Abstract
The convergence-confinement method via the ground reaction curve (GRC) is used as the common practice of tunnel design which demands accurate determination of the stress state and material strength behavior in different zones around the tunnel section. Besides, formation of the excavation/blast-induced damaged zone (EDZ/BDZ) adds more complexity to the problem due to variation of elasticity modulus of the rock mass in this zone. As a result, advanced numerical methods via finite element/difference commercial packages or user-coded, semi-numerical techniques are required to develop the GRC, which demands a high degree of proficiency and knowledge of computational plasticity and geomechanics. In this study, a new, simple, and accurate method is proposed for prediction of GRC of circular tunnels constructed in the damaged, elastoplastic rock masses obeying softening in the plastic zone. The effects of deterioration caused by the drilling/blast in the EDZ were taken into account by assuming a reduced and varying Young’s modulus using the disturbance factor, in the form of Hoek–Brown failure criterion and the Geological Strength Index (GSI). Besides, effects of intermediate principal stress and the exponential decaying dilation parameter are taken into account thanks to adoption of the unified strength criterion (USC) as the material strength criteria. To do so, genetic algorithm (GA) via the method of evolutionary polynomial regression (EPR) is used to find a relationship between a number of 19 affecting parameters on the GRC as the input, and the internal support pressure as the target of prediction. Verification analysis was performed to verify the validity of the results using field measurements data as well as other advanced numerical studies found in the literature. Lastly, variation of the support pressure with simultaneous changes in the affecting input parameters was investigated using multivariable parametric study.
Highlights
Academic Editor: Wei Shen e convergence-confinement method via the ground reaction curve (GRC) is used as the common practice of tunnel design which demands accurate determination of the stress state and material strength behavior in different zones around the tunnel section
A new, simple, and accurate method is proposed for prediction of GRC of circular tunnels constructed in the damaged, elastoplastic rock masses obeying softening in the plastic zone. e effects of deterioration caused by the drilling/blast in the excavation damaged zone (EDZ) were taken into account by assuming a reduced and varying Young’s modulus using the disturbance factor, in the form of Hoek–Brown failure criterion and the Geological Strength Index (GSI)
E proposed method is based on the development of an evolutionary polynomial regression (EPR) technique, which uses the best features of the genetic algorithm (GA) and the least squared method to predict new target functions based on their previous learning of the relationship between input and output parameters
Summary
A full description of the proposed algorithm along with the formulation and governing equations is presented in Ghorbani and Hasanzadehshooiili [26]. In order to calculate the stresses and strains of the points located in the elastic-plastic boundary, the Newton–Raphson (NR) method of root finding is used to calculate the radial stress (σr sR), which is equal to the critical support pressure (Pic). In summary, the following procedure has been taken for analysis of the plastic zone: first, the RungeKutta-Fehlberg method is put into practice to determine the radial stress on the on the plastic-EDZ boundary (srP−E). Determination of radial and tangential strains in the plastic zone σr(i) ≥ σrP-E No. Calculation of new initial boundary conditions for radial and tangential stresses, radial and tangential strains, radial and tangential elastic and plastic strains and their corresponding increments. Calculation of radius dependent Young’s modulus in EDZ and update radial and tangential elastic strain increments ω(i) = ωp – (ωp – ωr)(γ p(i)/γ p*). The support pressure (Pi) to generate the database of GRCs used in this study
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