Tunneling in heterogeneous rock masses with a block-in-matrix fabric

Abstract

Heterogeneous rock masses composed of strong rock blocks embedded in a weaker matrix belong to complex formations and are often referred to as bimrocks (block-in-matrix-rocks). Due to the high spatial, dimensional, geo-hydrological and lithological variability of such rock bodies, a common geotechnical engineering design practice is not to consider the presence of the blocks and assign the strength and deformability properties of the weaker matrix to the whole rock mass. However, over the last decades, several case histories and many relevant studies on bimrocks have demonstrated that neglecting the presence of the blocks can produce wrong forecasts, which often lead to serious technical problems and severe economic repercussions during the construction of engineering works on and in these complex formations.The aim of this study is to investigate the stability of a deep circular tunnel excavated in a heterogeneous rock mass with a chaotic block-in-matrix fabric. In order to determine how the presence of rock inclusions may influence the overall behavior of the bimrock during the excavation, different Volumetric Block Proportions (VBPs) are used. To take the inherent spatial and dimensional variability of the blocks into account, many heterogeneous tunnel configurations are generated for each VBP considered by means of a stochastic approach. The analyses are performed using the Finite Element code RS2.Moreover, in order to highlight potential inaccuracies caused by neglecting the presence of the blocks at the design stage, simplified approaches commonly used by geopractitioners, which assume bimrocks to be homogeneous equivalent geomaterials, are also applied by way of comparison.