The 1994 edition of IS 13946 Part 2 outlines the standardized methodology for assessing in-situ rock stresses by employing the USBM-type drillhole deformation gauge technique. This guideline provides detailed instructions on drilling, gauge placement, overcoring operations, and interpreting data to quantify secondary principal stresses perpendicular to the borehole axis. It is a vital reference for professionals engaged in evaluating rock mass stability and underground excavation design.
Overview
The 1994 edition of IS 13946 Part 2 outlines the standardized methodology for assessing in-situ rock stresses by employing the USBM-type drillhole deformation gauge technique. This guideline provides detailed instructions on drilling, gauge placement, overcoring operations, and interpreting data to quantify secondary principal stresses perpendicular to the borehole axis. It is a vital reference for professionals engaged in evaluating rock mass stability and underground excavation design.
Audience
Contents
Structure
This section defines the purpose and coverage of the standard, focusing on measuring in-situ rock stresses through the overcoring method using deformation gauges. It details information to be included in reports such as drillhole positioning, equipment specifications, and data presentation formats including field sheets and graphical plots. Equipment requirements for the deformation gauge, strain indicators, calibration devices, and modulus chambers are also described.
Presents the key mathematical relationships governing the stress-deformation behavior of rock under plane strain isotropic elasticity, including formulas to calculate principal stresses and orientations from gauge measurements, and the derivation of Young's modulus via biaxial chamber tests.
Clarifies essential terms such as diametral deformation under zero axial stress, the use of USBM-type gauges with three sensors spaced at 60 degrees, and mathematical expressions for principal stresses and their orientation angles.
Describes the necessary apparatus including calibrated deformation gauges, strain indicator readout systems, placement rods marked for depth and orientation, biaxial modulus chambers for elastic property measurement, and tools for core retrieval, emphasizing their functions and specifications.
Outlines procedures for drilling pilot and overcore holes, including diameter specifications, drill rod types, stabilizer placement, water swivel usage, and core extraction. Details on gauge insertion methods and overcoring parameters such as rotational speed, penetration rate, and data acquisition intervals are provided.
Details calculation techniques for deriving in-situ stresses from measured diametral deformations, including two-dimensional assumptions with zero axial stress, interpretation of readings from three sensor orientations, and three-dimensional plane strain elasticity considerations. Also covers estimation of Young's modulus from overcoring data.
Specifies the contents of comprehensive reports such as drillhole details, geotechnical logs, equipment descriptions, field data sheets, strain and stress plots, calculated elastic properties, statistical parameters, and explanations for any inconsistencies observed in data.
Provides templates and examples of data sheets capturing all relevant measurements including hole identifiers, deformation values (U1, U2, U3), elastic moduli, radial pressures, stress magnitudes and orientations, along with procedural notes and geological observations.
Lists members of the Rock Mechanics Sectional Committee and its subcommittees involved in preparing IS 13946 Part 2, highlighting affiliations and roles, reflecting the multidisciplinary expertise contributing to the standard.
Frequently Asked
The USBM-type deformation gauge method quantifies secondary principal stresses in the plane perpendicular to the borehole by measuring variations in the pilot hole diameter during the overcoring process. It employs cantilever-mounted electrical resistance strain gauges positioned at three orientations spaced 60 degrees apart to detect minute diameter changes (U1, U2, U3). These measurements are then used in formulas that incorporate Young's modulus and the pilot hole diameter to calculate the magnitudes and directions of principal stresses, enabling accurate in-situ stress determination.
The process begins by drilling a large diameter overcore hole to the target depth, followed by removal of the core. A pilot hole, typically about 38 mm in diameter and extending around 2 meters, is then drilled within ±14 mm of the overcore centerline, avoiding fractured zones to ensure instrument stability. Overcoring proceeds with a bit approximately 150 mm in diameter, drilling beyond the gauge's cantilever tips by at least 150 to 225 mm, with a total overcore length of 300 to 450 mm to stabilize readings. Optionally, a nearby diamond drill hole can be made to identify intact rock zones, improving test accuracy and reducing time.
Key equipment includes: drill rods and bits for creating pilot (38 mm) and overcore (approximately 150 mm) holes; USBM-type deformation gauges capable of detecting diameter changes in multiple orientations with high sensitivity; water swivels to allow signal cable passage through drill rods; strain indicator readout bridges and switchgear for data acquisition; placement rods marked with depth and orientation scales for accurate gauge positioning; and core retrieval tools such as wedges, shovels, and pullers for extracting rock samples. Additionally, a calibration device ensures gauge accuracy, and a biaxial modulus chamber is used to determine Young's modulus of the rock core.
Young's modulus (E) is derived from biaxial chamber testing using the relationship between applied radial pressure and measured changes in pilot hole diameter, expressed as E = (D² - d²)/(2 d P) × U, where D and d are diameters of overcore and pilot holes respectively, P is radial pressure, and U is the deformation. Poisson's ratio (ν) is determined through conventional laboratory testing on rock samples. Both parameters are fundamental to relate measured strains to in-situ stresses, facilitating iterative calculations to refine principal stress estimates and account for axial stresses when present.
Reports should comprehensively document drillhole location, orientation, length, and geotechnical core logs with detailed geological observations. Measurement data must include field sheets or plots depicting deformation values (U1, U2, U3), radial pressure versus borehole deformation, and calculated elastic properties (Young's modulus and Poisson's ratio). Tabulated results should present hole identifiers, stress magnitudes, directions, and statistical analyses such as standard deviations and correlations. Additionally, documentation should contain descriptions and illustrations of equipment and procedures, along with explanations for any discrepancies or anomalies observed, ensuring reproducibility and clarity in reporting.
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