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This paper uses tunnel stability factors to estimate the minimum supporting pressure required to maintain the stability of twin circular tunnels drive side-by-side in drained cohesive-frictional soil. Similar to the bearing capacity equation proposed by Terzaghi, the proposed method is based on the three stability factors (Fc, Fs and Fγ) that are functions of the soil internal friction angle and the tunnel depth ratio. Unlike the case of a single tunnel, the centre-to-centre distance between the tunnels appears as a new parameter that must be considered in twin tunnel stability. For various spacing ratios (S/D) between the tunnels, non-dimensional tunnel stability factors (Fc, Fs and Fγ) are established for various depth ratios (C/D) and soil internal friction angles (Ø) using two-dimensional finite element limit analysis. For practical suitability, the results are presented in the form of dimensionless stability charts with the actual tunnel stability factors closely bracketed from above and below using upper and lower bound methods. As an extra validation, the results are compared with available solutions reported in the literature. [ABSTRACT FROM AUTHOR]

This paper investigates the use of stability factors to estimate tunnel heading pressures. The primary method adopted is a conventional equation based on the soil property and stability factors, analogous to the bearing capacity factors (N c , N s and N γ) of strip footings. The finite element limit analysis (FELA) is employed to determine rigorous upper bound (UB) and lower bound (LB) solutions of stability factors (F c , F s and F γ), which are functions of the depth ratio (C/D) and soil internal friction angle (Ø). The obtained results are compared and validated by using the finite difference analysis as well as other available published results in the literature. A number of examples are illustrated on how to use the factors to estimate tunnel heading pressures. [ABSTRACT FROM AUTHOR]

This paper investigates the three-dimensional (3D) stability of a circular tunnel in drained cohesive-frictional soil. Numerical simulations are performed to study the face stability of a circular tunnel for various soil properties and tunnel diameter-to-depth ratios. The finite element limit analysis (FELA) is employed to determine rigorous upper and lower bounds solutions of tunnel stability factors (F c , F s and F γ), which are functions of the depth ratio and soil internal friction angle. These stability factors are used to estimate the critical heading pressures at collapse. The obtained results are compared with existing published solutions in the literature. The study aims to improve the available solutions by using the latest 3D finite element limit analysis technique. A number of examples are provided, which demonstrate the practical use of the three stability factors in estimating tunnel heading pressures. [ABSTRACT FROM AUTHOR]