Influence of Anisotropic Rock Mass Characteristics on Tunnel Stability
Rock Engineering 2009 - Rock Engineering in Difficult Conditions
Recent works in deep underground tunnelling have illustrated how the behaviour of the expected ground conditions can differ from those assumed (Kaiser 2005 & 2006). Two of the reasons for this deviation are (1) a result of structural features such as joints, bedding planes or foliations, facilitating the stress-driven rock mass disintegration process in areas that would not normally be considered; and (2) specifically due to rock mass degradation as a result of stress damage processes that were not properly taken into account in the lowering of rock quality of the near excavation boundary rock material. This paper discusses rock mass behaviour in design and focuses on the use of simple tools to assess the influence of structural features on excavation stability. From this work, an Orientation Reduction Factor (ORF) based on a modification to the Stress Reduction Factor (SRF) proposed by Barton et al. (1974) is proposed to assist in the design of circular tunnels in anisotropic rock.