When the Layer-Cake Bites Back: Tunnelling Challenges in Sedimentary Rock

ABSTRACT: Site Investigation, geomechanical design analysis, ground monitoring and support optimization are integral parts of the tunnel engineering process. The validity of the geotechnical approach to specific behaviour of the ground impacts constructability, excavation method selection, details of TBM design, support design, monitoring strategy and long-term performance of the project. This caveat is relevant for tunnelling within sedimentary rocks. Layering creates challenges for tunnelling if not considered during the design stage. Kinematics, structural and anisotropic response must be considered. 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. 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.