Deep-buried Tunnel Research Articles

Review on Early Warning Methods for Rockbursts in Tunnel Engineering Based on Microseismic Monitoring

Due to the different geological conditions and construction methods associated with different projects, rockbursts in deep-buried tunnels often present different precursor characteristics, bringing major challenges to the early warning of rockbursts. To adapt to the complexity of engineering, it is necessary to review the latest advancements in rockburst early warning and to discuss general early warning methods. In this article, first, microseismic monitoring and localization methods applicable under tunneling construction are reviewed. Based on the latest engineering examples and research progress, the microseismic evolution characteristics of the rockburst formation process are summarized, and the formation process and mechanism of structure-type and delayed rockbursts are analyzed. The different methods for predicting the risk and level of rockbursts using microseismic indices are reviewed, and the implementation methods and application cases for predicting potential rockburst areas and rockburst probability based on a mechanical model are expounded. Finally, combined with the new practice in early warning methods, development directions for the early warning of rockbursts are put forward.

Performance validation of the heat extraction performance by the natural ventilation with a board-coupled shaft in a deep buried tunnel

Experiments were conducted in a 1/20 scale model tunnel, a higher shaft (5 times the tunnel height) was applied to model the shaft in a deep buried tunnel. Researches have validated that the board-coupled shaft can efficiently eliminate the plug-holing phenomenon and improve the smoke and heat extraction performance when the board-coupled shaft is suitably applied in a shallow buried tunnel. The purpose of this paper is to explore whether the board-coupled shaft (BCS) is effective and has stable heat extraction performance in deep buried tunnels. The influence of heat release rate (HRR) and board location on the heat extraction performance was investigated. The experimental results show that the BCS is effective and has stable heat extraction performance for the deep buried tunnels applied in this study. Also, an analytical solution for predicting the dimensionless smoke layer thickness in the one-dimensional flow region was developed, and it can be found that the dimensionless smoke layer thickness in the one-dimensional flow region depends only on the longitudinal position and doesn’t rely on the HRR. Furthermore, based on the general condition that the first shaft cannot extract the entirety of the smoke, two empirical correlations were derived for predicting the dimensionless temperature difference (ΔTs∗) and the dimensionless heat extraction rate (qs∗) in the BCS under different HRRs and board locations. These studies may contribute positive significance for promoting the application of natural ventilation with vertical shafts in deep buried tunnels.

ANALYSIS AND THINKING ON STRUCTURAL PERFORMANCE EVOLUTION OF SHIELD TUNNEL LINING

Under the background of China's strategy to strengthen transportation industry, the mechanical performance and durability of shield tunnel ling face higher requirements with the increasing construction of ultra-long, large cross-section, deep buried and high-density developed shield tunnels. In this paper, the traditional shield tunnel structure design theory and method and the current development trend are summarized based on an extensive investigation of domestic and foreign literatures. And the status quo and shortage of researches on shield tunnel structure’s mechanical property and durability are discussed, the current shield tunnel structure performance detection technology and evaluation methods are further reviewed. In view of the limitations of the traditional reinforced concrete segment, the (steel) fiber concrete material, the new joint and the compression concrete lining have great advantages in improving the structural performance and construction efficiency of shield tunnel lining, which are the future development direction of shield tunnel lining structures.

Study on earth pressure of deep-buried tunnel in layered ground with centrifuge modelling

Deep-buried tunnels have been increasingly used for transportation infrastructure and underground space development in major metropolitan areas around the world. Evaluation of the earth pressure on deep-buried tunnels is crucial in the design of such urban infrastructure. This paper presents an experimental study using a series of centrifuge model tests undertaken in a beam centrifuge at 150g to provide insight into the earth pressure distribution on deep-buried tunnels after the construction in layered soil strata. The experimental and comparative results indicate the earth pressure at the invert of the deep-buried tunnel is slightly greater than the earth pressure at the crown of the tunnel, and the earth pressure at the springline is less than that at the crown. These results also show that the full overburden theory can reasonably predict the vertical earth pressure of a deep-buried tunnel in the clay layer. However, in the scenarios when sand layers are dominant in the overburden soil, the vertical earth pressure values calculated by the Terzaghi loosening earth pressure theory are more consistent with measured values from centrifuge tests. The insights obtained from this study can provide useful references for improved methods in the load consideration of deep-buried tunnel design.

Countermeasures against large deformation of deep-buried soft rock tunnels in areas with high geostress: A case study

Large squeezing deformation has been a main concern in the construction of deep-buried tunnels with soft surrounding rocks in high-geostress areas, threatening the safety of tunnel construction and delaying the construction progress. In this study, the Dapingshan tunnel, where a large deformation occurred, was taken as an example to study this issue. First, the geological and deformation characteristics of the large deformation area were analyzed. Subsequently, the creep model and creep parameters of the surrounding rock in the large deformation area were identified through laboratory tests. Finally, considering the time-dependent effect of tunnel construction, the geostress in the large deformation area was measured using the displacement back-analysis method, and the feasibility of the optimization scheme was verified. The results showed that: (1) The occurrence of large deformation is due to the combined action of the high geostress and poor self-stability of argillaceous sandstone. (2) The creep characteristics of argillaceous sandstone are consistent with the creep characteristics of the CVISC model. (3) The calculation result of the displacement back-analysis method considering the influence of the time-dependent effect of tunnel construction is found to be reliable. (4) The optimization scheme can help reduce the tunnel arch convergence and horizontal convergence by 50% and 60%, respectively, which confirms that the optimization scheme can help eliminate the large deformation of the Dapingshan tunnel.

Calculation of earth pressure distribution on the deep circular tunnel considering stress-transfer mechanisms in different zones

It is of great significance for ensuring the lining safety to reasonably determine the earth pressure on the tunnel, especially the deep-buried tunnel. In this study, the stress-transfer mechanisms of soils in different zones above the tunnel were analyzed firstly. A multi-arch model for calculating the distribution of the vertical earth pressure on a deep tunnel in dry sand was then proposed based on the limit equilibrium method. The model is composed of three parts: the upper end-bearing arch, the stability zone and the lower friction arch. The key parameters (i.e., width and height of friction arch zone, thickness of end-bearing arch zone and lateral stress ratio in friction arch zone) in the proposed model were suggested according to the existing literature, and a formula for predicting the height of the friction arch zone was deduced. The experimental and numerical results and Terzaghi’s solution were adopted to assess the validity of the proposed model, which indicates that the proposed model not only coincide with the test results of average vertical earth pressures on the tunnel, but also describe the nonuniform distribution characteristics of vertical stress on the circular tunnel, namely, the minimum vertical earth pressure is located at the centreline of the tunnel, and then gradually increases as it moves away from the centreline. Finally, the impacts of geotechnical and geometrical parameters on the earth pressure were discussed.

Numerical Simulation Research on Rockburst Prediction of Gaoloushan Deep Tunnel Based on Strength-stress Ratio Method

In view of the complex geological environment of deep buried tunnel, the great harm of rock burst, and the serious hidden danger to the underground engineering construction and the safety of the workers, it is necessary to adopt an effective method to reasonably predict the rock burst, so as to carry out the rock burst disaster warning. Based on the strength-stress ratio method, the possibility of rockburst in the deep-buried tunnel of Pingliang-Mianyang National Expressway is predicted in this paper. At the same time, the FLAC3D numerical simulation software was used to build a numerical model to simulate the excavation process of the deep buried tunnel, so as to analyze the stress state of the surrounding rock of the deep buried tunnel, and the ratio of the maximum principal stress σ 1 of the surrounding rock obtained from the simulation calculation to the uniaxial compressive strength Rc of the rock in the excavation process. According to the ratio, the possibility level of rockburst at different length sections along the deep buried tunnel is divided, analyzed and predicted, and compared with the actual situation of rockburst in the field, the results show that the location of rockburst and the rockburst level of the corresponding length are basically the same. This discriminant method can provide reference and relevant reference for rockburst prediction of similar engineering.

Modelling transient discharge into deep-buried tunnels in karst area based on a coupled discrete-continuum model

Karst aquifers are characteristic of complex hydrogeological conditions and highly developed void structures, often causing serious water and mud inrush, karst collapse and other geological disasters during construction of deep-buried tunnels. Accurate prediction of groundwater discharge into tunnels has become a key issue for tunnel engineering in karst formations. Motivated by safety assessment of tunnel construction in the Jiayan Water Diversion Project located in Guizhou Province, China, we perform groundwater flow simulations to predict transient discharge into the long, deep-buried tunnels. Two numerical models, an equivalent porous medium model and a coupled discrete-continuum model, are used. In the coupled discrete-continuum model, both the tunnels and karst conduits are treated as discrete channels, which are coupled with the surrounding matrix through water exchange. Based on hydrogeological data and site characterization, the model parameters are calibrated by using the PEST inversion algorithm. It is shown that numerical predictions of tunnel discharge during construction agree well with the measurements, indicating reliability of the numerical models. Comparison between the two models shows that the coupled discrete-continuum model can more accurately reflect the changes in the groundwater level and can better estimate the sources of groundwater discharge into the tunnels during the construction. This work suggests that the coupled discrete-continuum model is advantageous over the equivalent porous medium model as the former is a better representation of the karst system. It is demonstrated that the simulation methodology provides a reliable method for assessment of water discharge into tunnels and the optimization of construction scheme. The results are of practical significance for safe construction of tunnels in karst areas.

Deformation and failure analysis of surrounding rock mass in deep buried tunnels subjected to excavation

With the rapid development of underground engineering, the construction of hydraulic tunnel is faced with complex geological conditions. Rock burst and deformation have become the bottleneck of engineering construction. The study of deformation and failure mechanisms of surrounding rock mass during the excavation of deep buried tunnel has become important for the field of underground engineering. The traffic tunnel of Shuangjiangkou hydropower station is studied to identify the failure mechanism of surrounding rock mass of deep buried tunnel. We introduced microseismic monitoring technology to delineate potential risk areas and reveal the space-time evolution of microfractures. The failure type of the surrounding rock is determined by source parameters. The relationship between surrounding rock deformation and excavation dynamic response is analyzed based on the geological and microseismic data. The results show that the 3D microseismic visualization model can realize the advanced detection of the internal damage of surrounding rock. The source parameters reveal that tensile failure is the main form of failure in the tunnel surrounding rock. Strong unloading and poor geological conditions are the main factors affecting the stability of surrounding rock mass. The study combines microseismic monitoring with field investigation, and then comprehensively evaluates the stability of the tunnel surrounding rock providing a reference for similar underground engineering.

Propagation mechanism of fracture zones in single-hole rock mass under high in-situ stress

The blasting process is affected by in-situ stress in deep buried tunnels. The design of blasting parameters with consideration of in-situ stress is important in damage control for deep buried tunnel excavation. In this work, a two-dimensional theoretical model of single-hole blasting crack propagation is established, and the relationship between crack propagation and in-situ stress is studied from the perspective of fracture mechanics. Using the finite element dynamic analysis software LS-DYNA, the single-hole blasting failure process under high in-situ stress is numerically simulated, and the crack propagation mechanism of single-hole blasting under high in-situ stress is explored. Results show that the ultimate crack growth length depends on the interaction of detonation gas and in-situ stress. The internal pressure p of explosive gas and in-situ stress (σx , σy ) are linearly superimposed on the boundary to control the stress intensity factor at the crack tip. The range of the fracture zone decreases as in-situ stress increases. When the stress level is less than 20 MPa, the change in the crack growth length is obvious with the increase of in-situ stress. When the stress level is greater than 20 MPa, the change degree of crack growth length is relatively small as in-situ stress increases. This result is due to the nonlinear pressure of the gas escaping along the crack direction. The relationship between crack growth length and in-situ stress is nonlinear and thus makes the stress intensity factor at the crack tip reach the critical threshold.

A new evaluation index of rockburst tendency considering tunnel excavation and fracture distribution

Rockburst risk and tendency analysis of fractured rock mass after excavation and unloading has been gaining importance. Within the framework of a constitutive model based on internal variable thermodynamics, the energy dissipation rate is proposed as a new evaluation index of rockburst tendency. Taking the numerical simulation of a traffic tunnel in western China as an example to carry out the rockburst tendency analysis, the models with parallel fractures of different dip angles were established in FLAC3D by Monte-Carlo method and MATLAB programming. Combined with the estimated in-situ stress of the deep-buried tunnel and the inversion results of constitutive parameters of rock mass, the influence of fracture dip angles on rockburst tendency of the surrounding rock is analyzed. The analysis results under different calculation conditions show that with the increase of fracture dip angle, the possible location of rockburst gradually transfers from the vault to the left arch waist. In addition, the comparison with the stress index verifies that the energy dissipation rate index can effectively evaluate the potential dangerous areas and rockburst positions after excavation and unloading of fractured rock mass, which has a reference value for the rockburst risk assessment in deep-buried tunnels.

Multisource monitoring and early warning system of rock burst in the Gaoloushan deep-buried tunnel

The Gaoloushan tunnel is a two-hole, deep-buried, extra-long rocky cross-ridge tunnel with a maximum depth of approximately 1,680 m. Due to the dual effects of tectonic stress and self-weight stress field, 90% of the tunnel body is subject to extremely high-stress and high-stress, and strong-moderate rock burst likely occurs in hard rocks, such as metamorphic sandstone and schist. To prevent human casualties and equipment damage caused by rock burst, this article investigates the layout of the multisource monitoring and early warning system for disasters in the Gaoloushan deep-buried tunnel. Early warning methods for potential destructive disasters and other ground pressure phenomena are also proposed by adopting important monitoring methods, such as microseismic monitoring, electromagnetic radiation monitoring, ground sound monitoring, surrounding rock stress monitoring, and acoustic emission monitoring. Therefore, this study has important significance and engineering application value for the prevention and control of rock burst disasters in deep-buried long tunnels.

Ground settlement and fault deformation response to deep-buried tunnel excavation and groundwater drawdown

The groundwater drawdown caused by tunnel excavation may induce large-scale deformation, including uneven settlement deformation of rock mass and local deformation of faults, which poses a great risk to engineering safety, and requires the constraint of deformation stability. Based on the theory of deformation reinforcement and the effective stress principle, we study the nonuniform deformation and unbalanced force distribution of the rock mass and faults caused by the groundwater drawdown after the excavation and drainage of the deep tunnel. The results suggest that the groundwater level drawdown can increase the effective stress in a wide range, and significantly disturb the stress and deformation of the rock mass. It is noted that the fault crossing the tunnel is relatively dangerous under these disturbances, and deserves special attention. Moreover, the unbalanced force and the plastic zone can be transmitted to further distances along the fault, and reinforcement and support should be carried out in time. In addition, the greater the Biot coefficient can lead to the greater impact of groundwater drawdown on stress changes and deformation of the rock mass and faults.

Effect of high temperature and confining pressure on the mechanical behavior of granite from Batang fault zone

The mechanical behavior of rocks under high temperature and confining pressure contributes to the design and stability analysis of deep-buried tunnels. In this study, a series of triaxial compression tests with high temperature and confining pressure was conducted on granite samples through a self-developed high temperature true triaxial test system. The granite samples were collected from the Batang fault zone located at the eastern of the Qinghai-Tibet Plateau. Based on the actual geological conditions, three temperatures (room temperature/70°C/110°C) and the confining pressure of 30 MPa were used. The stress–strain output, peak stress, and peak strain were summarized, and the relationship between the peak stress and temperature was obtained based on the experimental data. The results of the test show that high temperatures can weaken the mechanical properties of granite. The peak stress of the granite is decreased with the increase in temperature. A linear function can describe the strength behavior of the granite being studies at different temperatures. The experimental data obtained in this study can be used as basic parameters for the construction and stability evaluation of the deep-buried tunnels near the Batang fault zone.

An analysis of the support loads on composite lining of deep-buried tunnels based on the Hoek-Brown strength criterion

The composite lining load of a deep-buried tunnel is one of the key problems in tunnel support design. This study establishes four support cases to discuss the loads on primary support and secondary lining. A support load solution method is proposed based on the Hoek-Brown strength criterion by considering the stress release coefficient and the distal positions of bolts. According to engineering measurement, the displacements and loads of support structure calculated by elastoplastic theory are consistent with field measurement and numerical simulation which verifies the reliability of the theoretical method and studies the influence of support parameters on support load and bearing ratio. Results show that GSI, support time and support thickness have a great impact on support load, among which the combination of several factors may lead to the change of the main bearing structure. The higher the surrounding rock grade is, the higher the secondary lining bearing ratio will be. The support bearing ratio based on the Hoek-Brown strength criterion is consistent with field monitoring. The theoretical work done in this research can provide suggestions for engineering practice.

Key problems on hydro-engineering-environmental geology along the Sichuan-Tibet Railway corridor: Current status and development direction

The Sichuan-Tibet Railway is a century project under construction. The complex geological evolution history has led to large differences in geo-environmental conditions along the railway. Hydrogeological, engineering geological, and environmental geological problems are complex and changeable, which deserves great attention in engineering construction and future operations. Based on a brief review of the past geological work on the Sichuan-Tibet Railway corridor, the hydro-engineering-environmental geological problems that may be faced during the railway construction have been described. The hydrogeological problems include 1) high-pressure water inrush from plateau tectonic karst, 2) high-pressure water inrush and mud outburst from faulted bedrock fissures, 3) high-temperature hot water thermal damage. The engineering geological problems include 1) active faults and strong earthquake disasters, 2) high in-situ stress and rock burst or large deformation in deep tunnel, 3) problem rocks or soils and their disaster effects, 4) high-position landslide disaster chain. The environmental geological problems include 1) acidic water corrosiveness from coal-bearing strata and hydrothermal deposits, 2) wetland ecological degradation, 3) interaction effects between the railway construction and the sensitive ecological environment. In the meanwhile, key scientific and technological issues, which should be deeply studied in the future, are put forward. In terms of hydrogeology, such as 1) sequence law of plateau karst development and types of tectonic karst water storage structures, 2) disaster-generating models and prediction methods of water and mud bursts in deep-buried tunnels, 3) active fault control mechanisms and geothermal water circulation models, 4) identification of high-temperature heat hazard risk and geothermal resources utilization technology. In terms of engineering geology and geohazard, such as 1) investigation of fine features of active faults and related engineering fault effects, 2) development features of deep tectonic stress fields in complex geological structures, 3) engineering geological properties of tectonic melange belts and related disaster effects, 4) surrounding rock stability and disaster effects of deep-buried tunnel under water-mechanical-thermal multi-field coupling condition, 5) mechanisms of high-position run-out landslide and risk control technology under internal and external dynamic coupling. In terms of environmental geology, such as 1) mechanisms of plateau multi-source water conversion cycle and the technology of ecological water demand control in the ecologically fragile area, 2) hydro-ecological environmental effect of tunnel construction, 3) key technology of ecological geological environment monitoring, evaluation and protection, 4) geo-ecological environmental effect of global climate warming, etc. From the view of combining non-profit geological surveys and commercial engineering surveys, a response strategy has been proposed that geological surveys are the foundation, scientific and technological research is the key, and the monitoring of potential hazards and engineering control should be promoted, which provides suggestions on developing direction for hydro-engineering-environmental geology work on major project planning areas.

Treatment technology for large deformation of tunnel passing through soft and fragmented zone

Large deformation is a major problem for tunnels, threatening engineering safety and increasing construction costs. In order to solve the large deformation problem of deep-buried tunnels passing through the soft and fragmented zone, relying on the Dafalang tunnel project under construction, the mechanism of the large deformation of the fracture zone tunnel is analyzed, and deformation control measures suitable for geological conditions are proposed accordingly. The large deformation of the Dafalang tunnel is the combined effect of many factors. The complex tectonic stress environment is the mechanical condition for the large deformation of the tunnel. The weak and broken surrounding rock is the material basis and internal condition of the large deformation of the tunnel. Optimizing construction parameters, grading deformation control, and combined length and short anchor rods are adopted to effectively reduce tunnel deformation and ensure construction safety and progress.

Study on the Geo-Stress Loading and Excavation Unloading Devices of the Large-Scale Photoelastic Model Test for Deep-Buried Tunnels

At present, theoretical analysis, numerical simulation, and other methods cannot be used to properly solve the problems associated with the stability and bearing capacity of the surrounding rock and its supporting system, the interaction between the supporting structure and surrounding rock, and the sharing role of each supporting structure, all of which commonly occur in deep tunnels. The model test method represented by the photoelastic test is still an important approach to study this kind of problem. In view of the deficiency of the current loading system of the photoelastic model test, we developed a geo-stress loading system for the photoelastic model test, which can simulate the in situ geo-stress environment of unidirectional loading, bidirectional equal pressure, bidirectional unequal pressure, and tridirectional unequal pressure. The universal retaining force loading rod can realize the stability and effective compensation of loading, which is an original design. According to the principle of umbrella-shaped expansion and contraction mechanism, an excavation unloading device for the photoelastic model test is developed, which can realize the simulation of various degrees of displacement release in the excavation process of deep tunnels and other underground projects. The loading simulation test and excavation unloading simulation test show that the geo-stress loading system and excavation unloading device developed in this paper are flexible, exhibit good performance, and can fully achieve their respective test functions. The combination of two devices can compensate for the insufficiency of the current photoelastic model test and will promote the application of photoelastic model tests in underground engineering applications such as deep tunnel projects.

Analysis and Research on Blasting Network Delay of Deep-Buried Diversion Tunnel Crossing Fault Zone Based on EP-CEEMDAN-INHT

The Empirical Mode Decomposition (EMD) of blasting seismic wave monitoring signal with noise can get IMFs with serious modal confusion and endpoint divergence. The Hilbert transform is constrained by the Bedrosian theorem, when it dealing with such IMFs will get negative instantaneous frequency, which leads to serious error in the identification of non-electric millisecond detonator initiation delay. EP-CEEMDAN-INHT is proposed and applied to the delay analysis of blasting network of deep buried diversion tunnel crossing fault zone. Comparing EP-CEEMDAN-INHT with EMD-HHT, it is found that EP-CEEMDAN-INHT can clearly display the time–frequency information contained in the measured blasting vibration signal, and EMD modal confusion and endpoint effect are well suppressed. The actual millisecond time interval obtained by EP-CEEMDAN-INHT can judge whether the detonator is in normal service. At the same time, the blasting millisecond interval with the best damping effect is 54.51–59.75 ms, which can realize the optimization of blasting network and has important practical significance for blasting safety control.

Shaking Table Test on the Tunnel Dynamic Response under Different Fault Dip Angles

Fault-crossing tunnels are often severely damaged under seismic dynamics. Study of the dynamic response characteristics of tunnels crossing faults is thus of great engineering significance. Here, the Xianglushan Tunnel of the Central Yunnan Water Diversion Project was studied. A shaking table experimental device was used, and four sets of dynamic model tests of deep-buried tunnels with different fault inclination angles were conducted. Test schemes of model similarity ratio, similar material selection, model box design, and sine wave loading were introduced. The acceleration and strain data of the tunnel lining were monitored. Analysis of the acceleration data showed that when the input PGA was 0.6 g, compared with the ordinary tunnel, the acceleration increases by 117% when the inclination angle was 75°, 127% when the inclination angle was 45°, and 144% when the inclination angle was 30°. This indicates that the dynamic response of the cross-fault tunnel structure was stronger than that of the ordinary tunnel, and the effect was more obvious as the fault dip angle decreased. Analysis of the strain data showed that the strain response of the fault-crossing tunnels was more sensitive to the fault dip. The peak strain and increase in fault-crossing tunnels were much larger than those of ordinary tunnels, and smaller fault dips led to larger increases in the strain peak; consequently, the tunnel would reach the ultimate strain and break down when the input PGA was smaller. Generally, the influence of fault inclination on the dynamic response of the tunnel lining should receive increased consideration in the seismic design of tunnels.