Shallow Urban Tunnelling Through Heterogeneous Rockmasses: Practical Experience From Small Scale Tunnels in Calgary, Alberta and the Influence of Rockmass Layering on Excavation Stability and Support Design
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Shallow excavations through variable rockmasses in urban centers present significant design challenges, whether considering small diameter tunnels for utilities or large span underground caverns. In designing shallow excavations in urban environs, it is especially critical to minimize the impact of the excavation on surface. In small diameter projects, minimal surface disturbance is often achieved by the employment of TBMs as the excavation method. While reducing the risk of surface subsidence due to displacements in front of the face, TBM progress is sensitive to variable ground conditions and the TBM design must be appropriately matched to the expected geology. Sufficient understanding of the geology and development of geological models are critical in the selection of an appropriate TBM and cutting tools. In this study, recent projects in Calgary, AB are used to highlight the challenges faced with using TBMs through sedimentary rock with distinct, variable units. In larger scale projects, long term excavation stability is critical in the reduction of surface disturbance. Due to the low confining stresses, structural failure is often the primary failure mode in shallow excavations, especially within fractured, heterogeneous rockmasses. In these cases, numerical methods are often used in excavation design. The ability of numerical methods to capture the expected failure modes of shallow excavations through layered rockmasses is explored, with an emphasis placed on the ability of support elements to reduce shear slip for increased stability. Passive bolt models are analysed using both 2D and 3D numerical models to adequately capture the behaviour of a passive support system in shear. The shortcomings of some current support models are discussed, and modifications are suggested.