Abstract: Prediction and visualization of the width and developing law of the key cracks are crucial to the invention of the standing prototype loading system and the further optimal design for special-shaped segmental structure. This paper conducted loading tests on a CF50 steel fiber reinforced concrete beam with a volume rate of 2% to improve the theoretical formula of the maximum crack width, and the non-contacting strain optical measuring technique was adopted for the first time in the field of prototype loading tests on segmental lining to monitor the cracking characteristics in key areas of the inner surface of the tunnel vault. The study shows that the determination method of crack initiation time and calculation method of crack width can be based on three-dimensional digital image processing technology (DIC-3D). The increment of the crack width experiences initial, stable and accelerated three stages, and the ultimate buried depth of special-shaped tunnel is finally determined to be 18.5m. The calculation parameters obtained from the beam loading tests for the theoretical maximum crack width are verified to be effective, and prediction of the crack width combined with the internal forces derived from shell-spring model is suggested.

Key words: special-shaped shield; prototype test; damage; concrete crack; digital image processing technology