Deep Rock Tunnels Research Articles

An Insight from Rock Bolts and Potential Factors Influencing Their Durability and the Long-Term Stability of Deep Rock Tunnels

Selecting and designing the most suitable support systems are crucial for securing underground openings, limiting their deformation and ensuring their long-term stability. Indeed, the rock excavations imposed by the erection of deep tunnels generate various harmful effects such as stress perturbation, damage, fractures, rockbursts, convergence deformation, and so on. To combat such effects by helping the surrounding rocks of these structures to hold up, rock bolts are typically utilized as pioneer support systems. However, the latter must be efficient and sustainable to properly fulfil their vital roles. A thorough understanding of the existing rock bolt types or models and the relevant factors influencing their failure is highly required for appropriate selection, design and applications. It is observed that, despite numerous studies carried out, there is a lack of comprehensive reviews concerning the advances in such rock support systems. This paper provides an insight into the most pertinent rock bolt types or models and describes the potential factors influencing their failure. Additionally, it discusses the durability of rock bolts, which has a huge impact on the long-term stability of deep rock tunnels. Furthermore, the paper highlights some proposals for future trends.

Creep Behavior of Rocks and Its Application to the Long-Term Stability of Deep Rock Tunnels

Since underground structures such as tunnels are inevitably surrounded by rocks, their long-term safety and stability are primarily governed by the comportment of these materials. Being able to express the time-dependent behavior of rocks, creep is one of the most interesting mechanical properties considered in the study of tunnels. Based on relevant research efforts, this article aims to provide a comprehensive review of pertinent information on rock creep and its potential influencing factors. It also presents the latest progress in constitutive models of rock creep and discusses their applicability to the long-term stability of deep underground structures. The results show that rock creep is significantly influenced by various potential factors both external and internal. These are mainly hydraulic pressure, stress level, water content, temperature, damage, and time-to-failure. For instance, the creep lifetime of andesite is drastically reduced by the presence of water. It is about 180 times shorter in wet conditions than in dry conditions, under the same stress conditions. By the combined influence of high stresses, high pressures, and high temperatures, creep rupture occurs in a semi-brittle manner for most types of hard rocks. The characteristics and installation period of the lining structures also have a strong influence on the evolution of creep in the rocks surrounding the underground structures. It is suggested that despite the colossal research efforts already made in this area, more accurate creep constitutive models are still needed for more adequate applications to the long-term stability of deep rock tunnels. Accordingly, key perspectives for future investigations are highlighted. This work can serve as a good reference in the establishment of new constitutive models of rock creep aimed at improving their accuracy, and facilitate appropriate actions to predict the long-term stability of deep tunnels in realistic situations.

Identifying Rock Loads on TBM Shields During Standstills (Non-Advance-Periods)

Tunnel boring machine (TBM) operational data is mostly analysed with respect to data that was recorded during the advance of the TBM. Focusing on data that was recorded during standstills of a gripper TBM, we analyse rock loads that were passively recorded in the cylinders of a small roof support shield. These roof support cylinders are situated beneath the TBM’s shield – extending it against the rock mass during non-advance periods. Equipped with pressure sensors, they enable the unique opportunity of logging rock load variations throughout the tunnel. Hence due to the big amount of resulting data, techniques of unsupervised machine learning (i.e. cluster analysis) are used to automatically pre-process the TBM operational data. Furthermore, regression analysis is used to determine sections of the tunnel where rock loads are mainly occurring on the left or right side respectively. The data driven analysis shows that the main rock loads are occurring on the right side of the TBM which is in good accordance with observation from the construction site, as well as numerical models from literature. This paper contributes towards the understanding of rock load conditions in anisotropic rock masses recorded during the drive of a deep hard rock tunnel.

Design of deep rock tunnels using hyperstatic reaction and convergence confinement methods

Tunnel lining is usually installed with delay behind the tunnel face excavation, which requires that a displacement is considered at the tunnel boundary before the support structure installation. This delayed installation comes with a release and redistribution of stress in the rock mass. Internal tunnel lining forces significantly rely on the stress relaxation process taking place in the rock mass surrounding the excavated tunnel. One of the difficulties when designing tunnel supports is, therefore, to analyse the rock–support interaction considering the tunnel lining convergence caused by the stress redistribution. Here, a simple and effective calculation process based on the combination of two methods, the hyperstatic reaction method (HRM) and convergence confinement method, is presented to analyse the interaction of rock mass and support structure. The rock mass is assumed to obey the Hoek–Brown criterion. The stress release is also taken into consideration. The present method is validated by comparing results of the HRM in terms of tunnel lining forces against the analytical ones. Thereafter, the effect of the stress release coefficient, the tunnel depth and the Hoek–Brown criterion parameters (the geological strength index and σci) on the lining internal forces are presented and discussed.

Probabilistic Stability Analysis of Deep Rock Tunnel Excavated by Mechanized Tunneling Considering Anisotropic Initial Stresses

Due to the complex geological conditions, the construction of underground tunnels in the rocks exhibiting creep behaviors is a great challenge and the structures’ long-term stability is of main importance. The excavation by tunnel boring machines (TBM) of a tunnel that is supported by a double flexible/concrete liner is considered a potential technical solution. However, the efficiency of such a support system on the stability at long-term of a tunnel must be verified in the real condition by considering both the effects of uncertainties of the time-dependent rock behavior and anisotropy of initial stress state. In this paper, the subset simulation and the Sobol global sensitivity analysis are chosen to estimate the failure probability of the tunnel supports and quantify the importance of different random parameters, respectively. The results reveal that considering the anisotropy of initial stresses increases the failure probability, especially in the concrete support elements. In addition, the parameters of initial stresses are the ones of main importance for both liners according to the Sobol indices. Therefore, the anisotropy of initial stresses and the related uncertainties should be considered for reliable tunnel designs.

Three-dimensional forward analysis and real-time design of deep tunneling based on digital in-situ testing

The mechanical behavior of a deep rock mass cannot be precisely evaluated with the traditional theory and model used for shallow tunneling. Moreover, the deep rock mass parameters are primarily determined from laboratory rock tests or field displacement-based back analysis. There is still a lack of a real-time, rapid, and dynamic forward analysis method for deep tunneling. In this study, three-dimensional (3D) numerical results based on the Mohr–Coulomb (MC), Drucker–Prager (DP), Hoek–Brown (HB), and generalized Zhang–Zhu (GZZ) strength criteria were compared and analyzed using physical tunnel models with different buried depths. Subsequently, practical models of shallow-, medium-, and deep-buried tunnels were established to investigate the stress and deformation evolution during tunneling, and a digital in-situ test-based forward analysis method was developed to dynamically update the rock mass parameters and perform subsequent 3D numerical simulations. (1) GZZ-based numerical results agreed well with the physical model tests; (2) large differences in the tunnel excavation responses (stress and deformation) between the 3D strength criterion (GZZ) and the 2D strength criteria (MC and HB) indicate the significant effect of the intermediate principal stress (σ2) on tunnel stability, and these differences are more significant in deep tunnels because of the widely distributed plastic zone; (3) steel arch distortion is closely related to the minimum principal stress (σ3) and σ2 of the surrounding rock near the tunnel face, and the effect of σ2 increases with the buried depth; however, steel arch distortion in the plane strain tunnel section can be equally attributed to both principal stresses (σ3 and σ2). This indicates that complex engineering problems cannot be accurately analyzed with the plane strain model, particularly in relation to deep tunnels; (4) overemphasis on the radial stiffness of the support system seems inappropriate, and more attention should be paid to the longitudinal and overall stiffness of the deep tunnel support. The digital in-situ test-based 3D forward method and design theory developed in this study may contribute further to the dynamic construction and design of deep and ultra-deep rock tunnels.

Numerical Simulation of Non-Stationary Parameter Creep Large Deformation Mechanism of Deep Soft Rock Tunnel

The accelerated creep plays an important role in the disasters of soft-rock tunnels under high stress. However, most of previous studies only involved attenuation creep and uniform creep. Large deformation disasters of soft rock occurred during the tunneling process in the Qianzhou–Sanyangchuan Tunnel, Gusu Province, China. In the paper, we developed the nonlinear generalized Nishihara rheological model with non-stationary parameter creep (NGNRM) to simulate the accelerated creep behaviors of soft rocks under high stress, and implemented it in ABAQUS, to reveal the mechanism of large deformation of soft rock. We proposed the multi-objective back analysis method of surrounding rock mechanical parameters based on the eXtreme Gradient Boosting and the non-dominated sorting genetic algorithm-II. In addition, the orthogonal test design method was used to determine the main parameters affecting the displacement of the tunnel. Using the proposed method, we can evaluate the large deformation mechanism of deep soft rock tunnels, and scientifically determine when to reinforce to prevent a large deformation disaster of the tunnel.

Roof stability in flat-ceiling deep rock cavities and tunnels

Three-dimensional stability of roofs in deep flat-ceiling cavities is analyzed. The stability number, factor of safety, and required supporting stress are used as measures of roof stability. Despite the simplicity of the flat roof geometry, the three-dimensional stability analysis presents some complexities owed to the shape of the failure surface geometry in the collapse mechanism. The failure mode assumes a rock block moving downward into the cavity, and the study aims to recognize the most critical shape of the failing block. Three specific block shapes are described in some detail, but more have been analyzed. Blocks defined by a special case of a 4th order conical surface (quartic) on a rectangular base, and a 2nd order elliptic surface (quadric) are found to be the most critical in the stability analysis. The kinematic approach of limit analysis was used, with the rock strength governed by the Hoek-Brown failure criterion. The parametric form of the Hoek-Brown function was employed. Interestingly, an absence of diagonal symmetries in the most critical failure mechanisms was observed in roof collapse of square-ceiling cavities. Computational results in terms of dimensionless measures of stability are presented in charts and tables.

Three-dimensional tunnel face extrusion and reinforcement effects of underground excavations in deep rock masses

The design experience and the shallow tunnel theory, which emphasize the plane strain assumption, two-dimensional (2D) strength theory, and indoor or back analysis-based parameters of rock mass, should not be directly adopted for deep tunnel design . This paper reports the development of a digital acquisition technique to quickly, automatically, and accurately obtain the in-situ strength parameters (e.g., GSI and D ), followed by the establishment of a three-dimensional (3D) numerical model for deep tunneling for a typical tunnel in western China to study the 3D and nonlinear spatial effects and mechanisms during excavation with incorporation of the 3D Hoek-Brown (HB) strength criterion. The reliability and correctness of the developed methods were verified by the field monitoring data , and the results of our study showed that: (1) The deformations based on 2D strength criteria (e.g., HB and MC) and 3D criterion (e.g. GZZ) are close for shallow tunnel, but significantly different for deep tunnel because of the widely distributed plastic zone in deep tunneling, thereby indicating the obvious effect of intermediate principal stress ( σ 2 ) on rock deformation ; (2) The face extrusion deformation exhibits obvious nonlinear trends with an increasing buried depth, and the σ 3 of the core rock reflects a tensile value in deep tunnel, thereby indicating the presence of an extremely unstable state. Reinforcement could increase the overall stress inside the core rock and reduce the stress unloading effect caused by the strong disturbances of deep excavations , thereby promoting a gradual change from tensile stress to compressive stress ; (3) A uniform relationship was found between pre-extrusion and pre-convergence deformations, indicating that the pre-convergence deformation of the surrounding rock could be controlled by constraining the pre-extrusion deformation of the core rock. This digital technique and such 3D HB-based (e.g., GZZ) numerical modeling may be promising for conducting the 3D forward and dynamic analyses of tunnel stability. Additionally, the core rock extrusion control can be treated as an active method to render the rock itself a natural support material and to realize its strength potential, thereby making a valuable contribution to the construction of deep and ultra-deep rock tunnels. • A digital acquisition method for in situ rock mass parameters was proposed and applied to deep tunneling analysis. • The reliability of 3D forward numerical analysis based on the 3D Hoek-Brown strength criterion was verified by the field monitoring data. • The 3D extrusion effects and mechanical mechanism of deep tunnel face extrusion were studied. • The reinforcement effects of the core rock mass under different GSIs were examined. • A uniform relationship was found between the pre-extrusion and pre-convergence of the core rock mass.

A simplified approach to reliability evaluation of deep rock tunnel deformation using First-Order Reliability Method and Monte Carlo simulations

abstract: The Monte Carlo simulation (MCS) and First-Order Reliability Method (FORM) provide a reliability analysis in axisymmetric deep tunnels driven in elastoplastic rocks. The Convergence-Confinement method (CV-CF) and Mohr-Coulomb (M-C) criterion are used to model the mechanical interaction between the shotcrete lining and ground through deterministic parameters and random variables. Numerical models synchronize tunnel analytical models and reliability methods, whereas the limit state functions control the failure probability in both ground plastic zone and shotcrete lining. The results showed that a low dispersion of random variables affects the plastic zone's reliability analysis in unsupported tunnels. Moreover, the support pressure generates a significant reduction in the plastic zone's failure, whereas the increase of shotcrete thickness results in great reduction of the lining collapse probability.

Real-time monitoring of the development of brittle fracture in hard rock tunnels based on physical model test

Brittle fracture in deep hard rock tunnels in the Jinping laboratory, China, can often endanger the stability of engineering structures. This study describes physical model testing and real-time observations on the evolution process of v-shaped brittle fracture, failure characteristics, onset conditions, and influencing factors in hard rock tunnels. Real-time observations are presented integrated a high-speed camera, acoustic emission system, a digital image correlation, and distributed fiber optic strain sensing technologies. The results show that the process of spalling failure can be divided into two main stages, namely, the instantaneous cracking process in the sidewall surface, and the spalling process of the thin rock chips developing progressively from the excavation boundary to the far field, finally forming stable v-shaped notches. The deviatoric stress of spalling failure in the physical model test satisfies the constant deviatoric stress criterion. The critical strain at spalling failure is consistent with that calculated by the Hoek–Brown strength criterion. Spalling is a form of brittle failure caused by deferential tensile strain. The working face effect is observed in the physical model excavation process. The highest strain caused by overloading occurs near the free surface of the sidewall. The relationship between stress and geometric similarity constants in deep rock engineering is derived. The failure characteristics revealed in the physical model tests are verified by numerical simulations based on cellular automaton theory, which agree well with the experimental results. The research findings can provide theoretical support for the prevention and control of spalling disasters in deep engineering practice.

Long-term degradation, damage and fracture in deep rock tunnels: A review on the effect of excavation methods

Rocks are frequently host materials for underground structures, particularly for deep Tunnels. Their behavior plays a fundamental role in the overall stability of these structures. In fact, the erection of deep tunnels imposes rocks excavations around the defined routes. These excavations are generally carried out by various methods of which the most used are Drill-and-Blast (DB) and Tunnel Boring Machine (TBM). However, regardless of the tunnelling method used, the impacts such as the perturbation of the initial stress field in rocks and the release of the stored energy are always significant. The impacts produce damage, fractures and deformations which are generally time-dependent and influence the long-term stability of deep tunnels built in rocks. Thus, by considering the aforementioned excavation methods, this paper identifies, reviews and describes the relevant factors generated during and after rock excavations. Interestingly, such factors directly or indirectly influence the long-term stability and therefore the structural integrity of deep rock tunnels. In addition, some recommendations and proposals for future works are presented. This paper can provide useful references in understanding the degradations, damage and fractures generated by tunnelling methods and facilitate suitable actions to ensure long-term stability of deep underground structures.

An extended numerical manifold method for simulation of grouting reinforcement in deep rock tunnels

In this study, a numerical manifold method (NMM) based simulator, which is able to simulate the whole process of grouting reinforcement, including the migration and coagulation process of injected slurries in rock fractures networks, is developed for evaluating the effectiveness of grouting reinforcement in deep rock tunnels. To implement the migration process, the grouting hole generation algorithm, the Bingham fluid model, the migration network searching algorithm as well as the framework for fluid–solid (FS) coupling process are introduced into the zero-thickness cohesive element (ZE) based NMM (Co-NMM). To reflect the reinforcement effect of the slurry coagulation after the migration process, a bonding reinforcement algorithm based on the ZE model is proposed. To improve the computational efficiency, an explicit integration scheme is introduced. To validate and illustrate the capability of the developed method, an uncoupled and a coupled numerical examples are conducted and the simulation results are in good accordance with the analytical solutions or laboratory observations. In addition, a series numerical direct tensile test and uniaxial compressive test are conducted to verify the capability of the proposed bonding reinforcement algorithm. Finally, to demonstrate the capability of the developed simulator in simulating problems with large scale, grouting reinforcement in a deep rock tunnel model is conducted and the predicted results show that the convergence deformation and fragmentation-bulking phenomenon induced by excavation in the deep rock tunnel are effectively controlled through the grouting reinforcement.

Influence of Unloading Conditions of Confining Pressure on the Compressive Strength and Permeability of Deep Mudstone

To investigate the compressive strength and permeability of deep mudstone under stress disturbance, a triaxial rheometer is used to conduct seepage experiments on mudstone specimens with different buried depths under triaxial compression and unloading conditions. The experimental results show that the compressive strength of mudstone specimen with a depth of 1000 m is much lower than that of specimen with a depth of 200 m, and the compressive strength of mudstone increases with the increase in confining pressure. Under constant axial pressure and unloading of the confining pressure, the mudstone with a depth of 200 m exhibits brittle failure, and the strain fluctuates in a pointwise manner with the increase in axial stress. In this case, the mudstone with a depth of 1000 m exhibits a transition from brittle failure to ductile failure, and the strain fluctuates linearly with the axial stress. Further, when the volumetric strain change reaches 0.01, it shows an oblique “Z” fluctuation. During the initial stage of unloading of confining pressure, the permeabilities of both the mudstone specimens (with depths of 200 and 1000 m) decrease gradually. As the confining pressure is unloaded, the permeability of mudstone with a depth of 1000 m increases. Until the specimen is completely destroyed, the permeability of mudstone increases rapidly. Overall, this study can serve as a useful reference for analyzing the engineering disasters associated with deep rock mass, tunnel ventilation, and gas storage.

Simulation of subway tunnel construction process based on fi-nite element analysis

Simulation of subway tunnel construction process based on fi-nite element analysis

Application of constitutive model in simulation analysis of tunnel construction

Application of constitutive model in simulation analysis of tunnel construction

A Geostatistical Solution to Estimating Groundwater Inflows in Deep Rock Tunnels with Validation Through Case Studies

Estimating groundwater inflows in rock deep tunnels is critical to the safety of personnel constructing the tunnel and the cost of construction. This paper presents two case studies for estimating steady-state groundwater inflows into deep rock tunnels. The case studies included the Chattahoochee and Nancy Creek tunnels. In this paper, statistical methods were applied to packer test data obtained from exploratory borings during geotechnical site investigations. A semi-empirical procedure was utilized for estimating groundwater inflows into these deep rock tunnels. It is the most widely used method/procedure in the tunnel industry. The Chattahoochee and Nancy Creek tunnels are deep rock tunnels which exhibit radial flow conditions. For the radial flow condition, a statistical analysis presented in this paper indicates that the semi-empirical procedure works well; however, variations in the procedure and/or an insufficient amount of testing could lead to underestimations of the inflow quantities. Packer test data plotted in histograms were observed in all cases to be log-normally distributed for the radial flow condition. Modeling using Monte Carlo simulations was observed to be an effective tool for removing irregularities in the distribution of the data and incorporating high-end permeability data to derive reasonable groundwater inflow estimates. This paper is based on the PhD thesis entitled “Improvements to the Current State of the Practice in Steady-State, Groundwater Inflow Estimates into Rock Tunnels based on the Application of Statistical Analysis Methods of Packer Test Data obtained from Exploratory Borings During Geotechnical Site Investigations.”

Identification of potential high-stress hazards in deep-buried hard rock tunnel based on microseismic information: a case study

Rockbursts, stress-induced collapse, and spalling often occur in the excavation process of deep-buried hard rock tunnel, which affect the stability thereof. The early identification of these potential high-stress hazards during tunnel excavation can provide reliable basis for the selection of preservative and controlled measures. During the excavation of a deep-buried hard rock tunnel in southwest China (a railway tunnel with a maximum burial depth over 2000 m), rockbursts, stress-induced collapse, spalling, and cracking occurred frequently. In this study, the microseismicity of various high-stress hazards in the genesis process of the tunnel was analysed. The difference of microseismicity characteristics of these high-stress hazards was revealed. A matter-element extension mathematical model was established to quantify the microseismicity of the rock mass during tunnel excavation. Based on microseismic (MS) information, an identification method of potential high-stress hazards in deep-buried hard rock tunnels was established. The method was then applied to a real case, and reliability analysis thereof performed. Thirty test case samples were selected for reliability analysis and case verification. Through practical application, it is found that 90% samples were identified correctly. The results show that the identification method of potential high-stress hazards in the deep hard rock tunnel constructed in this study is both applicable and reliable, and can identify potential high-stress hazard types in the process of tunnel excavation by using MS information.

Economic and Technical Viability of Using Shotcrete with Coarse Recycled Concrete Aggregates in Deep Tunnels

This work analyzes the technical and economic viability of using coarse recycled aggregates from crushed concrete in shotcrete, as a primary lining support in tunnels. Four incorporation ratios of coarse natural aggregate (CNA) with coarse recycled concrete aggregates from concrete (CRCA) were studied: 0%, 20%, 50% and 100%. The mechanical properties of the dry-mix shotcrete were obtained in an independent experimental campaign. Initially, the technical viability of CRCA shotcrete was validated for deep rock tunnels, based on the convergence-confinement method. Two cases were studied to determine the equivalent thickness for each combination of replacement ratio using CRCA shotcrete: (i) similar stiffness and (ii) similar yield stress. Subsequently, an economic assessment was performed. The stiffness criterion increased the thickness below 10% in both the 20% and 50% replacement ratios, which shows their technical viability with very marginal cost increase (<5%). On the other hand, the maximum pressure criteria required higher increments, close to 30% in the 50% replacement ratio. A full replacement was proven to be impracticable in both analyses.

Experimental Investigation of Unloading‐Induced Red Sandstone Failure: Insight into Spalling Mechanism and Strength‐Weakening Effect

To study the effect of excavation unloading on hard rock failure, a series of true‐triaxial compression tests, biaxial compression tests, and true‐triaxial unloading compression tests (two different unloading rates) at different confining pressures was conducted on red sandstone cube samples. The strength and failure characteristics and their relationship for red sandstone unloading at different unloading rates and confining pressures were analyzed. Based on the test results, the effects of the unloading rate and confining pressure on the strength and failure characteristics of hard rock were explored, and a reasonable explanation for unloading‐induced spalling in hard rock tunnels was presented. The results show the stress‐strain curve of highly stressed red sandstone exhibits a stress step during unloading, and the higher the unloading rate, the lower the stress level required for a stress step. The rock strength‐weakening effect induced by unloading was confirmed. The mechanical properties of red sandstone become more unstable and complicated after unloading. After the red sandstone is unloaded to a two‐dimensional stress state, with increasing confining pressure, the strength increases first and then decreases; the failure mode changes from a low‐confining pressure tensile‐shear failure to a high‐confining pressure tensile failure; and the geometries of the slabs change from large thick plates and wedges to medium‐ and small‐sized thin plates. At equal confining pressures, the higher the unloading rate, the lower the strength (i.e., the strength‐weakening effect is more pronounced), the thinner the slab, and the lower the confining pressure required for the failure mode to change from tensile‐shear failure to tensile failure. The unloading rate and confining pressure affect the strength and failure characteristics by affecting the crack initiation type and propagation direction in hard rock. For deep hard rock tunnels with high unloading rate and axial stress, neglecting the effects of unloading rate and axial stress will lead to a dangerous support design. For deep hard rock ore, if the maximal horizontal principal stress exceeds the critical confining pressure, the mining surface should be perpendicular to the direction of the minimal horizontal principal stress. The results of this study are of great engineering significance for guiding deep hard rock tunnel construction and mining.