Excavation Of Circular Tunnel Research Articles

Macro-micro analysis of ground response to circular tunnel excavation in granular media

Ground reaction in terms of stress and displacement analysis is one of the major challenges in tunnel design and is helpful for understanding the stability around the excavation zone. In this paper, the analysis of stress, strain responses and radial displacement around circular tunnel excavated in cohesive-frictional granular materials is discussed via an innovative multi-scale approach (FEM × DEM) that combined finite element (FEM) and discrete element (DEM) methods and compared to other solutions obtained with conventional constitutive laws. Thanks to simulations, (i) the Ground response curve is then established and provides a better understanding of the complex behaviour in the vicinity of the tunnel; (ii) the prediction by FEM × DEM approach differs considerably from conventional constitutive laws; and (iii) in a unique manner, macroscopic and microscopic analyses are simultaneously performed showing that at the macroscopic level (FE mesh), diffuse failure mode are observed, while at the microscopic level (DEM level), microscopic evolutions are observed to be strictly related to the macroscopic behaviour.

Elastic–plastic response of a deep tunnel excavated in 3D Hoek–Brown rock mass considering different approaches for obtaining the out-of-plane stress

The difficulty in the 3D analysis of a circular tunnel excavation lies in the selection of a reasonable softening equation for the out-of-plane stress (σz) in the plastic zone. This study developed and compared the semi-analytical solutions of a circular opening considering different approaches for obtaining σz, and the results were verified by the numerical results (1) The elastic approaches (e.g., generalized Hooke's law). cannot accurately reflect the mechanical properties of rock in the plastic zone and the results exhibit large differences compared with numerical results. (2) Plastic approaches demonstrate that the in-site axis stress (q) has a limited effect on the stability of surrounding rock, whereas the elastic approach-based results exhibit severe dependence on q. (3) The excavation responses based on the GZZ strength criterion and Hoek–Brown strength criteria have significant differences, reflecting the obvious effect of σz (σ2) on the stability of rock mass, and an unreasonable criterion may result in inappropriate support measures (4) Different post-peak behaviors of rock mass (e.g., constitutive model, residual strength, dilatancy, etc.). significantly affect the surrounding rock stresses, and the applicability of elastic theories and shallow tunnel experiences need to be further considered in deep tunnel where the plastic zone is widely distributed.

Study of presplit blasting under high in-situ stress

High in-situ stress generally results in difficulties in achieving crack interconnection in presplit blasting, which can further arouse challenges related to construction cost and schedule of rock engineering. In this study, the crack interpenetration in presplitting under high in-situ stresses is theoretically analysed according to the propagation and superposition of stress waves, and then the relationships between in-situ stress, hole spacing and charge diameter in presplitting are analytically established. Following, based on the theoretical results, the rock cracking and crack coalescence induced by presplitting under different in-situ stresses are numerically investigated with a two-hole planar model in LS-DYNA. The simulation results demonstrated the effectiveness of presplitting. Subsequently, to provide guidance for practical application, presplitting with multi-holes in circular tunnel excavation under high hydrostatic pressure and high anisotropic in-situ stress is numerically performed. The performances of presplitting under high in-situ stress using different methods such as changing hole spacing and adjusting charge diameter are analysed and compared. Based on the theoretical analysis and numerical findings, we suggest that drilling boreholes along the direction of the major principal stress, using less charge, and employing small and uniform hole spacing can be effective in presplitting under high in-situ stress. This study provides insights into the mechanisms of crack interpretation in presplitting and offers guidance for overcoming difficulties in controlled blasting under high in-situ stress.

Investigation of Volumetric Block Proportion (VBP) Effect on Excavation-Induced Ground Response of Talus-like Rock Mass Based on DEM Simulations.

Due to the complexity of the talus-like rock mass with different values of volumetric block proportion (VPB), it is thus crucial to explore the VBP effect on the excavation-induced ground responses. We conduct a series of 2D DEM (discrete element method) simulations on a common circular tunnel excavation in the talus-like rock mass with different VBPs (0%, 15%, 50%, 85% and 100%). For each VBP, two support scenarios, i.e., unsupported and supported by a rigid lining, are considered. The micro characteristics of the excavation-induced ground responses, including the contact force, force chain, coordination number and shear-slip contact, and the stress distribution and ground settlement are elaborated in detail. Accordingly, three types of talus-like rock masses are identified as soil-, hybrid- and rock-types, corresponding to VBP = 0-15%, 50%, and 85-100%, respectively. It is found that the lining support is essential for maintaining the ground stability of a tunnel excavation in the soil- and hybrid-type talus-like rock masses while the backbones formed by rock blocks in the rock-type talus-like rock mass can provide a certain support for the surrounding ground. Our findings have important implications for optimizing the construction scheme of tunnel excavation in different types of talus-like rock masses.

Improved analytical solution for ground movements induced by circular tunnel excavation based on ground loss correction

This paper presents an improved analytical solution for the prediction of ground movements based on the ground loss correction. The boundary function of ground displacement composed of radial contraction, vertical translation of tunnel and tunnel ovalization is derived. The displacement analytical solution is further optimized by the equivalent ground loss rate components corrected from both the vertical translation of the tunnel and the geological conditions. Field measurements from eight tunnel cases are used to validate the capability and applicability of the proposed solution. Finally, the influence of the three deformation components mentioned above on ground deformation is explored. The results demonstrate that in practice, while the focus is on the quality and timing of grouting to control ground loss, the vertical translation of the tunnel should also be strictly controlled. The research results in this paper can provide a new simple and reliable calculation method for predicting the ground movements induced by circular tunnel excavation.

Role of Particle Shape on Ground Responses to a Circular Tunnel Excavation in Sandy Soil: Consequences from DEM Simulations.

Due to the sensitivity of sandy soil’s mechanical behavior to the particle shape, it is thus of importance for interpreting the effect of particle shape on the ground response induced by tunnel excavation in sandy formations. We conducted a series of 2D DEM (discrete element method) simulations on a common circular tunnel excavation in sandy soil with variable-shaped particles, which are characterized as two descriptors, i.e., aspect ratio (AR) and convexity (C). The macroscopic responses and the microscopic characteristics of the sandy ground are elaborated in detail. The simulation results show obvious asymmetrical features of the excavated ground, which results from the ground heterogeneity caused by the irregular particle shape. In addition, we investigate the roles of AR and C on the ground response and find that reducing AR or increasing C will enlarge the ground settlement, i.e., the sandy ground deformation is more sensitive to the particles with more irregular shapes. However, elongated particles are beneficial for the generation of soil arching with stronger bearing capacity and thus reduce the soil pressure on the tunnel lining. Our findings have important implications for the safety assessment of the tunnel excavation, as well as other underground structure construction in sandy formations.

Energy Generation and Attenuation of Blast-Induced Seismic Waves under In Situ Stress Conditions

During blasting in deep mining and excavation, the rock masses usually suffer from high in situ stress. The initial seismic energy generated in deep rock blasting and its attenuation with distance is first theoretically analyzed in this study. Numerical modeling of the multiple-hole blasting in a circular tunnel excavation under varied in situ stress conditions is then conducted to investigate the influences of in situ stress levels and anisotropy on the blasting seismic energy generation and attenuation. The case study of the deep rock blasting in the China Jinping Underground Laboratory (CJPL) is finally presented to demonstrate the seismic energy attenuation laws under varied in situ stress levels. The results show that with the increase in the in situ stress level, the explosive energy consumed in the rock fracture is reduced, and more explosive energy is converted into seismic energy. The increasing in situ stress causes the seismic Q of the rock mass medium to first increase and then decrease, and consequently, the seismic energy attenuation rate first decreases and then increases. Compared to the condition without in situ stress, the blasting seismic energy decays more slowly with distance under in situ stress. Then the seismic waves generated in deep rock blasting are more likely to reach and exceed the peak particle velocity (PPV) limits stipulated in the blasting vibration standards. Under non-hydrostatic in situ stress, the generation and attenuation of the blasting seismic energy are anisotropic. The highest seismic energy density is generated in the rock mass in the minimum principal stress orientation. Its attenuation is dependent upon the in situ stress aligning the wave propagation orientation.

Elastoplastic semi-analytical investigation on a deep circular tunnel incorporating generalized Zhang–Zhu rock mass strength

The semi-analytical method based on finite difference method is widely used in the study of the mechanical response of surrounding rocks in a circular tunnel. The core of the three-dimensional (3D) mechanical analysis of the plastic zone is to solve nonlinear equations with a complex yield criterion, which leads to a long iterative solution time and low accuracy. In this study, a new 3D semi-analytical method is proposed for a circular tunnel excavation. The method is based on the finite difference method, wherein a linear equation is used to replace the complex yield surface equation. This significantly reduces the iterative solution process and effectively improves the calculation rate. The accuracy of the proposed method is verified using conventional and 3D numerical methods. The two-dimensional (2D) generalized Hoek–Brown criterion (GHB), 3D generalized Pan–Hudson criterion (GPH), and 3D generalized Zhang–Zhu criterion (GZZ) are used to analyze the applicability of the proposed method. The mechanical properties of a circular tunnel excavation are studied using these criteria. Our results show that the calculation rate of the plastic zone response of the proposed method (based on the GZZ criterion) is 12,000 times higher than that of the conventional iterative method. The overall calculation rate is increased by a factor of 65, and the calculation error is smaller than that of the conventional method. The GHB does not consider the intermediate principal stress, and the GPH cannot reflect the mechanical properties of rock masses with different tensile and compressive strengths, although it considers the intermediate principal stress. The convergence–constraint characteristics of the surrounding rock of the GHB and GPH are significantly different from those of the GZZ. There is an optimal initial axial stress value qopt that yields the smallest deformation of the surrounding rock and the best stability. The internal support pressure and residual strength of the rock mass have a significant effect on the mechanical response of the surrounding rock. The proposed method has acceptable applicability to elastic–brittle and elastic–perfectly plastic rock masses. Additionally, it can be used in the study of deep-buried tunnel excavation at high geo-stresses.

Inverse Analysis for the Convergence-Confinement Method in Tunneling

For the safety of tunnel excavation, the observation of tunnel convergence not only provides a technique for assessing the stability of the surrounding ground, but also provides an estimate of the constitutive parameters of geological materials. This estimation method belongs to an inverse algorithm process called the inverse calculation method (ICM), which utilizes the incremental concept in the convergence-confinement method (CCM) to solve the support-ground interaction of circular tunnel excavation. The method is to determine the mathematical solution of the intersection of the two nonlinear curves, the support confining curve (SCC) and the ground reaction curve (GRC) in the CCM by using Newton’s recursive method and inversely calculating the unknown parameters. To verify the validity of the developed inverse algorithm process, this study compares the results of the ICM with those of the published articles. In addition, the modulus of rock mass and unsupported span are inversely deduced using the values of convergence difference measured in the practical case of railway tunnels.

Verification and Comparison of Direct Calculation Method for the Analysis of Support–Ground Interaction of a Circular Tunnel Excavation

A direct algorithmic process can deal with the solution of the support–ground interaction in a circular tunnel excavation through the convergence-confinement method (CCM) with the concept of increment. This process is the so-called direct calculation method (DCM) that can find solutions, the mobilized support pressure and the convergence, in the analysis of CCM. To achieve the solution, using two linear equations in the elastic region and Newton’s recursive method to find the roots in the plastic region are proposed and realized by a calculated spreadsheet. The validity of the algorithmic process for the analytical solutions was investigated and verified by the finite element computation, and compared with the published results, Rocksupport (2004), Oreste (2009), and Gschwandtner-Galler (2012). The results obtained between DCM and related studies show no significant differences.

Time-dependent crack development processes around underground excavations

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.

Explicit analysis for the ground-support interaction of a circular tunnel excavation in anisotropic stress fields

ABSTRACTThe convergence-confinement method (CCM) applied to a circular tunnel drilled through the ground under, initially, an anisotropic stress condition is investigated in this paper by employing the explicit procedure for implementing the analytical solution for executable calculations that can be directly estimated using a simple computational spreadsheet. The solutions of ground-support interaction in the final equilibrium state are solved with, in the elastic state, the simultaneous equations between the support confining curve (SCC) and the ground reaction curve (GRC) and using the Newton recursion method for finding roots in the plastic state. The influence factor of ground reaction, the apparent shear modulus of the ground, the apparent stiffness of the support and the interaction curve (IC) were especially examined in this study to consider the influence of locations around the tunnel in anisotropic stress fields. The validity of the developed explicit procedure for analytical solution was verified by numerical analysis to study the effects of in-situ stress ratio on the stress path, SCC and GRC at the intrados of the tunnel. The comparison of results obtained between the proposed explicit analysis (EAM) and the numerical analysis (FEM) was found to be consistent in elastic and elastic-perfectly plastic media.

Influence of the softening stress path on crack development around underground excavations: Insights from 2D-FDEM modelling

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.

Analysis of Soil-Compacting Effect Caused by Shield Tunneling Using Three-Dimensional Elastoplastic Solution of Cylindrical Cavity Expansion

Soil squeezing effect and formation disturbance caused by tunnel excavation can be simulated by cylindrical cavity expansion due to the comparability between tunneling and cavity expansion. Although most of the existing theoretical derivation is based on simple constitutive model of soil foundation, not only the relation between principal stress components was simplified in the solution process, but also the stress history, initial stress anisotropy, and stress-induced anisotropy of structural soil were neglected. The mechanical characteristics of soil are closely related to its stress history, so there is a gap between the above research and the actual engineering conditions. A three-dimensional elastoplastic solution of cylindrical cavity expansion is obtained based on the theory of critical state soil mechanics and engineering characteristics of shield tunneling. In order to fully consider the influence of initial anisotropy and induced anisotropy on the mechanical behavior of soils, the soil elastoplastic constitutive relation of cavity expansion is described in the course ofK0-based modified Cam-clay (K0-MCC) model after soil yielding. An equation with equal number of variables is obtained under the elastic-plastic boundary condition based on the Lagrange multiplier method. By solving the extreme value of the original function, the analytical solution of radial, tangential, and vertical effective stresses distribution around the circular tunnel excavation is obtained. In addition, changes of elastic deformation area and plastic deformation area for soil during the shield excavation have been analyzed. Calculation results are compared with the numerical solutions which usually consider isotropic soil behavior as the basic assumption. In this paper, a constitutive model which is more consistent with the actual mechanical behavior of the soil and the construction process of the shield tunnel is considered. Therefore, the numerical solutions are more realistic and suitable for the shield excavation analysis and can provide theoretical guidance required for design of shield tunneling.

Thermal radiation characteristics of stress evolution of a circular tunnel excavation under different confining pressures

The infrared thermography has been tested herein for the study of the thermal radiation characteristics of stress evolution induced by a circular tunnel excavation during the biaxial compression tests of the similar material, together with the uniaxial compression test for the cylindrical specimen of the same material. The results showed that the stress curve of the cylindrical specimen displayed some larger fluctuations while the average infrared radiation temperature (AIRT) curve decreased with time. The correlations between the axial stress of the cylindrical specimen and its surface thermal radiation temperature presented distinctly two stages, namely negative nonlinear correlation in the front-peak curve and linear correlation in the post-peak curve. The cylindrical specimen demonstrated a tensile failure mode and showed the cooling effect. Under biaxial uniform compression, the temperature field and the stress field of the surrounding rock displayed a homogeneous state after the circular tunnel excavation. Maintaining the horizontal load constant and continuing to increase the vertical load, the thermal radiation temperatures on the specimen surface demonstrated a significant change after the circular tunnel excavation, which verified the consistency of the dynamic evolution process of the temperature field and the stress field during the circular tunnel excavation. The results obtained in the study lay the foundations for future studies focusing on the utility of the infrared thermography in the similar engineering.

Determination of tunnel support pressure under the pile tip using upper and lower bounds with a superimposed approach

This study aimed to develop upper and lower bounds to predict the tunnel support pressure under the pile tip during the circular tunnel excavation. Most previous studies on the upper and lower bound methods were carried out for the single ground structures, e.g., retaining wall, foundation, ground anchor and tunnel, in the homogeneous ground conditions, since the pile-soil-tunnel interaction problem is very complicated and sophisticated to solve using those bound methods. Therefore, in the lower bound approach two appropriate stress fields were proposed for single pile and tunnel respectively, and then they were superimposed. In addition, based on the superimposition several failure mechanisms were proposed for the upper bound solution. Finally, these upper bound mechanisms were examined by shear strain data from the laboratory model test and numerical analysis using finite element method.

A Study on the Vibration Frequency of Blasting Excavation in Highly Stressed Rock Masses

During blasting excavation in deep-buried tunnels and mines characterized by high in situ stress, the rock vibration is attributed not only to blast loading, but also to dynamic unloading caused by transient release of the in situ stress on excavation faces in the process of rock fragmentation by blasting. Understanding the vibration frequency characteristics under these two excitation sources is of important signification to determine appropriate vibration threshold limits for structure damage in deep-buried opening excavations. With a theoretical model developed for a deep-buried circular tunnel excavation by the millisecond delay blasting sequence, frequency characteristics and their influence factors are investigated and discussed for the vibrations induced by the blast loading, the dynamic unloading and the combined effects, respectively. The results show that the rising time of blast loading, the duration of dynamic unloading and the dimension of excavation boundaries are the main factors that affect the vibration frequency of blasting excavation in highly stressed rock masses. It is found that, the blast loading with a much shorter rising time accentuates higher vibration frequency than the dynamic unloading with a long duration, and it causes the blast loading vibration to be more readily attenuated as the propagation distance increases. Thus, the unloading vibration may become the main vibration component at far distances where its low-frequency vibration may exceed the vibration limits. The vibration induced by the combined effects has two distinctly dominant frequency bands corresponding to the two vibration excitation sources. The frequency analyses of the vibration records from two underground projects excavated by blasting are presented to demonstrate this finding. The findings of this study also clearly reveal that, reducing the dimension of excavation boundaries is one of the most effective means to prevent the vibrational damage to structures as it increases the vibration frequency and meanwhile reduces the peak particle velocity.

A new approach for analyzing circular tunnel in strain-softening rock masses

Although it is relatively a simple problem to study circular tunnels excavated in isotropic rock masses under hydrostatic stress fields, the results of stresses and displacements around circular opening can be insightful for ground support design and stability evaluation. Moreover, the solutions are useful in the validation of constitutive models and numerical codes. In the several decades past, many scholars studied this kind of problem in the Mohr–Coulomb (M–C) media, the Hoek–Brown (H–B) media and the generalized H–B media and considered the elastic–perfectly plastic, elastic–brittle–plastic and strainsoftening models with associated flow and non-associated flow rules. According to the existing literature surveys, most [1–13] of the researches are about elastic–perfectly plastic and elastic– brittle–plastic models because closed-form solutions (or numerical solution) can be easily obtained under certain assumptions. However, Hoek and Brown [14] suggest that the average quality rock masses behave in a strain-softening mode in the field of underground rock engineering. Therefore, it is of practical importance to study the mechanical behavior of normal rock masses with strain-softening behavior. By means of the classical theory of plasticity, materials with perfectly brittle–plastic and perfectly plastic constitutive model can be easily described and simulated in numerical methods such as the finite element method [15,16], etc. But it is more difficult to model the strain-softening material due to the negative slope in the stress–strain coordinates. Even for the above-mentioned excavation of circular tunnel in strain-softening materials, fewer attempts have been made [17–23] thus far. The reason is that the relationship among the stresses, plastic strain and the strength

EXPERIMENTAL STUDY ON THIN FLEXIBLE TUNNEL SUPPORT SYSTEM

The mechanical efficiency of a thin flexible tunnel support system such as shotcrete lining and rock bolts is investigated on the basis of experimental works. At first, model tests of tunnel excavation in which shotcrete lining and rock bolts were simulated by thin papers were carried out in a dry sand ground. It was found out that even if so flexible thin paper closed ring lining has a remarkable effect on the tunnel stability and that the effect of rock bolts appears only when they are placed to get into the outside of a plastic zone developed in the surrounding ground. A circular tunnel excavation was simulated by shrinking a metal ring in an aluminum rod mass and the development of a loosened zone in the ground due to tunneling was discussed. (Author abstract)