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 1: Using hydraulic fracturing technique
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.
Practical Notes
Accuracy is highly dependent on the quality of the slot cutting and ensuring the jack is properly seated without voids.
This method measures stress near a free surface (e.g., tunnel wall), which can be influenced by excavation-induced stress redistribution.
To determine the complete stress tensor at a point, tests must be conducted in a minimum of three, and preferably more, non-parallel slots.