This standard details the overcoring method for determining the in-situ 2D stress field in a rock mass using a USBM-type borehole deformation gauge. It specifies the testing apparatus, field procedure for drilling and stress relief, and the calculation methodology to convert measured borehole deformations into principal stress magnitudes and orientations. The results are critical for the design of underground excavations like tunnels and caverns.
Determination of rock stress-Code of practice, Part 2: Using USBM-type drill hole deformation gauge
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.