Improved polygonal constitutive model for columnar jointed basalt and its application in tunnel stability analysis

Abstract

The columnar jointed basalt (CJB) in the dam site of the Baihetan hydropower station exhibits noticeable geometric anisotropy. The current constitutive relationship based on a quadrangular columnar structure fails to truly represent the natural characteristics of the CJB. To address the stability problem of the diversion tunnel at the dam foundation, the mechanics of composite materials and Goodman superposition principle were employed to establish improved three-dimensional constitutive relations for pentagonal and hexagonal CJBs. The constitutive relations of three types, including the quadrangular prism CJB, were optimized by considering the impact of transverse joints and internal staggered zones. The mechanical parameters of the Baihetan CJB were appropriately evaluated using the three constitutive relations, and the achieved results demonstrated that the determined elastic moduli of the three improved constitutive models were consistent with field results. Subsequently, the three anisotropic constitutive models were employed for numerical simulations of the diversion tunnel excavation in the Baihetan CJB stratum. Strain field tests were conducted at different locations within the tunnel, and an empirical method was proposed that combined statistical data on the shape of CJBs at the site, which can effectively predict the deformation of the surrounding rock. The theoretical and numerical results provide crucial reference values for engineering optimization design.