Challenges in Tunnelling Through Layered Rockmasses

Engineers and engineering geologists today have an ever increasing toolbox of sophisticated analytical and numerical tools to simulate increasing mechanical detail within the rockmasses surround the tunnels we build. Sedimentary or volcaniclastic layering creates challenges for tunnelling if not considered during the design stage. While the classic classification systems for rockmass assessment can give useful information about the quality of a rockmass in general (based on an assumption of structural isotropy), kinematics, structural and anisotropic response must be considered for layered systems through the use of modern analysis tools. Monitoring strategies must account for delamination or localized shearing. Support strategies, design and sequencing, must be compatible with this anisotropic response and the interlayer deformations. Geological formation designation is often based on consistent environments during the deposition phase. A deltaic formation, for example, may contain hard cemented sandstone, weak cross-bedded sequences as well as silty or clay-rich interbedding or lensing. A layered volcaniclastic sequence can contain hard strong flow layers, sandwiched within softer, weaker or altered tuffs, breccias and lapilli layers with occasional thin layers of ash fallout, often turned to clay. Rockmass classification performed based on exposures or face mapping may not capture the behavioural character of such rockmasses. Caution is required when interpreting geological descriptions for project sites. Site investigation, selection of analysis model and monitoring plan must link to ensure that the mechanics of the sedimentary rock are considered in excavation strategy, support design and monitoring (alert levels and response). Case examples will be discussed highlighting pitfalls and solutions (TBM, Mechanical and D&B) in sedimentary rocks.