| dc.description.abstract | In the engineering of rock masses, such as tunnels, rock slopes, and foundation construction within fractured rock formations, discontinuity is a crucial factor influencing the behavior of the engineering structures. This influence is not only related to the orientation of the discontinuities but also closely tied to factors such as the excavation direction of tunnels, the slope of rock, and the orientation of foundation configurations. Despite the acknowledgment by previous engineers of the anisotropic behavior in rock engineering and the widespread application of empirical rules in practical projects, the lack of rigorous and comprehensive results from physical model tests, analytical analyses, or numerical analyses regarding anisotropic behavior in rock engineering has led to a long-standing deficiency in verifying the accuracy of empirical rules.
This paper aims to explore the anisotropic behavior of rock tunnels, focusing not only on validating the rationality of adjusting orientation scores in Rock Mass Rating (RMR) and Rock Structure Rating (RSR) but also emphasizing the data processing and interpretation of numerical analyses of three-dimensional synthetic rock mass tunnel models. Two-dimensional numerical simulation tools can not address the effects of different excavation directions, so this paper utilizes three-dimensional numerical simulation programs PFC3D and RS3 to investigate the influence of tunnel excavation direction on the stability of fractured rock masses. The verification of tunnel excavation using the Kirsch equations confirms that both PFC3D and RS3 are suitable for simulating tunnel excavation in fractured rock masses. A Bonded Particle Model (BPM) combined with a Discrete Fracture Network (DFN) is generated using PFC3D to construct synthetic rock mass tunnel models with a set of discontinuities. The dip angle (?j) variables include: ?j=0˚, 15˚, 30˚, 45˚, 60˚, 75˚, and 90˚. Three tunnel excavation scenarios are simulated: Scenario 1 (γ=90˚): Strike parallel to the excavation direction; Scenario 2 (γ=0˚): Strike perpendicular to the tunnel axis and direction of drive with dip; Scenario 3 (γ=180˚): Strike perpendicular to the tunnel axis and direction of drive against dip. RS3 also performs simulations with the same variables and scenarios. Through numerical simulation results, the paper investigates the influence of various variables on tunnel stability, and quantitative indicators such as displacement fields, radial displacements, strain invariants, strain energy, and potential energy are used to describe tunnel stability. By comparing simulation results with existing literature, the paper consolidates a scoring adjustment system for discontinuities concerning excavation direction and summarizes deformation patterns and distributions after tunnel excavation under various conditions.
The analysis of numerical results reveals that: (1) Displacement values around tunnels in RS3 are much smaller than those in PFC3D, especially when γ=0˚ and 180˚. RS3 shows extremely small displacement values around tunnels, with only slightly larger values when γ=90˚ and ?j=60˚ to 90˚, approximately 1/10 to 1/60 of the displacement values in PFC3D. (2) Theoretically, rock masses with a dip angle βj=90˚ should exhibit consistent stability when excavated in γ=0˚ and γ=180˚. However, RSR and RMR evaluations for excavations in γ=0˚ and 180˚ are ＂very favorable＂ and ＂fair,＂ respectively, deviating from theoretical expectations. The analysis results for βj=45˚ and βj=90˚ in γ=90˚ and 180˚ show significant differences, indicating distinct tunnel behaviors, and suggesting that these should not be grouped under the same dip angle. Therefore, the dip angle grouping is refined from three groups to six groups, providing a more detailed scoring system and addressing the discrepancies in RSR and RMR evaluations. (3) Using strain invariants, strain deviator invariants, strain energy, radial displacement, and potential energy, the paper performs optimization fitting with RMR discontinuity orientation score adjustments. Radial displacement and potential energy show the highest correlation with RMR; out of 18 combinations, 10 are identical, while the remaining 4 differ by one grade and 4 differ by two to three grades. Strain invariants, strain deviator invariants, and strain energy are based on the assumption of small strains and must filter out particles with large displacements, making them unsuitable as indicators for tunnel stability. (4) Rock masses with high dip angles (βj=60˚~90˚), when excavated parallel to the discontinuity orientation (γ=90˚), exhibit significant differences in orientation score adjustments proposed by RS3 (finite element method) and PFC3D (discrete element method) compared to RMR. Reviewing previous research results, three main approaches are identified: finite element methods, discrete element methods, and model tests. Finite element methods (including RS3 in this paper) show similarities to RMR, while discrete element methods and model tests show similarities to PFC3D results. Finite element methods based on continuum mechanics may underestimate deformation significantly due to their inability to simulate large displacements associated with discontinuity sliding. This indicates that different numerical methods can lead to variations in analysis results. (5) Relative to radial displacement and potential energy, strain invariants, strain deviator invariants, and strain energy are not suitable as indicators of tunnel stability. However, each of these invariants and strain energy has different physical meanings. The first strain invariant (I1) represents volume strain in the rock mass around the tunnel; the second strain invariant (I2) and the second strain deviator invariant (J2) are suitable for describing shear failure due to discontinuity sliding; strain energy (U0) represents the product of stress and strain and is useful for marking the location of failure.(6) The investigation of tunnel stability requires careful consideration of data collection range, and this study adopts radial displacement measurements taken one and a half times the tunnel radius outward. The suitability of this approach is examined at distances of 2m, 6m, and 18m from the excavation face. When collecting data 2m ahead of the excavation face, the region near the excavation face exhibits greater stability when γ=90˚, while the region with lower inclination angles (γ=180˚) appears less stable. As the data collection range increases, the results show instability for γ=90˚, while the radial displacement for γ=0˚ and 180˚ tends to converge. This convergence suggests that the stability of these two angles should be similar after the tunnel is completely penetrated. Therefore, the study recommends analyzing the data up to half of the model′s excavation as a more suitable approach to distinguish and examine the stability behavior for γ=0˚ and 180˚. | en_US |