Determination of rock stress-Code of practice, Part 4: Using flat jack technique
1994 Edition

The flat jack method for rock stress determination follows these essential steps: First, a thin slot is created in the rock by drilling overlapping holes, which causes stress relief and slot convergence. Second, initial displacement between fixed points across the slot is recorded. Third, a flat jack is inserted and cemented into the slot. Fourth, hydraulic pressure is applied incrementally to the flat jack while monitoring displacement until the original deformation is fully reversed, known as cancellation pressure. This pressure corresponds to the in-situ rock stress normal to the slot. Finally, using the pressure and displacement data, rock mechanical properties are calculated based on the relevant equations. This method presumes elastic behavior of the rock within the working stress range.

Cancellation pressure is the hydraulic pressure applied within the flat jack that counteracts the deformation caused by slot cutting, effectively restoring the original rock stress state. When the slot is cut, the rock stress is relieved, causing the slot sides to converge. By inflating the flat jack, the convergence is reversed, and the pressure at which displacement is neutralized is termed the cancellation pressure. Using cancellation pressures from two perpendicular slots (horizontal and vertical), principal rock stresses are calculated by solving simultaneous equations that incorporate calibration constants and rock parameters such as Poisson's ratio. This method assumes isotropic elastic properties and negligible stress concentration effects.

Accurate flat jack testing requires precision instruments for measuring both displacement and hydraulic pressure. All devices must be calibrated prior to each series of tests by an independent laboratory to ensure traceability and reliability. The test site should be a flat, sound rock surface verified by impact testing using a steel rod to exclude weathered or hollow rock zones. Equipment such as spacer bars (typically 300 mm in length) and gauge pins must conform to specified dimensions for precise measurement setup. Adhering to these calibration and equipment standards is critical to obtaining valid and consistent test results.

The optimal site for flat jack testing should have a relatively flat rock surface free from pronounced irregularities and be composed of sound, unweathered rock. The rock's soundness is verified by striking it with a steel rod to detect any hollowness or drumminess. Sites near large excavations, blasted areas, or points of stress concentration such as tunnel corners should be avoided. The rock mass should behave elastically within the intended stress range and not exhibit significant stress relaxation. Tests are generally conducted near the rock surface due to depth limitations of the method.

The flat jack technique assumes that cutting the slot causes stress redistribution, resulting in measurable displacement at gauge points, and that applying cancellation pressure in the flat jack counters this displacement to restore initial stress conditions. It presumes the rock behaves elastically with isotropic deformation modulus and Poisson's ratio, and that stress concentration effects from excavation are negligible. Limitations include the inability to measure stresses at significant depths within the rock mass, potential inaccuracies due to local stress concentrations, and assumptions of uniform rock properties. The method is best suited for small-scale, near-surface stress evaluations.