This paper addresses design challenges associated with the use of grouted bolt reinforcement in laminated ground at shallow to moderate depths for large span excavations. Flat-roofed cavern designs enable the inherent structure of the rockmass to be exploited in order to optimize the cavern design. In order to design a stable flat-roofed excavation with a large span, special consideration must be given to beam mechanics and the influence on support requirements. In addition, the shear resistance afforded by fully-grouted bolts across lamination interfaces is a key consideration. Modern analysis tools are capable of simulating both discrete laminations and structural bolting elements. While the support mechanics of axially stretched or tensioned bolts is validated and verified in these analyses, the incorporation of shear resistance (of a bolt across a sliding discontinuity) is less robust. Linear element numerical bolt models often rely on theoretical assumptions to relate joint slip to bolt resistance. Due to the complex bolt-grout-rock interactions in this case, three-dimensional Finite Difference Method (FDM) models were used to assess support capacity to resist small shear displacements and to explore the validity of these underlying assumptions.
Underground excavations are often designed with an arched roof which is either a horseshoe or near circular profile. These roof shapes provide optimal stress distribu-tions and result in a more stable excavation. Several natural geological conditions exist for which an arched roof design is not optimal; this includes laminated sedimentary rock, horizontally banded metamorphic rock and horizontally fractured isotropic rocks. In laminated ground conditions, an arched roof design can lead to destabilization of roof materials which leads to supple-mentary support requirements to prevent additional roof fall-out. In such settings, the structure of the material can be used to improve cavern design and prevent the need for excessive support systems.
