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An experiment to study the in situ fracture shear strength of fracture surfaces was performed at a depth of 437 m in the ONKALO® underground research facility as part of the POST-project, a joint research and development project performed by Posiva Oy, the Swedish Nuclear and Waste Management Company and the Nuclear Waste Management Organization of Canada. The test, termed the PUSH test, was primarily conducted to demonstrate and assess the methodology for conducting an in situ fracture shear test while also determining fracture behaviour. A loose block identified on the tunnel wall after excavation was used for the experiment, as it was formed due to the intersection of a natural fracture and excavations.
The block was attached to the wall with anchors to create a constant normal stiffness condition and LVDT sensors were installed to measure normal and shear displacement. Normal and shear forces were measured using load cells and a hydraulic pressure gauge. Out-of-plane shear forces were minimised using flat jacks. Slot drilling was performed to release the block. Once the instrumentation displacement limit had been reached, the block was removed. High-resolution photogrammetry was performed both before and after the experiment, as well as after the block was detached.
Back-analysis was performed using numerical simulations with 3DEC, using planar, fully-mated photogrammetry-based and unmated representations of the fracture surface. The unmated model yielded an acceptable match to measurements, with back-analysed friction angles 10 degrees greater than laboratory results, suggesting a possible scale effect.
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