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AbstractThe quantitative support design plays a vital role in tunnel design, while the conventional quantitative framework of tunnel support design through convergence-confinement method (CCM) generally focuses on the safety of the support structure element. Herein, a quantitative tunnel support design method considering the safety factor of the surrounding rock is proposed. First, the solution for the safety factor of surrounding rock element is derived under the CCM framework considering influences of the intermediate principal stress (IPS) and the dilatancy on rock-mass yielding. Second, the allowable safety factor of surrounding rock element is obtained based on the critical support line in the widely used rock quality (Q) system. Then, the framework and procedure of the quantitative tunnel support design method are outlined. Verification of the proposed method is conducted through comparisons with the analytical solution, laboratory model experiment and field monitoring data. Furthermore, a case study is employed to illustrate the proposed method, results indicate that the failure of the whole underground system is caused by the insufficient safety reserve of surrounding rock element. In general, the proposed method provides theoretical guidance for quantitative stability analysis and design optimization of tunnels. [ABSTRACT FROM AUTHOR]

There is a development trend from qualitative towards quantitative analysis for tunnel stability. The widely accepted convergence-confinement method (CCM) provides a quantitative framework for tunnel stability analysis, while the conventional CCM based on closed-form functions is essentially an ideal model without considering the tunnel section shape and rock weight in plastic zone. Herein, based on the CCM framework, a quantitative analysis procedure formulated through numerical simulations for more practical situations was systematically proposed to calculate the safety factor of spray-anchor support system in non-circular tunnel. Besides, similarities and differences between the most widely used Hoek-Brown (HB) and equivalent Mohr-Coulomb (EMC) criteria were considered for practical reference. The proposed method was utilized to conduct quantitative analysis of LDPs, mechanical responses of surrounding rock and safety factors for four typical tunnels, and the consistency between tunnels based on HB criterion (TBHBC) and EMC criterion (TBEMCC) was analyzed. The results indicated that the maximum radial convergence of LDP, plastic zone area, and safety factor of TBEMCC were 34.54%, 37.29%, and 47.71% lower than those of TBHBC respectively. The reasons for differences between results were revealed. The spatial effect induced by the face excavation of TBEMCC was weakened as compared to TBHBC, leading to a drop of LDP. Consequently, the safety factor was underestimated. The method systematically provided an effective tool for quantitative stability analysis and design optimization of support system for an underground excavation.

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Abstract The convergence-confinement method used for tunnel analysis is based on the simplifying assumption of axisymmetric conditions. This paper presents a method for tunnel support design which is not limited to the hydrostatic loading simplification. With knowledge of the unsupported tunnel displacements, a circular beam equivalent to the plane strain tunnel boundary is found. Then, the distribution of field stresses between the ground and support system can be readily computed. The proposed method allows utilizing existing solutions for plastic deformations around excavations under non-hydrostatic loading, such as the solution derived by Detournay (1986), in order to compute the non-uniform support response. Significantly less computational resources are required compared to the finite-element method; hence, this method is well suited at the preliminary stage of a project to conduct extensive sensitivity analyses and identify critical support scenarios. [ABSTRACT FROM AUTHOR]

There is a development trend from qualitative towards quantitative analysis for tunnel stability. The widely accepted convergence-confinement method (CCM) provides a quantitative framework for tunnel stability analysis, while the conventional CCM based on closed-form functions is essentially an ideal model without considering the tunnel section shape and rock weight in plastic zone. Herein, based on the CCM framework, a quantitative analysis procedure formulated through numerical simulations for more practical situations was systematically proposed to calculate the safety factor of spray-anchor support system in non-circular tunnel. Besides, similarities and differences between the most widely used Hoek-Brown (HB) and equivalent Mohr-Coulomb (EMC) criteria were considered for practical reference. The proposed method was utilized to conduct quantitative analysis of LDPs, mechanical responses of surrounding rock and safety factors for four typical tunnels, and the consistency between tunnels based on HB criterion (TBHBC) and EMC criterion (TBEMCC) was analyzed. The results indicated that the maximum radial convergence of LDP, plastic zone area, and safety factor of TBEMCC were 34.54%, 37.29%, and 47.71% lower than those of TBHBC respectively. The reasons for differences between results were revealed. The spatial effect induced by the face excavation of TBEMCC was weakened as compared to TBHBC, leading to a drop of LDP. Consequently, the safety factor was underestimated. The method systematically provided an effective tool for quantitative stability analysis and design optimization of support system for an underground excavation. [ABSTRACT FROM AUTHOR]

The excavation of twin tunnels is a process that destabilizes the ground. The stability of the tunnel lining, the control of ground displacements around the tunnel resulting from each excavation and the interaction between them must be controlled. This paper provides a new approach for replacing the costly 3D analyses with the equivalent 2D analyses that closely reflects the in-situ measurements when excavating twin tunnels. The modeling was performed in two dimensions using the FLAC2D finite difference code. The three-dimensional effect of excavation is taken into account through the deconfinement rate X of the soil surrounding the excavation by applying the convergence-confinement method. A comparison between settlements derived by the proposed 2D analysis and the settlements measured in a real project in Algeria shows an acceptable agreement. Also, this paper reports the investigation into the changes in deformations on tunnel linings and surface settlements which may be expected if the twin tunnels of T4 El-Harouche Skikda were constructed with a tunneling machine. Special attention was paid to the influence of the excavation phase shift distance between the two mechanized tunnel faces. It is revealed that the ground movements and the lining deformations during tunnel excavation depend on the distance between the tunnels' axis and the excavation phase shift. [ABSTRACT FROM AUTHOR]

The convergence-confinement method (CCM) is a method that has been introduced in tunnel construction that considers the ground response to the advancing tunnel face and the interaction with installed support. One limitation of the CCM is due to the numerically or empirically driven nature of the longitudinal displacement profile and the incomplete consideration of the longitudinal arching effect that occurs during tunnelling operations as part of the face effect. In this paper, the authors address the issue associated with when the CCM is used within squeezing ground conditions at depth. Based on numerical analysis, the authors have proposed a methodology and solution to improving the CCM in order to allow for more accurate results for squeezing ground conditions for three different excavation cases involving various excavation-support increments and distances from the face to the supported front. The tunnelling methods of consideration include: tunnel boring machine, mechanical (conventional), and drill and blast. [ABSTRACT FROM AUTHOR]

Construction of support characteristic curves is an important aspect of the implementation of the convergence-confinement method of tunnel support design. This paper presents a detailed analysis of the equations required to construct the support characteristic curve of a combined support system consisting of closed circular steel sets linked to the periphery of the tunnel by equally spaced prismatic wood blocks. This particular problem has been treated in the classical literature of underground excavation design in rock, although without providing details of the origin or validity of the equations. This paper fills this gap, providing the derivation and validation of all the equations required to construct the support characteristic curve for the combined support system. The paper shows that both the stiffness and maximum load that the combined support system can take increases as the angle between equally spaced wood blocks is decreased (or the number of blocks for the steel section is increased). In the limit, when the angle between wood blocks is assumed to be zero, the paper shows that the classical equations for construction of support characteristic curve for a steel set subjected to uniform radial load are recovered. The paper presents also a practical example illustrating the application of all equations needed to construct the support characteristic curve for the combined support system. [ABSTRACT FROM AUTHOR]

This paper deals with the use of field monitoring data to improve predictions of tunnel response during and after construction from numerical models. Computational models are powerful tools for the performance-based engineering analysis and design of geotechnical structures; however, the main challenge to their use is the paucity of information to establish input data needed to yield reliable predictions that can be used in the design of geotechnical structures. Field monitoring can offer not only the means to verify modeling results but also faster and more reliable ways to determine model parameters and for improving the reliability of model predictions. Back-analysis involves the determination of parameters required in computational models using field-monitored data, and is particularly suited to underground constructions, where more information about ground conditions and response becomes available as the construction progresses. A crucial component of back-analysis is an algorithm to find a set of input parameters that will minimize the difference between predicted and measured performance (e.g., in terms of deformations, stresses, or tunnel support loads). Methods of back-analysis can be broadly classified as direct and gradient-based optimization techniques. An alternative methodology to carry out the nonlinear optimization involved in back-analyses is the use of heuristic techniques. Heuristic methods refer to experience-based techniques for problem-solving, learning, and discovery that find a solution which is not guaranteed to be fully optimal, but good enough for a given set of goals. This paper focuses on the use of the heuristic simulated annealing (SA) method in the back-analysis of tunnel responses from field-monitored data. SA emulates the metallurgical processing of metals such as steel by annealing, which involves a gradual and sufficiently slow cooling of a metal from the heated phase which leads to a final material with a minimum imperfections and internal dislocations. During cooling, nature follows its own optimization path for the given conditions. Description of SA, its implementation in the computer code FLAC (Itasca Consulting Group in FLAC 5.0 manual. Itasca Consulting Group, Minneapolis, 2008), and use in the back-analysis of the response of a twin tunnel in China are presented. [ABSTRACT FROM AUTHOR]

Abstract: Assuming that grouted rockbolts increase internal pressure within a broken rock mass, a new procedure for computation of ground response curves for a tunnel reinforced with active grouted rockbolts is presented, while the effect of distance of bolted section to tunnel face has been also considered. This analytical solution for a circular underground excavation under hydrostatic stress field, and with consideration of a non-linear strength criterion for rock mass and on the basis of two material behavior models has been developed. In this work, the equation of the ground response curve for a tunnel which has been reinforced with passive grouted rockbolts is also derived. The proposed model allows one to take, the effect of the distance of the bolted section to the tunnel face, the effect of increasing rockbolts spacing, the influence of increasing pretension load in calculating of the ground response curve, and the effect of increasing the cross-section area of rockbolts, into account. The results show that decreasing rockbolts spacing increase the support system stiffness rather than preloading of them. [Copyright &y& Elsevier]

Abstract: This paper includes a presentation of a simplified three-dimensional numerical model for the prediction of soil movement induced during tunnel construction using tunnel boring machines (TBM). The model is based upon the generalization of the convergence-confinement concept to 3D tunnel construction. It uses two parameters (L dec and α dec) which stand for the length of the unlined zone and the partial stress release, respectively. The value of the parameter L dec can be taken equal to the tunnel diameter, while the value of α dec can be determined by fitting the model to empirical formula, and then adjusted based on settlement registered during tunnel construction. The capacity of the model is illustrated through an application to a shallow tunnel in soft soil. The comparison of the numerical results to those suggested by different authors shows good agreement. [Copyright &y& Elsevier]

Abstract: The paper presents a numerical back-analysis of the response of Shimizu Tunnel No. 3 during construction. The 1.1km long Shimizu Tunnel No. 3, which was completed in Japan in 1998, is a research tunnel along the proposed Tomei II highway. It was excavated in a region consisting mainly of soft to hard sedimentary rocks, such as locally weathered sandstone, underlain by interbedded sandstone and mudstone, using a sequential tunneling technique known in Japan as the “TBM pilot and enlargement method” along with NATM techniques. The tunnel response was analyzed using “ground characteristic curves” and the “convergence-confinement method” which were adapted to the distinct element method (DEM) and the computer code UDEC. Parametric analyses were performed to study the behavior of the wide tunnel geometry under various conditions. The results of the simulations using UDEC and the convergence-confinement method yielded results which are comparable to the monitored data on tunnel deformation and tunnel support loads. [Copyright &y& Elsevier]