The 1993 edition of IS 7317 lays out comprehensive guidelines for performing uniaxial jacking tests aimed at evaluating the deformation modulus of rock masses in their natural setting. It details the method of applying controlled hydraulic forces on rock surfaces within underground excavations and interpreting the resulting stress-strain responses to assess rock deformability, creep characteristics, and elastic modulus. This standard is indispensable for specialists in geotechnical engineering, rock mechanics, and underground structure design where precise rock deformation properties are vital.
Overview
The 1993 edition of IS 7317 lays out comprehensive guidelines for performing uniaxial jacking tests aimed at evaluating the deformation modulus of rock masses in their natural setting. It details the method of applying controlled hydraulic forces on rock surfaces within underground excavations and interpreting the resulting stress-strain responses to assess rock deformability, creep characteristics, and elastic modulus. This standard is indispensable for specialists in geotechnical engineering, rock mechanics, and underground structure design where precise rock deformation properties are vital.
Audience
Contents
Structure
IS 7317 defines the boundaries for displacement measurement beneath a circular loaded zone on rock surfaces and provides key formulae for interpreting displacement data. The principal formula calculates displacement (\delta) at depth (Z) under the center of a circular load, incorporating parameters such as applied stress, Poisson's ratio, modulus of elasticity, and loaded area radius. Typical displacement values versus depth and standard anchor depths for extensometers are tabulated, supporting design considerations in rock support and displacement monitoring.
The standard emphasizes choosing test sites within drifts, tunnels, or underground cavities, or alternatively adjacent to open excavation faces. Preparation of the test area should occur swiftly, ideally within 15 days, and within 30 days for rocks susceptible to weathering. Documentation of geological parameters such as Rock Quality Designation (RQD), joint characteristics, groundwater conditions, and rock mass ratings (RMR and Q) is required. Comprehensive reporting includes site coordinates, excavation details, in-situ stresses, and stress-deformation plots.
Preparation involves compiling all geological data to produce detailed 3D geological maps illustrating rock mass microstructures. Joint attributes, groundwater conditions, and rock mass quality indices must be recorded collaboratively by geologists and engineers. The test area's readiness should align with the stipulated timelines, ensuring the rock's freshness for accurate testing. Excavation methods and any blasting-induced damage should also be documented.
Preferred test setups are detailed, prioritizing the configuration shown in Fig. 4 for highest accuracy, with alternatives provided if the preferred is unfeasible. Equipment standards include steel plates with minimum dimensions, hydraulic jacks of 200-ton capacity, precision dial gauges, and properly prepared rock faces coated with cement mortar. Supporting structures ensure stability and positioning accuracy during testing.
The test procedure mandates adherence to the preferred setup for precise results, utilizing hydraulic jacks following specifications. Proper alignment and loading processes are essential. An updated formula for load and stress calculation supersedes earlier expressions and should be referenced from the latest edition. Flowcharts assist in selecting appropriate setups based on site constraints.
Data interpretation involves plotting deformation against stress, time, and depth to extract deformation and elastic moduli, and creep behavior. The critical displacement formula beneath a circular load informs calculations. The test report must comprehensively include geological context, test conditions, equipment details, and graphical representations aiding in the understanding of rock mass behavior.
Reports should cover the test environment, precise test site location, overburden, position on rock surface, elapsed time since excavation, and testing direction. Geological descriptions enriched with RMR and Q values, in-situ stress data, testing agency details, and apparatus specifications are mandatory. Comprehensive raw data tables and graphs illustrating stress-deformation relationships and creep factors must be included along with displacement formulae and creep factor calculations.
The deformation modulus (E_d) calculation relies on parameters including load, deformation, plate shape factor, and Poisson's ratio, with typical values varying by rock type. The elastic modulus (E_e) is similarly derived from recoverable deformation. The standard highlights the stress dependency of modulus values and emphasizes engineering judgment during design. Graphical plots and flowcharts illustrate the methodology for deriving and applying these moduli for stability analysis.
The standard provides guidance on the use of deformation moduli for static and dynamic analyses, including dam foundation design and concrete lining. It addresses the influence of confining pressure, saturation effects, and recommends nonlinear analyses for plastic behavior. Contact information for BIS and amendment listings are provided, alongside typical test setup figures and summarized application tables.
Frequently Asked
For narrow drifts (approximately 1.25 m width and 2.2 m height), the standard recommends minimizing drift size to reduce the size of packing plates or trusses. The distance between the loaded area and tunnel surface should be at least equal to the radius of the loaded area to prevent boundary restraints. The setup per Fig. 2B includes a hydraulic jack with a calibrated dial gauge, datum bar, hollow steel section, and pressure gauge, with excavation performed to minimize rock disturbance and no blasting allowed during final preparation. For wider tunnels, a larger configuration is utilized featuring restraint columns and a flat jack about one meter in diameter as per Fig. 3. Multiple MPBX anchors (five or more per hole) measure deformation, with core drilling extending approximately six times the flat jack diameter. The setup includes a calibrated hydraulic jack, load cell, concrete pedestal for the pump, and transducer wiring. These arrangements ensure reliable measurement of rock deformation while accounting for the influence of tunnel geometry.
The deformation modulus ( E_d ) is computed using applied pressure and measured displacements at two different depths beneath the loaded area. The procedure involves applying load through a flat jack with specified inner and outer radii, recording displacements ( \delta_1 ) and ( \delta_2 ) at depths ( Z_1 ) and ( Z_2 ), and calculating geometric factors ( K_1 ) and ( K_2 ) based on elastic half-space theory. The modulus is then calculated as ( E_d = \frac{P (K_1 - K_2)}{\delta_1 - \delta_2} ). The calculation accounts for all deformation components including elastic, plastic, and creep effects, excluding initial deformation caused by blasting or surface loosening. Stable modulus values are typically obtained from loading cycles two through five, recognizing the modulus's dependence on stress level.
According to the standard, the test site must represent the geological conditions relevant to the structure's influence zone. This requires comprehensive data gathering and analysis of both surface and subsurface geology, followed by preparation of a detailed 3D geological model of the exploratory drift. A 3D micro-geological map of the in-situ rock mass should be created, recording essential parameters such as Rock Quality Designation (RQD), Point Load Strength Index, joint number and orientation (dip and strike), spacing and condition, groundwater status, Rock Mass Rating (RMR), Rock Mass Quality (Q), modulus of elasticity, presence of shear or fault zones, excavation method, and any blasting damage. Tests should be conducted within 15 days of site preparation or within 30 days if the rock is prone to weathering to ensure fresh rock properties are assessed.
The standard recognizes rock mass deformation as comprising elastic (recoverable), plastic (permanent), and time-dependent creep components. It specifies that deformation measured in the tests includes these combined effects, with creep behavior analyzed through displacement versus time plots. The deformation modulus calculated from uniaxial loading implicitly accounts for these behaviors, although engineering judgment is necessary to select design values reflecting this complexity. The standard encourages time-dependent data interpretation to quantify creep and recommends adjusting the deformation modulus accordingly for realistic modeling of rock mass response under load.
The deformation modulus ( E_d ) derived from uniaxial jacking tests represents the stiffness of the rock mass under load and varies with stress magnitude. It is used to model the interaction between rock mass and structural elements, facilitating the estimation of displacements and stress distributions around tunnels, galleries, and dam foundations. Calculation involves parameters such as load, deformation, plate shape factor, and Poisson's ratio, with typical values depending on rock type. Design decisions rely on integrating geological data, test conditions, and stress-deformation behavior, often utilizing plots of ( E_d / E_r ) versus Rock Mass Rating (RMR) to select appropriate modulus values. This ensures stability, serviceability, and safety in structural design.
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