Geotechnical analysis for underground excavation design in complex tectonic environments requires an increased understanding and more rigorous consideration of the impact of healed ("intrablock") structure, such as veins, on rockmass behaviour. Intrablock structure occurs between blocks of rock defined by joints and other fractures (interblock structure). A new concept to better represent the behaviour of intrablock structure in explicit numerical models is proposed and tested in this paper by means of finite element method analysis and case study data from a 1200 m deep drift. This approach changes the stiffness and strength values of failed intrablock structural elements between pre-peak ("primary"), post-peak ("secondary"), and ultimate ("tertiary") states. The FEM models in the tertiary stage match 96% of overbreak patterns along the case drift, versus 80% in primary stage models. These findings suggest that the proposed method is a good option to more accurately model the influence of intrablock structure on rockmass behaviour.
Rockmass stiffness and strength parameters are determined by the characteristics of intact rock and rockmass structure. Empirical rockmass classification systems quantify components that contribute to rockmass strength, including intact rock strength, joint length, spacing, orientation, shape, aperture, and surface condition, into single values that can be used for ground support design. Advancement of numerical modelling tools has allowed for more detailed analysis of rockmass structure in modern geotechnical design than empirical classification systems can provide. As numerical methods grew from their early homogeneous continuum behaviour to include explicit joint elements, and to having fully discrete capabilities, additional input parameters were needed to better describe the rockmass behaviour. In particular, for capturing the behaviour of rockmass structure, the concept of normal and shear stiffness for joint elements was developed to describe the stress-deformation response before yield and sliding occur (Goodman et al. 1968).
During the early considerations of rockmass stiffness and strength, healed structures such as veins, veinlets and stockwork that are found in complex rockmasses (termed intrablock structure where joints are interblock structure) were disregarded as having an inconsequential influence on rockmass behaviour (Goodman et al. 1968). However, as both civil and mining underground excavations go deeper and enter into more high stress environments with complex excavation geometries and associated stress paths, healed structures have been found to have a significant impact on rockmass behaviour. Using case data, this study explores a new concept of primary, secondary, and tertiary values for stiffness and strength, defined by pre-peak, post-peak, and ultimate states of yield, for intrablock structure in complex rockmasses. Detailed Finite Element Method (FEM) numerical models of a drift with explicit intrablock structure are compared to overbreak profiles of a deep drift in an undercut level during development of a block cave mining operation in Chile.