Rock Tunnel Research Articles

Study on mechanical properties and acoustic emission characteristics of deep diorite under uniaxial compression

The research object is diorite in the Lingbei TBM section of the Hanjiang-To-Weihe River Qinling tunnel, with a buried depth of over 1 km. Using MTS-2000 microcomputer-controlled electro-hydraulic servo universal testing machine and DS5-16b acoustic emission (AE) monitoring system, uniaxial compression and acoustic emission monitoring tests were carried out on rock samples, to study the uniaxial compression mechanical properties and acoustic emission characteristics of the deep diorite. The results of the study indicate that: (1) During uniaxial compression, diorite undergoes four stages: initial compaction, elasticity, yield and failure, in which the curve of the initial compaction stage is significantly smoother. The uniaxial compressive strength is 41.95 MPã102.42 MPa, with an average of 74.07 Mpa; The axial peak strain ranges from 1 % to 1.4 %, and the failure mode belongs to brittle ductile splitting failure. (2) The cumulative ringing count and energy showed a very slow increase trend during the calm period; After entering a surge period (with the appearance of Kaiser points), both show a significant transition state; During the slow increase period, the overall growth rate of the two slowed down and remained almost silent. (3) On the basis of the analysis of RA-AF values during the deformation and rupture process of diorite, it can be seen that the damage type of diorite is tensile damage by the significant low RA value and high AF value characteristics, which coincides with the actual damage fracture characteristics of the rocks in the sample. (4) During the compaction stage, there are few acoustic emission location points, which correspond to low energy and are mainly distributed at the higher and lower ends of the sample; After entering the elasticity stage, the number of positioning points significantly increases and gradually expands towards the middle; Near Kaiser point, the number of location points and corresponding energy are both in a sharp increase state, and this trend is in good agreement with the changes in the ringing count-time and energy-time curves. (5) The damage time mainly starts at the end of the calm period, and the pattern of change in the damage curve coincides with the localization point and energy evolution. The results of the research can be used as a referential basis for the development of the excavation, protection and other construction plans for the Lingbei TBM section of the Hanjiang-To-Weihe River Qinling tunnel or similar surrounding rock tunnels, as well as for further conducting triaxial unloading tests on diorite.

Detection of Sub-surface Delamination and Moisture Penetration in Unlined Rock Tunnels Using Passive Thermography and Tapping

An unlined rock-blast tunnel constructed by the drill and blast method is susceptible to frequent deterioration, including sagging beds, water ingress and loose rock masses. Thus, assessing potential damages above the roadway that may impact the safety of tunnel users is of utmost importance. While tapping, a conventional nondestructive method, is typically used to predict sub-surface delamination, water ingress can be identified with the naked eye. In this research, we utilized a modern-technology approach called passive thermography, together with a high-resolution digital camera and tapping. The study encountered two primary challenges; namely, the absence of direct solar radiation during the inspection and the groove-exposed surface of the unlined tunnel. The results demonstrated that passive thermography was able to detect delaminated areas and water infiltration on the tunnel's walls and ceiling, even without direct sunlight. Large delaminated areas can be detected with just a 2°C change in atmospheric temperature twelve hours before testing. Additionally, the results of image post-processing significantly contributed to enhancing the results of passive thermography. The thermal image was processed into a grayscale image prior to HE processing, which enhances contrast by over 50%. The combination of tapping, digital camera and passive thermography was proven to be effective in periodically inspecting unlined rock tunnels, while significantly reducing time and cost. KEYWORDS: Tunnel safety, Passive thermography, Old rock-cut tunnel, Hammer sounding, Non-destructive evaluation, Image post-processing.

Pre-treatment influence of liquid nitrogen and microwave on the mode I fracture characteristics of granite

Microwave and liquid nitrogen have shown great potential applications in the mining, geological, petroleum, and particularly metal mining fields. In order to further promote their application in rock tunneling engineering, this paper conducted a series of I-model fracture tests on hard rock (granite) to explore the impact of liquid nitrogen, microwave, and liquid nitrogen and microwave pretreatment on the fracture characteristics of hard rock. The results showed that compared with the control group, all pretreatment groups had varying degrees of reduction in wave speed and fracture toughness, especially the microwave and liquid nitrogen group.

Optimization of Adaptive Active Support Parameters for Large Deformation Prestressed Anchor Ropes in Soft Rock Tunnels Using Genetic Algorithm Based Computer Applications

The large deformation of soft rock tunnel is an important problem to be solved urgently with the development of engineering technology. In order to improve the application effect of prestressed anchor cable adaptive active support system, this paper puts forward an optimization method based on adaptive genetic algorithm, reviews the research status of prestressed anchor support in soft rock tunnel, and analyzes and evaluates the existing methods. This paper introduces the construction process of AGA model, establishes the problem description model and genetic optimization algorithm selection model, and constructs the adaptive active support model of prestressed anchor cable. The model adopts an optimization algorithm with a total of 100 iterations. The Simple Genetic Algorithm and Parallel Genetic Algorithm are selected to evaluate the anchoring force, the control effect of surrounding rock displacement, the erosion of supporting structure and the construction time by comparative test. The test results show that AGA has a good effect on the anchoring force, the minimum anchoring force is 457 kN, and the displacement of surrounding rock is controlled within 1.21-1.59mm, which has a good influence on the quality of supporting structure and the construction completion time. Finally, the application of AGA model in parameter optimization of prestressed anchor cable in soft rock tunnel is summarized, and the future research direction is put forward.

Approximation method for yielding support analysis in high ground stress soft surrounding rock tunnels.

In solving the whole process of interaction between soft rock and yielding support in high-stress environments in tunnels using mechanical analysis methods, it is challenging to simultaneously satisfy both displacement coordination and static equilibrium at the contact surface between the rock and the support structure. This paper, based on the mechanical analysis of rock and rigid support, considers the impact of the circumferential installation of yielding elements on radial displacement, and proposes displacement approximation and support force approximation methods using displacement coordination and static equilibrium as approximation conditions. The study fits curves of numerical simulation results and laboratory test results of yielding elements, and attempts to directly use the laboratory test data set of yielding elements as computational data. By calculating two circular tunnel examples and comparing the effects of the trisection method, bisection method, and substitution method on the convergence of the displacement approximation method, the effectiveness of the methods proposed in this paper is verified. The research results show that the two approximation algorithms proposed in this paper have good accuracy and reliability in calculating the relative displacement of rock and yielding support structure contact surfaces, and the support force of yielding support. The bisection method outperforms the trisection and substitution methods in terms of stability and convergence. However, there are certain limitations in this study, such as the effectiveness of the algorithm may be influenced by geological conditions; the complexity of actual geological conditions may exceed the assumptions of the current rock-support mechanical analysis model.

Mechanical characteristics of large-section tunnel in soft rock based on various rock conditions and excavation footages

Taking the V-class surrounding rock section at the entrance and exit about Qianqi Tunnel as an example, the deformation rules as well as mechanics about the tunnel in different rock conditions and excavation footages are compared by numerical calculation, and the suitable excavation footage of the relied engineering is also proposed. The results show that the displacements about surrounding rock are greatly influenced by the excavation footage, namely, the arch settlements, horizontal convergences as well as surface settlement of the tunnel increase significantly with the excavation footage. For the preferred rock condition, the maximum bending moments about the preliminary linings at middle step after excavation occurs in the vault, and its increase amplitude has little correlation with the excavation footage. For the poor surrounding rock, the maximum bending moment is transferred from the vault to the arch waist with the increase of excavation footage, and it is basically manifested as the inner tension of the lining. The relationship between excavation footage and maximum deformation as well as mechanical index of vault is also revealed, respectively. The excavation footage of shallow-buried tunnel sections in V-grade surrounding rock is suggested, that is, 1.8m for the preferred surrounding rock condition and 1.2m for the poor surrounding rock condition.

Classification method of surrounding rock of plateau tunnel based on BP neural network

Due to the unique high-altitude geological conditions of the railway in the cold region, the problem of high ground stress in the construction process is very prominent. In constructing high ground stress tunnels, accurately evaluating the surrounding rock grades is important in rock mass engineering. Based on this, based on a plateau tunnel under construction, this paper selects the classification index of the surrounding rock, which can accurately reflect the geological characteristics of high ground stress tunnel around the geological environment elements of the surrounding rock of high ground stress tunnel. Based on the rapid classification method of surrounding rock of the BP neural network, the classification method of the surrounding rock suitable for high ground stress tunnel is constructed, and the tunnel engineering data is introduced into the BP neural network classification method of surrounding rock for training and testing. It is found that the classification results of surrounding rock obtained by the classification method of surrounding rock of high ground stress tunnel are in good agreement with the actual situation, which provides an important guarantee for the accurate and rapid determination of the surrounding rock grade of high ground stress tunnel and the safe and efficient construction of the tunnel.

Large deformation characteristics and the countermeasures of a deep-buried tunnel in layered shale under groundwater

Deep-buried tunnels constructed through layered soft rocks undergo large deformation, which threatens their safety and leads to construction delay. When a soft rock tunnel is excavated below the water table, the rock mass will deteriorate due to the presence of groundwater, aggravating the large deformation issue. In this work, the Baozhen tunnel excavated in layered carbonaceous shale was taken as a case study to investigate the mechanical behavior and support countermeasures of deep-buried tunnels with a high geostress considering the effect of groundwater. First, laboratory tests were performed to study the groundwater effect on the mechanical properties of the shale. Subsequently, the geological and large deformation characteristics of selected study areas using the original support scheme were analyzed. Using the tested constitutive parameters combined with the field-monitored deformation, a UDEC numerical model was established to investigate the tunnel deformation behavior under different bedding angles, groundwater depths, and shear stresses. Finally, the optimized support scheme was verified through numerical and on-site monitoring results. The tunnel responses at three typical monitoring cross-sections, such as the deformation distribution and normal pressures acting on the primary support and secondary lining, were compared to investigate the effect of groundwater and the optimized support scheme on the tunnel responses. An asymmetric large deformation was induced by the combined action of the soft layered rock and shearing action under the effect of groundwater. The groundwater reduced the rock strength and increased the tunnel deformation, and it also strengthened the unsymmetrical loading effect, which was reflected in the distributions of the tunnel deformation and support pressures. As a countermeasure, asymmetrically arranged rockbolts could help reduce the full-ring tunnel deformation and support pressure by 50.3 % and 22 %, respectively, which could also alleviate the unsymmetrical loading effect, thus enhancing the synergistic performance of the support structures. The insights gained in this study can help improve the effectiveness of countermeasures against layered rock instability while tunneling in the presence of groundwater.

Analysis of the interaction between bolt-reinforced rock and surface support in tunnels based on convergence-confinement method

To investigate the interaction of the bolt-reinforced rock and the surface support, an analytical model of the convergence-confinement type is proposed, considering the sequential installation of the fully grouted rockbolts and the surface support. The rock mass is assumed to be elastic-brittle-plastic material, obeying the linear Mohr-Coulomb criterion or the non-linear Hoek-Brown criterion. According to the strain states of the tunnel wall at bolt and surface support installation and the relative magnitude between the bolt length and the plastic depth during the whole process, six cases are categorized upon solving the problem. Each case is divided into three stages due to the different effects of the active rockbolts and the passive surface support. The fictitious pressure is introduced to quantify the three-dimensional (3D) effect of the tunnel face, and thus, the actual physical location along the tunnel axis of the analytical section can be considered. By using the bolt-rock strain compatibility and the rock-surface support displacement compatibility conditions, the solutions of longitudinal tunnel displacement and the reaction pressure of surface support along the tunnel axis are obtained. The proposed analytical solutions are validated by a series of 3D numerical simulations. Extensive parametric studies are conducted to examine the effect of the typical parameters of rockbolts and surface support on the tunnel displacement and the reaction pressure of the surface support under different rock conditions. The results show that the rockbolts are more effective in controlling the tunnel displacement than the surface support, which should be installed as soon as possible with a suitable length. For tunnels excavated in weak rocks or with restricted displacement control requirements, the surface support should also be installed or closed timely with a certain stiffness. The proposed method provides a convenient alternative approach for the optimization of rockbolts and surface support at the preliminary stage of tunnel design.

Analysis on particle flows with flaky shape and influencing factors on particle conveying capacity in roadheader

Spherical and flaky particles are mixed during hard rock tunneling. In particular, flakey particles are difficult to load, resulting in conveying machines that have difficultly operating efficiently. Based on the characteristics of flakey particles and the present conveying structures, such as star wheels, three new conveying structures are introduced in this paper, and the parameters related to the conveying ability are evaluated with theoretical analysis. Then, the flow and trajectory of particles under different conveying structures and the difference in their working capabilities were investigated with the help of the discrete element method (DEM). Finally, conveying machines with different conveying structures are made with 3D printing technology and experiments for validation of the conveying ability are performed. The results show that the conveying structures can be divided into two categories: those with a rotational speed proportional to the loading rate and those with the opposite condition. The propulsion status has a stable loading phase with a long time period relative to the non-propulsion status. The findings can be used to guide conveying machine designs, and they can be helpful for the enhancement of the working ability in conveying particles with flakey shapes.

Inversion of Surrounding Red-Bed Soft Rock Mechanical Parameters Based on the PSO-XGBoost Algorithm for Tunnelling Operation

In constructing hydraulic tunnels, construction disturbances and complex geological conditions can induce variations in the surrounding rock parameters. To navigate the complex non-linear interplay between rock material parameters and tunnel displacement during construction, this study proposes a hybrid learning model. It employs particle swarm optimization (PSO) to refine the hyperparameters of the eXtreme Gradient Boosting (XGBoost) technique. Sensitivity analysis and inversion of rock parameters is performed by using orthogonal design and the Sobol method to analyze the sensitivity of environmental and rock material factors. The findings indicate that the tunnel depth, elastic modulus, and Poisson ratio are particularly sensitive parameters. Mechanical parameters of the rock mass, identified through sensitivity analysis, are the focal point of this research and are integrated into a three-dimensional computational model. The resulting tunnel displacement calculations serve as datasets for the inversion of the actual engineering project’s surrounding rock mechanical parameters. These inverted parameters were fed into the FLAC3D software (version 7.0), yielding results that align closely with field measurements, which affirms the PSO-XGBoost model’s validity and precision. The insights garnered from this research offer a substantial reference for determining rock mass parameters in tunnel engineering amidst complex conditions.

An overview of the research trends on fiber-reinforced shotcrete for construction applications

Abstract In this study, data mining, followed by the scientometric analysis of fiber-reinforced shotcrete (FRS), was carried out for knowledge mapping, co-citations, and co-occurrence. The information needed for the analysis was retrieved from the Scopus search engine. Important publishing sources, keyword analysis, writers with the most contribution in citations and publications, the most-cited articles, and the regions most actively engaged in FRS research were identified throughout the data review process. Moreover, the need for FRS, the major constraints associated with their usage, and their possible solutions were discussed. The analysis of the bibliographic data showed that research publications on FRS progressed inconsistently till 2015, and over the past 6 years (2016–2021), publication numbers increased steadily, which exhibited the interest of academics in fiber-reinforced materials. The analysis of keywords in the field showed that the most common FRS research keywords are shotcreting, shotcrete, steel fibers, FRS, and fiber-reinforced materials. Keyword analysis showed that FRS is typically used for tunnel rock support and lining. Based on the review of relevant literature, research gaps have been identified, and future research has been suggested.

Study on Instability Change Law of Surrounding Rock of Tunnel Crossing Fault Fracture Zone Based on Cusp Point Mutation Theory

The instability process of surrounding rock is a nonlinear mutation. To truly reflect the nonlinear mutation in the failure of surrounding rock and grasp the opportunity of instability of surrounding rock, it is necessary to study the stability of surrounding rock by combining the existing methods with nonlinear scientific theory. Based on the numerical simulation results of a tunnel project crossing the fault fracture zone, the mutation theory is introduced to study the instability law of surrounding rock during tunnel excavation. The dynamic excavation process of surrounding rock tunnel in fault fracture zone is simulated by the finite difference software FLAC3D. Based on the cusp point mutation theory, the deformation law of each excavation step to the working face under different dip angles, strike angles and thicknesses of fault fracture zone is studied. Combined with the dichotomy method, curve fitting is used to analyze the instability threshold of surrounding rock and predict the opportunity of surrounding rock instability. The results indicate that (1) As the dip angle and strike angle of the fault gradually increase, the settlement of the surrounding rock arch first decreases and then increases, while as the thickness of the fault increases, the vertical displacement of the surrounding rock gradually increases. (2) When the dip angle of the fault is less than 90°, the sudden instability of the surrounding rock is advanced, while when it is greater than 90°, the time for sudden instability is delayed. (3) As the strike angle and thickness of the fault gradually increase, the excavation step for the tunnel to undergo sudden instability is gradually advanced. The research conclusion can provide theoretical guidance and suggestions for similar engineering construction in the future.

Thermal Insulation Properties and Simulation Analysis of Foam Concrete Regulated by Mechanical and Chemical Foaming.

To address, mitigate, or prevent thermal environmental issues arising from the heat dissipation of high-temperature surrounding rocks in deep hot tunnels, a research proposal is put forward based on previous studies and the team's initial experiments. The proposal involves using mechanical and chemical foaming to enhance the thermal insulation properties of foamed concrete, and this will be tested through engineering verification. Different proportions of cementitious materials, latex powder, polypropylene fiber, and self-made composite foam materials were designed using an orthogonal approach for testing the macroperformance and microstructure of foamed concrete. The pore structure of foamed concrete was quantitatively analyzed by Image-Pro Plus 6.0 software, and a fitting expression was established between thermal conductivity and the number of pores (1-2 mm). Characteristics of heat transfer inside the foam concrete were simulated and analyzed using COMSOL software, and the transmission path of heat streamline was found to be ″concave-convex form″, illustrating the blocking effect of foam concrete on heat. A thermal insulation engineering model was created using Fluent software to investigate the effects of thermal insulation layer thickness, water gushing heat release, seasonal factors, and other working conditions on the airflow temperature in the roadway before and after the application of foam concrete. The simulation results demonstrate that foam concrete can effectively reduce the airflow temperature in the roadway and weaken the surrounding rock heat dissipation. Additionally, it is found that the decreasing rate of heat dissipation of surrounding rock increases with the increase of insulation layer thickness, proving the engineering applicability of foam concrete for roadway insulation. The research results provide a theoretical basis and practical guidance for heat damage control of deep mining roadway.

Extrusion 3D printing circular and horseshoe tunnel physical models: A comparative study of deformation and brittle failure

The instability of high-stress hard rock tunnels mainly presented with the deformation and brittle failure, the tunnel brittle failure’s location and range are dissimilar under different stress and shape conditions. In this study, similar 3D printing materials are developed according to the brittleness index of the hard rock in an underground cavern. The physical models of circular and horseshoe tunnel with the equal area are printed, and overloading tests are carried out. The test results reveal that the horseshoe tunnel’s peak strength and residual strength are decreased by 17.2% and 6.6% compared with that of the circular tunnel, the circular tunnel’s vertical displacement is greater than the horseshoe tunnel’s, and the depth and range of brittle failure in the circular tunnel are bigger than the horseshoe tunnel’s. The circular tunnel brittle failure is mainly in the middle of both sides’ walls, while the horseshoe tunnel brittle failure is mainly in the bottom of both sides’ walls. The location and range of the tunnels’ brittle failure can be well simulated by using the rock mass deterioration model (RDM) combined with the rock fracture degree (RFD) index, and the numerical simulation results are consistent with the physical simulation experiment results. According to the results of laboratory physical model experiment and numerical simulation, the brittle failure evolution model of hard rock tunnel is proposed, which can provide a reference for the excavation and support design of hard rock tunnel under high stress.

Numerical Analysis of Low-Cost Recognition of Tunnel Cracks with Compressive Sensing along the Railway

Currently, the use of microseismic detection technology for crack detection and localization in rock masses has great potential in detecting structural damage. As engineering safety has always been a very important issue, this study investigated the problem of multi-crack identification in rock masses within the environment of track tunnels using transient waves. A tunnel rock was modeled using MIDAS GTS NX software (2019.v1.2) and a crack transient wave model in the frequency domain was obtained through data analysis and simulation. Then, this was combined with compressive sensing techniques to locate and detect multiple cracks in tunnel rock. The performance of the proposed approach was validated through experimental simulations, which included experiments on differences in the number of cracks, as well as spatial samples. The experimental results indicate that the technique performs well for single-crack localization in tunnel rock mass, where the average localization error is 4 m. Meanwhile, the localization error is larger in multi-crack localization, and the number of spatial sample points set using compressive sensing also has a large impact on the experimental results.

Retracted: Image Recognition about Stability of Soft Surrounding Rock in Tunnel Based on ILBP Algorithm

Retracted: Image Recognition about Stability of Soft Surrounding Rock in Tunnel Based on ILBP Algorithm

The deformation and failure characteristics of tunnels in layered rock with gentle dip angles after freeze–thaw cycles: Physical model tests and numerical investigation

When a tunnel passes through layered rock with a gentle dip angle in cold regions, the occurrence of multiple freeze–thaw (F–T) cycles can easily cause fatigue damage to the rock bedding surfaces and seriously affect the rock stability. One of the engineering problems that needs to be solved is to determine the deformation and failure mechanisms of such surrounding rock under F–T cycles. Based on the similarity principle, the mechanical response of the tunnel lining structure and the progressive failure process of the surrounding rock under long-term F–T action were studied by performing physical model tests and numerical simulations. The results show that the model tunnel lining partially separates from the surrounding layered rock with a gentle dip angle under the action of F–T cycles. Moreover, the load bearing capacity of the model weakens after undergoing F–T cycles. Additionally, the bending moment and axial force evolution characteristics of the tunnel and surrounding rock model are consistent with the deformation characteristics, which agree with the actual mechanical and deformation characteristics of a real tunnel under F–T cycles. After establishing numerical calculation models for surrounding rock with three bedding angles (0°, 10°, and 20°) and verifying the numerical models based on the test results, the horizontal and vertical displacements of the surrounding rock in the tunnel entrance section under ground stress conditions at a depth of 400 m were calculated. The research results accurately reflect the long-term deformation characteristics of layered rock with gentle dip angles surrounding tunnels in cold regions and can provide theoretical guidance for the application of corresponding tunnel support measures in cold regions.

Experimental investigation on biaxial mechanical properties of laminated rock and floor heave mechanism of tunnel based on DIC

Floor heave at the bottom of a tunnel is a common defect during tunnel service. To study the deformation behavior and failure characteristics of the floor, laminated sandstone specimens with precracked U-shaped grooves were prepared, and a series of biaxial compression tests were carried out in the laboratory. Two typical floor structures of a tunnel (with and without inverts) and four lateral stress levels (σ2 = 0, 5, 10 and 15 MPa) were considered. The effects of lateral stress and inverted structure on the deformation and failure characteristics of the floor were investigated. The displacement, strain evolution and failure extent of the floor were analyzed based on the digital image correlation (DIC) technique. The testing results show that the stress concentration at the U-shaped grooves of the specimens can be effectively reduced by the inverted structure. The specimen with an invert can bear more axial deformation and axial stress than that without an invert. The V-shaped notch failure behavior occurred near the tunnel sidewalls for the specimen without an invert, and it occurred in the middle of the tunnel floor for the specimen with an invert. The failure intensity of the V-shaped notch was positively related to the lateral stresses. With the increasing of axial stress, the heave displacement at the tunnel floor gradually increased, while the settlement displacement of the sidewall showed a trend of first increasing and then decreasing. The tensile strain concentration area of the floor undergoes a process of increasing and then decreasing and finally transforms to compressive strain. The failure characteristics of the sandstone specimen observed in the laboratory tests are consistent with the phenomena in real tunnel engineering. In addition, the deformation and failure mechanism of the tunnel floor was analyzed. The key points for the treatment of tunnel floor heave are put forward based on the testing results.

The Use of Slag in Accelerated Shotcrete for Rock Support: A Pilot Study on the Influence on the Early Compressive Strength

Abstract Today, the world is facing an enormous challenge where the emissions of greenhouse gases must be reduced significantly to stop the increasing mean temperature. For the construction industry, this challenge means that structures must be designed more carefully, and materials must be used and produced more efficiently. Concrete is one of the most widely used materials, and cement production alone contributes to around 8 % of the world’s total CO2 emissions. This paper presents a pilot study in which part of the cement clinker content in accelerated shotcrete is replaced with ground granulated blast furnace slag (GGBS). Shotcrete is widely used as part of the rock support for tunnels in hard rock and has a large cement content. Thus, the environmental impact must be reduced significantly to meet future demands. However, introducing GGBS will slow down strength development, which is a crucial part of shotcrete. Thus, this paper aims to investigate the early strength development, i.e., after one and seven days. The results show that an average compressive strength of 18 MPa can be reached after 24 hours when 35 % of the cement clinker content is replaced with GGBS. Finally, some important topics for further studies are pointed out.