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IS 13946:2000 (Part 4) is the Indian Standard (BIS) for determination of rock stress-code of practice, part 4: using flat jack technique. This standard details the procedure for determining the in-situ stress in rock near an exposed surface using the flat jack technique. It outlines the equipment, method for cutting a slot, installing the jack, applying pressure to cancel deformation, and calculating the stress component perpendicular to the jack.
Determination of rock stress-Code of practice, Part 4: Using flat jack technique
Standard Test Method for Determination of the In-Situ Stress in Rock Using the Hydraulic Fracturing Method
Both standards provide a detailed procedure for determining in-situ rock stress using hydraulic fracturing in a borehole.
ISRM-SM-2003-Part 3International Society for Rock Mechanics (ISRM)
HighCurrent
ISRM Suggested Methods for rock stress estimation—Part 3: hydraulic fracturing (HF) and/or hydraulic testing of pre-existing fractures (HTPF)
Provides a comprehensive methodology for hydraulic fracturing, forming the theoretical basis for many national standards including IS 13946.
Key Differences
≠IS 13946 specifies a prescriptive pumping rate of '10 to 20 litres/min', which is unusually high for typical test setups. In contrast, ASTM D4645 uses a performance-based criterion, specifying a rate sufficient to cause breakdown in 30 to 60 seconds.
≠The ISRM Suggested Method explicitly includes procedures for Hydraulic Testing of Pre-existing Fractures (HTPF) to determine the stress state, a technique not covered in IS 13946.
≠ASTM D4645 and the ISRM method provide more detailed guidance and criteria for selecting suitable, intact, and homogeneous rock sections for testing, whereas the guidance in IS 13946 is more general.
≠International standards like ASTM D4645 provide more detailed descriptions of graphical techniques for interpreting the pressure-time curve to accurately determine the shut-in pressure (Ps), a critical parameter for stress calculation.
Key Similarities
≈All standards are based on the same fundamental principle established by Hubbert and Willis, using breakdown, shut-in, and re-opening pressures to calculate the magnitude of principal stresses.
≈The basic field equipment, including the use of a straddle packer system to isolate the test interval, a high-pressure pump, and pressure transducers, is functionally identical across the standards.
≈All standards require the use of an impression packer or similar downhole tool (like a borehole televiewer) after the test to determine the orientation of the induced fracture.
≈The general test sequence is consistent: sealing the test interval, a primary pressurization cycle to induce breakdown, and subsequent pressurization cycles to determine the fracture re-opening pressure.
Parameter Comparison
Parameter
IS Value
International
Source
Test Interval Length to Diameter Ratio (L/D)
Approx. 5 to 10 (based on 30-50 cm length in AX/BX boreholes)
4 to 10
ASTM D4645-18
Pumping Rate for Breakdown
Prescriptive: '10 to 20 litres/min'
Performance-based: Rate sufficient to cause breakdown in 30 to 60 seconds
ASTM D4645-18
Number of Post-Breakdown Cycles
At least two cycles
Several cycles are normally run (typically 3 to 5)
ASTM D4645-18
Impression Packer Setting Pressure
7 to 10 MPa
Slightly above the previously measured shut-in pressure (Ps)
ASTM D4645-18
Impression Packer Setting Time
10 to 15 minutes
5 to 10 minutes
ASTM D4645-18
Assumed Fracture Orientation
Assumes induced tensile fracture is vertical and parallel to the maximum horizontal stress.
Assumes induced fracture is perpendicular to the least principal stress.
ISRM-SM-2003-Part 3
Pore Pressure Assumption in Calculation
The standard calculation formula explicitly includes the pore pressure (Po) term.
The calculation formula also explicitly includes the pore pressure (Po) term, emphasizing its direct measurement if possible.
ASTM D4645-18
⚠ Verify details from original standards before use
Key Values4
Quick Reference Values
Typical slot width5 to 6 mm
Recommended distance between measurement pinsAbout twice the jack width
Number of pressure cyclesAt least two cycles of loading and unloading (Clause 5.5.1)
Maximum test pressureShould not exceed one-third of the uniaxial compressive strength of the rock (Clause 5.5.1)
Key Formulas
σ_n = P_c * K_m * K_a — Stress normal to jack, where Pc is cancellation pressure, Km is jack constant, Ka is partial slot factor.
Tables & Referenced Sections
Key Tables
No tables data
Key Clauses
Clause 4 - Equipment
Clause 5 - Procedure
Clause 5.4 - Installation of Flat Jack
Clause 5.5 - Pressurization of the Flat Jack
Clause 6 - Calculation and Interpretation of Results
The test determines the magnitude of the in-situ compressive stress acting perpendicular to the plane of the jack at the measurement location.
What is 'cancellation pressure'?+
It is the hydraulic pressure applied to the flat jack that is required to restore the rock mass around the slot to its original state of strain before the slot was cut.
Why is this method considered a 'stress relief' technique?+
Because the first step involves cutting a slot, which relieves the stress in the rock immediately surrounding it. The test then measures the pressure needed to reverse this relief.
Can this test measure tensile stress?+
No, the standard flat jack technique is designed to measure compressive stress. If the pins move apart after slot cutting, it indicates a tensile or very low stress field, but the method cannot quantify it.