True-triaxial characterisation of the cement-based grout for application in underground space construction

Cement grouts are extensively used in underground space construction. They fill the Karst in the rock mass, improve the rock mass quality, and stop water inflow. During underground construction, the excavation alters the stress state of the rock mass, which even means the grout-filled Karst. In most practical cases, the altered stress state is either biaxial or true-triaxial. However, the mechanical characterization of cement-based grout is typically reported using triaxial and direct shear tests, with limited data on its behaviour under biaxial or true-triaxial conditions. To address this gap, the present study investigates the mechanical response of cement-based grout under biaxial and true-triaxial stress conditions. The results demonstrate enhanced hardened cement grout strength with increasing intermediate principal stress, while the failure pattern reveals fracture planes nearly parallel to the intermediate principal stress and inclined toward the minor principal stress. Additionally, seventeen true-triaxial failure criteria based on strength parameters obtainable from conventional triaxial compression test data are assessed to predict the strength of hardened cement grout. The choice of using conventional triaxial test data is due to the limited availability of true-triaxial testing facilities globally. Among the considered criteria, the Jiang-Zhao criterion outperforms the others. Moreover, using the four similitude criteria, hardened cement grout behaviour was similar to that of natural sandstone. It is concluded that the hardened cement grout corresponds to sandstone with a low strength-low modulus ratio (DL, Deere and Miller classification).