Tunneling in complex soft bimrock formations: Numerical analyses

Geotechnically complex formations include materials such as conglomerates, agglomerates, glacial tills and melanges, which are very common geological units and are often defined to be “bimrocks” (block in matrix rocks), “bimsoils” (block in matrix soils) and “SRM” (soil-rock mixtures) in the literature. The term bimrock (block in matrix rocks) was coined by Medley in order to identify heterogeneous materials with hard rock blocks enclosed in a softer matrix of finer texture. Bimrocks are of great importance as the strength contrast that exists between the rocks and the matrix deems them quite significant in the geotechnical context. Their presence has often caused severe economic challenges during construction works, because of their poor characterization. A geometric and mechanical characterization of these formations in the development stages of building works is vital to avoid facing unnecessary economic and safety risks in the future. While there has been previous analysis done on Bimrocks, not much work has been done on the presence of an interface between the rock and matrix. While analyzing Bimsoils also, the presence of such interfaces and how their properties can affect underground excavations and other earthworks has not been taken into account. In this dissertation to understand the differences in the behavior of bimsoils with respect to bimrocks, numerical analyses were carried out by introducing an interface at the block-matrix contacts represented by Joint boundaries simulated through FEM software RS2. The underground excavation of a tunnel having a diameter of 10 m in complex heterogenous bimsoil formations with different VBP is analysed. An arbitrary extraction of the Block distribution that characterizes the geomaterial is made via a statistical approach based on the Monte Carlo strategy. For 25% VBP, 8 random block distributions were extracted via a Matlab code in which the blocks are elliptical, and hypothetical in-situ conditions are defined for the analysis with results then compared with bimrock models without interfaces, investigated in previous works. Moreover, the properties of the joints are decreased gradually from 100% to 25% of the matrix properties with the aim of analyzing bimsoils with different block-matrix interface strength. Results showed that the joint boundaries had little to no effect on the tunnel excavation. The same analysis is also carried out for 1 model each of 40% and 70% VBP in order to evaluate if the results were different if the VBP was increased, but it was not the case. The increase in VBP did not produce significant differences in the results. Since many heterogeneous block-in-matrix formations are characterized by the presence of joints/schistosity, one model for each VBP was modified to incorporate a Joint Network, and analyse both with and without the rock-matrix interfaces. An increase in the overall radial displacements along the tunnel contour was observed in the case in which along with the Joint network, rock-matrix interfaces were also present. The difference in the displacement was more pronounced for less VBP percentage at 25% as compared to 70%. Lastly, the equivalent homogenous approach proposed by Lindquist (1994) was modelled assuming different VBPs (25%, 40%, 55% & 70%) with and without the presence of a Joint network, to compare the results. As expected, an anisotropic behavior is observed in the models with joints, with a decreasing trend of displacements and yielding zones with the increase of VBP.