Authors: Naseri, S; Bahrani, N

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Naseri, S & Bahrani, N 2019, 'Stability assessment of initial shotcrete lining using two-dimensional continuum numerical modelling', in J Hadjigeorgiou & M Hudyma (eds), Proceedings of the Ninth International Symposium on Ground Support in Mining and Underground Construction, Australian Centre for Geomechanics, Perth, pp. 311-326.

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The convergence–confinement method (CCM) is a simple tool for the preliminary design of ground support for circular excavations. The influence of excavation advance rate on the design of a support system is often ignored using the CCM. In this paper, a two-dimensional (2D) continuum numerical modelling approach for support design is introduced, which integrates the excavation advance rate into the CCM. The numerical simulations are carried out based on the results of a previously calibrated 2D finite element model of an instrumented section of a 10 m diameter shaft in an average-quality rock mass. First, the internal pressure reduction approach is used to simulate the three-dimensional shaft advance and to generate the ground reaction curve (GRC). The longitudinal displacement profile (LDP) is calculated using a semi-empirical approach. The LDP is then integrated with the GRC to determine the radial displacements corresponding to the locations of the shaft face and initial shotcrete liner. Knowing the shaft advance rate allows determination of the appropriate mechanical properties of early-age shotcrete based on empirical equations. The paper presents three approaches for analysing the stability of the shotcrete liner. The first approach is based on calculating the load Factor of Safety using support-capacity diagrams. In the second approach, the load Factor of Safety is calculated using the conventional CCM. In the third approach, the strain Factor of Safety is calculated by considering the plastic deformation of early-age shotcrete. The minimum allowable shotcrete thickness is then determined by calculating the load and strain Factors of Safety for different lining thicknesses. It is suggested that a combination of these approaches provides a deeper insight into the complex behaviour of early-age shotcrete lining during excavation advance compared with the conventional CCM.

Keywords: initial shotcrete lining, convergence–confinement method, ground reaction curve, longitudinal displacement profile, 2D numerical modelling, support-capacity diagram, strain Factor of Safety

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