Abstract: A biaxial compression test is conducted on a real sandstone transverse elliptical tunnel; moreover, the characteristics of the progressive crack propagation process of the transverse elliptical tunnel are analyzed with acoustic emission and a digital image correlation realtime acquisition system to investigate the failure process and evolution warning of the elliptical tunnel. The results show the following: (1) Tensile cracks first appear in the tunnel′s crown and bottom; subsequently, shear and farfield cracks appear on both sides of the tunnel. Furthermore, a Vshaped groove is formed on the right sidewall of the tunnel, and the farfield cracks extend to the Vshaped groove, leading to the brittle failure of the tunnel. (2) The strain concentration area on the tunnel surface appears before the crack, and the crack′s location can be predicted. (3) The fractal dimension gradually increases from zero in the stable crack propagation stage. When the tunnel shear crack starts, the growth rate of the fractal dimension accelerates. The fractal dimension shows a sudden increase before reaching the peak tunnel load. (4) At a quiet period, the sudden drop of the 〖WTBX〗b〖WT５《TNR》〗 value, a sudden increase in the entropy value, a sudden increase in strain field variance, and a sudden increase in the crack fractal dimension can be used as early warning information before tunnel failure. The early warning time of different physical fields′ monitoring data is not synchronized, and the early warning load is 87% to 97% of the peak load. The sudden drop of the 〖WTBX〗b〖WT５《TNR》〗 value is the earliest and the sudden strain field variance increase is the latest.

Key words: transverse elliptical tunnel, crack propagation, acoustic emission, digital image correlation, fractal dimension, damage warning