This code of practice outlines the procedure for assessing in situ rock stresses employing a CSIR or CSIRO type strain cell fitted with 9 or 12 strain gauges. It involves pilot hole drilling, strain cell installation, overcoring to relieve stresses, and computation of the stress tensor from recorded strain data. The standard is tailored for professionals engaged in geotechnical and rock mechanics fields requiring precise stress evaluations in elastic, uniform rock masses.
Overview
This code of practice outlines the procedure for assessing in situ rock stresses employing a CSIR or CSIRO type strain cell fitted with 9 or 12 strain gauges. It involves pilot hole drilling, strain cell installation, overcoring to relieve stresses, and computation of the stress tensor from recorded strain data. The standard is tailored for professionals engaged in geotechnical and rock mechanics fields requiring precise stress evaluations in elastic, uniform rock masses.
Audience
Contents
Structure
This section defines the applicability of the method for in situ rock stress evaluation using the overcoring technique with strain gauges. It specifies drilling equipment requirements such as using NXC core barrels (minimum 86 mm diameter) for the main hole and 38 mm diameter bits for the pilot hole. The strain cell contains three rosettes with 3-4 gauges each, positioned closely to reduce rock volume influence. Gauge length must be at least 10 mm with orientations following diagrams for capturing six independent strain components. Measurement protocol includes taking two readings per gauge, averaging, and completing at least two full reading cycles. Reporting mandates tabulation of orientations and strain relief, listing Young’s modulus and Poisson’s ratio with their determination methods, and providing strain relief tensor components with associated error estimates.
Key formulae relate measured strain values to stress tensor components under the assumption of isotropic rock behavior. The stress-strain relationships depend on Young’s modulus, Poisson’s ratio, and angular orientations of gauges. Data reporting instructions include documenting all strain relief readings and elastic constants, while measurement procedures require multiple averaged readings per gauge. Notes address the necessity of homogeneous, elastic rock and caution against using this method in water-saturated holes due to bonding difficulties.
Defines critical parameters such as Young’s modulus (E), Poisson’s ratio (ν), components of the stress tensor (σ and τ), and orientation angles of strain gauges. Presents the core elasticity equations linking strain measurements to stress components, referencing illustrative figures for precise angular definitions. Reporting requirements reiterate the need for detailed documentation of readings, elastic properties, and error margins.
Details equipment needed for drilling and installing the strain cell, including NXC-type drills with 86 mm or larger barrels for the main hole and 38 mm pilot hole bits with centralizers. Safety measures for underground operations include exhaust scrubbers and anti-spark devices. The strain cell features multiple closely spaced gauge rosettes oriented to capture six independent strain components. Installation tools must allow electrical connection, orientation, and secure placement of the strain gauges against pilot hole walls, often using gas-operated mechanisms. Additional requirements include cleaning materials for preparing the hole and plugs to seal the pilot hole during overcoring.
Covers the process of inserting the strain cell using specialized tools that hold and orient the device, ensuring the gauges contact the pilot hole wall. Measurement instruments include precision strain bridges, temperature sensors, and calibrated electrical connectors. The procedure mandates taking two readings per gauge in at least two complete measurement rounds, averaging the results for accuracy. Reporting must include gauge orientations, elastic constants with determination methods, strain relief tensor components, and error analysis. The section also provides typical strain calculation formulas and schematic diagrams illustrating strain cell installation.
Describes the mathematical relationship between measured strain and stress components in isotropic rock using elasticity theory. Defines parameters such as Young’s modulus, Poisson’s ratio, and gauge orientation angles. Instructions cover tabulating orientation and strain relief readings, calculating six stress tensor components with precision, and estimating error through regression analysis. Methods for determining principal stresses and directions using matrix eigenvalue analysis are also outlined.
Specifies the comprehensive reporting format including drillhole location, geometry, geotechnical core logging, procedures and equipment descriptions with visuals, and detailed per-depth measurement data. Reports must present strain relief readings, elastic constants with derivation methods, six components of the strain relief tensor, and statistical error assessments. Emphasizes the necessity of taking multiple balanced readings per gauge and averaging them to ensure reliable data.
Provides a template for recording field data, encompassing drillhole identification, coordinates, length, core condition, and measurement depth. Includes pre- and post-overcoring strain readings, calculated strain relief values, geological observations, and equipment used. Summarizes formulas for computing strain relief and highlights quality control measures for core integrity. The annex serves as a practical guide for systematic data collection in field conditions.
Lists the members of the Rock Mechanics Sectional Committee (CED 48) responsible for this part of the standard. Members include academic experts from universities, representatives from government departments (irrigation, power, geological survey), research institutions (CSIR, NTPC), industry professionals from construction and instrumentation firms, and BIS officials. The chairman and member-secretary are named, and the diversity of expertise ensures comprehensive standard development.
Frequently Asked
The installation requires drills equipped with NXC core barrels of at least 86 mm diameter to form the main overcore hole, complemented by 38 mm diameter coring bits and centralizers to create the pilot hole concentrically. Installation tools are designed to hold and orient the strain cell inside the pilot hole, provide electrical connections to multi-conductor cables, and push the strain gauge rosettes firmly against the hole walls, often operated by gas pressure. Additional materials include alcohol sprays for cleaning and priming the pilot hole and plugs to seal it during overcoring. For underground settings, safety features like exhaust scrubbers and anti-spark devices are mandatory to mitigate hazards.
The strain gauges are arranged in rosettes mounted on the cylindrical surface of the strain cell inserted into the pilot hole. For nine gauges, three rosettes each containing three gauges are positioned at specific azimuth angles, commonly at 0°, 45°, and 90°, enabling full multi-axis strain capture. In the case of twelve gauges, four rosettes with three gauges each are spaced evenly around the circumference, typically at 90° intervals. This systematic orientation ensures measurement of all six independent strain components necessary to compute the stress tensor accurately.
Initially, strain gauges attached to the rock core record baseline strain values before overcoring. The overcoring process then relieves in situ stresses by drilling around the strain cell and extracting the core. Following this, multiple sets of strain readings are collected until the strain values stabilize, indicating complete stress relief. For each gauge, two readings per measurement round are taken and averaged, with a minimum of two full rounds performed. The strain relief is calculated by subtracting the averaged post-overcoring strain from the pre-overcoring baseline. This process ensures reliable strain data for subsequent stress calculations.
Young’s modulus (E) and Poisson’s ratio (ν) are primarily derived through laboratory testing of rock core samples, employing uniaxial or triaxial compression tests to characterize elastic properties under controlled conditions. Alternatively, biaxial or triaxial tests may be conducted directly on cores containing the strain cell to capture elastic constants under relevant stress states. These values are critical inputs for elasticity equations that convert measured strain data into stress components. Documentation of the testing methods and results is required in the final report to validate the calculated stress tensor.
This method presumes isotropic elastic rock behavior; therefore, it is unsuitable for significantly anisotropic rocks where elastic constants vary with direction, as the fundamental stress-strain relations become invalid. Additionally, water-saturated conditions can alter elastic moduli and Poisson’s ratio due to pore pressure and fluid interactions, leading to erroneous strain measurements. The technique also demands intact, unfractured cores; fractured or saturated samples compromise strain gauge bonding and data integrity. For such cases, specialized approaches incorporating anisotropic elasticity models or pore pressure corrections should be employed to obtain accurate stress assessments.
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