual}}{P_{measured}} ]

Where:

• (K) = Calibration constant
• (P_{actual}) = Known applied pressure
• (P_{measured}) = Pressure reading from test equipment

flowchart TD
    A[Drill Core Logging] --> B[Test Equipment Setup]
    B --> C[Pressure Measurement]
    C --> D[Calibration of Equipment]
    D --> E[Calculation of Calibration Constants]
    E --> F[Accurate Test Results]

Summary: Use calibrated pressure gauges/transducers with ±2% sensitivity, appropriate fluid, and detailed core logging for reliable testing under IS 12955 Part 2.

IS 12955 Part 2 — Test Procedures: Key Points

Pressure Measurement (Clause 5.5.2)

• Use a pressure measuring system (Bourdon gauge or electric transducer).
• Accuracy: Better than ±2% of full scale.
• Pressure Range: ≥ 20 MPa for hard rocks.
• Pressurizing fluids: Glycerin, ethylene glycol, water, or hydraulic oil.

Test Reporting Requirements (Clause 8.2)

• Tabulate raw and corrected pressure readings with depth and direction.
• Present deformability parameters with calculation methods and assumptions.
• Show deformability variation logs vs. depth or distance from drill-hole collar.
• Provide anisotropy graphs perpendicular to the drill hole, showing dilation and deformability.

Test Execution (Clause 6.2.7)

• Plot pressure-dilation curves during cyclic testing (see Fig. 2 in the standard).

Typical Pressure-Dilation Curve (Conceptual)

graph LR
A[Applied Pressure] --> B[Dilation Response]
B --> C[Hysteresis Loop during Cycling]
C --> D[Deformability Parameters]

Summary Table for Pressure Measurement

This ensures reliable measurement and comprehensive reporting of rock deformability under pressure per IS 12955 Part 2.

IS 12955 Part 2: Analysis of Test Data – Key Points

1. Data to Report (Clause 8.2)

• Tabulated readings: Raw & corrected values, depths, directions.
• Graphs: Pressure-dilation curves (see Fig. 2).
• Deformability parameters: Derived values, methods, assumptions; tabulated & graphed vs applied pressure.
• Deformability logs: Variation with depth or distance from drill-hole collar.
• Anisotropy graphs: In plane perpendicular to drill-hole, showing directional differences.

2. Test Procedure (Clause 6.2.8)

• For a single displacement direction probe:
  • Rotate probe at same depth to measure deformability in other directions.
  • Relocate probe for next test.

3. Analysis (Clause 7.2.4)

• Use pressure-dilation diagrams to determine:
  • Short-term rock mass response.
  • Time-dependent behavior.

Typical Pressure-Dilation Curve (Fig. 2 schematic):

graph LR
A[Applied Pressure] --> B[Measured Dilation]
B --> C[Plot Curve]
C --> D[Analyze Short-term & Time-dependent Response]

Important Notes:

• Deformability parameters often include modulus of deformability (E) and Poisson’s ratio (ν).
• Corrected values account for probe and equipment calibration.
• Anisotropy assessment is critical for rock mass characterization.

For detailed formulas and graphical methods, refer directly to IS 12955 Part 2, Fig. 2 and clauses mentioned.

IS 12955 Part 2: Reporting of Results - Key Points

Clause 8: Reporting Requirements

For each test, report the following:

• Tabulated readings: Raw and corrected values with depths, measurement directions; include graphs (e.g., pressure-dilation diagrams as per Fig. 2).
• Deformability parameters: Derived values with methods and assumptions; tabulate and plot deformability vs. applied pressure.
• Deformability logs: Variation with depth or distance from drill-hole collar.
• Anisotropy graphs: Show dilation and deformability anisotropy in the plane perpendicular to the drill-hole.

For the site as a whole (Clause 8.1):

• Drilling details: agency, method, equipment.
• Geotechnical logs: core characteristics, casing, groundwater, test section locations.
• Discontinuity characteristics within ±0.5 m of test sections (refer IS 11315 Parts 1-11).
• Calibration and testing methods, noting any deviations.
• Complete calibration results.

Important Reference Standards for Discontinuities (IS 11315 Series)

Typical Reporting Table Format (Example)

Summary Diagram: Reporting Flow

flowchart TD
    A[Start Test] --> B[Record Raw Data]
    B --> C[Apply Corrections]
    C --> D[Calculate Deformability Parameters]
    D --> E[Plot Graphs: Pressure

IS 12955 Part 2: List of Referred Indian Standards (Annex A, Clause 2.1)

This part of IS 12955 references essential IS codes for quantitative description of rock mass discontinuities:

Key Notes:

• These standards provide methods for quantitative description of rock mass discontinuities crucial for geotechnical design.
• Used collectively, they help assess rock mass behavior for foundations, tunnels, slopes, etc.
• The Standard Mark ensures compliance and quality control under BIS supervision.

Summary Table for Quick Reference: