Laboratory study on the anisotropic behaviour of extremely thin-layered rocks

Anisotropic failure of extremely thin-layered rocks (bedding thickness ≤10 mm) often leads to severe tunnel squeezing. However, direct laboratory studies on this behaviour remain scarce, as standard samples for conventional testing cannot be readily prepared, and suitable loading devices for small-sized samples are lacking. To overcome this challenge, a micro-mechanical device was developed and used to conduct uniaxial compression tests on 8 mm cubic miniature shale samples. The results demonstrate that the variations in strength and failure modes of the miniature samples with respect to bedding angles align with those observed in standard samples derived from medium-thick layered rocks. Notably, the strength of miniature samples is generally 50%–60% lower than that of the standard samples. The minimum strength ratio (σα/σα=0°) of 0·45 occurs at 60°, which is 16·7% lower than that of the standard samples. These findings indicate that the extremely thin-layered rocks exhibit lower overall strength and more pronounced anisotropies than medium-thick layered rocks. This study fills the gap of directly obtaining the mechanical parameters of extremely thin-layered rocks and provides a reliable basis for the design of tunnel support in such stratum.