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The rheological deformation of soft rock resulting from tunnel excavation can lead to significant construction and safety challenges. In this study, a multiphase numerical model was developed to simulate the rheological deformation of soft rock surrounding a tunnel after excavation. The developed model considers the coupled meso/macroscale creep and damage processes of the rock using the coupled discrete element method–finite element method (DEM–FEM). In particular, the damage and deformation accumulation at the mesoscale (i.e., initial phase before excavation, loading phase due to the disturbance of the excavation and creep-induced damage phase leading to large deformation) were incorporated into the model. The model predictions were validated using field monitoring data. By incorporating the coupled meso/macroscale deformation process of the rock into the model, the predicted time-dependent displacements of the tunnel face agree reasonably well with the monitoring data. In addition, the results demonstrate that tunnel brittle damage accumulated in mineral clusters severely leads to instantaneous deformation, which becomes less important in the creep evolution stage. Furthermore, the results indicate that the final deformation is characterized by a high sensitivity to the value of mesoscale modeling parameters. Highlights: A multiphase numerical model was developed to simulate the rheological deformation of soft rock surrounding a tunnel after excavation. The developed model considers the coupled meso/macroscale creep and damage process of the rock using the discrete element–finite element coupling method (DEM–FEM). By incorporating the coupled meso/macroscale deformation process of the rock into the model, the predicted time-dependent displacements of the tunnel face agree reasonably well with the monitoring data. Bedding joints were considered in the model, and the influences of joints on the local movement of grains as well as the final converged deformation were assessed. The influences of mesoscale tensile strength, mesoscale cohesive strength and the fraction of grains experiencing creep on macroscale deformation were discussed. [ABSTRACT FROM AUTHOR]

The “yield-resist” combined support measure is a widely employed control measure in soft rock tunnels for controlling large deformation, particularly in high geostress conditions. For strain-softening rock masses, the strength parameters in the plastic zone are coupled with the support reaction. Due to the complexity of the interaction mechanism between yielding support and strain-softening surrounding rock, the majority of current solutions are based on the plane–strain model. However, the advancement of the tunnel is a three-dimensional problem. Therefore, the longitudinal effect is worthy of discussion when analyzing the mechanical behaviors of strain-softening rock and yielding support. A new two-stage method is proposed to describe the interaction between strain-softening rock mass and yielding support based on the generalized Zhang–Zhu (GZZ) strength criterion. Firstly, a simplified mechanical model of the yielding support structure is suggested to describe the mechanical response of the surrounding rock and support. Subsequently, a semi-analytical solution to three-dimensional ground–support interaction is proposed, taking into account the longitudinal effect. The results of the proposed solution are compared with those of a finite element simulation, and a high degree of agreement is observed. Finally, the mechanical behaviors of different yielding supports are discussed. The findings indicate that postponing the support timing by means of yielding technologies is essential, as otherwise the support would bear a very large load. The second stage of support reaction can be significantly reduced by implementing the “control-yield-resist” (CYR) support. The research offers novel insights and methodologies for investigating the three-dimensional interaction between the surrounding rock and various tunnel supports in high geostress conditions. [ABSTRACT FROM AUTHOR]

To date, numerically simulating the Fluid–Solid Interaction (FSI) and the entire process of host rock damage and water inrush involved in tunnel construction relies intensively on hybrid methods of two or more numerical codes, which commonly encounter deficiency in learning, modelling and computation. A modified framework of Smoothed Particle Hydrodynamics (SPH), the Coupled Discontinuous SPH (CDSPH) method, was developed to simulate cracking, contacting and large deformation of rock blocks, free surface flow and the FSI for water/mud inrush disasters in tunnel construction through water–rich stratums. The failed SPH particles were transformed to discontinuous particles and new contacts between these particles were formed to simulate the frictional slip and separation/compression between fracture walls. The improved SPH method was employed to a water inrush case of a tunnel and the simulation results indicate that the entire process of progressive rock damage, water inrush and flooding are precisely reproduced by the improved method. The greater kinematic viscosity of fluids and rock wall thickness can increase the flow velocity and hence increasing the impact pressure of floods, and the failure pattern of the rock wall changes from partial outburst to complete failure. Vulnerability analysis of the water inrush reveals a complete damage status for people, vehicles and Non-RC frame buildings, and a moderate to complete damage to RC frame buildings at the tunnel portal. This single code is capable of simulating the behaviour of rocks, fluids and their interactions in catastrophic disasters that is potentially applicable to a wide spectrum of surface and subsurface problems involving FSI. [ABSTRACT FROM AUTHOR]

Water inrush is one of the most serious threats to the construction safety of karst tunnels. Numerical simulation is one of the important means of underground engineering research, which is of great significance for revealing the evolution process of water inrush. In this study, an improved peridynamic model is established to simulate the formation process of water inrush channels due to rock mass progressive failure in karst tunnels. The results show that the surrounding rock between the karst cave and the tunnel excavation area is gradually damaged under the integrative action of excavation disturbance and karst cave water pressure during tunnel excavation. The initiation, propagation, and coalescence of cracks in the surrounding rock are spontaneous, and the water inrush channels are finally formed by the cracks. Based on the improved peridynamics, a total of 21 simulations were performed to study the influence law of different factors on the evolution process of water inrush channels. The influencing factors mainly include water pressure and radius of karst cave, distance between karst cave and tunnel, elastic modulus, and tensile strength of rock mass. Through the analysis of the morphological difference of water inrush channels and the damage degree of the surrounding rock under different calculation conditions, the formation mechanism and evolution process of water inrush channels are revealed. The research methods and results can provide some guidance for disaster prevention and mitigation of karst tunnel water inrush. [ABSTRACT FROM AUTHOR]

• Visco-elastic solutions for tunnel excavation considering supporting delay are presented. • Three-dimensional effect of tunnel faces is considered. • The interaction between liner and geomaterial is considered. • Different visco-elastic models are applied to simulate geomaterial. This paper proposes complex variable solutions for stress and displacement fields for tunnel excavation at great depth in a visco-elastic geomaterial, considering the equivalent three-dimensional effect, liner installation, supporting delay, and the interaction between liner and geomaterial. The geomaterial is simulated by three typical visco-elastic models: the three-parameter solid model, the Poyting–Thomson model and the Burgers model. The proposed solutions can simulate both tunnel excavation and liner installation stages, which are continuous in the time dimension. In the derivation, the variable substitution, the Laplace transform, and their inverse computations are applied. The proposed solutions are verified in detail by comparing to a numerical solution and a set of field data. Good agreements between the analytical solution and the numerical solution/field data are observed, indicating the validity of the proposed solutions. Subsequently, a parametric study is performed to investigate the influences of tunnel geometry (including tunnel size and liner thickness), material parameters of liner and geomaterial (including Poisson's ratio, shear modulus and viscosity of both elements), tunnel advance rate, and liner installation time moment (denoting supporting delay) on the stress and displacement fields in liner and geomaterial. The proposed solutions may serve as an alternative method for the conceptual and preliminary designs in tunnel engineering. [ABSTRACT FROM AUTHOR]

This paper presents a semi-analytical solution as well as finite element numerical simulations for the tunnel excavation problem, based on an extended Drucker–Prager model and under the drained condition. The adopted plasticity model is capable of accounting for the strain-hardening behavior of soils/rocks through both the internal friction angle and the cohesion. The analytical formulations show that this problem can be reduced to solving a system of ordinary differential equations, with the radial, tangential and vertical stresses, as well as the specific volume being the basic unknowns. Parametric studies are conducted to illustrate mainly the tunnel support pressure–displacement curve and the distributions of stress components, and, in particular, the critical support pressure required to stabilize the tunnel on the basis of three different collapse criteria. With regards to the numerical aspect, an integration algorithm has been developed by employing the return mapping scheme for the extended Drucker–Prager model and subsequently implemented into ABAQUS through the user subroutine UMAT. The agreement between the ABAQUS numerical results and the analytical solution predictions is overall excellent, which demonstrates the accuracy and reliability of the proposed integration scheme. [ABSTRACT FROM AUTHOR]

Highlights • Pre-embedding intelligent multi-point extensometers before excavation. • Estimating loss displacement with 3 LDP curve equations, Hoek equation is optimal. • Internal displacement passes through initial growth, rapid growth, stable convergence. • Temporal curve is similar to a creep test curve for rock subjected to graded loading. • Measuring points farther from the side wall are less affected by the spatial effect. Abstract It is very difficult to observe the deformation of the surrounding rock within a very short time after excavation in hard rock such as granite. This paper reports an in situ monitoring test of the internal displacement of the surrounding rock conducted using multi-point extensometers that were pre-embedded before excavation in the Beishan Exploration Tunnel (BET) in China's Gansu Province. The internal deformation characteristics of the surrounding rock throughout the whole tunnel excavation process were analyzed, and an analysis of the spatial effect of the tunnel excavation face and an estimation of the loss displacement were carried out with 3 typical longitudinal deformation profile (LDP) curve equations: the Lee equation, the Hoek equation and the Vlachopoulos and Diederichs (V-D) equation. The results show that under the geological conditions of Beishan granite, the internal displacement of the surrounding rock throughout the whole process of blasting excavation passes through 3 stages, including initial growth, rapid growth and stable convergence. The temporal curve of the internal displacement of the surrounding rock near a measuring section shows stepwise characteristics, which are similar to the morphological characteristics of a creep test curve for rock subjected to graded loading. Measuring points farther from the side wall of the tunnel are less affected by the spatial effect of the tunnel excavation face and experience a larger displacement release ahead of the tunnel face relative to the total displacement. The loss displacement calculated with the Hoek equation is closest to the measured value; thus, the Hoek equation is the optimal estimation equation. When the measuring section is located 2 m from the tunnel face, the loss displacement accounts for more than 50% of the total displacement. [ABSTRACT FROM AUTHOR]

Risk assessments during tunnel excavation are a constant concern for researchers and practitioners. During excavation in water-rich and karst regions, an assessment of the risk of water inrush mediated by coupled hydrological–mechanical effects is crucial. However, as the major channel for water transport, few published studies have analysed the effects of the fracture microstructure on the risk assessment under the combined effects of in situ stress, matrix deformation and water pressure. Based on fractal theory, this study proposes a highly coupled fractal analysis model for tunnel excavation that is capable of simultaneously and quantitatively investigating the interaction between the distribution and fracture evolution of the surrounding rock, the risk of water inrush, and the tunnel stress. We adopt the maximum fracture length (l max) and fracture fractal dimension (D f) to quantitatively characterize the fracture density and fracture size of the surrounding rock. A comparison with field data and analytical solutions is conducted to verify the correctness. The rock stress, water pressure, deformation and microstructural evolution of rock under different excavation processes of the tunnel have been comprehensively analysed. The contributions of the evolution of various microstructural parameters during tunnel excavation to the risk of water inrush are assessed. This provides a new micro-macro solution for the risk assessment of tunnel water inrush at the engineering scale and provides technical guidance for underground engineering practitioners [ABSTRACT FROM AUTHOR]

Large-scale columnar jointed basalt (CJB) developments have had serious adverse effects on the stability of the project in progress to create the diversion tunnels at the Baihetan hydropower station in China. An in situ microseismic (MS) monitoring experiment was carried out to help understand the fracturing process in the CJB rock mass as a result of tunnel excavation. MS monitoring performance (accuracy of location estimation and system sensitivity) is analyzed first. Then, the spatiotemporal evolution of the MS activity associated with the Baihetan CJB rock mass fractures along the tunnel axis and at the tunnel sidewalls during the entire excavation unloading process is obtained. Finally, the forms and opportunities to deploy appropriate support are suggested according to the characteristics of the CJB microseismicity. Meanwhile, the MS monitoring methods required for CJB or jointed hard rock engineering (e.g. sensor types, sensor spacing, sensor array forms, system protection, and selection of analysis regions) resulting from this exploration experiment are summarized. [ABSTRACT FROM AUTHOR]

[Display omitted] • A model is proposed to study interaction between supports and rheological rock. • Impact of shotcrete hardening feature on tunnel mechanical response is identified. • Effect of rock rheological behavior on shotcrete mechanical behavior is examined. • Supporting time of secondary lining in rheological rock is recommended. The shotcrete stiffness varies over time in the hardening period, which complexes the interaction between support structures and rheological rock. This study presents an analytical approach to examine the time-dependent interaction between support structures and rheological rock of a deep-buried tunnel, where shotcrete progressive hardening feature, tunnel excavation effect, and sequential installation of linings are considered. The time-dependent stiffness and fictitious support pressure are introduced to qualify hardening process of shotcrete and spatial effect of tunnel excavation. The Generalized Kelvin and Burgers models are adopted to represent two different rheology behaviors of surrounding rock. Results show that shotcrete hardening characteristics and excavation speed significantly influence tunnel convergence and support pressure. Ignoring shotcrete hardening characteristic leads to underestimating tunnel convergence and pressure of secondary lining, but overestimating pressure borne by shotcrete. Furthermore, mechanical response of two different types of primary support is investigated and the supporting time of the secondary lining is recommended. Investigations show that secondary lining should be applied suitably to bear subsequent rheological load when tunneling in surrounding rock with a notable rheological effect. Finally, the proposed analytical model is applied to tunnel engineering to prove its applicability. The main contribution of this work to literature is establishing an approach to identify hardening characteristics of shotcrete on interaction mechanism between support structures and rheological rock. [ABSTRACT FROM AUTHOR]

The islands and coasts in the South China Sea have developed coral reefs, which are the basis for the development of ocean resources and the construction of ocean engineering. However, the biogenetic coral reef limestone in the sea has the physical properties of much porosity, low strength and low density, which is obviously different from the traditional geological rock mass during the underground engineering construction. So in this paper, surrounding rock stability of coral reef limestone stratum in tunnel excavation is studied by physical model tests and numerical simulation. Firstly, according to the similarity theory, quartz sand, barite powder, high strength gypsum, calcareous sand and cement are selected to configure the similar materials of coral reef limestone by the orthogonal ratio test and physical mechanics test. Secondly, the triaxial geomechanical model tests of tunnel excavation in coral reef limestone stratum are carried out. The influence of tunnel shape, support method and rock strength on the tunnel stability is studied by analyzing the variation laws of displacement and stress of the surrounding rock. Finally, the numerical calculation of tunnel excavation in coral reef limestone stratum is conducted and the results of physical model tests are verified. The influence of tunnel depth on the surrounding rock stability is further studied. The results show that: (i) Compared with the horseshoe shaped tunnel, the displacement of the circular tunnel at the haunch and springer is significantly increased, and the stress concentration phenomenon occurs. (ii) The stress release rate and displacement of surrounding rock decrease significantly when the anchor support is added or the strength of reef limestone increases. (iii) The stress release rate and displacement of surrounding rock increase with the tunnel depth, so the support strength of surrounding rock should be increased. • The similar materials of coral reef limestone are configured by the orthogonal ratio test and physical mechanics test. • The triaxial geomechanical model tests of tunnel excavation in coral reef limestone stratum are carried out. • The numerical calculation of tunnel excavation in coral reef limestone stratum is conducted. • The physical model tests are verified by numerical calculation, and the influencing factors of tunnel stability are analyzed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Construction techniques of the tunnel in water-rich sand fine strata were provided. • Dewatering and reinforcing tunnel face will increase the pressure monitored of lining. • Deformation characteristics of support of three excavation methods were determined. The surrounding rock of the Taoshuping tunnel is fine silty sand in a plastic flow state after encountering water, which leads to inferior stability of the surrounding rock and extremely high construction risk. Therefore, the Taoshuping tunnel project is considered to be a rare worldwide problem in China. This article aims to review the engineering problems, construction techniques, and technical points faced in Taoshuping tunnel's construction, as well as reveal the evolution characteristics of surrounding rock pressure and evaluate structural safety. First, through the analysis of the characteristics and difficulties of the project, the combined dewatering scheme of vacuum wellpoint dewatering and collecting wells was formulated. Three kinds of tunnel excavation methods were developed: the double-heading at bottom method, both-side heading method with 9 steps, and ADECO-RS (Analysis of COntrolled DEformation in Rocks and Soils). Second, the variation law of the water content for the surrounding rock during excavation was studied through experiments. The water content that can ensure the stability of the surrounding rock is approximately 6.5%. Then, the evolution characteristics of the surrounding rock pressure of the first lining, the liner contact pressure, and the axial force of the reinforcement during construction were monitored and analyzed. The safety of the liner was evaluated by the pressure ratio of the liner and the strength-based safety factor. Finally, the deformation characteristics of the surrounding rock in the context of different excavation methods were investigated by numerical simulation. The lessons learned from the Taoshuping tunnel can provide insights for the increasing number of similar projects. [ABSTRACT FROM AUTHOR]

We perform a numerical study of the spatio-temporal evolution of rock damages and fracture displacements around underground excavation in fractured rocks. We model fracture networks based on a power law model and explore different values of the length exponent and fracture intensity. An elasto-brittle constitutive relation is employed to characterise the deformation/failure of intact rocks, while a non-linear stress–displacement relation is used to mimic the normal/shear displacement of pre-existing fractures. Fluid flow through fractured porous media is simulated based on Darcy's law. Both direct coupling (via poroelasticity) and indirect coupling (via changes of rock mass properties) are considered, with excavation-induced stress alteration, pore pressure variation, damage evolution, and fracture deformation realistically captured. It is observed that, fracture systems dominated by large fractures in general accommodate significant sliding along pre-existing fractures but minor damage in intact rocks, while fracture systems governed by small fractures can have profound damage and slippage simultaneously. Excavation-induced stress variation near the tunnel can instantly cause pore pressure fluctuations in low-permeability rock matrix, promoting fracture shear and rock damage. In addition, we investigate the microseismicity generated during and after the excavation as well as analyse the mechanisms of excavation and drainage-induced disturbances. Our results have important implications for underground excavation activities involved in many geoengineering applications. • Excavation-induced coupled hydro-mechanical processes in fractured rock masses are studied. • Poroelasticity and drainage effects lead to spatio-temporal variation of pore water pressure. • Fracture network distribution plays a crucial role in damage evolution, pressure perturbation and microseismicity occurrence. [ABSTRACT FROM AUTHOR]

• The similarity model test was conducted to investigate the failure mechanism of rock mass. • The model test was verified by numerical simulation. • Different cases were designed to analyze the effect of parameters on failure mechanism in numerical simulation. • The designed model test can be a useful method to study the failure mechanism of tunnel beneath a mined-out area. Tunnel construction beneath a mined-out area may inevitably break the existing balance of the surrounding rock mass, which would result in the collapse of the inner cavity of the mined-out area and even the deformation of the tunnel structure. Based on the engineering background of the Yudushan tunnel of the major expressway guarantee project for the 2022 Beijing Olympic Winter Games, both the similarity model test and numerical simulations were conducted to investigate the failure mechanism of the rock mass around the mined-out area above the highway tunnel. In the model test, we studied the progressive failure process of the surrounding rock during tunnel excavation and loading. The information obtained by using a variety of monitoring methods indicated that the displacement changes at different positions in the mined-out area were generally concave, and the stress variations detected by the stress sensors were different for each position in the tunnel. At the same time, by comparing the results with those of the numerical simulation, the validity and accuracy of the results in the model test were verified. Furthermore, the effects of the relative parameters of the mined-out area on the failure mechanism were discussed in terms of the length, location and distance to the tunnel in the numerical simulations. The test and simulations revealed the failure law of the surrounding rock and tunnel deformation characteristics during the excavation beneath a mined-out area, which had beneficial significance for guiding similar engineering construction. [ABSTRACT FROM AUTHOR]

We present a numerical study of spatio-temporal damage evolution and fracture displacements around underground excavation in fractured rock masses. We conduct a comparative analysis of this problem based on a mechanical (M) model by assuming an invariant pore pressure and a hydro-mechanical (HM) model by solving the coupling between the fluid and solid fields. In both models, an elasto-brittle constitutive law is employed to mimic the deformational and failure behavior of intact rocks, while a non-linear stress-displacement relationship is used to account for the normal compression and shear dislocation of natural fractures. In the HM model, we simulate excavation-induced transient groundwater flow in fractured porous rocks based on Darcy's law. The hydraulic and mechanical fields are linked based on the coupling mechanisms of poroelasticity (direct HM coupling) and stress-dependent material properties (indirect HM coupling). Many important HM phenomena such as stress perturbation, pore pressure fluctuation, damage evolution and fracture deformation are realistically captured in our simulation. We find that both direct and indirect couplings play important roles, and the excavation-induced disturbance in rock varies with the coupling degree characterized by the Biot coefficient. One interesting observation is that excavation-induced stress redistribution around the tunnel instantly causes perturbed pore pressure in low-permeability matrix, which gradually becomes dissipated during the post-excavation drainage. We also highlight the role of natural fractures in the tunnel inflow process and elucidate the consequences of spatial and temporal pressure variations. In addition, we analyze the microseismicity occurrence during and after the excavation to gain insights into excavation and drainage-induced responses in the fractured rock. The results of our simulation and analysis have important implications for underground excavation involved in many geoengineering applications such as civil infrastructure, nuclear waste repository and oil/gas/geothermal wellbore systems. • Excavation-induced hydro-mechanical processes in fractured rocks are studied. • Poroelasticity and drainage effects lead to spatio-temporal pore pressure variation. • Hydro-mechanical coupling results in enhanced rock damage and fracture displacement. [ABSTRACT FROM AUTHOR]

The deformation capacity of tunnel will be overestimated by small strain theory especially in high geostress and heavily squeezing rocks, the calculated wall convergence of tunnel may even exceed the excavated radius. Based on the G eneralized Z hang- Z hu (GZZ) strength criterion, a new large strain solution for tunnel excavated in strain-softening rocks under high geostress is proposed by a semi-analytical approach. During calculation, the material nodes in the plastic zone of surrounding rock are generated automatically in each unloading step. The locations of these nodes are updated after each step and the governing equations are established on the updated state. The proposed procedure allows one to predict the rock stresses, strains, and deformations in a rational and rigorous manner, and it is validated by numerical simulation. Results show that the ground displacement calculated by proposed approach is in good agreement with that obtained by GeoFBA3D, which indicates that the proposed approach is capable in predicting the large deformation of surrounding rock. As long as the unloading radial pressure of each step δp is sufficient small, i.e., the number of material nodes in the plastic zone exceeds 40, the computational error can be neglected. Compared to HB strength criterion, the GZZ criterion can take into account the intermediate principal stress, which is beneficial to tunnel stability. The applicable conditions for large strain theory and small strain theory are given by parametric studies. Parametric studies show that the calculation error produced by small strain theory can reach 30% when the deformation of surrounding rock exceeds about 10% R 0 , therefore, a large strain theory is necessary for calculation. • A new semi-analytical large strain solution for tunnel excavated in strain-softening rocks based on GZZ strength criterion. • Nodes are generated and updated automatically during calculation. • When the deformation of surrounding rock exceeds about 10% R 0 , a large strain theory is necessary for calculation. [ABSTRACT FROM AUTHOR]

The stress path during underground excavation is critical to the mechanical response of rock masses, and the excavation effects in front of a tunnel face cannot be ignored. In this study, a realistic stress path model was built to capture the actual behaviour of the surrounding rock during the entire excavation process. The key technical steps of the two-dimensional excavation simulations with this method were presented. This approach was applied to characterise the deformation and damage evolution of an experimental tunnel, where the soft rock under high stress experienced severe damage. The research results revealed the evolution of the excavation-induced deformation and damage, which was in good agreement with the results of field investigations and previous research. The cause of support failure was found to be that the growth process of the excavation damage zone (EDZ) was different from the convergence displacement, and the evolution of the EDZ did not receive sufficient attention during the design of the support. Corresponding suggestions for the improvement of support designs were put forward. The proposed method could provide key information for predicting the long-term performance and determining a reasonable design for the support of tunnels exposed to a high risk of destruction. [ABSTRACT FROM AUTHOR]