Size effects on the strength and cracking behavior of flawed rocks under uniaxial compression: from laboratory scale to field scale

The failure behavior of field-scale rock masses has long been studied indirectly through laboratory compression tests on rock specimens with preexisting flaws. However, little to no attention has been paid to size effects on these cracking processes that may be governed by the relative size between the fracture process zone and the rock structure. Here, we investigate such size effects on the compressive strength and cracking behavior of flawed rocks through high-fidelity simulations of mixed-mode fracture in quasi-brittle materials. We perform a series of numerical uniaxial compression tests on geometrically similar gypsum specimens with single and double flaws, across a wide range of sizes from 0.25 times a standard laboratory specimen size to 16 times. The results suggest strong size effects on both the uniaxial compressive strength and cracking patterns. The size effect on the compressive strength appears qualitatively similar to Bazant's size effect law derived for the tensile strength of notched structures. However, the quantitative changes in the strength deviate from the existing size effect law which does not account for mixed-mode fracture. As for the cracking behavior, three types of cracking patterns per flaw configuration are identified as the specimen size changes. Remarkably, the cracking patterns that emerge at the field scale, where the size of the fracture process zone is negligible, are analogous to those observed from laboratory experiments on highly brittle materials. The findings of this work provide important insights into how to bridge observations on rock fracturing processes across scales.