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Many methods have been proposed to predict the field excavation performance of tunnel boring machine (TBM). They can usually predict the general trends of the field rock cutting force and efficiency when cutter penetration depth increases. However, in many cases, the predicted values are not so close (sometimes even very different) to the field values. This study is to check, compare and then correlate the prediction results of three widely used methods in predicting the field TBM excavation performance. These three methods are the laboratory LCM (linear cutting machine) method, the semi-theoretical CSM (Colorado School of Mines) method and the empirical NTNU (Norwegian University of Science and Technology) method. First, the basic procedures of these three methods are reviewed, and their prediction results are obtained concerning the TBM excavation performance in Chongqing Yangtze River Tunnel Project. Then, the quantitative comparison and correlation between the field measured result with the laboratory, semi-theoretical and empirical prediction results are conducted. After that, combined with the previous studies, some empirical formulas are proposed to predict the field TBM excavation performance at the optimum rock cutting condition more reliably and accurately. These empirical formulas can take the laboratory, semi-theoretical and empirical prediction results into comprehensive consideration. Finally, the south lot of the Qinling Tunnel in Yinhanjiwei Water Diversion Project is set as the case study, and the machine parameters and TBM excavation performance in this project are predicted and compared to the field values to verify the applicability and accuracy of the new proposed empirical formulas. Results show that the use of these new proposed empirical formulas can offer more acceptable prediction results for TBM cutterhead design and operation optimization. [ABSTRACT FROM AUTHOR]

• A novel thermo-mechanical coupling scheme is developed based on the combined finite-discrete element method (FDEM) to capture the mechanical response of the rock to the isotropic/anisotropic thermal conduction process. • The mechanisms of the creation and development of the excavation damaged zone (EDZ) around the wellbore subject to excavation unloading, drilling mud support, and thermal change in the surrounding rock are analyzed. • The influences of the thermal-mechanical properties of the rock mass, such as the isotropic/anisotropic rock strength and thermal conductivity, thermal expansivity, as well as drilling mud-rock thermal interaction process, on the EDZ development, are discussed. Insights into fracture development and excavation damaged zone (EDZ) formation around wellbore excavation sites are essential for understanding wellbore stability. In this study, a novel thermo-mechanical scheme is implemented in the 2D combined finite-discrete element method (FDEM) to investigate the fracturing process around a wellbore in a high-temperature subsurface environment. The developed scheme captures the isotropic/anisotropic thermal conduction characteristics within the rock formation. The coupled scheme and stress distributions around the wellbore subject to in-situ stress, drilling mud pressure, and temperature changes are validated by closed-form solutions. Fracture development and progressive EDZ formation by various mechanisms are analyzed. The results show that for an unsupported wellbore, fractures initiate in the region of the most severe stress concentration due to excavation unloading and constitute an EDZ that resembles the logarithmic-spiral rupture zone captured by experimental observations and Mohr-Coulomb models, resulting in potential water inrush due to the significantly increased fracture transmissivity of the EDZ and wellbore collapse. The shape and extent of the EDZ for wellbore excavation in the layered rock mass are dominated by the stress redistribution and the presence of low-strength bedding planes favorably oriented for bedding slippage. The excavation unloading-induced stresses can be effectively counteracted by properly applying drilling mud pressure, and the disturbance of the stress field occurs only within a small region, greatly reducing the extension of the EDZ. Thermal contraction of the surrounding rock due to its convective interaction with the cold drilling mud induces extra fractures around the wellbore. The extension of the EDZ is greater with increased thermal expansivity of the rock formation. Overall, the results indicate that the adopted thermo-mechanical FDEM simulation can provide unique geomechanical insight into wellbore stability behavior analysis, in which explicit consideration of the fracturing and fragmentation processes is of great significance. [ABSTRACT FROM AUTHOR]

In this study, determination of some machine parameters and performance prediction for tunnel boring machine (TBM) are conducted based on laboratory rock cutting test. Firstly, laboratory full-scale linear cutting test is carried out using 432-mm CCS (constant cross section) disc cutter in Chongqing Sandstone. Then, the input parameters for TBM cutterhead design are extracted; some TBM specifications are determined and then compared to the manufactured values. Finally, laboratory full-scale linear cutting test results are compared with the field TBM excavation performance data collected in Chongqing Yangtze River Tunnel. Results show that laboratory full-scale linear cutting test results, combined with some engineering considerations, can be used for the preliminary and rough design of TBM machine capacity. Meanwhile, combined with some modification factors, it can also well predict the field TBM excavation performance. [ABSTRACT FROM AUTHOR]

Basic Quality (BQ) method is a basic national standard of rock mass classification suitable for different industries in geomechanics and geotechnical engineering in China. Referring to the relevant provisions of BQ method, Hydropower Classification (HC) method, a specialized engineering geological classification system widely used in China, was compiled for evaluation on overall stability of surrounding rock and guide of excavation and support design of underground engineering in water conservancy and hydropower industry. As the input parameters of BQ or HC method are quite different with those used in RMR or Q system, which indirectly limits the applicability of the foreign developed TBM performance prediction models for the China’s TBM tunnelling projects. In order to develop an empirical model for hard rock TBM performance prediction with more suitable applicability in China, 49 valid datasets were collected from a water conveyance tunnel mostly excavated in medium to hard igneous rocks, and the empirical relationships between TBM performance and each parameter in the database were studied. The results showed that the prediction accuracies of TBM performance based on HC or BQ are very limited as the effects of the input parameters of HC method on field penetration index ( FPI ) are different and their weights assigned are improper. TBM penetration rate ( PR ) reaches its maximum value in the HC range 40–60 and BQ range 350–450, respectively. Boreability of the rock mass in class III is higher than that in class II. Ridge regression, principal component regression and partial least-squares regression methods were employed to solve the multicollinearity between uniaxial compressive strength of intact saturated rock, intactness index of rock mass, angle between discontinuity plane and tunnel axis, and average overburden of tunnel section in the database. Comparisons between the measured FPI and predicted FPI showed good agreement. This highlights the powerful potential of multiple regression analysis model based on HC method in TBM performance prediction. However, it deserves to emphasize that the developed empirical relationships should be considered valid only for new projects with geological conditions similar to the studied tunnel in this study, and more field data from different projects need to be collected to develop a universal model in the future. [ABSTRACT FROM AUTHOR]

• The long-term degradation behavior of rock strength was systematically summarized. • The long-term degeneration laws of cohesion and internal friction angle were proposed. • FDEM was used to study the long-term crack propagation of tunnel surrounding rock. • The conclusion of this paper was verified by engineering case. The time-dependence of rock strength is an issue that cannot be ignored in the rock engineering field. This paper summarizes previous study results on the degradation of the uniaxial compressive strength of rock (including component cohesion and internal friction angle), tensile strength and Young's modulus over time. Based on previous studies, the degeneracy formulas of intact rock cohesion and internal friction angle are derived. Taking circular tunnel excavation under hydrostatic pressure as an example, the surrounding rock response is studied under different degradation degrees of rock strength and stiffness, including the radius of the excavation damage zone, damage degree of surrounding rock, lining support pressure and tunnel boundary displacement. The results show that with the increasing degree of degradation of the surrounding rock compressive strength, the above four parameters increase in the form of an exponential function and that weakening of the tensile strength had little effect on the surrounding rock response. In addition, as the degradation degree of rock stiffness increases, the radius of the excavation damage zone remains unchanged, whereas the other three parameters values are reduced. An actual engineering case simulation using the combined finite-discrete element method shows that results consistent with field observations can be obtained if the rock strength degradation law proposed in this paper is adopted. [ABSTRACT FROM AUTHOR]

Core material softening method has been widely used in two-dimensional tunnel excavation simulation by the combined finite-discrete element method (FDEM). In numerical studies, softening stress paths (generally described by critical kinetic energy (CKE), softening time step (STS) and softening curve (SC)) usually have a significant influence on simulation results and computational efficiency. There is not a direct way to determine these three parameters. In the present study, a series of numerical studies by the FDEM are performed to discuss the effects of these three parameters on the simulation results and calculation efficiency in a circular tunnel excavation project with a 4 m diameter. The results show that the CKE is closely related to the model size and rock density. The CKE can be obtained from the model kinetic energy curve, and a selection method is proposed. The STS value of 50 000 steps is optimal. To obtain a uniform and stable propagation process of the surrounding rock crack, the SC should be in an exponential function. The engineering case shows that the simulation results are consistent well with the field observations as the softening stress paths are calibrated by the proposed method. [ABSTRACT FROM AUTHOR]

In this study, an enhanced numerical manifold method (NMM) simulator is developed to determine the combined impacts of reinforcement range and grouting timing on the effectiveness of grouting reinforcement for preventing the large deformation geo-hazard during the tunnel excavation through a fault rupture zone. To accomplish this objective, the limitations of the current two-dimensional simulation method, which lacks connection to the actual tunnel excavation process, are overcome by proposing a novel softening path function. Using this novel softening path function, each simulation step is given a specific physical meaning, establishing a connection to the actual tunnel excavation process. This improvement enables more accurate and analyzable simulation outcomes. To verify the accuracy of the proposed softening path function, a series of simulations of tunnel excavation are conducted using this softening path function. These results align closely with the results obtained by other reliable numerical methods and previous studies. After validation, several sets of numerical cases are conducted, considering 6 different geo-stress conditions, 10 different grouting reinforcement ranges, and 7 different grouting timings. Based on these simulation results, a critical value for reinforcement range, which equals to the diameter of tunnel face, is identified, and it is founded that delaying the grouting timing can increase the slurry migration range up to 94.6%. Furthermore, delaying the grouting timing can also reduce the convergence displacement of reinforced surrounding rocks after excavation up to 18.2% within a geo-stress range of 10 to 20 MPa. • A novel softening path function with clear physical meanings for tunnel excavation simulations is proposed. • A critical value of the grouting reinforcement range is identified. • Influences of grouting timing on the slurry migration range are revealed. • Influences of grouting timing on the convergence displacement of tunnels are revealed. [ABSTRACT FROM AUTHOR]