Dynamic Cracking Evolution and a Damage Mechanics Model of Shield Tunnelling Spoil Solidified by Alkali-Activated Ground Granulated Blast-Furnace Slag

Abstract

Tunnelling construction requires cautious consideration of the disposal of shield tunnelling spoil, as this involves environmental, economic, and governmental requirements. This paper uses alkali-activated ground granulated blast-furnace slag (GGBS) to solidified the shield tunnelling spoil and conducts a case study on the microcracks and their evolution characteristics during structural damage in solidified spoil. Specifically, the real-time X-ray computerized tomography (X-CT) scans under uniaxial compression loading, along with image processing technology are used to analyze two-dimensional (2D) and three-dimensional (3D) features of microcracks. Based on X-CT results, the porosity and cracks connectivity at different axial strains were calculated to summarize the evolution law of structural damage factors. Furthermore, a damage equation was proposed to describe the evolution of structural damage in the specimen. Results show that the stress–strain curve of solidified spoil under uniaxial loading exhibits strain–softening characteristics, with an unconfined compression strength of 1.85 MPa. The 2D slices and 3D reconstructed models effectively visualize the dynamic expansion and connection of internal cracks throughout the deformation process. The distribution of cracks is significantly heterogeneous in the specimen, with the cracking degree in the middle is obviously higher than that at both ends. The failure of the specimen follows a varying pattern of initial emergence, rapid expansion, slow extension, and eventually stable development of load-induced cracks. During the compressive process, the load-induced cracks in the original specimen continuously expand and penetrate, leading to the formation of the major crack surface that induces structural damage in the late stage of deformation. The predicted stress-strain curve plotted by the mechanics model by incorporating damage variables based on the 3D X-CT data is in good agreement with the experimental one. ​The findings from this study provide a new perspective on the knowledge of structural damage and failure mechanisms and facilitate the utilization of shield tunnelling spoil.

Keywords: shield tunnelling spoil, ground granulated blast-furnace slag, uniaxial compression test, x-ray computed tomography, mechanics model