DETERMINATION OF NOMENCLATURE, MECHANISTIC BEHAVIOUR, AND NUMERICAL MODELLING OPTIMIZATION OF UMBRELLA ARCH SYSTEMS
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Surface limitations in terms of infrastructure and the need for more direct transportation routes often call for the development of underground construction works in geotechnically unfavourable conditions, which oftentimes require a temporary pre-support system prior to excavation in order to avoid tunnel collapse or surface settlement. One such pre-support system is the umbrella arch system, which has been gaining popularity due to its lower cost and reduced time to employ when compared to other pre-support systems. However, the umbrella arch system has been employed without a complete understanding of its mechanical response. The system has been found in the literature to be either under- or over- designed with negative impacts on safety and/or cost. This study makes three major contributions with respect to increasing the understanding of the relevant geomechanical aspects and the use of the umbrella arch system. The first major contribution is the development of a relevant nomenclature standard in order to categorize and define umbrella arch elements and systems, which lead to the creation of an empirically driven Umbrella Arch Selection Chart (UASC) with a view to aiding initial design of tunnel support. The second major contribution is the establishment of a design methodology, based upon advanced numerical modelling techniques to aid in the optimized design of the umbrella arch arrangement for site-specific conditions. These techniques were based primarily on robust parametric analyses utilizing advanced two-dimensional and three-dimensional numerical models complemented with field data from select case studies for validation purposes. The third major contribution is the development of a semi-analytical model for umbrella arch systems employed in squeezing ground conditions by incorporating elastic foundation theory and Convergence-Confinement Method (CCM). The model addresses the deficiencies associated with the CCM when supports are installed close to the tunnel face. As umbrella arch systems are always employed within close proximity of the tunnel face, an investigation and subsequent improvement to the CCM was conducted. These three major contributions are introduced and explained in detail; including a discussion of how they are incorporated into the tunnel support design process and throughout the construction and monitoring phase of the project.